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Somatic Sensation Flashcards

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1
Q

Somatosensory system involves

A

cutaneous sensation
proprioception
kinesthesis

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2
Q

proprioception

A

the sense of limb position

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3
Q

kinesthesis

A

the sense of limb movement

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4
Q

skin

A

epidermis ( dead layer of cells )
dermis ( living layer beneath epidermis )

in both layers are the mechanoreceptors located

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5
Q

glabrous

A

hairless

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6
Q

four principal mechanoreceptive afferent systems

A

slowly adapting type 1 ( SAI) affarents that end in Merkel cells

rapidly adapting ( RAI) affronts ending in Meissner’s corpuscles

rapidly adapting Pacinian corpuscles ( PC ) type II

and slowly adapting type 2 ( SAII) affarents that end in Ruffini endings

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7
Q

mechanoreceptive affarent systems

A
  • each serve a distinctive perceptual function , type of stimulation responded to best determined by accessory structures
  • tactile perception: sum of activity of four mechanoreceptive systems
  • respond to mechanical stimulation by producing a depolarising receptor potential
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8
Q

Pacinian corpuscle ( PC )

A

composed of:
concentric layers of cellular membranes alternating with fluid filled spaces
distributed widely
extremely sensitive - responding to 10 nm of skin motion at 200Hz

–> role in perception of events through an object held in hand

receptive fields: central zone of max sensitivity surrounded by large continuous surface

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9
Q

distribution of Pacinian corpuscle

A

wide

eg skin,
connective tissues in muscles,
periosteum of bones
mesentery of the abdomen

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10
Q

Meissner’s corpuscles

A

small receptive fields (averaging 3-5 mm), up to 20 receptors per neuron

150/cm^2

rapidly adapting structure

respond to low frequency vibration i.e 10-15 Hz

initial contact and motion

enhanced sensitivity and poorer spatial resolution ( like scotopic vision )

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11
Q

Merkel cells

A

dense innervation of the skin- small epithelial cell found under fingerprint ridges

pressure: firing frequency proportional to pressure applied

small receptive field --> high spatial resolution, 
decreased sensitivity ( like photopic vision )

10 times more sensitive to dynamic stimuli than static

sensitive to points, edges and curvature

spike discharge invariant ie very good at discrimination

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12
Q

rapidly adapting affarents respond to

A

change in stimulus

ie burst of APs at onset / offset but non if maintained

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13
Q

slowly adapting afferent fibres ( activation )

A

tonic activation if stimulus is continuous

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14
Q

phase locking in rapidly adapting affarents

A

if stimulus intensity ( ie. skin indentation) is sinusoidal, phase locking occurs with AP around peak stimulus as response has then decayed enough by next peak to detect a “change”
–> RA useful for sensing vibration

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15
Q

accessory structure

A

not directly involved in transduction but aids sensory process eg through protection, conduction, concentration, analysis, sensitisation, inhibition

eg pacinian corpuscle lamellae , cornea + sense of eye , basilar membrane cochlea, intrafusal muscle fibres

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16
Q

overall sensitivity to vibration determined by

A

combination of Meissner’s and Pacinian corpuscles - can be altered by changing the responsiveness of the two receptors

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17
Q

intensity of sinusoidal stimulus

A

encoded by the number of sensory fibres that are active
( not the frequency of firing )

numb of active fibres linearly related to amplitude of vibration

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18
Q

Pacinian corpuscle rapidly adapting

A

onset of step pulse/ turning off the stimulus

receptor potential rises + decays ( adapts ) rapidly

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19
Q

desheathed pacinian corpuscle adapting

A

lamellae removed –>increase in Receptor potential with increase in stimulus intensity, become slowly adapting

as receptor potential plateaus before repolarising

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20
Q

how to reduce sensitivity in Meissner’s corpuscle/ raise threshold

A 
local anasthetics ( lie close to skin) 
preadapting to low frequency stimulus ( 30 Hz) m
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21
Q

how to reduce sensitivity in Pacinian corpuscle/ raise threshold

A

pre-adapting receptor to 250 Hz stimulation ( high frequency)

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22
Q

Merkel cells linear dynamic range

A

AP firing rate and. perceived stimulus intensity both increase linearly with stimulus intensity from 200-2000 nanometers

Ouput of neuron and psychophyisical intensity match

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23
Q

dynamic range comparison Merkel cell vs visual auditoy systems

A

Merkel: less than one order of magnitude

vision/audition: many orders of magnitude

Vision hearing:
Non-linear –> saturated sigmodial input output functions, in vision: rods and cones ( light adaptation shift input output function )

Merkel: linear

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24
Q

Ruffini ending

A

little known
thought to contribute to:

motion perception ( respond to skin stretch )
information about hand shape and finger position
relatively high threshold : deep in skin

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25
Ruffini ending transduction mechanism
tension applied to collagen tightens axon spirals
26
spatial event plots show
AP responses of fibres in response to spatial stimuli
27
reading Braille
Spatial even plots used to this SAI ( attached to Merkel cells ) fibres responsible for reading Braille as most similar spatial event plot to the actual Braille pattern - also makes sense because smallest receptor field and only receptor in epidermis i.e most superficial
28
tactile acuity determined by
two-point limen : smallest discriminable distance between two points of contact increases with higher mobility of body parts eg 20 fold from shoulder ( 40 mm ) to finger ( 2 mm ) small receptive fields : if two points contacting skin touch same receptive field cannot differentiate
29
fingertips
highest density of RAI and SAI fibres with small receptive fields to high tactile acuity
30
labelled lines theory
individual receptors and individual afferent fibres give information about a single type of stimulus evidence provided by warm + cold spots evidence through nociceptors microneugraphy experiments in human subjects
31
warm vs cold spots
many more cold than warm with the relative proportion varying across the body both few mm in diameter
32
spatial summation warm spots
if only warm spots convey information about warmth then large proportions of the body should be insensitive to heat however, this is not the case : hypothesis: many more warm receptors exists than spots , requires simultaneous activation of many receptors to elicit the sensation of warmth --> spatial summation
33
Trpv1 channels
active ingredient of child peppers, capsaicin and painful increase in temp above 43 degrees
34
Trpm 8
menthol | non-painful decreases in temp below 25
35
cold receptors connected to
A-delta | C-fibres
36
warm receptors connected to
C-fibres ( subpopulation )
37
TRP
transient receptor potential | Na+ and Ca+ channels
38
skin thermoreceptors are
free nerve endings - no accessory organs
39
paradoxical cold
sensory illusion when heat stimulus over 45 degrees applied to cold spot --> perceived as cold applied to diffuse area of skin --> perceived as painful activity of cold fibre experienced as cold irrespective of physical nature of stimulus
40
pain mediated by
nocireceptors
41
nociceptors
free nerve endings with no accessory organs (specialised endings) two different affarent fibres ( A-delta and C ) respond to different components of pain : early ( first ) sharp pain second , dull, burning pain
42
A-delta fibre nociceptor
first pain, initial sharp pain | myelinated
43
free nerve endings nociceptors consequence
particularly sensitive to : | chemicals produced or released at site of injury
44
C-fibre nociceptor
second pain i.e throbbing unmyelinated polymodal ie one can respond to noxious hot, cold, mechanical ( strong ) , chemical stimuli ( Chilli peppers acid )
45
C-fibre tactile affarents ( CT )
fibres respond to light touch, low-velocity stroking --> pleasant stimulation low conduction velocity ( 1m/s) found only in hairy skin
46
microneurography
how does it work: what are we trying to measure: eg response of myelinated A-delta affarent shorter latency than CT affarent
47
CT - emotional feeling not touch?
there is a non-linear relationship between velocity and action potential firing non linear relationship between pleasantness of stimulation and stroke velocity but a linear relationship between CT output ( mean impulse rate ) and rating of pleasantness
48
peripheral nerves ( bundles of fibres ) grouped by
fibre diameter
49
large myelinated axons
A-alpha , A-beta
50
small unmyelinated axons
C-fibres
51
intermediate thinly myelinated axons
A-delta
52
compound action potential
if stimulating median nerve action potentials are summed
53
fibre diameter influences
conduction velocity
54
pain evoked with
stimulation of A-delta and C-fibres If these are removed with anasthetic , no pain felt even if A-beta fibres are stimulated strongly mechanically
55
diabetes, multiple sclerosis
the myelin sheath of large diameter fibres can degenerate resulting in a slowing of nerve conduction or failure of impulse transmission
56
role spinal chord and bilateral spinal nerves
receive affarent fibres from sensory receptors of the trunk and limbs control movement of trunk and limbs provide autonomic innervation for the viscera
57
the spinal chord has two enlargements
wide portions due to many nerve fibres to and from limbs : lumbar cervical
58
lumbar
CSF can be removed in lumbar puncture for diagnosis anaesthetics can be injected into epidural space to induce epidural block does not contain actual spinal chord but elongated spinal roots
59
Cervical
may fill with fluid , resulting in syringomyelia --> loss of fibres which cross to contralateral side die A-delta and C but ipsilateral A beta fibres retained
60
dermatome
area of skin innervated by a single dorsal root dermatomal boundaries overlap by mixing fibres from several dorsal roots in the peripheral nerve
61
spinal chord anatomy
two symmetrical halves divided into by: dorsal median sulcus ventral median fissure around central canal : H-shaped grey matter - -> nerve cell bodies - -> divided into functionally distinct laminae ( Rexed's laminae) white matter: affarent and efferent axons 3 regions: dorsal, lateral, ventral
62
dorsal column- medial lemniscal ( DC-ML ) system
main pathway for information about touch and proprioception : tactile, vibratory, proprioceptive sensations consists of large diameter myelinated fibres ( A-alpha, A-beta)
63
primary affarent entering spinal chord
``` bifurcates into: dorsal horn ( short branch ) dorsal columns ( long branch ) ```
64
long branch axons ( spinal chord )
enter spinal chord below the mid-thoracic level ascend in the fascicles gracilis terminate in fragile nucleus
65
entering above mid-thoracic level
enter fascicles cuneatus | terminate in cuneate nucleus
66
dorsal column nuclei
cuneate and gracile nuclei organised according to somatic origin : leg medially arm laterally
67
pathway is organised
somatopically
68
leaving dorsal column nuclei
axons cross brainstem and ascend to the thalamus int the medial lemniscus
69
spinothamalic tract
major ascending nociceptor pathway in spinal chord | axons from neurons in layers I and V-VII of dorsal horn
70
anterolateral system
spinothalamic tract as the axons ascend in contralateral, anertolateral white matter
71
lesions of anterolateral system
reduce pain sensations for contralateral side of body however, pain relief often only temporarily
72
spinoreticular tract
projects from laminae VII and VIII , terminates in reticular formation and thalamus some axons travel ipsilaterally ( do not cross mid-line)
73
spinomesencephalic tract
projects from laminae I and V , via anterolateral quadrant of the spinal chord to the mesencephalic reticular formation and the periaqueductal gray
74
sensation in face
cranial nerves not spinal nerves
75
periphery
skin, joint, muscles
76
sensory fibres
dorsal side of spinal chord
77
lesion info used to deduce location of
A-delta and C vs A-beta fibres in spinal chord
78
Hemisection: Brown Sequard
effects below site of lesion: loss of pain and temperature sensation contralaterally ( A-delta and C loss of fine touch and proprioception ipsilaterally ( A-beta mechanoreceptors )
79
Syringomyelia
caused by fluid filled cavity within the spinal chord disrupts anterolateral system ( a-delta and C fibres ) - -> loss of pain and temperature sensation in upper limbs and trunk - -> preservation of touch and pressure sensation
80
posterior column syndrome
dosal lesion: bilateral abscence of touch below lesion
81
complete transection
impairment of all sensory modalities below level of transection
82
trigeminal nerve
cranial nerve V branches intracranially into three divisions : ophthalmic, maxillary ( purely sensory) , mandibular ( mixed sensory and motor ) provides general sensory innervation providing motor fibres for muscles of mastication + smaller muscles
83
Tic doulourex
trigeminal neuralgia gentle stroking of face or mouth --> massive stabbing pain example of allodynia
84
allodynia
ophthalmic and maximiliary branches
85
lateral inhibition
receptive fields show centre surround lateral inhibition ( analogous to that of the visual system ) --> enhances contrast between fibres hence point-point discrimination ( excitation of neurons between two points surpassed )
86
thalamus
contains synapses for both the touch sensitive pathway ( ventral posterior nucleus ) and pain pathway ( ventral medial nucleus )
87
receptive fields and gate control
receptive fields can change for a cell depending on gate control eg dogs: neurons may have a receptive field on a foot but this is only due to descending inhibition if the inhibition is removed ( by cooling the relevant are of the spinal chord ) the receptive field moves to the flank
88
Location of primary somatosensory cortex
S1 in post-central gyrus - which is located posterior of the central sulcus in parietal cortex Brodman map : area 3, 1, 2
89
cortex organised into
series of vertical columns | layers
90
columns in somatosensory cortex preserve stimulus location | modality
stimulus location modality all neurons receive input from same are of skin about a particular modality eg touch, temperature
91
where do thalamic affarents terminate
Layer Iv
92
Which layer projects back to the thalamus
Layer VI
93
Layer II and III project to
cortical regions
94
Layer V projects to
subcortical structures: basal ganglia, brainstem, spinal chord
95
Homunculus
somatotopic map of sensory inputs to cortex with exaggeration of regions according to how much of the cortex they occupy. Face big for humans, nose for star-nosed mole
96
Direction sensitivity
some cortical neurons are direction sensitive by combining several lower receptive fields, just as occurs in the visual system
97
What explain direction sensitivity
spatial arrangement of excitatory and inhibitory inputs --> whether they overlap and in what direction the motion is: are the excitatory and inhibitory areas stimulated simultaneously ( weak output ) , what is stimulated primarily
98
Attention- cortex
in S2, firing rate of neurons is modulated by attention i.e reduced if the person is distracted by visual task
99
cortex plasticity
receptive fields not fixed- can be modified by experience or injury ( amputations ) eg if monkey trained to keep finger on rotating disk ( receives reward ) expansion of representation of finger eg homunculus of guitar players has unusually large representation of fingers of left hand
100
phantom limb pain
plasticity of cortex responsible for this phenomenon area of cortex receives ascending inputs from a different area of skin often leads to perception of pain and doesn't appear to be useful
101
Phantom limb not restricted to somatosensory cortex
tinitus in audition
102
Regions in cortex responding to pain
S-I , the anterior cingulate cortex, the insula
103
Anterior cingulate cortex ( ACC )
Part of limbic system | Emotional element of pain
104
Insular cortex
Processes information of internal state of body  autonomic component of overall pain response
105
ACC experiment
ACC neuron responded with more vigour to the experience of pain (receiving) than by simply watching the delivery of painful stimuli to the examiner (watching)
106
pain
“an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”
107
Projection neurons in lamina I
narrow dynamic range ie only tespond to noxious stimuli ( specificity theory of pain ) input from myelinated Aδ nociceptive fibres and both direct and indirect input from ‘C’ fibres
108
Projection neurons in lamina V
receive input from both large diameter fibres (Aβ) from mechanoreceptors as well as input from other nociceptors.  respond to innocuous stimuli at low intensity  And noxious stimuli at high intensities  wide dynamic range
109
Wide dynamic range neurons
can signal changes in stimulus intensity by increases in spike discharge rate over a wide range of intensities
110
Single neuron with narrow dynamic range
only signal changes in intensity over a limited range of amplitudes
111
Gate control theory
A-delta and C fibres stimulated by injury and directly excite transmission cells in the spinal chord Transmission cell also receives input from los-threshold myelinated affarents ( L )  A-beta fibres  whether this inhibits or excites depends on the intensity of the stimulus
112
Decending systems
Pain, a heavily modulated sensation: |  integrated with other body systems eg skin reflexes, emotion, attention, autonomic regulation
113
Location of descending systems
The periaqueductal gray ( PAG ) in mid-brain The raphe nucleu + other nuclei in medulla
114
Electric stimulation of PAG- effects
produce sufficient analgesia
115
PAG respnsibilites
control the ‘nociceptive’ gate in the dorsal horn by integrating inputs from the cortex, thalamus and hypothalamus
116
How can morphine induced analgesia be blocked
injection of naloxone (an opiate antagonist) into the PAG | Bilateral transection of the dorsolateral funiculus blocks both this stimulation- and morphine-induced analgesia
117
Placebo analgesia
situation where administration of a substance known to be non-analgesic produces an analgesic response when the subject is told that it is a pain killer
118
Where does the placebo effect work
asthma, cough, diabetes, ulcers, multiple sclerosis and Parkinsonism
119
Neural analgesic placebo
Is locally induced and opiate dependent as blocked by naloxone ( an opiod antagonist) Only works for areas to which topical cream is applied  seems to involve attentional mechanisms Can be measured experimentally when measuring pain threshold when capsacin is infused into distal limbs

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