S3: somatosensory system

Question 1

List the modalities of the spinothalamic system

Answer 1

Temperature
Pain
Pressure/crude touch

Question 2

List the modalities of the dorsal column-medial lemniscus system

Answer 2

Vibration
Proprioception/joint position sense/kinaesthetic sense
Fine touch
Two point discrimination

Question 3

Describe primary sensory neurones

Answer 3

Receive information from receptors and are responsible for the initial encoding of sensory information
Each individual primary neurone receives input from a single receptor type
Has their cell body in the dorsal root ganglion & collect information from a single dermatome along their peripheral axon
Project into the spinal cord along their central axon

Question 4

Compare rapidly adapting and slowly adapting receptors

Answer 4

Rapidly adapting – respond best to changes in strength of stimulation, however their frequency of firing diminishes rapidly after the initial stimulus (i.e. not aware of clothes on your skin)
Slowly adapting – change their frequency of firing very little after the initial stimulus (explains why pain can be so persistent and you never really get ‘used’ to pain)

Question 5

Describe receptive fields

Answer 5

Receptive field = single primary sensory neurone supplies a given area of skin
If an area of skin is supplied by sensory neurones with relatively large receptive fields, this area will have low sensory acuity eg. skin of back
If an area of skin is supplied by sensory neurones with relatively small receptive fields, this area will have high sensory acuity eg. skin of fingertip
Overlap of receptive fields of primary sensory neurones from adjacent dermatomes = this is why dermatomes can have ‘fuzzy’ boundaries

Question 6

Describe the somatosensory system (chain of three neurones)

Answer 6

First order sensory neurones – have their cell bodies in the DRG, communicate with a receptor, their central axon projects ipsilateral to the cell body
Sensory order sensory neurones – have their cell bodies in the spinal cord dorsal horn or medulla, decussate
Third order sensory neurones – have their cell bodies in the thalamus, project to the primary sensory cortex

Question 7

Describe the dorsal column-medial lemniscus system (upper half of body)

Answer 7

Axons of first order neurones ascend ipsilaterally through the dorsal columns of the spinal cord
Upper half of the body (T6 and above) first order neurones ascend through the cuneate fasciculus to the cuneate nucleus in the medulla
Neurones in the cuneate nucleus project to the contralateral thalamus in the medial lemniscus
Thalamic neurones receiving information ultimately from the upper half of the body project to the lateral part of the primary sensory cortex

Question 8

Describe the dorsal column-medial lemniscus system (lower half of body)

Answer 8

Axons of first order neurones ascend ipsilaterally through the dorsal columns of the spinal cord
Lower half of the body (T7 and below) first order neurones ascend through the gracile fasciculus to the gracile nucleus in the medulla
Neurones in the gracile nucleus project to the contralateral thalamus in the medial lemniscus (second order)
Thalamic neurones receiving information ultimately from the lower half of the body project to the medial part of the primary sensory cortex (third order)

Question 9

Outline the topographical organisation of the dorsal columns

Answer 9

Axons from the lower parts of the body run most medially

Axons from progressively superior body segments are added laterally to the dorsal columns

Question 10

Describe the spinothalamic pathway

Answer 10

Axons of first order neurones project to the ipsilateral dorsal cord, but the spinothalamic tract supplies the contralateral half of the body
First order neurones project onto second order neurones in the ipsilateral spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root
Second order neurones: decussate in the ventral white commissure & then go on to form the spinothalamic tract, projects to the thalamus
Third order neurones: inferior parts of the body project to medial part of primary sensory cortex and vice versa

Question 11

Describe the topographical organisation of the spinothalamic tract

Answer 11

Axons from the lower parts of the body run most laterally

Axons from progressively superior body segments are added medially/deeper onto the spinothalamic tract

Question 12

Compare how a central cord lesion would affect the dorsal column and spinothalamic tracts

Answer 12

Dorsal column – loss of lower limb (more medial)

Spinothalamic tract – loss of upper limb (more medial)

Question 13

Describe the signs of Brown-Sequard syndrome

Answer 13

Ipsilateral complete segmental anaesthesia affecting a single dermatome (due to destruction of dorsal root and dorsal horn)
Ipsilateral loss of dorsal column modalities below the destroyed segment
Contralateral loss of spinothalamic modalities at and below the destroyed segment (although level can be up to a couple of segments lower due to ascent of some primary afferents in Lissauer’s tract)

Question 14

Describe the descending modulation of pain (gate control theory)

Answer 14

A fibres carry impulses from mechanoreceptors in the skin
C fibres carry pain
Activation of mechanoreceptors (via a-beta neurones) alleviates pain (rubbing a painful area helps) by exciting inhibitory encephalinergic interneurons (inhibit the C fibres)
Additionally, these encephalinergic interneurons can also be activated by descending inputs from higher centres such as periaqueductal matter and the nucleus raphe magnus (in extreme pain, less pain can be felt due to this)