3. Somatosensory system 2

Question 1

name the primary sensory neurones which carry pain impulses from nociceptors

Answer 1

C fibres

Question 2

why does rubbing alleviate pain

Answer 2

i. Impulses from mechanoRs transmitted via primary sensory A fibres…
ii. synapse in dorsal horn with inhibitory enkephalinergic interneurones…
iii. synapse and inhibit secondary neurones that normally transmit pain impulses from primary C fibre

Question 3

how can the brain moderate pain

Answer 3

Psychological inhibition (eg stress) of pain via descending inputs from brain which activate inhibitory interneurones to prevent secondary sensory neurone activation by pain impulses from C fibres

Question 4

what is the pathophysiology of Brown-Sequard Syndrome

Answer 4

Trauma/ischaemia resulting in spinal cord hemisection with unilateral loss of:

• dorsal and ventral hornes and all other cord grey matter
• all white matter pathways
• dorsal and ventral roots

Question 5

describe the 3 features of the sensory loss seen in Brown-Sequard Syndrome

Answer 5

1. ipsilateral anaesthesia affecting a single dermatome (due to destruction of dorsal root and dorsal horn)
2. ipsilateral loss of dorsal column modalities below lesion
3. contralateral loss of spinothalamic modalities below lesion

Question 6

what type of injury would cause complete sensory loss (all modalities) of a specific dermatome, eg S1

Answer 6

Injury to dorsal root of relevant level (e.g. S1) - only place where sensory nerves of both modality types are together and isolated
(spinal nerve also has motor efferents, in spinal cord the 2 modalities diverge)

Question 7

bilateral loss of pinprick and temp sensation suggests a lesion where in the spinal cord

Answer 7

In white ventral commissure as this would affect the spinothalamic tract fibres from both sides of the body

Question 8

how can severe vitamin B12 deficiency (e.g. vegan diet) affect the nervous system

Answer 8

Causes demyelination of nerve fibres, esp. in posterior columns (and occasionally in lateral columns) of spinal cord - so loss of peripheral sensation/paraesthesia (i.e. loss of dorsal column pathway modalities: light touch, vibration, joint position sense and 2 point discrimination)

Question 9

describe the pathophysiology of syringomyelia

Answer 9

Development of a syrinx (fusiform cyst) in or beside the central canal, usually in cervical region (uncertain aetiology). Initial symptoms arise from obliteration of spinothalamic fibres decussating in white commissure.

Question 10

which modalities will be lost in syringomyelia

Answer 10

spinothalamic tract modalities: loss of sensitivity to pain, crude touch and temperature

(with fine touch and proprioception retained as dorsal column-medial lemniscus pathway is retained)

Question 11

where will syringomyelia symptoms first appear

Answer 11

Neck and arm fibres are located more medially than trunk and leg fibres so symptoms will first affect the former.