Somatosensory System 2217 – 2006 Week 7 Dr Avinash Bharadwaj
Jan 06, 2018
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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