Somatosensory System 2217 – 2006 Week 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 1 2

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 1 2 3