Somatosensory System

2217 – 2006Week 7

Dr Avinash Bharadwaj

Somatosensory…

“General” sensations Touch

“Simple” and discriminative Pain Temperature Musculoskeletal sensations

Proprioception / kinaesthetic “Somatic” structures

Sensations arising in somatic structures Referred visceral sensations

Destinations…

Conscious perception Awareness Analysis

Reflex activity Somatic – reflexes in response to pain/temperature Skeletal reflexes Visceral reflexes (with ANS)

Others… E.g. input to cerebellum

Conscious Perception To cortex via thalamus Mainly contralateral Cortical areas Pathways

Beautiful in simplicity, interesting in complexity! Three long neurones Two main synapses Complex circuitry around synapses

Short neurones, information processing Pathway vs tract Influence of descending pathways

The Three Neurones First neurone

Body in a sensory ganglion Peripheral and central processes

Second neurone Body in spinal cord / Brainstem Crossing Termination in thalamus – VP nucleus

Third neurone Body in thalamus Termination in cortex

Two Major Pathways

Spinothalamic Ventrolateral system

Ventrolateral (spinothalamic) tracts thalamus cortex

Touch, Pain, Temperature Dorsal Column (Lemniscal) system

Fasciculus cuneatus, f. gracilis medulla oblongata. Medial lemniscus thalamus. Thalamus cortex Discriminative touch, musculoskeletal sensations

Ventrolateral System

First neurone Body in a sensory ganglion

Second neurone Body in nucleus proprius

(Laminae IV, V, VI) Crossed Spinothalamic tracts Ventral posterior nucleus

Third neurone Sensory cortex (postcentral)

Ventrolateral System

Information processing “Gate” mechanism Reflex activity

Spinothalamic Tracts Ventral Lateral

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Ventrolateral System

Alternative pain pathway Multisynaptic Reticular formation Intralaminar &c nuclei Widespread cortical projection

Dorsal Column System

Tracts formed by first neurones Ipsilateral to medulla oblongata Internal arcuate fibres Medial lemniscus V-P nucleus Some complexities Cerebellar inputs

Discrimination Circuitry

Cuneate and gracile nuclei

Forward and feedback inhibition

Amplification of contrast+- -

Cortical Areas First somaesthetic area (primary somatosensory

area) Postcentral gyrus and paracentral lobule Brodmann 3, 1 and 2 Contralateral representation Proportions – “Homunculus”

Second somaesthetic area Lateral sulcus / insula Bilateral representation Less discriminative

Association cortex Parietal lobe Integration overall picture and interpretation

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