Somatosensory system

Question 1

Define modality. (2)

Answer 1

A “flavour” of sensory information determined by the type of receptor detecting the information.

Question 2

Describe the modalities of the dorsal column-medial lemniscus pathway and the receptor type that interprets them. (8)

Answer 2

Vibration - mechanoreceptors
Fine touch - mechanoreceptors
Proprioception - muscle spindles
Two point discrimination - mechanoreceptors

Question 3

Describe the modalities of the spinothalamic tract and the receptor type that interprets them. (6)

Answer 3

Temperature - thermoreceptors
Pain - nociceptors
Pressure / crude touch - mechanoreceptors

Question 4

Describe the location of the somatosensory tracts within the spinal cord. (5)

Answer 4

DCML system - dorsal funiculus with the cuneate and gracile funiculi.
Spinothalamic tract - ventral and lateral, around the lateral curve of the ventral horn.

Question 5

Describe the differences between rapidly adapting and slowly adapting receptors. (7)

Answer 5

Strong activation causes a high frequency of action potentials, and weak activation causes a low frequency of action potentials.
Rapidly adapting receptors (mechanoreceptors) respond best to changes in strength of stimuli, but their frequency of firing drastically reduces after initial stimuli, which is why you don’t feel your clothes on you.
Slowly adapting receptors (nociceptors) don’t change their frequency of firing after the initial stimuli passes, so you don’t get “used to” pain.

Question 6

Describe the relationship between acuity and the size of receptive field. (2)

Answer 6

Acuity is proportional to the inverse (1/x) of receptive field size, meaning acuity is best when the receptive field is small.

Question 7

Describe the three types of neurones present in the somatosensory pathways. (9)

Answer 7

First order neurones:
Have cell bodies in the dorsal root ganglion
Communicate with a sensory receptor
Connect to second order neurones
Project into the ipsilateral side of the spinal cord.
Second order neurones:
Have cell bodies in the medulla or spinal cord (dep on pathway type)
Decussate
Project onto the third order neurone
Third order neurones:
Have a cell body in the thalamus
Project onto the primary sensory cortex in the postcentral gyrus.

Question 8

Describe the path of first order neurones in the dorsal column-medial lemniscus pathways. (

Answer 8

First order neurones enter at their spinal level and are ordered with lower level medially, higher level laterally. This means T7 and below run in the Gracile fasciculis, and T6 and above run in the Cuneate fasiculus.
In the medulla, they synapse onto second order neurones.

Question 9

Describe the path of second order neurones in the dorsal column-medial lemniscus pathway. (4)

Answer 9

First order neurones synapse in the medulla.
Decussates, then runs through the medial lemniscus brainstem pathway.
Synapses with the third order neurones in the thalamus.

Question 10

Describe the path of third order neurones in the dorsal column-medial lemniscus pathway. (3)

Answer 10

Synapses with second order neurones in the thalamus, projects onto the primary sensory cortex.
Lower spinal nerve = more medial on the homunculus.

Question 11

Describe the path of first order neurones in the spinothalamic tract. (1)

Answer 11

Enters the spine at the spinal level and synapses immediately at that level.

Question 12

Describe the path of the second order neurones in the spinothalamic tract. (5)

Answer 12

Synapses on the first order neurones at the spinal level and decussates immediately across the ventral white commissure. Ascends the cord to the thalamus, passing through the spinal lemniscus of the brainstem, where it synapses in the third order neurone. Lower spinal nerves run more laterally.

Question 13

Describe the path of the third order neurones of the spinothalamic tract. (3)

Answer 13

Synapses on the second order neurones in the thalamus, and project to the primary sensory cortex.
Lower spinal nerve = lateral cord = medial cortex.

Question 14

Explain what would happen if a lesion occurred at the ventral white commisure at L5. (3)

Answer 14

Bilateral signs because both sides pass through here, so bilateral loss of spinothalamic. Only at the L5 level though, because levels below don’t pass through here.

Question 15

Describe Lissauer’s Tract. (3)

Answer 15

The tract through which first order neurones of the spinothalamic tract can ascend the cord before synapsing, meaning the lesion would be higher than you think.

Question 16

Describe Brown-Sequard Syndrome. (5)

Answer 16

A complete cord hemisection leads to unilaterally destroyed dorsal and ventral horns, white matter paths, and the dorsal and ventral roots.
This leads to:
Ipsilateral complete anaesthesia of the single dermatome of the lesion (dorsal root and horn lost).
Ipsilateral loss of DCML modalities below the lesion because they can’t ascend (level spared because this can)
Contralateral loss of spinothalamic modalities at and below the lesion.

Question 17

Explain why rubbing a bruise helps relieve the pain. (3)

Answer 17

Activation of mechanoreceptors (by rubbing) alleviates pain by acting on interneurones in the cord that prevent pain-impulse transmission of second order neurones.

Question 18

Explain why a vegan diet can cause nerve damage.

2

Answer 18

B12 deficiency can lead to demyelination of large neurones which leaves them open to nerve damage.