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Brain and Behaviour > The Somatosensory Pathway > Flashcards

The Somatosensory Pathway Flashcards

1
Q

what are sensory nerve that mediate discriminative touch perception

A

A alpha and A beta

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2
Q

where do ensory nerve that mediate discriminative touch perception project into

A

sensory nerve fibres mediating discriminative touch perception (A alpha and A beta) project into the dorsal fasciculi (also known as posterior columns or dorsal columns)

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3
Q

what are the dorsal columns divided into

A

divided into the Fasciculus gracilis and fasciculus cutaneous

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4
Q

what does the Fasciculus gracilis contain

A

contains afferents from the lower limb and genitalia

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5
Q

what does the fascicles cutaneous contain

A

contains afferents from the upper limb

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6
Q

describe fibres in the dorsal columns

A

they are not post-synaptic

- they are branches of the primary afferent fibres in the peripheral nerves

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7
Q

dorsal column fibres ascend the….

A

spinal cord on the same side they enter it

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8
Q

describe the pathway of the dorsal column from the forelimb

A
  • transmits the sensations of vibration, proprioception and discriminative touch from the periphery to the brain via the dorsal column and medial leminiscus
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9
Q

where does the Fasciculus gracilis and fasciculus cutaneous synapse into

A

two nuclei called the nucleus gracilis and nucleus cuneatus which are in the lower medulla of the brainstem

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10
Q

describe the pathway once the axons leave the dorsal column

A
  • The axons leave the dorsal column nuclei and cross the midline
  • they then form the contralateral medial lemniscus
  • they ascending to the VPL nucleus (ventro-postero-lateral nucleus) of the thalamus
  • Then they project into the somatosensory cortex
  • they desucate in the medulla
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11
Q

what are the dorsal column nuclei

A

nucleus cuneatus & nucleus gracilis

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12
Q

describe the nucleus cuneatus & nucleus gracilis

A

They are long columns of cells extending several mm rostrally into the medulla.

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13
Q

what is the medial lemniscus

A

This is the tract from the dorsal column nuclei to the somatosensory thalamus

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14
Q

what is the dorsal pathway called the dorsal column medial lemniscal pathway

A

Because the pathway starts with the dorsal columns and continues as the medial lemniscus it is often called the dorsal column-medial lemniscal (DCML) pathway

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15
Q

describe where the DCML pathway is crossed and uncrossed

A

The DCML pathway is UNCROSSED in the spinal cord, ie the information passes up the spinal cord on the same side as the peripheral stimulus. It crosses over in the medulla

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16
Q

where does DCML have collaterals

A

the primary afferents entering the DCML pathway have collaterals that synapse in the dorsal horn

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17
Q

what are the medial leminiscal fibres know as in the region where they cross over the midline

A

The medial lemniscal fibres are known as internal arcuate fibers in the region where they cross over the midline
- this occurs in the lower medulla

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18
Q

what are medial leminsical fibres from the body joined by

A

Medial lemniscal fibres from the body are joined by the cutaneous afferents from the face. These come from the contralateral trigeminal nucleus

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19
Q

where do the fibres from the trigeminal nucleus synapse

A
  • fibres from the trigeminal nucleus synapse in the VPM thalamus ventro-postero-medial nucleus)
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20
Q

describe the structure of the thalamus

A

the thalamus lies in the midline, the black parts are the fluid filled ventricles, the part in the middle is called the third ventricle, therefore the thalamus forms the wall of the third ventricle

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21
Q

what is the thalamus

A

The thalamus is the relay station for information from the periphery to reach the cortex

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22
Q

all inputs to any part of the cortex….

A

(except for inputs from other cortical areas) must come via the thalamus.

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23
Q

what is the VPM (ventro-postero-medial nucleus)

A
  • this is where afferents from the face synapse and where visceral afferents from the body synapse and where third order neurone travel up to the somatosensory cortex
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24
Q

what is the organised body map caused

A

called the homonculus

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25
Q

Describe the homunculus

A
  • cells in the thalamus send their axons to the somatosensory cortex in an organised manner but that twist round 180 degrees, so that in the somatosensory cortex the leg is medial-most, the arm is in the middle and the head and face most lateral
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26
Q

describe the thalamic nuclei and where parts of the body are placed

A
  • In the thalamic nuclei, the face is represented medially (VPM) and the upper limb more laterally (VPL) , and then the lower limb most laterally (VPL)
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27
Q

areas with the highest density of touch and proprioceptive receptors….

A

Areas with the highest density of touch and proprioceptive receptors occupy the largest areas of the map

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28
Q

what is the parts of the Body that have the larger parts of the homunculus map

A

The hands, lips & tongue have the highest density of touch receptors and have the largest areas in the homunculus.

29
Q

where does the primary somatosensory cortex run

A

The primary somatosensory cortex is a strip of cortex running medio-laterally just posterior to the central sulcus

30
Q

what is a characteristic of the homunculus map

A
  • it has plasticity
31
Q

describe the idea of plasticity of the homunculus map

A

If a limb is amputated the area devoted to that limb shrinks and the area devoted to surrounding parts of the body expands.

32
Q

what does the somatosensory cortex tell you

A

The somatosensory cortex tells you where something is on your body and its touch and texture.

33
Q

what does the somatosensory cortex not tell you

A

It does NOT tell you about its temperature or whether it is painful. These modalities are processed in other cortical regions

34
Q

What are the parts of the clinical assement of the DCML pathway

A
  • fine touch
  • vibration
  • barognosis
  • graphesthesia
  • kinaesthesia and proprioception
  • proprioception
35
Q

how do you test the parts fo the clinical assessment in the DCML pathway

A

Fine touch/stereognosis tested by asking the patient to recognize common objects hidden within a cloth bag using touch alone and recognsie the object by touch - this is a key role of the dorsal column system this is called sterognosis

Vibration sense tested using a tuning fork placed along a bony prominence of the desired corresponding spinal level(s) to be tested. – pancian corspules, these are more or less exlccusive in the dorsal column

Barognosis tested by asking subject to determine the approximate weight of an object placed in the hand.

Graphesthesia can the subject recognise writing on the skin by touch.

Kinaesthesia & proprioception tested using the subject’s ability to detect an externally imposed passive movement, or the ability to reposition a joint to a predetermined position.

Proprioception can also be assessed using Romberg’s test.
In the Romberg test, the patient is stood up and asked to close their eyes. A loss of balance is interpreted as a positive Romberg sign.

36
Q

Describe how Rombergs test works

A
  • The Romberg test assesses proprioception from leg muscles and joints. This is the ability to sense the position of our limbs and thus our position in space.
  • The standing patient is asked to stand upright and close his or her eyes. An instability (wobble) followed by a loss of balance is interpreted as a positive Romberg’s test.
  • This inability to balance with eyes closed is called sensory ataxia: it often occurs with damage to the dorsal columns
37
Q

What happens if there is dorsal column damage

A

loss of proprioceptive input leading to sensory ataxia when visual input is removed

38
Q

describe the pathway of the spinothalamic tract

A
  • all the afferents are in the peripheral nerve that comes into the dorsal root and synapse in the laminae of the dorsal horn
39
Q

what are the laminae

A

this is when there is a bunch of cells of a particular morphology
- different modalities of afferents synapse on cells in different laminae

40
Q 
where do 
- cutaneous afferents 
- proprioreceptors 
- nociceptors 
synapse in which laminae
A

Cutaneous afferents (Pacinian corpuscles etc) synapse in lamina III & IV,

Proprioceptors (muscle spindles, Golgi and joint afferents) synapse in lamina V &VI

Nociceptors (free nerve endings) synapse in lamina I & II.

41
Q

how many cell layers are there in the dorsal horn

A

6 layers of cells in the dorsal horn

42
Q

what is the lateral part of the spinothalamic tract associated with and what is the anterior part of the spinothalamic tract associated with

A
  • it is associated with pain whereas the anterior part of the spinothalamic tract is located in the lower part of the dorsal so is associated with proprioceptors
  • the lateral spinothalamic tract is temperature and pain and the anterior part is crude touch
43
Q

describe where the spinothalamic tract crosses over

A

The cells in the dorsal horn are called ‘second order afferents’. Their axons cross the spinal cord within one segment of the cell body and then project up the contralateral spinal cord in the spinothalamic tract

44
Q

what are the two parts of the spinothalamic tract

A
  • a phylogenetically older lateral part

- a newer anterior part.

45
Q

what is crude touch

A

aware that you are being touch but not sure where you are being touched or what type of touch it is for example is it light touch, light pressure or deep pressure

46
Q

describe the pathway of the spinothalamic tract

A

The spinothalamic tracts arises from cells in the contralateral spinal dorsal horn. The axons cross (decussate) in the spinal cord

47
Q

what type of sensation does the spinothalamic tract covey

A

the sense of pain, temperature and ‘crude’ or ‘passive’ touch.

48
Q

What are the two types of pain

A

Normal pain

neuropathic pain

49
Q

what is pain

A
  • this is a nueornal program that is normally activated by tissue damage
50
Q

What are the receptors that are associated with tissue damage and therefore pain

A
  • the receptors specific for tissue damage called nociceptors
  • nociceptors are free nerve endings found at the end of A delta (small myelinated) and C(unmyelinated) nerve fibres
51
Q

what are the receptors for fast and slow pain

A
  • A delta – fast pain

- C – slow pain

52
Q

were does neuropathic pain arise from

A

neuropathic pain – arising from epileptic like seizures

- it is when the pain programmes become spontaneously active and pain is felt but there is no injury present

53
Q

what can normal pain be treated with

A

normal or nociceptive pain can be treated by conventional analgesics, NSAIDs, steroid anti-inflammatory agents, opiates

54
Q

why do we have fast pain

A

Fast pain (Ad afferents) triggers withdrawal reflexes to minimise injury.

55
Q

what is the difference between fast pain and slow pain

A

Fast pain

  • delta fibres
  • goes away after the initial injury

slow pain

  • type C fibres
  • this may continue until the wound has healed
56
Q

what are the special sodium channels in the free never endings called

A

called ‘TTX resistant’ channels

57
Q

How does inflammation produce continuous slow pain

A
  • Free nerve endings have special sodium channels. (called ‘TTX resistant’ channels)
  • Tissue damage releases pro-inflammatory chemicals into the extracellular space.
  • The resulting inflammation opens the TTX resistant channels leading to a continuous depolarisation of the ending and (a continuous receptor potential) & thus tonic generation of action potentials.
  • Thus inflammation produces continuous slow pain
58
Q

where do affernets form nociceptive C fibres and Adelta fibres terminate

A

terminate in laminae I and II of the dorsal horn

59
Q

where do the cells in lamina I and II project

A
  • Afferents from nociceptive C fibres and Adelta fibres terminate in laminae I and II of the dorsal horn.
  • Cells in lamina I and II project across the midline (decussate) in the spinal cord and ascend in the contralateral anterolateral part of the cord as the lateral spinothalamic tract.
  • This tract DOES NOT project to the VPL but to various nuclei in the brainstem
60
Q

why does the spinothalamic tract and pain can go to the VPL in the nucleus

A

A painful stimulus will activate touch receptors as well as nociceptors.
The touch receptors project in the anterior spinothalamic tract.
This DOES end in the ventroposterolateral (VPL) nucleus of the thalamus along with the medial lemniscus.

61
Q

where does the anterior spinothalamic tract go

A

VPL

62
Q

where does the lateral spinothalamic tract go

A
  • it contains temperature sensing afferents these go to the hypothalamus where temperuatre regulation takes place
  • goes to he midbrain reticular formation – this is the desnciend feedback control of pain
  • goes to the rticular nuclei of the thalamus – leads to arousal, and attention to pain
  • goes to the parabrachial nucleus of pons – leads to limbic system (cingulate and insula cortex) this gives the perception of pain
63
Q

what is the limbic system

A

The limbic system includes a group of cortical and subcortical nuclei found on the medial aspect of the frontal, parietal and temporal lobes

64
Q

what is the cingulate cortex

A

The cingulate cortex is part of the limbic system and this is where are perception of pain occurs

65
Q

where does the perception of pain occur

A

cingulate cortex

66
Q

what is referred pain

A
  • no map for internal organs
  • therefore the pain is felt in the region of the body surface that has its afferent input in the same spinal segment (dermatome) that the active visceral pain fibres terminate
67
Q

how do you treat neuropathic pain

A

antiepipletic and anti seizure drugs

68
Q

what kind of lesion is..

  • brown sequard
  • stroke
  • diabetic neuropathy
A
  • spinal
  • cortical
  • PNS

Brain and Behaviour (34 decks)

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