Somatosensory System

Question 1

What are the two pathways in the somatosensory system and what modalities do they sense

Answer 1

Spinothalamic pathway - temperatue, pain, pressure/crude touch

Dorsal column pathway - vibration, fine touch, proprioception, 2-point discrimination

Question 2

Name and describe the two types of sensory receptors

Answer 2

Rapidly adapting receptors - initially fire signals very rapidly but as time progresses, receptors adapt and frequency of APs decreases

Slowly adapting receptors - receptor frequency does not change with time and the receptors do not adapt

Question 3

Describe receptive fields

Answer 3

Receptive fields - area of skin that a single sensory neurone innervates. A neurone can have multiple receptors supplying one receptive field

Neurones can have overlap between receptive fields -> boundaries of dermatomes is unclear due to overlap

Large receptive field = low acuity

Question 4

Describe the general pathway of the somatosensory system

Answer 4

Primary sensory neurone has its cell body in the dorsl root ganglion. It collects information from a given sensory neurone and then projects into the ipsilateral side of the spinal cord/medulla

Secondary sensory neurone has its cell body in the spinal cord/medulla. The neurone decussates and projects to the thalamus

Tertiary sensory neurone has its cell body in the thalamus. It projects to the primary sensory cortex

Question 5

What signs are seen in an isolated lesion of the dorsal column and what signs are seen in an isolated lesion of the spinothalamic pathway

Answer 5

Isolated lesion of the dorsal column leads to ipsilateral signs below the lesion

Isolated lesion of the spinothalamic pathway leads to contralateral signs below the lesion

Question 6

Where is the dorsal column pathway found in the spinal cord and what is it split into

Answer 6

Dorsal column pathway is found in the dorsal funiculus which is split into:

Cuneate fasciculus - upper half of body

Gracile fasciculus - lower half of body

Question 7

In the dorsal column, where do the primary neurones synapse with the secondary neurones

Answer 7

In the dorsal column pathway, primary neurones synapse in the medulla at two nuclei:

Cuneate nuclei - upper half of body

Gracile nuclei - lower half of body

Question 8

In the dorsal column pathway, what structure do the secondary sensory neurones go through in the brainstem

Answer 8

Medial lamniscus

Question 9

Where does decussation occur in the spinothalamic and dorsal column pathways

Answer 9

In the dorsal column pathway, decussation occurs in the medulla

In the spinothalamic pathway, decussation occurs in the ventral white commissure of the cord at the level where the primary neurone synapses

Question 10

Where does the primary neurone synapse in the spinothalamic pathway and what do the axons of the secondary neurones run through in the brainstem

Answer 10

Primary neurone in the spinothalamic pathway synapses in the ipsilateral dorsal horn of the spinal cord

Axons of the secondary neurones run through the spinal lemniscus

Question 11

What is Brown-Sequard syndrome and what are the signs/symptoms

Answer 11

Brown-Sequard syndrome - complete hemi-section of the cord causing destruction of one lateral half of a single cord segment

Can occur following trauma or ischaemia

Leads to:

• Ipsilateral complete segmental ischaemia affecting a single dermatome
• Ipsilateral loss of dorsal column modalities below destroyed segment
• Contralateral loss of spinothalamic modalities below destroyed segment