somatosensation I

Question 1

what does the somatosensory system do?

Answer 1

connects the body to the CNS via peripheral nerves

Question 2

what sensations does the somatosensory system convert from the body?

Answer 2

• heat, cold
• pain, itch
• proprioception
• touch

Question 3

what is proprioception?

Answer 3

being aware of the position of the body (like sixth sense)

Question 4

what types of peripheral nerves exist?

Answer 4

spinal and cranial

Question 5

structure of peripheral nerves?

Answer 5

a nerve is a bundle of axons, covered in a connective sheath called an epineurium

• the nerve consists of several fascicles, all separate by connective tissue called perineurium
• the fascicles contain axons, and individually myelinated axons are wrapped in endoneurium

Question 6

what does the dorsal root contain?

Answer 6

cell bodies of incoming sensory neurones

Question 7

what does the ventral root contain?

Answer 7

cell bodies of outgoing motor neurones

Question 8

what us the dorsal root ganglion?

Answer 8

occurs just as the dorsal and ventral roots fuse, located just outside of the spinal cord
-contain cell bodies of the sensory neurones that are part of the somatosensory system

Question 9

dorsal and ventral root reunite to form what?

Answer 9

spinal nerve

Question 10

white matter

Answer 10

ascending and descending axons tracks

Question 11

grey matter

Answer 11

cell bodies, dendrites, synapses, glial cells

Question 12

what is the spinal cord divided into

Answer 12

incoming sensory info and outgoing motor info

Question 13

what are dorsal root ganglion cells?

Answer 13

sensory receptors of the somatosensory system

Question 14

explain the 2 functionally distinct systems of the somatosensory system

Answer 14

2 different white matter tracts

1. (DCML)
   - mechanosensory afferent fibres
   - a-beta and a-alpha afferent fibres
   - large fibres, large diameter, myelinated, fast conduction
   - touch, vibration, tactile, proprioception
   - enter the spinal cord and enter the medulla and then cross
2. (STT)

• thin fibres
• a-delta and C fibres
• small diameter, thinly myelinated/unmyelinated, medium/slow conducting
• pain, itch, crude touch coarse, temperature
• enter the spinal cord, make contact with second order neurones in the spinal cord - these cross to the other side, then ascend up to the brain
• the receptor endings/axon terminals are in areas of the body eg. the fingers
• the axon travels centrally via the dorsal root
• axon can then terminate or they may ascend the spinal cord all the way to the brainstem

Question 15

crude touch

Answer 15

crude touch is touch without knowing the location

Question 16

what does the quality of sensation depend upon?

Answer 16

the afferent fibre type

Question 17

if the afferent fibre is a member of the small class of fibre what will the sensation be?

Answer 17

temperature related, pain or crude touch

Question 18

if the afferent fibre is a member of the large class of fibre what will the sensation be?

Answer 18

proprioception

Question 19

cutaneous receptors of the somatosensory system?

Answer 19

larger diameter afferents - 4 major classes in the skin:

2 are cutaneous, receptor endings between the dermis and epidermis - tactile afferents, sensitive to light touch - Meissners Corpuscle and Merkels discs

2 are deeper in the dermis, or between dermis and hypodermic - Ruffini’s and Pacinian corpuscles

also there are free nerve endings

Question 20

proprioceptors of the somatosensory system?

Answer 20

group 1 and 3 afferent axons detect changes in length

• muscle spindles are stretched and increase their firing rate
• muscle spindle itself can be made to contract by the gamma motor neurone - recalibration of muscle spindle length

Question 21

what are muscle spindles?

Answer 21

specialised muscle fibres, detect changes in muscle length

Question 22

receptors of the somatosensory system?

Answer 22

1. proprioception
   - muscle spindles, A-alpha afferent fibres, large diameter, fastest conduction
2. tactile afferents - A-beta afferent fibres, large diameter, second fastest conduction
   a) superficial
   Meissners Corpuscle and Merkels discs
   b) deep
   Ruffini’s and Pacinian corpuscles
3. free nerve endings
   - end in the dermis/epidermis
   - A delta fibres - thin, myelinated, moderate conduction
   - C fibres - thin, unmyelinated, slow conduction

Question 23

what do Pacinian Corpuscles detect?

Answer 23

vibration

Question 24

what do Ruffini Corpuscles detect?

Answer 24

stretch

Question 25

what are small receptive field needed for?

Answer 25

precise detail

Question 26

having a large receptive field mean?

Answer 26

the stimulus cannot be localised apart from the fact that its within that large receptive field

Question 27

larger areas of the cortex are devoted to?

Answer 27

areas of the body where receptive fields are small