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Somatosensory function Flashcards

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1
Q

what are the 4 somatosensory modalities?

A

touch
proprioception
temperature
pain

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2
Q

what comes under touch?

A

light touch
pressure
vibration

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3
Q

what comes under proprioception?

A

joint position
muscle length
muscle tension

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4
Q

what are mechanoreceptors used for?

A

touch and proprioception

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5
Q

what receptors detect temperature?

A

thermoreceptor

  • slow adapting
  • transient receptor potential channels (e.g TRPVs)
  • TRPVs are also activated by spicy food e.g. capsaicin
  • heat detection: deep in the dermis
  • cold detection: superficial dermis (e.g TRPV8)

at extreme cold temperatures, thermoceptors are inactive and nociceptors become active

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6
Q

what receptors detect damaging stimuli?

A

nociceptor

  • trigger pain as a protective mechanism
  • respond to extremem temperatures
  • respond to excess pressure and deformation
  • respond to a mix of both

pain sensitive people have a reduced pain tolerance (not pain threshold)

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7
Q

3 main receptor types

A

mechanoreceptors
thermoreceptors
nociceptors

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8
Q

examples of mechanoreceptors

A

plexuses
peritrichial endings
end bulbs

1) Proprioceptors:
- muscle spindles
- Golgi tendon organ
- joint receptors

2) Mechanosensors: touch (pressure and skin deformation)
- Merkel endings (epidermis; slow adapting)
- Pacinian corpuscles (dermis and subcutaneous; fast adapting)
- Meissner’s corpuscles (finger tips; fast adapting)
- Ruffini endings (joints; slow adapting)

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9
Q

describe a mechanreceptor detecting vibration

A

rapidly-acting
each fibre with a different threshold
generate cycles of APs
when thresholds overlap, the fibres may fire simultaneously

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10
Q

what Hz is the body most sensitive to?

A

250Hz
Pacinian peak at 250
Meissner peak at 20-50

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11
Q

what happens to vibratory threshold in neurodegeneration?

A

vibratory threshold increase

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12
Q

what leads to tickle detection?

A

relatively mild stimulation to areas of the body with naked unmyelinated afferent nerve fibres

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13
Q

what leads to itch detection?

A

o Caused by local mechanical stimulation or chemical agents (e.g. histamine).
o Relieved by scratching – stimulates large nerve fibres that overwhelm spinal transmission.

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14
Q

where in the body is temperature detection most sensitive?

A

face and chest

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15
Q

when do temperature gated channels open?

examples of TGCs

A

they open and close at different ranges of temperatures

TRPV channels are triggered by heat and capsaicin : TRPm8 and TRPv1

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16
Q

what does TRPM8 respond to?

A

cold e.g. menthol

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17
Q

what does TRPV1 respond to?

A

heat and capsaicin

includes V1-4

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18
Q

what do nociceptors respond to?

A

noxious or harmful stimuli

require high threshold for activation

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19
Q

how are nociceptors triggered?

A

direct activation of ion channel proteins, TRP, neurotrophin and GPLRs

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20
Q

what nociceptor detects a pH below 7

A

Acid Sensing Ion channels

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21
Q

what nociceptor senses intense pressure?

A

K+ channels

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22
Q

what are the commonest nociceptors

A

polymodal C fibres (cutaneous)- pressure, temperature, chemical

chemoreceptors (skeletal) - lactic acid

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23
Q

why are inflammations painful?

A

there is no single pain neurotransmitter so substances modulate nociception
e.g. prostaglandin, substance P, histamine, serotonin, bradykinin etc

there are associated with inflammation hence painful inflammation

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24
Q

definition of stimulus threshold

A

weakest stimulus detectable

adequate stimulus required to elicit a specific response/reflex

minimum stimulus detected at least 50% of the time

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25
how is stimulus intensity gauged? | 2 ways involving APs and receptors
1) frequencies of APs generated | 2) number of separate receptors activated (recruitment)
26
what is a receptive field?
area that a sensory nerve innervates a small receptive field has high resolution and picks up stimuli more precisely
27
what allows the increased sensation of stimulus intensity?
the receptive fields overlap, so the recruitment of adjacent fields can increase stimulus intensity
28
how is increased number of APs interpreted?
it is seen as increased intensity by the brain
29
what enables pinpoint accuracy in location of the stimulus?
lateral inhibition: activation of one neural unit inhibits the activation of other neural units - strong stimuli in a receptive field could mildly stimulate nearby neurones - the receptor receiving the strongest stimuli sends inhibitor signals via interneurones to adjacent 1st order neurones to suppress the activity - this enables point discrimination as boundaries are being defined
30
what mediates lateral inhibition?
interneurones within the dorsal horn of the spinal cord
31
what is two-point discrimination?
the ability to detect two stimuli as distinct from each other there is two-point threshold
32
what is the two-point threshold?
minimum distance required between 2 stimuli in order to perceive that they are two separate stimuli
33
what does two-point discrimination depend on?
peripheral mechanoreceptors spinal posterior column cortical fuction location e.g. back (65mm) vs fingers (2mm)
34
what causes the difference in two-point discrimination in e.g. the back and the fingers?
density of innervation area of receptive field sensory homunculus
35
what is neural adaption and how does neural adaption occur?
- a strong stimulus increases the firing of 1st order neurones - when a stimulus of constant strength is maintained for a period of time, the frequency of APs diminished as the e. g. after sitting on the chair, you don't feel the chair anymore after the initial sensation of the chair - this uses fast adapting (phasic) and slow adapting (tonic) receptors
36
what affects the time taken for neural adaption?
due to the type of neurones and receptors: 1) phasic- rapidly adapting - initially very active when the stimuli first starts, but stops firing eventually 2) tonic- slowly adapting - keep firing without loss of sensitivity
37
A fibre alpha
stimulus: proprioception, somatic motor largest, myelinated fastest non-noxious
38
A fibre beta
stimulus: touch, pressure - large-medium, myelinated - fast non-noxious stimuli
39
A fibre gamma
motor to muscle spindle small slow
40
A fibre delta
stimulus: cold temperature, gross touch, sharp pain - small myelinated - slowest noxious
41
B fibre
postganglionic autonomics
42
C fibre dorsal root
stimulus: burning pain, hot temperature and mechanoception - non-myelinated - slowest noxious
43
C fibre sympathetic
postganglionic sympathetic
44
what nerve fibres are involved in pain (noxious)
A delta and C fibres
45
what nerve fibres are involved in touch and pressure (non-noxious)
A beta
46
which neurones are for perception and interpretation of pain?
cortical neurones
47
which neurones are for modulation of nociceptive signals
thalamic neurones | ventrobasal complex and nucleus reticular
48
which neurones process nociceptive signals
superficial dorsal horn neurones (sensory pathway)
49
what is the pathway for touch and proprioception?
via dorsal column (medial leminiscal pathwayP - uses A-alpha and A-beta fibres (large myelinated) - up the cuneate fasciculus (arms and chest); located laterally - up the gracilis fasisculus (trunk and legs); located medially - decussation occurs in the caudal medulla (where 2nd order neurone begins) - synapses at ventroposterior nucelus (VPN) in the thalamus where the 3rd order neurone starts - lateral inhibition occurs in the dorsal column nuclei
50
what is the pathway for pain and temperature?
via spinothalamic tract - sharp pain and cold via small myelinated A-delta fibres - burning pain and heat via non-myelinated C fibres - decussation occurs at the same level in spinal cord (about 2 vertebral levels up) where the 2nd order neurone starts - run up the lateral and anterior funiculi
51
Brown-Sequard syndrome | what are the effects of hemi-section of the spinal cord?
caused by damage to one half of the spinal cord, i.e. hemisection of the spinal cord i.e below medulla: - ipsilateral paralysis (corticospinal) and loss of touch+ proprioception (dorsal column) - contralateral loss of pain and temperature (spinothalamic) sensation on the opposite
52
how can cortical response mismatch the peripheral input? | i.e. the brain doesn't do accordingly to the stimulus
Modulation: - spinal neurones respond to peripheral stimuli - but are modulated by : 1) interneurones 2) descending inhibitory controls
53
what is glutamate and what receptors does it activate?
excitatory synaptic neurotransmitter | activates AMPA, NMDA, mGluR, Kainate
54
activation of NMDAr
removing Mg2+ plug (no longer inactive) causes large Ca2+ influx intracellular action
55
what is the effect of long term potentiation of NMDAr
hypersensitivity to pain
56
effect of ketamine
NMDAr antagonist painkiller dissociative analgesia (pain that is there but you don't notice)
57
what is the gate control theory?
non-painful/non-noxious stimuli can inhibit the transmission of pain from peripheries to the brain e.g. rubbing elbow after hitting it small A delta and C fibres transmit noxious stimuli large A beta fibres transmit non-noxious stimuli stimulating the large fibres you can reduce neuropathic pain
58
what effect do the large fibres have in transmission of pain?
- prevent/reduce transmission of the smaller fibres within the dorsal horn of the spinal cord that are used to stimulate painful stimuli - instead, the larger fibres are being used (e.g. A-beta) so transmission in noxious fibres is reduced this "closes" the gate inhibitory interneurones have been activated
59
how can the dorsal root ganglia be targetted to treat neuronal dysfunction
X-ray guided techniques to reduce the sensitivity of the DRG electrical stimulation leads to reduced frequency and amplitude of APs which can compensate for neuronal dysfunction
60
what happens in neuronal dysfunction?
sensitisation even to minor stimuli due to channelopathy or secondary messengers enabling depolarisation
61
What rexed laminae are involved in noxious and non-noxious stimuli detection?
1) These respond to non-noxious stimuli: o Rexed Lamina LIII-LVI = Large myelinated (A-beta, A-alpha) - respond to mechanical stimuli. 2) These respond to noxious stimuli: o Rexed Lamina LI-LII = Small myelinated (A-delta) - respond to Cold and fast pain nociception. o Rexed Lamina LI-LII = Small unmyelinated (C fibres). - respond to pain and temperature. 3) Rexed Lamina LII = Substantia Gelatinosa - involved in pain perception noxious use small fibres (C and A-delta) non-noxious use large fibres (A-beta, A-alpha) all connected by interneurones
62
descending modulation
pain pathway neurones are active even without the noxious stimulus descending pathways have inhibitory or excitatory influences on spinal nociceptive transmission mediated by 5HT3, NA, GABAa and glycine receptors
63
where is the primary somatosensory cortex located?
postcentral gyrus
64
where is the secondary somatosensory cortex located
parietal operculum, posterior parietal cortex
65
association cortex
complex mental functions surrounds primary areas most developed part of the brain
66
what areas of the brain are active in pain?
o Cerebral cortices; SI, SII, Insular cortex, Anterior cingulate and prefrontal cortex. o Brainstem. o Amygdala. o Cerebellum.
67
what are the types of somatosensory fibres based on myelination?
1) non myelinated: Type C; slow (hot temperature, burning pain) 2) small myelinated: Type A-delta; faster (cold temperature, gross touch, sharp pain) 3) large myelinated: Type A-alpha and Type A-beta; fastest (fine touch, proprioception, vibration)

LCRS Neuro (19 decks)

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