somatosensory system

Question 1

what are the functions of the somatosensory system?

Answer 1

perception of events on skin surface, pressure and pain in joints and internal organs, position of limbs in space, sensory guidance of movement

Question 2

what parts of CNS does somatosensory system consist of ?

Answer 2

primary sensory neurones, ascending spinal tracts to brainstem and thalamus, somatosensory cortex

Question 3

where are the cell bodies of the neurones in the somatosensory system

Answer 3

peripheral axon cell bodies in DRG

central axon cell bodies in spinal cord

Question 4

which receptors are triggered by touch and what is their firing frequency range and perception of these mechanoreceptors

Answer 4

merkel: pressure, 0.3-3 Hz
meissner: flutter, 3-40 Hz
Ruffini: 15-400 Hz, stretching
pacinian: 10-500Hz, vibration

Question 5

what is a difference between endings of mechanoreceptors and nociceptors?

Answer 5

mechanoreceptors have specialised endings, nociceptors do not

Question 6

what type of somatosensory neurones have their cell bodies in DRG

Answer 6

large cells, myelinated Abeta fibre axons

medium cells, thinly myelinated Adelta axons

small cells, unmyelinated C axons

Question 7

what is the order of thickness of myelination in afferents?

Answer 7

Aa- C

Aa is thickest

Question 8

which spinal segments innervate which areas of the body?

Answer 8

cervical: upper limbs
thoracic: trunk
lumbar: lower limbs
sacral: pelvic

Question 9

which laminae of the dorsal horn do different sensory neurones synapse onto?

Answer 9

Ad nociceptors: 1
C and Ad peptidergic fibres: 1 and 2
C non peptidergic fibres: 2
Ad hair follicle afferents: 2 and 3
Ab hair follicle and tactile afferents: 2-5

Question 10

how does the ascending pain pathway differ from the touch pathway

Answer 10

pain starts in nociceptors, primary neurone with cell body in DRG synapses in the dorsal horn onto second order neurones which synapse onto third order neurones at thalamus which project to primary somatosensory cortex

touch primary neurone has cell body in DRG does not synapse till it reaches dorsal column nuclei, the second neurone passes through the decussation of the medial lemniscus and then back through the dorsal column nuclei before synapsong onto a third order neurone at the thalamus which projects to the primary somatosensory cortex

(touch crosses pain does not)