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NEUR3905 > Weeks 4 to 6 > Flashcards

Weeks 4 to 6 Flashcards

1
Q

What is the purpose of the vestibular system?

A

o Provides sense of balance with respect to placement of head in space, and designed to sense motions that arise from head movements as well as the inertial effects due to gravity
oEssential for co-ordination of motor responses, eye movements and posture

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2
Q

Is the vestibular system mostly conscious or unconscious?

A

o Generally unconscious

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3
Q

What is the general location of vestibular sensory receptors and what do they do?

A

o Sensory receptors lie in vestibular labyrinth in inner ear and convey information to primary vestibular sensory neurons

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4
Q

Describe where the primary vestibular sensory neurons relay information and where they synapse/ reach and what comes of this signal transmission

A

o Primary vestibular sensory neurons relay info via vestibulocochlear nerve (CNVIII) to brainstem and cerebellum where they synapse with projection neurons responsible for eye movements and posture
o Vestibular signals also reach thalamus and cortex, where, with convergence of visual and proprioceptive info, a sense of head position in space is constructed

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5
Q

What is the result of unnatural movements of the head and why?

A

o Unnatural movements of head, produce a conflict between visual and vestibular input which leads to disorientation and nausea (reticular formation)

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6
Q

What are the main structures of the vestibular system?

A

 Semicircular canals
• Receptive organ: Cristae
 Otolith organs
• Receptive organ: Maculae
o Vestibular sensory receptors (vestibular hair cells)
o Vestibular primary sensory neurons, ganglion and nerve
o Vestibular nuclei in brainstem

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7
Q

What are the 4 projection pathways of the vestibular system?

A

o Vestibulo-cerebellar projections
o Vestibulo-spinal projections
o Vestibulo-ocular projections
o Vestibulo-thalamo-cortical projections

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8
Q

What are the 2 vestibular reflexes?

A

o Vestibulo-spinal reflex

o Vestibulo-occular reflex

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9
Q

What two parts is the vestibular labyrinth made of?

A

o Bony labyrinth

o Membranous labyrinth

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10
Q

What is the bony labyrinth of the vestibular labyrinth made of and filled with?

A

 Convoluted hard walled cave of canals constructed by the skull within petrous portion of temporal bone. Filled with perilymph (low potassium/high sodium, similar to CSF)

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11
Q

By what is the perilymph in the bony labyrinth of the vestibular labyrinth made?

A

• Perilymph secreted by arterioles in periosteum surrounding labyrinth

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12
Q

Where does the perilymph from the bony labyrinth in the vestibular system drain into?

A

o Drains into subarachnoid space via perilymphatic duct which runs through cochlear aqueduct in temporal bone

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13
Q

What is the mebranous labyrinth of the vestibular labyrinth made of and filled with?

A

 Convoluted delicate walled sac of ducts floating in perilymph and following shape of bony labyrinth canals. Filled with endolymph (high potassium/low sodium, similar to intracellular fluid)

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14
Q

By what is the endolymph in the membranous labyrinth of the vestibular labyrinth made?

A

• Endolymph is secreted by tissue in cochear duct

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15
Q

Where does the endolymph from the membranous labyrinth in the vestibular system drain into?

A

o Drains into an extradural sac via endolymphatic duct which runs through vestibular aqueduct in temporal bone

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16
Q

Is it important for the perilymph and endolymph to pressure to be balanced in the vestibular system? Why/why not?

A

• Pressure balances in perilymph and endolymph important for functioning of vestibular system. Excess pressure causes vestibular disturbance: Meniere’s disease/labyrinthitis

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17
Q

What are the sensory organ components in vestibular labyrinths?

A

• Sensory organ components in vestibular labyrinths
o 1 vestibular labyrinth on each side of the head
o In each inner ear there are:
 Semicircular canals
 Otolith organs

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18
Q

How many semicircular ducts are in the semicircular canals? What are they?

A

3
o Anterior semicircular canal
o Lateral semicircular canal
o Posterior semicircular canal

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19
Q

What is the general role of semicircular canals?

A

Involved in detecting rotation
• Respond to angular accelerations (rotations) of the head and are maximally sensitive to rotational motions that lie in the plane of the canal

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20
Q

What is the role of the anterior semicircular canal? Give examples

A

 Detects rotations of head in sagittal plane
 Nodding head forward and back as in ‘yes’
 Doing a somersault

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21
Q

What is the role of the lateral semicircular canal? Give examples

A

o Lateral semicircular canal-
 Detects rotations of head in transverse plane
 Turning head side to side as in ‘no’
 Doing a pirouette

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22
Q

What is the role of the posterior semicircular canal? Give examples

A

 Detects rotations of head in coronal plane
 Tilt head towards shoulder as in ‘maybe’
 Doing a cartwheel

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23
Q

What is the structure of semicircular canals and what are they filled with? What do they contain?

A
  • Endolymph-filled ring-like structures

* Contain sensory receptors that detect endolymph movement

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24
Q

How are semicircular canals oriented? What is the benefit of this?

A

• Oriented orthogonally

o At right angles to each other- can detect every plane of movement in the rotational accelerations

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25
Q

How are the different semicircular canals organised in regards to each other? What is the benfit of this?

A

• Organised into 3 functional pairs (based on orientation)
o One half of each pair on each side of the head allow for detection of which way movement is occuring
 Left anterior and right posterior
 Left lateral and right lateral
 Left posterior and right anterior

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26
Q

Where are the sensory receptors in semicircular canals located?

A

• Sensory receptors in semicircular canals are located in the ampullae
o Dilated ends of each canal near their attachment to utricle inside vestibule

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27
Q

Describe exactly where the endolymph is located/directed in the semicircular canal

A

• Endolymph in semicircular canals is continuous with endolymph in utricle but at end where the ampulla is, endolymph is partitioned off by flexible septum of receptive tissue called the crista ampullaris

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28
Q

What is the crista ampullaris and what is it covered by?

A

flexible septum of receptive tissue

o Crista ampullaris covered by neuroepithelium made of sensory hair cells and supporting cells

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29
Q

Where does the ridge of crista ampullaris project into?

A

o Ridge of crista ampullaris projects into lumen of ampulla of semicircular canal and is bathed in endolymph

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30
Q

Where are the sensory hair cells of the cilia embedded in?

A

 Sensory hair cells’ cilia embedded in gelatinous mass called the cupula

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31
Q

Describe the role of the cupula

A

 Hair cells are either excited or inhibited when cupula sways in endolymph and moves in respect to neuroepithelium

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32
Q

Describe what happens to the semicircular canals during head rotation and why this head rotation is percieved

A

o At the beginning of head rotation within the plane of a semicircular canal, endolymph is subjected to inertia and lags behind in canal and so endolymph temporarily moves in opposite direction to head rotation
o Cupula has same specific gravity as endolymph and so ‘sways’ in same direction as endolymph as it experiences inertia
o Swaying cupula effectively bends the sensory hair cells embedded in it in the opposite direction to the direction of head rotation – this is what makes you sense you are turning
o Eventually, the endolymph/cupula will catch up and moving sensation will no longer be felt as moving direction and endolymph/cupula are in line

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33
Q

How many otolith organs are there in the vestibular system? What are they?

A

2
o Utricle
o Saccule

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34
Q

What is the function of the utricle? Where is its receptive tissue patch positioned?

A

 Detects tilt/linear movements of head in transverse plane as receptors are oriented in such manner
 Forward and back movement
 Left and right movement
 Receptive tissue patch positioned horizontally on the floor

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35
Q

What is the function of the saccule? Where is its receptive tissue patch positioned?

A

 Detects linear movements of head in sagittal plane
 Up and down movement
 Receptive tissue patch positioned vertically on medial wall

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36
Q

What are the otolith organs and what do they contain?

A
  • Endolymph-filled sac-like structures

* Contain sensory receptors that detect movements dependent on gravity

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37
Q

What is the role of otolith organs?

A

• Respond to changes in angle (tilt) and linear movements of the head and are maximally sensitive to straight line changes in acceleration and direction

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38
Q

What are sensory receptors in otolith organs maximally sensitive to?

A

• Sensory receptors in otolith organs are maximally sensitive to changes in linear accelerations that occur in the plane along which receptive tissue is oriented

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39
Q

Where are receptors embedded in otolith organs?

A

• Receptors are embedded in receptive tissue (neuroepithelial) patches which are positioned on either the floor or the wall of the otolith organs

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40
Q

What are maculae?

A

Maculae- Comma shaped receptive tissue patches of neuroepithelial tissue in otolith organs

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41
Q

What is the role of maculae utriculi and where is it located?

A

 One located on floor of utricle (macula utriculi)

• Wobbles in transverse plane

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42
Q

What is the role of maculae sacculi and where is it located?

A

 One located on wall of saccule (macula sacculi)

• Wobbles in sagittal plane

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43
Q

Are maculae bathed and moved by endolymph as cristae are? Why/why not?

A

 Bathed but not moved in endolymph
 Although bathed in same endolymph that circulates in semicircular canals, unlike the cristae, maculae are heavier than endolymph due to the presence of calcium crystals on their surface and so are more responsive to the pull of gravity rather than changes in endolymph current

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44
Q

What are maculae composed of?

A

 Composed of neuroepithelium made of sensory hair cells and supporting cells

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45
Q

Where are the hair cells in the maculae found?

A

• Sensory hair cells’ cilia embedded in gelatinous mass called the otolithic membrane and this is studded with ‘weighty’ calcium carbonate otoliths

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46
Q

Describe what happens in the otolith organs if head is tilted in the transverse plane and why this head rotation is perceived

A

o If head is tilted in the transverse plane (wobbles macula utriculi horizontally), the heavy gelatinous otolithic membrane is subject to gravity and flops towards the direction of the tilt

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47
Q

Describe what happens in the otolith organs if head is subjected to linear acceleration and why this head rotation is perceived

A

o If head is subjected to linear acceleration (that is, acceleration in sagittal plane= wobbles macula sacculi vertically), the heavy gelatinous otolithic membrane is subjected to inertia and lags behind in otolith organ

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48
Q

Describe the relationship between the direction of the sensory hair cells during a head tilt in contrast to during linear acceleration

A

o Upon head tilt or linear acceleration, it’s flop and lag characteristics effectively bend sensory hair cells embedded in it in same direction as tilt, and in opposite direction to direction of acceleration

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49
Q

Where are the sensory vestibular hair cells located in the vestibular system?

A

• Sensory hair cells (vestibular hair cells) are accumulated in the ridge of the cristae within the semicircular canals and in the patches of the maculae within the otolith organs.

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50
Q

What are 2 types of sensory hair cells? Describe:

  • Shape
  • Relationship to neurons around it
A

o Type 1:
 Flask shaped
 Completely surrounded by receptive end of bipolar primary sensory neurons
o Type 2:
 Cylinder shaped
 Contacted directly (no encasement) by receptive end of bipolar primary sensory neurons and motor neurons
 Can be modulated

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51
Q

What are sensory hair cells in the vestibular system surrounded by and accumulated in?

A

• Sensory hair cells are surrounded by supporting cells and accumulated in sheets of neuroepithelium (cristae or maculae)

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52
Q

Describe the structure of a vestibular sensory hair cell

A
  • Both types have hair tuft made of 30-50 stereocilia protruding from surface closest to endolymph
  • Vestibular hair cells are similar to auditory hair cells except that within each hair tuft of stereocilia there is also a single, longer, kinocilium projecting on only one side of the cell
  • The stereocilia and kinocilium in the hair tuft of a single hair cell all connect via linkages and are embedded in the gelatinous mass that makes up the cupula (semicircular canals) or otolithic membrane (otolith organs)
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53
Q

What can displace a vestibular hair cell?

A

• Movement of gelatinous mass either in response to endolymph flow (semicircular canals) or gravitational pull (otolith organs), bends/displaces the hair tuft

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54
Q

What determines whether or not sensory hair cells release neurotransmitter that stimulates the receptive ends of bipolar primary (vestibular) sensory neurons

A

• Displacing of hair tuft towards or away from the kinocilium determines whether or not the sensory hair cells release neurotransmitter that stimulates the receptive ends of bipolar primary (vestibular) sensory neurons

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55
Q

What is the result of displacement of hair tuft in a vestibular sensory neuron towards kinocilium?

A

 Displacement of hair tuft towards kinocilium excites hair cell, increases neurotransmitter release, resulting in increased firing rate of bipolar primary sensory neurons contacting hair cell

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56
Q

What is the result of displacement of hair tuft in a vestibular sensory neuron away from kinocilium?

A

 Displacement of the hair tuft away from the kinocilium inhibits hair cell, decreases amount of neurotransmitter release, resulting in a decreased firing rate of bipolar primary sensory neurons contacting hair cell

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57
Q

What type of acceleration are vestibular hair cells in cristae of semicircular canals designed to detect?

A

o Vestibular hair cells in cristae of semicircular canals are designed to detect rotational acceleration

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58
Q

In how many orientations are hair cells arranged over an entire crista?

A

o Over the entire crista, hair cells are arranged in a single orientation

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59
Q

What kind of signal will endolymph flow from the ampulla into the utricle cause?

A

o Endolymph flow from the ampulla into the utricle will cause depolarisation (excitation) of the hair cells (bending hair tuft towards kinocilium).

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60
Q

What kind of signal will endolymph flow from the utricle into the ampulla cause?

A

Endolymph flow from the utricle into the ampulla will cause hyperpolarisation (inhibition) of the hair cells (bending hair tuft away form kinocilium)

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61
Q

What is the push-pull concept of vestibular function?

A

o Semicircular canals come in function pair- kinocilia on crista in each half of the pair are arranged in opposite orientation to the other. Any rotational head movement affects each half of the functional pair in an opposite manner

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62
Q

What kind of acceleration are vestibular hair cells in the maculae designed to detect?

A

o Vestibular hair cells in maculae of otolith organs are designed to detect linear acceleration

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63
Q

Over a maculae, how many orientations are hair cells arranged in and how are they arranged so? What is the advantage of this?

A

o Over the maculae, hair cells are arranged with kinocilium placement in 2 different orientations depending on the side of the macula they are located on in relation to a central dividing line called the striola
o Striola curves through macula, so kinocilia are arranged in many different orientations and maculae capable of detecting many different head movements
 Tilt head to one side- some hair cells will be excited, other will be inhibited on the same macula

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64
Q

On which side is the kinocilia hair cell located in macula utriculi?

A

 Macula utriculi- kinocilia located on side of hair cell towards striola

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65
Q

On which side is the kinocilia hair cell located in macula sacculi?

A

 Macula sacculi- kinocilia located on side of hair cell away from striola

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66
Q

Starting from excitation/ inhibition of hair cells in the vestibular system, describe the vestibular system pathway

A

• Excitation or inhibition of hair cells in the cristae or maculae depolarises or hyperpolarises bipolar primary vestibular sensory neurons
• Axons of bipolar primary vestibular sensory neurons project towards the brain in the superior and inferior branches of the vestibular nerve
• Within the internal auditory meatus, and proximal to the vestibular ganglion, the superior and inferior branches of the vestibular nerve merge with the cochlear nerve to form the vestibulocochlear nerve (CNVIII)
• The merged vestibulocochlear nerve enters the anterior-lateral surface of the brain stem at the ponto-medullary junction
• Upon entering brainstem, central processes of primary vestibular sensory neurons travel posteriorly towards an area just beneath the floor of the 4th ventricle called the vestibular area (vestibular nuclear complex) containing vestibular nuclei
• The majority of primary vestibular sensory neurons terminate in ipsilateral vestibular nuclei and synapse with projection neurons which give rise to a number of vestibular pathways (and reflexes).
• Projections from vestibular nuclei go to
o The cerebellum (also direction connection from primary vestibular sensory neurons)
o The spinal cord
o The brainstem nuclei controlling eye movements (oculomotor, trochlear, abducens)
o The thalamus (and onto the cortex for conscious perception)

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67
Q

What is the vestibular area divided into?

A

o Vestibular area divided into 4 major groups of cell bodies called the vestibular nuclei

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68
Q

What are the 4 vestibular nuclei and where do they get their information from?

A

 Superior vestibular nucleus
 Lateral vestibular nucleus
 Inferior vestibular nucleus
 Medial vestibular nucleus

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69
Q

What is the role of vestibular nuclei?

A

o Vestibular nuclei are the main location at which information from semicircular canals and otolith organs regarding position and movement is processed

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70
Q

Where does the lateral vestibular nucleus get its information from?

A
  • Gets info from macula in utricle

* Gets info from macula in saccule

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71
Q

Where does the superior vestibular nucleus get its information from?

A

• Gets info from crista in semicircular canals

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72
Q

Where does the inferior vestibular nucleus get its information from?

A

• Gets info from macula in saccule

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73
Q

Where does the medial vestibular nucleus get its information from?

A

• Gets info from crista in semicircular canals

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74
Q

What is the vestibulo-cerebellar pathway and what is the purpose of the pathway?

A

-From vestibular nuclei to the cerebellum
 Vestibulo-cerebellar pathway
• Involved in regulatory control of eye movements and head movements, and modulation and coordination of muscle activity for maintaining basic tone and posture

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75
Q

Describe the lateral vestibulo-spinal pathway. What is its function?

A
  • Involved in the Vestibulo-spinal reflex which is the principle route by which the vestibular system brings about postural changes to compensate for tilts and movements of body
  • Responsible for stabilizing body’s center of gravity and preserving upright posture
  • Arises from projection neurons synapsing with primary vestibular sensory axons in the lateral vestibular and inferior vestibular nuclei.
  • Topographically organised and projects to all levels of the ipsilateral spinal cord
  • Projection neurons course caudally through lateral medulla and then through anterior funiculus of spinal cord and give off collaterals in different spinal cord segments.
  • Collaterals terminate directly on alpha and gamma motor neurons and interneurons in the cord ensuring different muscle groups (antigravity/ extensor leg muscles in particular) will be co-ordinated for postural control
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76
Q

What is the function of the cerebellum in the vestibular system?

A

 Fine tune muscle movements of head, eyes and those responsible for posture and basic tone either via direct primary afferent synapse or second order projection

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77
Q

What is the function of the spinal cord in the vestibular system?

A

 Allows quick reactions of extensor muscles of limbs and trunk necessary for maintaining balance. Influence muscle tone and postural adjustments of head and body.

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78
Q

What is the medial vestibulo-spinal pathway and its function?

A

 Medial vestibulo-spinal pathway
• Involved in the vestibulo-collic reflex which triggers an upward and backward neck movement, when the body falls forward, in an effort to protect head from impact
• Arises primarily from projection neurons synapsing with primary vestibular sensory axons in the medial vestibular nucleus. Projects to cervical spinal cord.
• Projection neurons course bilaterally through medial longitudinal fasciculus to terminate on both sides of the cervical spinal cord and specifically on neck flexor and extensor motor neurons as well as on propriospinal interneurons
• Responsible for controlling neck muscles that stabilise head whilst moving head in space, co-ordinate head movements with eye movements and serve to protect head and neck in situations of imbalance

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79
Q

What is the vestibulo-spinal reflex and its function?

A
  • Serves to alter muscle tone in neck, trunk and limb muscles and to change the position of the limbs and head with the goal of supporting posture, maintaining visual focus and maintaining balance with respect to centre of gravity
  • If body tilts to right, muscles contract to left and vice versa
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80
Q

What is the function of brainstem nuclei controlling eye movements in the vestibular pathway?

A

 Allows eyes to fix on movement object while staying in focus-

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81
Q

Describe the vestibulo-ocular pathway and its function

A

 Vestibulo-ocular pathway
• Involved in the vestibulo-ocular reflex which brings about yoked eye movements to compensate for movements of the head and neck. Responsible for maintaining eye fixation during head movement, preserving visual focus
• Arises primarily from second order neurons synapsing with primary vestibular sensory axons in the medial vestibular and lateral vestibular nuclei
• Projection neurons course rostrally through the medial longitudinal fasciculus to terminate in the brainstem nuclei of the extraocular muscles (VI, IV and III)
• Motor neurons from brainstem nuclei project to extraocular muscles to either activate or inhibit their contraction in pairs so that the eyes can move together te or inhibit their contraction in pairs so that the eyes can move together

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82
Q

What is the vestibulo-ocular reflex and its function?

A

• Serves to stabilise mages on retina during head movement by producing yoked and compensatory eye movements equal in magnitude and opposite in direction to the head movement perceived by the vestibular system
o If head moves to one direction, eyes move the other direction whilst they are focused

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83
Q

What is the role of the thalamus in the vestibular system?

A

o The thalamus (and onto the cortex for conscious perception)
 Allows for head and body motor control and are responsible for conscious awareness of body position

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84
Q

Describe the vestibulo-thalamo-cortical-pathway. What is its role?

A

 Vestibulo-thalamo-cortical-pathway
• Involved in cognitive perceptions of motion and spatial orientation through convergence of information from vestibular, visual and proprioceptive systems
• Arises from projection neurons synapsing with primary vestibular sensory axons in the superior, lateral, medial and inferior vestibular nuclei
• Second order neurons course rostrally and bilaterally to ventral posterior lateral nucleus and the posterior nuclear group of the thalamus
• Third order neurons project from thalamus to areas of cortex: base intraparietal sulcus (area 2v) posterior to somatosensory cortex and base of central sulcus (area 3a) adjacent to motor cortex

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85
Q

What is blood fMRI? Which relaxation state does it use and why?

A

• Most common form of fMRI and relies on the finding that neuronal activity is associated with changes in regional blood flow
• T1 relaxation- not related to changes in haemoglobin
• T2 relaxation- affected to changes in haemoglobin
o Magnetic susceptibility of deoxygenated haemoglobin is about 20% greater than haemoglobin
o Magnetic susceptibility of deoxygenated haemoglobin affects rate of T2 relaxation

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86
Q

What do the magnetic properties of haemoglobin depend on?

A

 Magnetic properties of a blood cell (haemoglobin) depends on whether it has an oxygen molecule
• With oxygen-> zero magnetic moment
• Without oxygen-> sizeable magnetic moment (paramagnetic)

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87
Q

Does fMRIs use absolute or relative measures? What are the implications of this?

A

o fMRI is a relative measure so you need to compare signal changes relative to a baseline period during the same scan
 Can only be used to measure evoked activity

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88
Q

What are the principles behind fMRI?

A

o When blood vessel is full of deoxyhaemoglobin, disrupts magnetic field
o When blood vessel is full of oxygenated haemoglobin, there is no disruption of magnetic field
o In a resting neuron, the amount of oxygenated haemoglobin entering the capillary bed is about equal as the amount of deoxygenated haemoglobin leaving the capillary bed
o In an activated neuron, there is oversupply of oxygenated haemoglobin and hence leaving capillary body has more oxygenated haemoglobin than it would have at rest
 Oversupply results in less overall distortion of magnetic field-> measured as an increase in blood oxygen level dependent signal
o However, increased blood flow to meet oxygen demands in activated neurons slightly delayed (4-6 seconds)

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89
Q

How would you perform a BOLD fMRI experiment?

A

• Performing an experiment with BOLD fMRI
o Place subject in fMRI scanner
o Perform experiment
o Place model into software and the analysis will reveal which voxels in the brain have signal intensity changes that match the input model- also take slight delay into account

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90
Q

What are the advantages of fMRI?

A
  • Good spatial resolution
  • Good temporal resolution
  • Non-invasive, not requiring injection of radioactive materials like PET. Subject can be repeatedly scanned
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91
Q

What are the disadvantages of fMRI?

A
  • Noisy
  • Susceptible to motion artefacts
  • Areas near bone tissue interfaces are susceptible to artefacts
  • Metal implants can be dangerous
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92
Q

What is the general role of the proprioceptive pathway?

A

o Provides sense of body position in space
o Proprioception monitors muscle length and tension, joint angle position and associated movement of musculoskeletal system. Responsible for subconscious coordination and finesse of motor responses, balance and posture

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93
Q

In which tract does conscious proprioception occur?

A

o Conscious proprioception via lemniscal system (dorsal column tract)

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94
Q

In which tract does subconscious proprioception occur?

A

o Subconscious proprioception via spinocerebellar system

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95
Q

Where are the proprioceptors located and how do they convey information?

A

o Sensory receptors- proprioceptors- lie in muscles, tendons, ligaments and connective tissue coverings of bond and muscle and convey information via primary sensory/primary afferent/first order sensory neuron axons (type Ia and type Ib)

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96
Q

What do proprioceptive primary afferents from muscles and tendons relay information through and to?

A

o Proprioceptive primary afferents from muscles and tendons relay info via dorsal root through dorsal root ganglion and into spinal cord where they synapse with second order neurons at various levels of the neuraxis which then project to the ipsilateral cerebellum via spinocerebellar tracts

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97
Q

What does damage to spinocerebellar tracts result in?

A

o Damage to spinocerebellar tracts result in lack of coordination during walking and moving-ataxia

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98
Q

What is exteroception?

A

 Exteroception- sensations originating outside the body- touch, pressure, temperature

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99
Q

What is interoception?

A

 Interoception- sensations originating inside the body- visceral movement, vessel expansion

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100
Q

How are sensory receptors responsible for exteroception, interoception and proprioception classified?

A

o Sensory receptors responsible for exteroception, interoception and proprioception have been classified on the basis of their location and the type of stimulus that activates them

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101
Q

Give examples of exteroreceptors

A
  • Free nerve endings of sensory neurons
  • Modified free nerve endings: tactile (merkel) discs
  • Hair follicle receptors
  • Tactile (Meissner’s) corpuscles
  • Lamellar (Pacinian) corpuscles
  • Bulbous corpuscles (Ruffini endings)
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102
Q

Give examples of interoreceptors

A
  • Free nerve endings of sensory neurons
  • Lamellar (Pacinian) corpuscles
  • Bulbous corpuscles (Ruffini endings)
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103
Q

Give examples of proprioceptors

A
  • Free nerve endings of sensory neurons
  • Lamellar (Pacinian) corpuscles
  • Bulbous corpuscles (ruffini endings)
  • Muscle spindles
  • Tendon organs
  • Joint Kinesthetic receptors
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104
Q

What is the role of muscle spindles?

A

Detect muscle stretch

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105
Q

What is the role of golgi tendon organs?

A

Detect muscle tension

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106
Q

What are the 4 proprioceptive projection pathways?

A

o Dorsal (posterior) spinocerebellar tract
o Ventral (anterior) spinocerebellar tract
o Cuneo-cerebellar tract
o Rostral spinocerebellar tract

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107
Q

Describe A sensory axons in terms of:

  • Axons from muscles
  • Diameter (um)
  • Speed (m/sec)
  • Role
A
  • Axons from muscles: Ia and Ib
  • Diameter (um): 13-20
  • Speed (m/sec): 80-120
  • Role: Proprioceptors of skeletal muscle
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108
Q

Describe AB sensory axons in terms of:

  • Axons from muscles
  • Diameter (um)
  • Speed (m/sec)
  • Role
A
  • Axons from muscles: II
  • Diameter (um): 5-12
  • Speed (m/sec): 35-75
  • Role: Mechanoreceptors of skin
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109
Q

Describe ADelta sensory axons in terms of:

  • Axons from muscles
  • Diameter (um)
  • Speed (m/sec)
  • Role
A
  • Axons from muscles: III
  • Diameter (um): 1-5
  • Speed (m/sec): 5-30
  • Role: Pain, temperature
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110
Q

Describe C sensory axons in terms of:

  • Axons from muscles
  • Diameter (um)
  • Speed (m/sec)
  • Role
A
  • Axons from muscles: IV
  • Diameter (um): 0.2-1.5
  • Speed (m/sec): 0.5-2
  • Role: Temperature, pain,itch
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111
Q

What is required for skeletal muscles to perform effectively?

A

• For skeletal muscles to perform effectively, the brain must be continually informed of their state and they must be continually informed of their ‘state’ and they must have a healthy ‘tone’- be in an optimal position/length to either contract or relax

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112
Q

What kind of receptors are in the muscle spindle?

A

o Stretch receptors

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113
Q

How do the muscle spindles communicate?

A

o Communicate via type Ia axons

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114
Q

Where are muscle spindles found?

A

o Embedded in muscles amongst extrafusal muscle fibres

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115
Q

What are muscle spindles made of?

A

o Made up of connective tissue capsule containing specialised muscle fibres and various (sensory and motor) myelinated axons
 Muscle spindles consist of a capsule in which there are 8-10 specialised intrafusal muscle fibres lying parallel to, and attached at either end to, extrafusal skeletal muscle fibres

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116
Q

What are two types of intrafusal fibres in muscle spindles? Describe them

A

• Two types of intrafusal fibres
o Nuclear chain fibres which are narrow and have a single row of central nuclei
o Nuclear bag fibres which are wider and have a central cluster of nuclei at the central bag-like dilation

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117
Q

What are two types of afferents that arise from intrafusal fibres? Describe what they innervate and what they are activated by

A

o Annulospiral endings innervate the central part of both chain and bag fibres- wrapped around like a spiral
 Primary afferents (type Ia axons) which are activated by brief stretch or vibration of the muscle as well as by a sustained stretch on the muscle
 Tonically active, ensuring optimal muscle strength
o Flower-spray endings innervate the ends/poles of both chain and bag fibres, either side of the centrally positioned annulospiral endings
 Secondary afferents (type II axons) which are only activated when there is a sustained stretch on the muscle

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118
Q

Why is the muscle spindle stretched whenever the muscle is also stretched?

A

• Because intrafusal muscle fibres are attached at either end to the extrafusal muscle fibres, whenever the muscle stretches, it also stretches the muscle spindle

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119
Q

What happens when a muscle is stretched? Describe the whole pathway

A

o Muscle spindles detect and respond to both active and passive changes in muscle stretch (length)
 When a muscle is stretched, the annulospiral endings and flower-spray endings are stimulated and send action potentials long their primary afferent (sensory) axons to the spinal cord
 In the spinal cord, the primary afferent axons (type Ia axons) of annulospiral endings make direct synapses with alpha motor neurons which provide motor input to extrafusal muscle fibres
 Activation of alpha motor neurons causes contraction of extrafusal muscle fibres (shortening of the muscle)

120
Q

What do muscle spindles prevent?

A

o Prevent damage from excessive stretch

121
Q

What happens in the muscles when a weight is picked up and what adjustments are made to keep the muscle spindle responsive after such an action? Describe the role of both alpha motor neurons and gamma motor neurons

A

 Extrafusal and intrafusal muscle fibres are pulled downwards by the weight- the muscle is stretched
 Type Ia axons wrapped around intrafusal muscle fibres in muscle spindle sense the stretch and send action potentials into spinal cord
 Alpha motor neurons in spinal cord are excited by type Ia axons and trigger extrafusal muscle fibre contraction- the muscle contracts
 When a muscle is contracted, it shortens and this effectively reduces the length of the muscle spindle as well
 A shortened muscle spindle contains shortened intrafusal fibres resulting in slack and less responsive type Ia axons- so there is an inbuilt mechanism to keep the muscle spindle at an optimal length of for optimal type Ia axons responsiveness at all time
 Intrafusal muscle fibres also receive motor input from spinal gamma motor neurons originating in the spinal cord
 Gamma motor neurons regulate the intrafusal fibres sensitivity to stretch by triggering the two poles of the muscle spindle to contract to effectively tighten the centre portion
 This intrafusal fibre contraction keeps type Ia axons tight and responsive even when extrafusal fibres are contracted

122
Q

What is the role of gamma motor neurons?

A

 Gamma motor neurons- provide motor input to the poles of intrafusal fibres to keep them tight when extrafusal fibres are contracted

123
Q

What is the role of alpha motor neurons?

A

 Alpha motor neurons provide motor input to extrafusal muscle fibres, triggering muscle contraction/muscle shortening

124
Q

What sensors do golgi tendons have?

A

Tension sensors

125
Q

What axons do golgi tendon organs communicate via?

A

type Ib axons

126
Q

Where are golgi tendon organs found?

A

o Embedded in tendons amongst fascicles of collagen/connective tissue
 Located at junction between muscle and tendon

127
Q

What are golgi tendon organs made up of?

A

o Made up of large myelinated axons which split and intermingle with and encircle the collagenous fascicles
 Consist of a connective tissue capsule ensheathing a meshwork of collagen fibres, intermingled with and encircled by type Ib primary afferent axons

128
Q

What do golgi tendon organs prevent?

A

o Prevent damage from excessive contraction

129
Q

How does the mytoactic reflex occur?

A
  • When patellar tendon is tapped it pulls on quadriceps (extensor) muscle, stretching both extrafusal muscle fibres and intrafusal muscle fibres inside the muscle spindles
  • Type Ia axons wrapped around intrafusal muscle fibres in muscle spindle are activated by stretch and send action potentials to spinal cord
  • Alpha motor neurons in spinal cord are excited directly (monosynaptic) by type Ia axons and trigger extrafusal muscle fibre contraction in the stretched quadriceps
  • This, contraction of the quadriceps (in concert with relaxation of the hamstring (flexor) muscles due to inhibition of extrafusal muscle fibres of the hamstring (due to inhibitory interneuron involvement)) results in a forward kick of the lower leg and signifies that proprioceptive connections in the spinal cord are intact and functioning
130
Q

How does the inverse myotactic reflex occur?

A
  • Muscle contraction shortens muscle belly, increasing tension along the long axis of the associated muscle tendons
  • The increased tension in collagen bundles in the muscle tendon Golgi Tendon Organ squashes type Ib axons stimulating them to send action potentials to the spinal cord
  • In the spinal cord type Ib axons make an excitatory synapse with inhibitory interneurons which release inhibitory neurotransmitter in their synapse with alpha motor neurons
  • Inhibition of alpha motor neurons causes a dampening, or cessation all together, of extrafusal muscle fibre contraction in the muscle linked to the tensed tendon
131
Q

What happens as muscle tension increase and the role of the golgi tendon organ in this?

A

o As muscle tension increases, firing of type Ib axons in the Golgi Tendon Organ increases and inhibition of the alpha motor neuron increases such that it slows (decreases) muscle contraction. This can progress to the point where muscle contraction can be stopped

132
Q

What happens as muscle tension decreases and the role of the golgi tendon organ in this?

A

o As muscle tension decreases, firing of type Ib axons in the Golgi Tendon Organ decreases and inhibition of the alpha motor neuron also decreases, increasing muscle contraction

133
Q

To what and through what is information from muscle spindles and golgi tendon organs transmitted?

A

• Information from muscle spindles and golgi tendon organs is conveyed to the spinal cord via myelinated type Ia and type Ib axons respectively (proprioceptive primary afferents) within the mixed spinal nerve

134
Q

What do the proprioceptive pseudounipolar axons project towards and through what?

A
  • The pseudounipolar axons of proprioceptive primary afferents project towards the spinal cord in the dorsal root (motor neurons project out of the spinal cord in the ventral root)
  • The myelinated axons project into the spinal cord where they either synapse in various laminae (I-VII) of the dorsal horn or project to higher levels of the neuraxis without synapsing
135
Q

Where are the cell bodies of pseudounipolar proprioceptive primary afferents?

A

• The cell bodies of the pseudounipolar proprioceptive primary afferents are clustered in the dorsal root ganglion located just outside of the vertebral column in the intervertebral foramen

136
Q

On what laminae of the dorsal horn do nociceptive neurons synapse?

A

o Nociceptive synapses synapse on lamina II

137
Q

On what laminae of the dorsal horn do proprioceptive neurons synapse?

A

o Proprioceptive synapses synapse around lamina VII

138
Q

How can proprioception be improved?

A

• Proprioception can easily be improved with specific exercises that train muscles, tendons and joints to adjust more quickly to unstable movements

139
Q

How can proprioception be impaired?

A
  • Proprioception can just as easily be impaired by injury to muscles, tendons and joints which reduces stability and by alcohol consumption
  • Disease and infection can also damage proprioceptors
140
Q

What is Friedreich’s ataxia and how does it occur?

A

• Friedreich’s ataxia- spinocerebellar dysfunction
o The importance of proprioception becomes apparent when it is lost
o Friedreich’s ataxia is caused by atrophy of spinocerebellar tracts and hypoplasia of spinal cord and dorsal root ganglia
 Dorsal columns and Clarke’s nucleus also get damaged
o Characterised by cerebellar ataxia (un-coordinated walking, wide gait, disturbed balance) that occurs when cerebellum doesn’t receive sensory feedback to regulate movement

141
Q

Describe the conscious proprioception pathway

A
  • Information conveyed from type Ia and Ib axons of primary afferent proprioceptors in periphery to the spinal cord
  • Primary afferents ascend the spinal cord in dorsal column tracts to synapse with second order neurons in the dorsal column nuclei in the medulla
  • Second order neurons cross to the contralateral side of the medulla in medial lemniscus and project to synapse with third order neurons in the thalamus
  • Third order neurons project in thalamocortical tracts to terminate in the somatosensory cortex of the cerebrum that is contralateral to the proprioceptive input
142
Q

Describe the start of the subconscious proprioception pathway (until the second order neurons) and in what pathways these second order neurons project

A

• Information conveyed from type Ia and Ib axons of primary afferent proprioceptors in periphery to the spinal cord
• Primary afferents either ascend the spinal cord in a dorsal column tract and synapse with second order neurons in a dorsal column nucleus in the medulla- or do not ascend the spinal cord but synapse with second order neurons within the dorsal horn
• Ultimately, second order neurons project in various (4) spinocerebellar tracts to terminate in parts of the cerebellum that are ipsilateral to the proprioceptive input
o Cuneocerebellar tract
o Rostral spinocerebellar tract
o Dorsal spinocerebellar tract
o Ventral spinocerebellar tract

143
Q

What are the subconscious proprioception spinocerebellar pathways for the upper body (above T6)?

A
  • Cuneocerebellar tract

* Rostral spinocerebellar tract

144
Q

What are the subconscious proprioception spinocerebellar pathways for the lower body (below T6)?

A
  • Dorsal spinocerebellar tract

* Ventral spinocerebellar tract

145
Q

Describe the cuneocerebellar tract and its function

A

o Proprioceptive information from trunk and upper limb is conveyed from type Ia and Ib primary afferents to the spinal cord above C8 (no Clarke’s nucleus at this level)
o Primary afferents DO NOT terminate and synapse in dorsal horn of cord but ascent to medulla in the cuneate fasciculus (part of dorsal column tract that only exists above T6) to synapse with second order neurons in the lateral/accessory cuneate nucleus
o Second order neurons in lateral/accessory cuneate nucleus project in the cuneocerebellar tract (CCT) to the cerebellum via the inferior cerebellar peduncle
o Responsible for conveying both muscle stretch and muscle tension information from the arms

146
Q

Describe the rostral spinocerebellar tract and its function

A

o Proprioceptive information from trunk and upper limb is conveyed from type Ib primary afferents to the spinal cord above C8 (no Clarke’s nucleus at this level)
o Primary afferents terminate in laminae VII in the base of the dorsal horn of the spinal cord where they synapse with second order neurons
o Second order neurons in dorsal horn traverse the ipsilateral lateral funiculus and collect on the lateral surface of the cord in the rostral spinocerebellar tract
o Neurons in the rostral spinocerebellar tract project to the ipsilateral cerebellum via the inferior cerebellar peduncle and the superior cerebellar peduncle
o Responsible for conveying muscle tension information from the upper limb

147
Q

Describe the dorsal spinocerebellar tract and its function

A

o Proprioceptive information from trunk and lower limb is conveyed from type Ia (and some type Ib) primary afferents to the spinal cord below T6
o Primary afferents terminate in the medial part of lamina VII of the dorsal horn of the spinal cord within Clarke’s nucleus where they synapse with second order neurons
 Clarke’s nucleus runs within the dorsal horn from C8-L2 spinal cord segments
o Second order neurons originating in Clarke’s nucleus traverse the ipsilateral lateral funiculus and collect on the dorsolateral surface of the cord in the dorsal spinocerebellar tract (DSCT)
o Neurons in the dorsal spinocerebellar tract project to the ipsilateral cerebellum via the inferior cerebellar peduncle
o Responsible for conveying information about muscle stretch and muscle tension from lower limb muscles to allow for adjustment of posture

148
Q

Describe the ventral spinocerebellar tract and its function

A

o Proprioceptive information from trunk and lower limb is conveyed from type Ib primary afferents to the spinal cord below L3 (no Clarke’s nucleus at this level)
o Primary afferents terminate in Laminae V to VII in the base of the dorsal horn of the spinal cord where they synapse with second order neurons
o Second order neurons immediately cross the midline in ventral white commissure and traverse the contralateral lateral funiculus to ascend to the pons in the ventral spinocerebellar tract
o In the pons, neurons in the ventral spinocerebellar tract project to enter the contralateral superior cerebellar peduncle before recrossing to terminate in the cerebellum ipsilateral to input
o Responsible for conveying muscle tension information from the lower limb, but related to attempted movement as well

149
Q

What is two-point discrimination?

A

• Distance that two different mechanical stimuli can be detected as separate stimuli

150
Q

What is the distance for two point discrimination on the back?

A

o On back, about 8 cm

151
Q

Describe two point discrimination on the fingers vs the palm of the hand

A

o Two-point discrimination is variable in hand
 Small in fingertips
 Larger in palm of hand

152
Q

How can spatial resoution be determined using mechanoreceptors?

A

o Density of Meissner’s corpuscles and Merkel cell units per cm2, can is proportional to the spatial resolution (inverse of 2 point discrimination)- the higher the spatial resolution (and lower the 2 point discrimination), the more Meissner’s and Merkle units there are per cm2

153
Q

Where are mechanoreceptors found and what is their structure?

A

• Mechanoreceptors are found in the skin

o Contain mechanosensitive ion channels which are gated/will cause action potentials by skin perturbations

154
Q

How are mechanoreceptors classified?

A 
•	Classified according to general properties
o	Depth
o	Skin type
o	Receptive field size
o	Encapsulation 
o	Adaptation properties
155
Q

What is receptive field size?

A

 Area within the skin where a mechanical stimulus can be detected by a single mechanoreceptor

156
Q

What is microneurography and who developed it?

A

 Ake Vallbo and colleagues developed a technique called microneurography
• Microneurography- record from a single axon

157
Q

How was mechanoreceptor receptive field size detected?

A

o Microneurography from the median nerve is how mechanoreceptor receptive field was detected according to movement of stimulus probe and axon activation
 Different mechanoreceptors have different field sizes

158
Q

What determines response profile of mechanoreceptors?

A

o Accessory structures of mechanoreceptors determine response profile

159
Q

What does it mean when a mechanoreceptor is rapidly adapting? What is the function of a slow adapting mechanoreceptor?

A

o Rapid adaptation
 When the stimulus is maintained, it rapidly stops firing the action potential- firing only happens at onset and offset of action potential
• Better at detecting changing stimuli (e.g. vibration)

160
Q

When is peak activation for a Meissner’s corpuscle achieved?

A

• Meissner’s corpuscle
o Peak activation is 50 Hz at 20 micron skin indentation (can detect 10-1000 micron range of skin indentation- very sensitive)

161
Q

What does it mean when a mechanoreceptor is slowly adapting? What is the function of a slow adapting mechanoreceptor?

A

o Slow adaptation
 When stimulus is maintained, get continuous firing of action potential-> also get firing at onset and offset of action potentials
• Better at detecting constant stimuli (e.g. pressure)

162
Q

When is peak activation for a Paccinian corpuscle achieved?

A

• Paccinian corpuscle
o Peak activation is at 300 Hz at 1 micron skin indentation (can detect 1 to 1000 micron range of skin indentation- very sensitive)

163
Q

Describe a Merkel cell in terms of:

  • Structure
  • Size
  • Location
  • Receptive field
  • Adaptation
  • Function
A

-Structure: Single epithelial cell
-Size: Small
-Location: Hairy skin and glabrous skin, Superficial
-Receptive field: Small
-Adaptation: Slow
-Function:Localisation,detail,
Sustained light touch and pressure. Most important for Braille reading

164
Q

Describe a Meissners corpuscle in terms of:

  • Structure
  • Size
  • Location
  • Receptive field
  • Adaptation
  • Function
A
  • Structure: Stack of flattened schwann cells wrapped around unmyelinated nerve terminals, Tough capsular outer layer, Flexible
  • Size: 140 microns
  • Location: Glabrous skin, Superficial
  • Receptive field: Small
  • Adaptation: Rapid
  • Function: Sensitive, vibration, localisation, Kinetic-> electric phenomena, Stroking, changes in texture, flutter
165
Q

Describe a Paccinian corpuscle in terms of:

  • Structure
  • Size
  • Location
  • Receptive field
  • Adaptation
  • Function
A

-Structure: Single unmyelinated nerve terminal wrapped around by up to 60 layers of fluid filled epithelial
cells, Tough capsular outer layer, Flexible structure
-Size: 2mm (largest)
-Location: Deep dermis, all skin, Found in joints
-Receptive field: Large
-Adaptation: Rapid
-Function: Sensitive, vibration, poor localisation

166
Q

Describe a Ruffini ending in terms of:

  • Structure
  • Size
  • Location
  • Receptive field
  • Adaptation
  • Function
A
  • Structure: Rows of collagen surrounding unmyelinated fibre, stiff
  • Size: 1 mm in length
  • Location: Deep dermis, all skin
  • Receptive field: Large
  • Adaptation: Slow
  • Function: Poor localisation, pressure and stretch, deep in the skin
167
Q

Describe a free nerve ending in terms of:

  • Structure
  • Location
  • Receptive field
  • Function
A
  • Structure: Single unmyelinated fibre+ Lots of branches
  • Location: Superficial
  • Receptive field: Large
  • Function: Important in crude touch but not discriminative touch, Touch, pressure, stretch, itch, pain, temperature
168
Q

Describe a hair follicle receptor in terms of:

  • Structure
  • Location
  • Function
A
  • Structure: Wraps around base of hair within skin
  • Location: Hairy skin
  • Function: Detects deflection within the hair,, Touch: movement of hair
169
Q

What determines speed of transmission in primary afferent fibres?

A

• Degree of myelination and diameter of the axon determines the speed of transmission

170
Q

What are the primary afferent fibres from skin?

A
  • Aa
  • AB
  • Aδ
  • C
171
Q

Describe the Aa primary afferent fibre in terms of:

  • Axons from muscles
  • Myelination
  • Diameter (um)
  • Speed (m/sec)
  • Sensory receptors
A
  • Axons from muscles: Groupp I
  • Myelination: Heavily myelinated
  • Diameter (um): 13-20
  • Speed (m/sec): 80-120
  • Sensory receptors: Proprioceptors of skeletal muscle- Cell bodies in dorsal root ganglia (pseudounipolar)
172
Q

Describe the AB primary afferent fibre in terms of:

  • Axons from muscles
  • Myelination
  • Diameter (um)
  • Speed (m/sec)
  • Sensory receptors
A

-Axons from muscles: Group II
-Myelination: Moderately myelinated
-Diameter (um): 6-12
-Speed (m/sec): 35-75
-Sensory receptors: Mechanoreceptors of skin
+ Cell bodies in dorsal root ganglia (pseudounipolar)

173
Q

Describe the Aδ primary afferent fibre in terms of:

  • Axons from muscles
  • Myelination
  • Diameter (um)
  • Speed (m/sec)
  • Sensory receptors
A
  • Axons from muscles: Group III
  • Myelination: Lightly myelinated
  • Diameter (um): 1-5
  • Speed (m/sec): 5-30
  • Sensory receptors: Pain, temperature
174
Q

Describe the C primary afferent fibre in terms of:

  • Axons from muscles
  • Myelination
  • Diameter (um)
  • Speed (m/sec)
  • Sensory receptors
A
  • Axons from muscles: Group IV
  • Myelination: No myelination
  • Diameter (um): 0.2-1.5
  • Speed (m/sec): 0.5-2
  • Sensory receptors: temperature, pain, itch
175
Q

How are the spinal segments divided?

A 
•	Spinal segments (31)- with paired spinal nerves
o	Cervical (C1-C8)
o	Thoracic (T1-12)
o	Lumbar (L1-5)
o	Sacral (S1-5)
o	Coccygeal (Co)
176
Q

How are dematomes arranged in the spinal cord?

A

• Each segment represents one dermatome

177
Q

Describe the relationship of the spinal cord length to vertebral canal length, the consequence of this and why its so useful therapeutic treatments

A

• Spinal cord is shorter than vertebral canal-> spinal cord itself ends around the vertebral level L2
o Below this level, spinal nerves have to go in vertebral canal at correct intervertebral foramen but then they have to ascend up to reach their correct spinal cord segment
o Spinal punctures done below vertebral level L2 so as not to puncture spinal cord

178
Q

Describe the pathway of mechanoreceptor signals to the spinal cord and what happens in the spinal cord

A

• From mechanoreceptor, Ab fibre (pseudounipolar neurons) travels in peripheral nerve, then in mixed spinal nerve
• Goes into spinal cord via the dorsal root, with the cell body of the neuron residing in the dorsal root ganglion cell and go into dorsal horn and branch
• Synapse onto second order neurons in dorsal horn (III-V)
o Some branches will terminate in dorsal horn
 Important for unconscious reflexes
o Some branches will ascend in the dorsal column tract
 Discriminative touch
 Conscious proprioception

179
Q

What are the two tracts making up the dorsal column?

A

 Dorsal column made of two tracts
• Gracile fasciculus
• Cuneate fasciculus

180
Q

What is the function of the gracile fasciculus tract?

A

• Gracile fasciculus

o Discriminative touch/proprioception for lower limbs (below T6)

181
Q

What is the function of the cuneate fasciculus tract?

A

• Cuneate fasciculus

o Discriminative touch/proprioception for upper limbs (above T6)

182
Q

Describe the pathway of the dorsal column tracts from the medulla to the postcentral gyrus

A
  • At level of medulla, gracile fasciculus tract from spinal cord synapses into gracile nucleus
  • At level of medulla, cuneate fasciculus tract from spinal cord synapses into cuneate nucleus
  • The second order neurons decussate across to contralateral medial lemniscus through internal arcuate fibres (sensory decussation)
  • Synapses to lateral ventroposterior nucleus of the thalamus
  • Third order neuron projects to the postcentral gyrus
183
Q

What are the three dermatomes of the face/head?

A

• 3 dermatomes
o Ophthalmic zone
o Maxillary zone
o Mandibular zone

184
Q

Is tactile sensation of the tongue contralateral or ipsilateral? Why?

A

• Tactile sensation of the tongue is ipsilateral because touch sensation of the tongue maps with taste sensation (which is ipsilaterally projected)

185
Q

Describe the trigeminal touch pathway

A

o Ab fibres from mechanoreceptors travel past the trigeminal ganglion
o Information enters into the pons and synapse within the principal nucleus of trigeminal (chief sensory nucleus of trigeminal in pons)
 Proprioceptive information is processed in the mesencephalic nucleus of trigeminal (superior in midbrain)
o Decussates to contralateral side of brain
o Second order neuron synapses into medial ventroposterior nucleus of the thalamus
 In ventroposterior part of the thalamus
• Face information- medial part of ventroposterior nucleus
o Third order neuron projects to postcentral gyrus

186
Q

Describe the somatotopic and functional/projection organisation of the ventroposterior nucleus of the thalamus

A

• Somatotopically organised ventroposterior lateral thalamus (VPL) vs ventroposterior medial thalamus (VPM)
o VPL- body- receives information from medial lemniscus
o VPM- face- receives information from trigeminal nerve
• Each further divided into
o Somatosensory-specific ‘core’ region
 Touch
 Projects to Brodmann area 3b within the primary somatosensory cortex
o Proprioceptive-specific ‘shell’ region (VPS)
 Conscious proprioception
 Projects to Brodmann area 3a within the primary somatosensory cortex

187
Q

What areas are in the somatosensory cortex?

A
  • Postcentral gyrus (areas 3b, 3a, 1 and 2)
  • Parietal operculum (S2)
  • Posterior parietal cortex (areas 5 and 7)
188
Q

Describe what kind of information the primary sensory cortex recieves and its location

A

o Primary somatosensory cortex (Area 3)

 Receives information from the contralateral side of the stimulus

189
Q

What are the parts of the primary somatosensory cortex and their general respective functions?

A

 Area 3a- for conscious proprioception
 Area 3b- for touch perception
o Compound sensation (areas 1 and 2)

190
Q

What inputs to area 3a and with what kind of information?

A

• Receives dense input from ventroposterior nucleus concerned with proprioception

191
Q

What inputs to area 3b and with what kind of information?

A
  • Receives dense input from the ventroposterior nucleus of thalamus
  • Neurons selectively responsive to touch
192
Q

What is the impact of lesions to 3b?

A

• Lesions impair somatic sensations

193
Q

How is finger somatosensation organised in area 3b?

A

o Each finger represented by an adjacent area of cortex
o Thalamic projections terminate in layer IV of cortical laminar
o Cells that respond to similar inputs stacked vertically across cortical layers
 Rapidly adapting
 Slowly adapting
o Labelled-line maintained at each level of neuraxis

194
Q

Why do area 3 cells have receptive fields?

A

 Area 3 cells have receptive fields which represent activity in specific receptor types

195
Q

Describe what inputs to area 1 and what kind of information it receives?

A

• Area 1 receives a dense input from area 3b, primarily concerning rapidly adapting mechanoreceptors and is critical in the perception of texture

196
Q

Describe what inputs to area 2 and what kind of information it receives?

A

• Area 2 receives input from areas 3b, area 1 and areas 3a-> integration of hand posture, grip force and tactile stimulation to help assess the shape and size of an object

197
Q

What happens if there is a lesion in area 1 or 2?

A

• Lesioning areas 1 or 2 leads to predictable deficiencies in discriminating texture, or size and shape

198
Q

What is the parietal operculum and where is it located?

A

• Parietal operculum (S2)
o Secondary somatosensory cortex
o Parietal operculum- buried in lateral sulcus

199
Q

How is the parietal operculum organised?

A

o Somatotopically organised: starting with the face region, and the toes on the insula
 Face region- most lateral
 Toes region- more medial

200
Q

Describe what inputs to the parietal operculum and from what side these inputs come from

A

o S2 has bilateral receptive fields
 Stimulus on one side of the body will stimulate activation on both sides of the brain
o Inputs from S1 and the VP thalamus

201
Q

What is the purpose of the secondary somatosensory cortex and how does it achieve this purpose/ what is the activation of the parietal operculum dependent on?

A

o Neurons in S2 show proportional firing rates in discriminative tasks, rather than a high-fidelity copy of signal intensity
 Discriminating properties of multiple sets of stimuli
o Activation intensity in S2 is purely dependent on memory rather than copying signal from peripheral stimulus
 S2 doesn’t just copy the signal- it has a trace of a previous stimulus which can affect the level of activation of a second stimulus which follows shortly afterwards
• Interconnected with neurons in the prefrontal cortex that preserve a memory of first stimulus
• S2 may concern higher cognitive processes
o Important for memories of different mechanosensitive information which can affect behaviour- comparison between two mechanosensative feelings
 E.g. trying to compare the best way to grip the tennis racket for best result

202
Q

From where does the posterior parietal cortex receive input from?

A

• Posterior parietal cortex (areas 5 and 7)

o Receives input from area 2

203
Q

Describe the somatotopic mapping of the somatosensory cortex?

A

• Somatotopic mapping of somatosensory cortex
o Foot position and genitals- in medial wall of gyrus (in the sulcus)
o Knee position- medial lateral boundary
o Upper parts of the lower limb/trunk- top of S1
o Hands and arms- more laterally to trunk
o Face – most lateral

204
Q

What is labeled-line processing in terms of the somatosensory system?

A
  • Information from each mechanoreceptor is kept separate at every level of the system until the primary somatosensory cortex
  • Area 3a and area 3b still part of labelled line system
205
Q

What is pain?

A

• Pain- an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage

206
Q

What is nociception?

A
  • Nociception is the neural encoding of noxious or potentially noxious stimuli
  • Nociception is one of the four modalities of the somatosensory system
207
Q

What is the function of nociception?

A

• Function of nociception
o Enables detection of noxious stimuli
o Prevent tissue damage

208
Q

What is congenital insensitivity to pain, the consequences of it and how it occurs?

A

o Congenital insensitivity to pain
 SCN9A gene mutation-defective Nav 1.7 channel
 Have extremely high level (upregulation) of endogenous opioids in pain system
• Body’s pain-relieving mechanism
 Normal touch sensation
 Can’t experience pain- leads to dire consequences
• Lots of injuries- don’t tend to live very long as have a lot of injuries subjects are not aware of

209
Q

What are the mechanoreceptors that detect nociception and where are they located?

A

• Free nerve endings are the only structures that detect nociceptive/noxious stimuli
o Found at the epidermal/dermal border

210
Q

Through what afferent fibres is nociception sent?

A

o Pain transmission is via the Aδ or C afferent fibres

211
Q

Where are nociceptors found?

A

• Nociceptors found at terminal of free nerve endings

o Equallly distributed throughout the body

212
Q

What modalities of nociceptors are there?

A 
	Mechanical pain
•	Mechanical specific nociceptors
	Thermal pain
•	Thermal specific nociceptors
o	Detect temperatures above 40 degrees and below 15 degrees
	Chemical pain
•	Chemical specific nociceptors
213
Q

What are modality specific nociceptors, where are they found and what is their role?

A

o Modality specific nociceptors
 Nociceptors contain ion channels which are specific for different types of noxious stimuli- determines what kind of pain free nerve ending will detect
• Response profiles depend on specific ion channels present on free nerve ending
 Found mostly on Aδ fibres
 No pronounced morphological differences between modality specific nociceptors

214
Q

What are examples of modality specific nociceptors?

A

 Examples:
• Heat nociceptors
o TRPVI channel-specific for hot stimulus
 Capsaicin activation
• Cold nociceptors
o TRPV8 channel- specific for cold stimulus
• Chemical stimuli
o TRPA1 channel- specific for noxious chemical stimuli

215
Q

What are polymodal nociceptors, their role and their location?

A

o Polymodal nociceptors
 Can detect a variety of noxious stimuli due to varied number of ion channels within them
 Tend to be found on C fibres

216
Q

What are the two different types of C fibres

A

o Peptidergic

o Nonpeptidergic

217
Q

What neurotransmitter do peptidergic C fibres release?

A

 Potential to release neuropeptide called substance P

218
Q

What is the role of substance P?

A

• Substance P- important for driving plasticity which can lead to chronic pain
 Most associated with intense pain- contribute to chronic pain

219
Q

Where are silent nociceptors found and what is their role?

A

o Silent nociceptors-
 Free nerve endings found in visceral structures
 Become active when detect inflammation or organ issues
 Get referred pain because viscera normally don’t signal pain, but in case of visceral structure pain, can get pain on skin which is innervated by same dorsal root ganglia to transmit its pain signal

220
Q

Are nociceptors slowly adapting or rapidly adapting?

A

o Nociceptors are always slowly adapting

 Continuously fire action potentials as stimulus is maintained

221
Q

What are the stages of acute somatic pain and the fibres responsible for each stage?

A

• Delta fibres-first pain
o Sharp, well localised
o Modality specific- specific information about pain
• C fibres- slow pain
o Aching, longer lasting, poorly localised
o Polymodal- general information about pain
• Hyperalgesia

222
Q

When there is external pressure to the nerve, does it block Aδ fibres or C fibres?

A

Aδ fibres

223
Q

Does anesthetic block C fibres first or Aδ fibres first?

A

C fibres

224
Q

Why is C fibre pain alone more unpleasant than Aδ fibre pain alone?

A

o C fibre pain are more unpleasant, maybe because of emotional aspect or inability to localise the pain

225
Q

What is hyperalgesia and its role?

A

o Where a person develops an increased sensitivity to pain due to previous exposure to noxious stimulus
 Stops person from putting more pain on that area- protects the vulnerable area

226
Q

Describe the pain pathway at the level of the periphery and the spinal cord?

A
  • Information travels in Aδ or C fibres and enters into the dorsal root similarly to the discriminative touch system- would have cell bodies inside the dorsal root ganglia
  • Enters into spinal cord
  • Second order neurons cross over the midline (decussate) in the ventral white commissure and ascend in contralateral spinal cord in spinothalamic tract (anterio-lateral system)
227
Q

When C fibres enter the spinal cord where do they synapse exactly?

A

o C fibres enter the spinal cord and ascends or descends up to 2 spinal cord segments/levels and synapses there- it does this through the Zone of Lissauer
 C fibre input is not tightly somatotopically organised
 Synapse in substantia gelatinosa
• Peptidergic C fibres terminate in Lamina I and outer lamina II of the spinal cord
• Nonpeptidergic C fibres terminate in inner lamina II of the spinal cord

228
Q

When Aδ fibres enter the spinal cord where do they synapse exactly?

A

o Aδ fibres synapse at the same level of the spinal cord as they go in
 Aδ fibre information somatotopically organised
 Aδ fibres mostly terminate in nucleus proprius (though some will terminate in substantia gelatinosa)
• Aδ fibres predominantly terminate in lamina V (most important transmission there) but can terminate in Lamina I

229
Q

What are the neurotransmitters and mediators that are the basis of the primary hyperalgesia-axon reflex?

A

• Primary hyperalgesia- axon reflex
o Based on release of neuropeptides (such as substance P and glutamate)
o Release of mediators from damaged skin area itself (such as bradykinin, potassium)
 Mediators released by damage can activate nociceptors and contribute to hyperalgesia
o Resident immune cells in skin (mast cells) may degranulate when get damage to skin- can release histamine (which can act on nociceptors to drive hyperalgesia)
 Substance P can contribute to mast degranulation

230
Q

Describe how primary hyperalgesia- axon reflex occurs?

A

 Damage area of skin
 Activate nociceptor and cause action potential
 As free nerve endings have branched like structure it will travel:
• Towards nervous system
o In dorsal horn, at synapse, there is glutamate and substance P (from peptidergic C fibre terminals which contribute to plasticity) release
• Down other branches of same neuron- away from nervous system
o Causes peripheral release of glutamate and substance P back into the skin
o This has the effect of driving the mechanisms of hyperalgesia
o Both area of injury and neighbouring regions become more sensitive
 This is because glutamate and substance P make the blood vessels more leaky so get more immune cells leaking out into the skin-> release inflammatory mediators which make nerve terminals more sensitive to pain
• When get leaky blood vessels, macrophages can enter into area of swelling where they will release cascade of inflammatory mediators (prostaglandins, cytokines)-> sensitive nociceptors to make them more activated (prostaglandins reduce threshold for nociceptor activation)

231
Q

What is the role of ibuprofin?

A

o Ibuprofin acts on prostaglandin to lessen nociceptor sensitivity

232
Q

What drugs can be used to treat rheumatoid arthritis and why?

A

o In some chronic pain conditions, particularly rheumatoid arthritis, drugs that block the TNF alpha and IL-1 beta produced by macrophages are commonly used to treat these because TNF alpha and IL-1 beta can directly activate nociceptors

233
Q

What is the purpose of spinal cord wind-up? What is spinal cord wind-up?

A

Spinal cord wind-up-another mechanism of hyperalgesia
• Process of plasticity within the dorsal horn of spinal cord at the termination of a peptidergic C fibre (in lamina I or outer lamina II of spinal cord)

234
Q

What is the process of spinal cord wind-up?

A

• When have noxious input, action potential fires, depolarisation of nerve terminal and release of glutamate and substance P into synapse
o Initially, there is only glutamate released
 Will activate AMPA receptors
 Transmit noxious input up to central nervous system
o If the pain is really intense and maintained, will get higher level of firing
 Get more glutamate released
• Activate AMPA receptors and through an intracellular signalling cascade involving sodium, which enters through AMPA receptor, will prime NMDA receptors to be active (normally not active under physiological conditions due to magnesium ion blocking them)
o When NMDA receptors primed, magnesium plug removed and start to see glutamate activation on NMDA receptors which leads to a dramatic increase in intracellular calcium
 Substance P will start to be released
• Acts on NK-1 receptor
• Through an intracellular signalling mechanism which involves cAMP and PK-A, will have priming effect on NMDA receptors so more NMDA receptors are opened
o Due to increased intracellular calcium, will get even greater effects on postsynaptic cell
• Increased Intracellular calcium, cAMP and PK-A lead to changes in postsynaptic cell
o Upregulation of certain enzymes and proteins
o Phenotype of neuron changes so that it becomes more sensitive to future noxious stimuli

235
Q

What is allodynia?

A

• Allodynia- Where touch can become painful

236
Q

What is the process of allodynia?

A

 Low intensity input activates both normally separate low threshold mechanoreceptor A beta and sensitised nociceptor A delta and C
 These both input signals to hyperexcitable dorsal horn neurons, which lead to pain

237
Q

What neurons facilitate both allodynia and the gate theory of pain?

A

o Wide dynamic range neurons

238
Q

What are dynamic range neurons, how can they be activated and where are they found?

A

o Wide dynamic range neurons-neurons where there is convergence of mechanosensitive information and nociceptive information. Can be activated by both low threshold mechanoreceptive information (touch) and noxious input

239
Q

What is the gate theory of pain and who developed it?

A

o Gate theory of pain- Melzack and Wall
 Based on observation that if you have a noxious stimulus and non-noxious touch is applied to it, it can inhibit pain system

240
Q

How does the gate theory of pain work?

A

 Process
• Noxious input from C/Aδ fibre goes into the spinal horn and terminates on second order neuron, which transmits pain up spinothalamic tract
• Touch system is able to integrate with this system- if touch is coming at the same time as noxious stimulus, it can activate an GABAergic inhibitory interneuron which is able to inhibit/ dampen noxious signal

241
Q

What is the purpose of the neo-spinothalamic tract?

A

• Sensory nociception transmitted to the brain via neo-spinothalamic tract (found anterior laterally)

242
Q

Describe the body neo-spinothalamic tract (nociception for the body)

A

• Pathway for the body
o Enters into dorsal horn
 C fibre- will ascend of descend in Zone of Lissauer and terminate in substantia gelatinosa
 Aδ fibre- will terminate in nucleus proprius
o Second order fibre decussates across ventral white commissure and ascends in contralateral spinal cord within spinothalamic tract
o Ascends to thalamus
 Nociception involving the body synapses in lateral ventroposterior nucleus of the thalamus
o Will arrive in primary somosensory cortex
 Aδ fibre information will go to 3b
 C fibre information will go to 3a

243
Q

Describe the face neo-spinothalamic tract (nociception for the face

A

• Pathway for the face/head
o Pain information from trigeminal nerve will enter through the pons and will descend in the spinal tract to synapse in the spinal nucleus of trigeminal
 Spinal nucleus of trigeminal receives pain and temperature information
o Second order neuron decussates at the level of the medulla
o Joins with pain and temperature information from the spinothalamic tract and will travel through the brainstem contralaterally
o Will synapse in the thalamus
 Nociception involving the body synapses in the medial ventroposterior nucleus of the thalamus
o Will arrive in primary somosensory cortex
 Aδ fibre information will go to 3b
 C fibre information will go to 3a

244
Q

Describe the somatotopic mapping of the Aδ fibres and C fibres in the ventroposterior thalamus. Why is this somatotopic mapping important?

A

• Ventroposterior thalamus
o Somatotopically organised-
 Aδ fibre specific -core regions of ventroposterior lateral/ ventroposterior medial nucleus of thalamus
 C fibre specific- terminates in ventral medial posterior portion of the thalamus
o Aδ system and C fibre system separated
 Important because:
• Aδ fibre system is more important for sensory dimension of pain
• C fibre system is more important for emotional aspects of pain

245
Q

Compare the dorsal column medial lemniscal pathway and the spinothalamic pathway

A

• Mechanosensation- dorsal column medial lemniscal pathway
o Abeta fibres and Aalpha fibres enter the dorsal horn
 Branches which drive reflexive response
o Branch ascends in dorsal column tract- cuneate and gracile fasciculi
o Synapse in the gracile and cuneate nuclei
o Second order neuron crosses in internal arcuate fibres which forms the medial lemniscus
o Synapses in the ventroposterior lateral thalamus
o Goes up to primary somatosensory cortex
• Nociception- Spinothalamic pathway
o Information from Adelta and C fibres terminate in dorsal horn
o Immediately decussate in ventral white commissure
o Ascend contralaterally in spinal cord
o Synapse in ventroposterior lateral thalamus
o Projects up to primary somatosensory cortex

246
Q

What is Brown-Sequard syndrome?

A

• Condition where spinal cord injury is complete hemisection
o Spinal cord injury that only damages half spinal cord
• Get damage to spinothalamic tract and dorsal column tract
• Lose pain and temperature detection below the injury on the contralateral side of the injury
• Lose discriminative touch below the injury on the ipsilateral side of the injury
• Paralysis on the ipsilateral side of the injury

247
Q

Compare mechanosensation vs nociception in terms of:

  • Receptors
  • Afferent fibres
  • Transmission speed
  • Ascension side
A 
Mechanosensation:
-Receptors
Meissner’s corpuscle
Pacinian corpuscle
Merkel’s disc
Ruffini’s endings
Free nerve endings
Hair follicle receptor
-Afferent fibres
Abeta fibres
(Adelta and C fibres for free nerve endings)
-Transmission speed
Fast transmission
-Ascension side 
Ascends ipsilaterally
Nociception:
-Receptors
Free nerve endings
-Afferent fibres
Adelta (first pain)
C fibres (slow pain, itch)
-Transmission speed
Slow transmission
-Ascension side 
Ascends contralaterally
248
Q

In the nociceptive system, where does plasticity occur?

A

• Plasticity occurs peripherally (axon reflex) and centrally (spinal wind up)

249
Q

Where does integration of nociceptive and mechanosensitive fibres occur in the spinal cord?

A

• Some integration of nociceptive and mechanosensitive fibres occurs in the spinal cord

250
Q

What are the functions of the superior colliculus/ what does it do/ how does it do these functions?

A

o General function of the superior and inferior colliculi in humans to direct behavioural responses towards specific points in egocentric space
o In primates, most research into superior colliculus function concerns superior colliculus involvement in visual reflexes
o While the input to the superior colliculus is strongly visual and the output strongly directed towards eye movement and head movement/posture, the superior colliculus is a centre of subcortical multimodal integration and has input from multiple senses (auditory, visual, somatosensory (tactile, proprioceptive, vestibular))
o Superior colliculus has motor outputs to organising areas and the spinal cord
o The superior colliculus contains topographic maps of egocentric space coordinates- relating sensory (especially retinal/visual) to motor output, particularly for the eyes and head as well as the upper body
o Sensory input from the space around us is in register with our body position and movement
o Supports rapid timing of sensory input with the reflexive motor output

251
Q

Is superior colliculus input/output conscious or subconscious?

A

Subconscious/not under volitional control

252
Q

Where is the superior colliculus located?

A

• The superior colliculus is located in the tectal area of the brainstem
o Sit under the pineal gland over the mesencephalic aqueduct

253
Q

What is the optic tectum?

A

o Optic tectum- non mammalian equivalent to the superior colliculus

254
Q

What is the function of the optic tectum?

A

 Function: Contains simple visual maps and reflexively outputs to simple behaviours

255
Q

What abilities did the optic tectum evolve?

A
  • Increased mapping of visual input beyond just light and shadow- topographical (but still no image perception)
  • Increased ability to detect motion
  • Expansion of connections with motor organising centres for rotation of eyeballs and lens focusing via cranial nerves, refinement of connections with motor organising centres for head and neck movement
  • Although proportionally smaller in humans, humans have retained the reflexive tectal (superior colliculus) non-conscious response and integration with fear response and behaviours
256
Q

Describe the evolution of our visual map and what structures make a complete visual map possible

A

 Evolution of the visual system
• Visual map has gained increased territory in visual cortex in occipital and temporal lobes, motor/premotor cortical areas for eye movement, brainstem areas for eye movement, sensory input to register head, eye, face position and adjust posture, body orientation in response

257
Q

What modality of inputs does the superior colliculus receive?

A
  • Receives strong topographical visual input, but no image perception
  • Also receives auditory and somatosensory input
258
Q

What does the superior colliculus do with the various inputs it receives and how does it use these inputs to our benefit?

A
  • Integrates sensory information (visual, auditory, somatosensory)
  • Initiates motor signals to orient the eyes and head and upper body toward stimulus (motor system)
  • Initiates saccades (eye movement) to scan the visual world
  • Subconscious input, reflexive output
259
Q

How does the superior colliculus interact with the emotional motor system?

A

• Interacts with emotional motor system via the periaqueductal grey area in stereotypical (patterned) behaviours

260
Q

What is the term for the combined inferior and superior colliculi?

A

o The inferior and superior colliculi together are termed the corpora quadrigemina (L quadruplets)

261
Q

What are the three components of the midbrain (describe them)

A

 Midbrain components
• Tectum- above the mesencephalic aqueduct. Generally includes colliculi
• Tegmentum- below the ventricles.
• Basis- cerebral peduncles, basis pons, pyramids

262
Q

What is griseum?

A

 Griseum- grey matter layers, contain neurons

263
Q

What is album?

A

 Album- white matter layers, axon tracts

264
Q

Describe the anatomy of the superior colliculus and a description of the roles of the components

A

• Specific anatomy of superior colliculus-
o Superior colliculus is laminated
 Each layer has a specific connectivity
 3 layers
• Superficial layer- mostly visual
• Intermediate layer- multisensory information (visual/somatosensory/auditory) and origin of most motor output (to regulate eye and head movement)
• Deep layer-multisensory information (visual/somatosensory/auditory) and origin of most motor output (to regulate eye and head movement)

265
Q

What are the layers in the superficial layer of the superior colliculus?

A

o Layers-
 SGS- stratum griseum superficiale
 SO- Stratum opticum

266
Q

What is the input to the superficial layer of the superior colliculus?

A

o Input:
 Visual input direct from retina and processed input from the visual cortex
 Brainstem (pretectal nucleus and parabigeminal nucleus)

267
Q

What are the outputs of the superficial layer of the superior colliculus and what is the role of these outputs?

A

 Motor output to areas controlling eye movement and head position-> reflexive orientation of eyes and head to take in visual stimuli
 Thalamus (lateral geniculate nucleus, pulvinar)
• Lateral geniculate nucleus- Visual relay
• Pulvinar- vision and body position, influences eye movement
 Cortex (frontal eye fields and premotor areas for head position)
• Frontal eye fields- cortical pre-motor area for eye movement
 Brainstem (pretectal nucleus and parabigeminal nucleus)
• Pretectal nucleus- pupillary light reflex, lens accommodation, eye movement
• Parabigeminal nucleus- parabigeminal nucleus interconnected with the amygdala.

268
Q

What are the layers in the intermediate layer of the superior colliculus?

A

 SGI-stratum griseum intermedium

 SAI- stratum album intermedium

269
Q

What are the inputs to the intermediate layer of the superior colliculus and what modality are these inputs responsible for?

A 
o	Input-
	Spinal and trigeminal nucleus areas
•	Somatosensation 
	Superficial superior colliculus
•	Vision 
	Inferior colliculus
•	Audition
	Cerebellum
•	Balance, eye position, proprioception 
	Cortex
•	Processed sensory information
270
Q

What are the outputs of the intermediate layer of the superior colliculus and what role are these outputs responsible for?

A

o Output
 Cortical areas controlling eye movement (frontal eye fields)
 Pulvinar nucleus of the thalamus
• Influences eye movement via cortex and brainstem- integrates body position in relation to multiple senses

271
Q

What are the layers of the deep layer of the superior colliculus?

A

o Layers-

 SGO- stratum griseum profundum

272
Q

What are the inputs to the deep layer of the superior colliculus and what modalities are these inputs responsible for?

A 
o	Input-
	Spinal and trigeminal nucleus areas
•	Somatosensation 
	Superficial superior colliculus
•	Vision 
	Inferior colliculus
•	Audition
	Cerebellum
•	Balance, eye position, proprioception 
	Cortex
•	Processed sensory information
273
Q

What are the outputs of the deep layer of the superior colliculus and what role are these outputs responsible for?

A

o Output
 Pulvinar nucleus of the thalamus
• Influences eye movement via cortex and brainstem- integrates body position in relation to multiple senses
 Brainstem reticular formation and spinal cord ventral horn
• Descending projection direct to the ventral horn of the spinal cord via tectospinal tract for head movement
• Descending projections to the reticular formation which contains coordinating centres for head and neck position, eye position, posture and complex patterned actions, muscle tone for axial and proximal limbs
o Reticular formation projects to the spinal cord ventral horn via the reticulospinal tract
 Paramedian pontine reticular formation (PPRF)- coordinates eye movement via CN III (oculomotor) , IV (trochlear) and VI (abducens) nuclei

274
Q

Where does motor output from the superior colliculus originate from?

A

• Motor output in superior colliculus circuitry

o Motor directed output originates mainly from intermediate/deeper layers of the superior colliculus

275
Q

Describe the mapping of sensory inputs and motor outputs in the superior colliculus? Describe the implications of this at a neuronal level

A

o Topographic map information from a particular part of the visual field is received by a corresponding region of the superficial layers of the superior colliculus
o All layers of the superior colliculus have a similar topographic arrangement
o Motor areas of the superior colliculus have the same topographic arrangement as the sensory areas
o At a neuronal level: activation of neurons at single points in the superior colliculus map can evoke a response directed towards the corresponding point in space
o Single neurons in the superior colliculus can receive multiple sensory inputs (auditory information, somatosensory information and visual information) and have direct motor output
 Vector coordinates- input coordinated to output

276
Q

Why is information in the superior colliculus topographically mapped?

A

o Allows at a subconscious level for the rapid integration and enhancement of signals that arrive via multiple sensory modalities
arrangement as the sensory areas, it allows for the rapid initiation of motor responses to incoming sensory information

277
Q

When does multisensory integration in the superior colliculus fully develop?

A

o Multisensory integration (touch, hearing, sight) in the superior colliculus takes about 4 weeks after birth to develop (develops in early birth)

278
Q

Why is the superior colliculus linked to the periaqueductal grey, the amygdala and the thalamus?

A

• Superior colliculus links to survival mechanisms of fear (fight, flight, freeze)
o Superior colliculus links to periaqueductal grey, amygdala and thalamus for fear memories and generation of emotional motor system responses

279
Q

How are primary sensory cortical areas mapped?

A

• Primary sensory cortical areas are mapped topographically

280
Q

Why are sensory inputs integrated with motor output?

A

• Sensory inputs are integrated with motor output for volitional control and reflexive responses and behaviour at many levels in the hierarchy

281
Q

What happens when coordinating input from different sensory channels match?

A

• Registering- matching coordinated input from different sensory channels can enhance the percept and motor output

282
Q

What does it mean when different sensory inputs are in register?

A

• When different sensory inputs are in register, they are aligned in time and space

283
Q

What does it mean when different sensory inputs make sense?

A

• When different sensory inputs make sense, they are aligned with prior learning

284
Q

Why is integration of multisensory inputs a challenge?

A

• Problem- our experience is derived from multiple sensory modalities arising through different sensory receptors
o The information arrives independently through different pathways from the periphery to the CNS
• Yet our perception of experiences seems singular and unified
• Illusion and brain lesions show that this unified perception can be unbound

285
Q

When is multisensory integration the strongest?

A

• Multisensory integration is strongest when each modality stimulus is presented at the same time and place- results in increased activation of particular areas in cortex and brainstem
o If stimuli match, there is more neuronal activity

286
Q

Is somatosensation unimodal or multimodal? Why?

A

• Somatosensation is multimodal-
o Each somatosensory receptor in skin conveys a separate quality of touch, but these all bind into a unified perception of the face and body

287
Q

What is haptic perception?

A

o Haptic perception- understanding object shape/position through touch and proprioception

288
Q

Describe the classical view of hierarchical sensory processing and debate its accuracy

A

• Classical view of hierarchical sensory processing
o Sensory receptor-> brainstem/spinal cord/retinal ganglion cells-> thalamus-> primary sensory (unimodal) cortex-> multimodal association cortex-> high level association cortex
 This is true as a basis but is more complicated than already displayed
• Spinal and brainstem reflex arcs
o Integration of sensory input with motor output
• Superior colliculus- vision/audition/somatosensation-> integration with reflexes for head and eye movement
• Pulvinar of the thalamus- vision/somatosensation-> integrates with position of body in space, vision, eye movement
• Unimodal cortex- receives multisensory input from other unimodal and polymodal cortex although inputs outside of its primary sense are not its priority

289
Q

When is neuronal activity the strongest when integrating information? When is neuronal activity the weakest when integrating information?

A

• Activity is magnified when audition and vision match in time and space
o Evidence from neuronal recordings
o The multisensory experience is more unified when the stimuli spatially and temporally align: match in time and space
o If signals come from the same place at the same time
 Sum response is greater than the response parts- magnification of signals
 Response is larger in some areas for congruent input
 Some areas show reduced activity when there is a conflict (error monitoring)
• Neuron activity is magnified when multiple sensory inputs match with prior learning and expectations
o Neuron activity is lessened when multiple sensory inputs don’t match with prior learning and expectations

290
Q

Are all modalities equally perceived? Why/why not? What are the consequences of this?

A

• There is competition between the modalities and individual sensory input can overshadow other modalities
• Different aspects of the input influences the perception
• There is also conflict and competition between the sensory dominant impression
o This impression can be manipulated

291
Q

Describe the two anatomical streams of visual information that determine the ‘where’ and ‘what’ of visual information ?

A

o Posterior brain/dorsal stream determines where visual stimuli are (movement, contrast, relation to egocentric location)- receptive fields respond to position in space and movement
o Inferior brain/ventral stream- receptive fields respond to colors and shapes, object recognition and face recognition

292
Q

What are the two main pathway functions of the visual dorsal stream?

A

 Dorsal stream pathway-directing eye movement

 Dorsal stream pathway- directing hand movement

293
Q

Describe the dorsal stream pathway in direction of eye movement

A

• Primary visual cortex inputs to posterior parietal cortex
• Posterior parietal cortex inputs to the frontal eye fields and the superior colliculus
o Posterior parietal cortex is the cortical multisensory area (somatosensation/vision/audition)
• Frontal eye fields input to posterior parietal cortex and superior colliculus
o Frontal eye fields (premotor area for eye movement)
• Superior colliculus (subcortical multisensory area, vision, somatosensation and audition) inputs to paramedian pontine reticular formation (coordinating centre for eye movement), which projects to motor nuclei (CNIII, IV and VI) for eye movement
o Superior colliculus- subcortical multisensory area/vision/somatosensation and audition
o Paramedian pontine reticular formation- coordinating centre for eye movement
o Eye/head movement controlled through superior colliculus and through cortex

294
Q

Describe the anatomical structures involved in the dorsal stream pathway in direction of hand movement and the motor responses output by this pathway

A

• Requires integration of somatosensation, vision and audition
• Involves parietal cortex (S1 somatosensation and association somatosensory areas PPC), occipital visual areas, premotor arm region, premotor eye movement areas
• Involves direction-selective (vectors) registration (alignment)
• Responds more to the location of objects
• Motor responses
o Eye movement direction
o Reaching behaviour
o Hand/eye coordination eye movement registration with hand movement

295
Q

Why is the parietal cortex multisensory? Which part especially?

A

o Contains S1 and somatosensory association areas
o Borders visual areas in occipital lobe
o Borders auditory areas in temporal lobe
• Parietal cortex, especially posterior parietal cortex, is multisensory
o Contains association areas with multisensory input required to perceive where your body fits into the spatial world
• Parietal association cortex- combines and integrates somatosensory, visual, temporal, auditory information

296
Q

What is the impact of posterior parietal lesions?

A 
o	Neglect syndromes
	Hemispatial neglect-
•	Neglects a part of their world 
	Spatial agnosia
•	Inability to process the spatial layout of an environment 
	Grasping dysfunction 
•	Cannot grasp at objects handed

NEUR3905 (5 decks)

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