Neuro4&5 - Somatosensory System

Question 1

5 features of receptors in the somatosensory system

Primary Sensory Neurone
Information Conversion
Rapidly Adapting Receptors
Slowly Adapting Receptors
Receptive Field

Answer 1

1. ) Primary Sensory Neurone - aka first-order neurone
   - can contain info from multiple identical receptors
2. ) Information Conversion - analogue –> digital
   - occurs between the receptor and the axon terminal
   - information goes from continuous to discrete
3. ) Rapidly Adapting Receptors - signal frequency decreases as time goes on so you lose awareness
   - e.g. cutaneous mechanoreceptors in the skin
   - e.g. wearing clothes, sitting on chairs
4. ) Slowly Adapting Receptors - signal frequency remains constant (stays high) e.g. pain receptors (nociceptors)
   - pain subsides once stimulus is removed
5. ) Receptive Field - a region of sensory space that can produce a neuronal response when stimulated
   - smaller receptive fields have greater sensory acuity
   - autonomous region in a dermatome is the area w/ the least receptive field overlap so the region best to test

Question 2

3 types of sensory neurones in the somatosensory system
- communications, locations, projections

First Order
Second Order
Third Order

Answer 2

1. ) 1st Order - receptor –> cell body of 2nd order
   - psuedounipolar neurone
   - cell body in dorsal root (sensory) ganglion (DRG)
   - projects to spinal cord ipsilaterally
   - involved in the monosynaptic reflex arc
2. ) 2nd Order - projects to cell body of 3rd order
   - cell body in dorsal horn or the medulla
   - decussates, so crosses the midline
3. ) 3rd Order - projects to primary sensory cortex
   - cell body in the thalamus
   - projects through the internal capsule

Question 3

4 features of somatotopy/topographical representation

Definition
Language Translation
Upper Limb
Lower Limb

Answer 3

1.) Definition - 1:1 correspondence of an area of the body to a specific point on the primary somatosensory cortex

2. ) Language Translation - dermatome –> homunculus
   - occurs in the spinal cord and thalamus
   - at the homunculus, all modalities converge
3. ) Upper Limb - lateral aspect of the sensory cortex
   - face and hands take up largest area so most susceptible to lesions
   - most lateral aspect represents the face

4.) Lower Limb - medial aspect of the sensory cortex

Question 4

6 features of the dorsal column medial lemniscus pathway (DCML)

Modalities x4 
Gracilie Nucleus
Cuneate Nucleus
Decussation Level
Relation to Spinal Cord
Isolated Lesions

Answer 4

1. ) Modalities - important for manual dexterity:
   - vibration, proprioception, fine touch, 2 point discrimination
   - lesion affecting DCML affects these modalities
2. ) Gracile Nucleus - lower body (T6/7 and below)
   - gracile fasciculus –> gracile nucleus (medulla)
3. ) Cuneate Nucleus - upper body (T5/6 to C1)
   - cuneate fasciculus –> cuneate nucleus (medulla)
   - added onto the lateral aspect of the gracile fasciculus

4.) Decussation Level - 1stON ascends to the medulla (nuclei) before the 2ndON decussates

5. ) Relation to Spinal Cord - medial to spinal cord
   - ascends straight away, no decussation
   - ascends through the dorsal funiculus
6. ) Isolated Lesions - symptoms on ipsilateral side
   - e.g. lesion on L side produces symptoms on the L side
   - generally not affected by midline lesions

Question 5

5 features of the spinothalamic tract

Modalities x3
Decussation Level
Relation to Spinal Cord
Lissauer's Tract
Isolated Lesions

Answer 5

1. ) Modalities - important for survival:
   - temperature, pain, pressure/crude touch
   - lesion affects these modalities
2. ) Decussation Level - 1stON does not ascend so 2ndON decussates at the same spinal cord level
   - 1stONs synapse at the nucleus proprius
3. ) Relation to Spinal Cord - lateral to spinal cord
   - decussates through the ventral white commissure
   - ascends through the lateral funiculus
4. ) Lissauer’s Tract - sometimes, 1stON can ascend a couple of segments via Lissauer’s tract before synapsing
   - e.g. C6 and C7 can ascend and synpase at C5 level
5. ) Isolated Lesions - symptoms on contralateral side
   - e.g. lesion on L side produces symptoms on the R side
   - midline lesions can cause bilateral symptoms

Question 6

4 features of Brown-Sequard Syndrome

Definition
Mechanism
Signs x3
Lissauer’s Tract

Answer 6

1. ) Definition - lesion in spinal cord causing paralysis on one side and a loss of sensation on the other side
   - hemiparaplegia and hemianesthesia on opposite sides
2. ) Mechanism - hemisection through spinal cord
   - following structures are destroyed unilaterally:
   - DH and DR, VH and VR, all white and grey matter
3. ) Signs
   - ipsilateral loss of all modalities in dermatome of affected SC only (due to loss of DH/DR)
   - ipsilateral loss of dorsal column modalities at SC level and below
   - contralateral loss of spinothalamic modalities at SC level and below
4. ) Lissauer’s Tract - 1stON of spinothalamic system can ascend a couple segments before synapsing in DH
   - sensory level of spinothalamic modalities (pain, temp, pressure) are sometimes a couple segments lower
   - e.g. C5 lesion but C5 and C6 preserved

Question 7

2 features of descending control of pain

Mechanoreceptors
Higher Brain Centres

Answer 7

1. ) Mechanoreceptors (A fibres) - alleviates pain
   - stimulates inhibitory interneurones in the cord which release enkephalins, inhibiting 2ndON of the spinothalamic system (pain (C) fibres)
   - rubbing a painful area helps alleviate pain
2. ) Higher Brain Centres - e.g. periaqueductal grey matter or the nucleus raphe magnus
   - can inhibit the spinothalamic pathway, stopping pain
   - occurs if in the right state (hypnosis)

Question 8

Effect of B12 deficiency on the somatosensory pathways

what is syringomyelia?

Answer 8

B12 deficiency can affect the DCML –> loss of proprioception –> sensory ataxia

Synringomyelia - fluid-filled cyst (syrinx) in the SC

• often midline so spinothalamic system affected first
• lesion can continue to expand until it affects DCML