31 & 32- Somatosensation

Question 1

define somatosensation

Answer 1

sensory perceptions that arise from stimuli received by the body’s sensory receptors

Question 2

describe the two divisions of the somatosensory system

Answer 2

1. mediated by low threshold mechanoreceptors - perceive touch, tactile sensations, proprioception
2. mediated by high threshold mechanoreceptors - perceive pain, temperature and crude touch

Question 3

describe the differences between fast and slow adapting mechanoreceptors

Answer 3

fast-adapting = e.g. Meissner’s corpuscles, Pacinian corpuscles
- respond quickly to stimulus, adapt quickly = AP firing rate decreases shortly after stimulus is applied
- suited for detecting changes in stimulus intensity

slow-adapting = e.g. Merkel’s discs, Ruffini corpuscles
- respond slower to stimulus, maintaining sustained AP firing rate for a longer period even during continuous stimulus
- suited for detecting the prolonged aspects of stimuli = continuous pressure/touch for maintaining grip and posture

Question 4

ideal cutaneous mechanoreceptors for reading Braille?

Answer 4

superficial receptors, fast-adapting = Meissner’s corpuscles give general indication of Braille

superficial, slow-adapting = Merkel’s discs capture fine details of Braille

deeper cutaneous receptors are less useful

Question 5

importance of receptor receptive fields

Answer 5

important in localising stimuli

small receptive fields (e.g. superficial cutaneous receptors) provide a higher resolution map of the skin surface. small indentations can produce a threshold response for AP firing.

large receptive fields (e.g. deep cutaneous receptors) provide a lower resolution map of the body.

receptive fields smaller at the tips of fingers, larger on other parts of the hand

Question 6

describe features of Merkel’s discs as cutaneous receptors

Answer 6

superficial
small receptive fields = higher resolution map of body surface. small indentations generate a threshold response for AP firing.
slow adapting = respond slowly to the onset of a stimulus, sustained AP firing rate even with continuous stimuli

respond to pressure and light tough

Question 7

describe features of Meissner’s corpuscles as cutaneous receptors

Answer 7

superficial
small receptive fields
fast adapting = responds quickly to the onset of a stimulus, AP firing rate decreases as it adapts to intensity of stimulus

respond to low frequnecy vibrations and light touch

Question 8

describe features of Pacinian corpuscles as cutaneous receptors

Answer 8

deep
large receptive field
fast adapting

respond to high frequency vibration

Question 9

describe features of Ruffini corpuscles as cutaneous receptors

Answer 9

deep
large receptive field
slow adapting

respond to skin stretch

Question 10

superficial vs deep cutaneous receptors

Answer 10

superficial - e.g. Merkel’s discs and Meissner’s
- close to skin surface
- high threshold mechanoreceptor afferents
- small receptive fields
- sensitive to discriminative touch

deep - e.g. Pacinian and Ruffini
- deeper in skin surface/ hypodermis
- low threshold mechanoreceptor afferents
- large receptive fields
- sensitive to vibrations and stretch

Question 11

describe tactile afferents as somatosensory receptors

Answer 11

A-beta = second fastest conduction and largest diameter, myelinated
- have sensory receptors with cutaneous receptors for discriminative touch and tactile sensation

cutaneous receptors - two subgroups:
- superficial, high-threshold mechanorecs. = Meissner’s corpuscles, Merkel’s discs
- deep, low-threshold mechanorecs. = Pacinian corpuscles, Ruffini endings

Question 12

describe proprioceptors as somatosensory receptors

Answer 12

A-alpha afferents = fastest conducting, largest diameter, myelinated

involved in proprioception = provide info. about limb and body position through two types of fibres = group Ia & golgi tendon organs

group Ia sensory endings = associated with muscle spindles embedded in contractile muscle fibres
- changes in muscle fibre length through contraction/ lengthening
- motor axons send APs to muscle spindles causing weak contractions
- group Ia sensory endings are sensitive to the change in muscle spindle length

golgi tendon organs = acts as a proprioceptor - sensory endings near joints of muscles with tendons, sensitive to muscle tension

Question 13

describe free nerve endings as somatosensory receptors

Answer 13

C-fibres and A-delta afferents

A-delta afferents = smallest diameter of myelinated axons, thinly myelinated, moderate conduction

C-fibre afferents = smallest diameter, unmyelinated, slow conducting

have bare nerve endings – without specialised structures encapsulating them

involved temperature and pain perception, sense mechanical pressure = high-threshold stimuli

Question 14

explain the concept of the somatotopic map in cerebral cortex

Answer 14

somatotopic map = representation of different body areas in specific cerebral cortical regions

primary somatosensory cortex in the postcentral gyrus of the parietal lobe involved in mapping = receives sensory info from afferent fibres

Brodmann areas - areas 1,2,3A &B = involved in sensory processing for forming the somatotopic map
- adjacent Brodmann areas represent adjacent body areas
- organised based on subtle differences in cortical cytoarchitecture - e.g. thickness of layers, cell size and density
- information from Brodmann areas is duplicated four times for the somatotopic map = map is a highly distorted representation

somatotopic map/ homunculus is based on receptive fields which determine tactile discrimination resolution
- represents relative size and arrangement of areas based on levels of sensitivity and innervation
- areas with smaller and more numerous receptive fields = higher resolution and tactile discrimination
e.g. fingers and lips = more cortical expansion
- relies on primary somatosensory cortex and higher order structures like S2, cerebral cortical connections, amygdala and hippocampus

Question 15

describe the effects of spinal cord lesions on the DCML and STT pathways - effects on somatosensation?

Answer 15

DCML pathway spinal cord lesions affect tactile sensations on the same side as the lesion
e.g. lesion on the left = disrupts mechanosensory afferents ascending from the left = disrupt tactile sensation on the left

STT pathway spinal cord lesions affect pain and temperature sensations on the opposite side of the lesion
e.g. lesion on the left = pain/temperature afferents ascend and cross over contralaterally to the left = interrupt sensations on the right

Question 16

A-delta and C fibres in nociception

Answer 16

nociceptors are sensitive to painful stimuli

thermal nociceptors are specifically sensitive to harmful/ tissue-damaging temperatures (e.g. higher than 55 degrees)

A-delta fibres responsible for initial, sharp pain sensation

C-fibres = slower, dull second wave of pain that’s more sustained

A-delta fibres conduct faster than C fibres

Question 17

describe the transduction mechanism of noxious heat in C fibres

Answer 17

thermosensitive C fibres have TRPVI receptors at their peripheral ends

capsaicin molecule in chilli peppers can activates TRPVI receptor

opening of ion channels = cation influx

noxious heat transduction

Question 18

mechanism of maintaining pain even after injury

Answer 18

inflammatory response from damaged tissue - release of various inflammatory mediators, cytokines, prostaglandins which maintain depolarisation and hypersensitivity of C fibres

local inflammatory action keeps C fibres hypersensitive - continued AP firing to spinal cord dorsal horn interneurons and brain leading to hyperalgesia and allodynia

Question 19

mechanism/pathway behind referred pain

Answer 19

referred pain = pain perceived at a location other than the actual site of tissue damage

A-delta and C fibres enter via dorsal root, terminate in dorsal horn

2nd order neurone crosses and ascends contralaterally to spinal cord - STT pathway

multimodal neurons receive convergent non-nociceptive and nociceptive inputs which enhance pain sensation and increase dorsal horn interneuron excitability = convergence of different visceral and somatic afferents into a bundle causes referred pain

inputs all converge onto the same 2nd order neurons in the STT system

Question 20

describe descending modulation of pain pathways

Answer 20

modulated by endogenous opioids – e.g. enkephalins and endorphins as peptide NTs

endogenous opioids bind to opioid receptors in the CNS - spinal cord expresses enkephalinergic interneurons, brainstem expresses opioids and their receptors

can inhibit descending pain pathways by inhibiting the transmission of pain signals from C-fibres to second order neurons = activate enkephalin-containing interneurons of SC/ dorsal horn projecting neurons = interrupts STT pathway

Question 21

describe how epidurals work

Answer 21

synthetic opiates injected into spinal cord epidural space - activate opioid receptors and reduce pain

Question 22

describe acute pain

Answer 22

well understood

driven by nociceptor stimulation, associated with tissue damage

responds to treatments like NSAIDs with an effective resolution

Question 23

describe chronic pain

Answer 23

poorly understood

persists beyond tissue damage - could be central/ CNS related, malfunctions in pain circuitry

neuropathic pain is associated with nerve damage from a previous injury OR trauma affecting brain structures of the medial pain pathway

treatments: experiments, neurosurgery, antidepressants as constant pain can take a mental toll

Question 24

pain as both a sensory and affective response

Answer 24

sensory = discriminating features of pain such as location, intensity

affective-motivational =affects mood and behaviour, unpleasantness or arousal

two systems are involved as central pain processing systems - medial and lateral

lateral: DCML and STT pathways come together to VP thalamic nuclei to S1 and higher order structures & S2 for further pain processing
- DCML is responsible for tactile sensations and discrimination of pain, involves the somatosensory map with S1
- STT is responsible for nociception, receives input from A-delta and C fibres

medial: medial thalamic nuclei sends signals to insular cortex and anterior cingulate cortex = affective response
- STT ascending axons branch to VP thalamic nuclei, to S1 & S2 = somatotopic maps for discrimination of pain
- amygdala, hypothalamus PAG grey matter of brainstem involved too