0812 - Somatosensory System

Question 1

Describe the structure of a sensory neuron

Answer 1

Unipolar or pseudounipolar neuron. Dendrites with sensory receptors at periphery. Cell body in non-synapsing ganglia in PNS (e.g. dorsal root ganglion), and central processes enter CNS and make synapses.

Question 2

What are the different cutaneous receptors in terms of type, location, response properties and function? Which Brodmann area to they go to?

Answer 2

FA1 (Meissner); Glabrous skin (upper dermis); responds to change in pressure - fine touch. BA1.
FA2 (Pacinian); Glabrous skin; responds to change in pressure - event detection/vibration. BA1.
SA1 (Merkel); Glabrous/Hairy skin (upper dermis); responds to pressure states/differences - fine touch/shape. BA3b.
SA2 (Ruffini); Glabrous/hairy skin (lower dermis); responds to skin stretch/deformation - unconscious proprioception. Does not go to Brodmann area in S1.
Free nerve endings; Glabrous/hairy (upper); responds to mechanical, temp, damage, sensitisation - noxious temp, mechanical/trauma, itch, chemicals etc.
Last 3 most important.

Question 3

Summarise the trajectory of dorsal column pathways to the cortex

Answer 3

Dorsal root axon (A-alpha, beta, delta) enters dorsal horn and then column (cell body in DRG) and ascends to dorsal column nucleus. There it crosses to enter medial lemniscus, and goes to VPN nucleus of thalamus. Synapses again and enters cerebral cortex.
Carries - touch, vibration, texture and proprioception.

Question 4

Summarise the trajectory of spinothalamic pathways to the cortex

Answer 4

Dorsal root axon (A-delta/C) enters dorsal horn, synapses, and ascends up spinothalamic tract directly to thalamus (VPN). Synapses again and is projected to cortex.

Question 5

Summarise the major divisions of the thalamus and their functional significance.

Answer 5

Anterior - Attention/learning
Medial - Planning/active memory
VA/VL - Motor
VP - Somatosensory
Medial Geniculate - Hearing
Lateral Geniculate - Vision
VPL and VPM are relevant for somatosensation.

Question 6

Summarise the major inputs and outputs to the VPM and VPL regions of the thalamus

Answer 6

VPL - Medial lemniscus (body) and spinothalamic tract (body) are inputs
VPM - Medial lemniscus (face) and trigeminal system are inputs.
Both output to somatosensory cortex.

Question 7

Explain the organisation of sensory information in Brodmann areas 1-3 in S1 neocortex.

Answer 7

BA3a - Mainly muscle spindles (some cutaneous)
Ba3b - Mainly cutaneous (RA1, SA1)
BA1 - Cutaneous (FA1, FA2)
BA2 - Deep receptors from muscles/joints, some cutaneous, direction-sensitive, complex responses.
BA’s are arranged over the S1 Gyrus - BA1 is ‘top of the hill’, with 2 on the posterior slope, and 3a, 3b on anterior slope. Anterior to posterior is 3a, 3b, 1, 2.

Question 8

Summarise the cortical representation of the sensory homunculus.

Answer 8

Medial half from VPL - Spinothalamic and dorsal column. Knee represents the corner into median longitudinal fissure, with genitals below.
Lateral part from VPM (trigeminal).
Pharynx/abdominal as you go towards the insula.