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Published byCharlotte Hunter Modified over 8 years ago
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Somatosensory System 2217 – 2006 Week 7 Dr Avinash Bharadwaj
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Somatosensory… “General” sensations Touch “Simple” and discriminative Pain Temperature Musculoskeletal sensations Proprioception / kinaesthetic “Somatic” structures Sensations arising in somatic structures Referred visceral sensations
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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
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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
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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
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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
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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)
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Ventrolateral System Information processing “Gate” mechanism Reflex activity Spinothalamic Tracts Ventral Lateral 1 2
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Ventrolateral System Alternative pain pathway Multisynaptic Reticular formation Intralaminar &c nuclei Widespread cortical projection
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Dorsal Column System Tracts formed by first neurones Ipsilateral to medulla oblongata Internal arcuate fibres Medial lemniscus V-P nucleus Some complexities Cerebellar inputs
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Discrimination Circuitry Cuneate and gracile nuclei Forward and feedback inhibition Amplification of contrast + - -
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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
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