Sensory Systems: Somatosensation (Touch) Lecture Notes PDF

Sensory systems: somatosensation (touch) Prof. Pavan Ramdya Image credit Reading: Purves Chapter 9 (pp 193 – 211)  What is the somatosensory system? Mediates a range of sensations includi...

Sensory systems: somatosensation (touch) Prof. Pavan Ramdya Image credit Reading: Purves Chapter 9 (pp 193 – 211)  What is the somatosensory system? Mediates a range of sensations including Touch Pressure Vibration Position Heat Cold Itch Purves, Figure 9.1 Pain  Sensory transduction is mediated by mechanoreceptors Purves, Figure 9.2 David Julius used capsaicin, the compound in chili pepper that elicits the sensation of heat, to identify the ion channel, TRV1, the first temperature sensor. Ardem Patapoutian discovered a family of pressure-sensitive PIEZO ion channels. © The Nobel Committee for Physiology or Medicine. Illustrator: Mattias Karlén  Different receptors have different sensory functions Purves, Table 9.1 What are somatosensory neurons sensitive to? 7  Receptive fields Receptive field: The "part" of the outer world to which a given sensory neuron is most responsive. This can be determined in functional mapping experiments Audition: The sound frequency to which a neurons responds most strongly. "tonotopic map“ Somatosensation: The part of the body which, when touched, drives a sensory neuron most strongly. "somatotopic map" Vision: The area of the visual field which drives a visual system neuron most strongly. "retinotopic map"  Receptive fields and the two-point discrimination threshold Purves, Figure 9.3  Slowly and rapidly adapting mechanoreceptors provide different information This helps explain why you don‘t feel your clothes on your body Purves, Figure 9.4  The skin harbors a variety of morphologically distinct touch mechanoreceptors Free nerve endings (esp. important for pain sensation) Meissner‘s corpuscles 3-40 Hz (low frequency) Merkel cell afferents slowly adapting; highest spatial resolution Rufini‘s endings Pacinian corpuscles (250 -350 Hz) High sensitivity (10 nm) Purves, Figure 9.5  The skin harbors a variety of morphologically distinct touch mechanoreceptors In Kenshalo DR [ed]: The Skin Senses. Springfield, IL, Purves, Figure 9.5 Charles C Thomas, 1968.  Different types of mechanoreceptors in the skin ("cutaneous mechanoreceptors") Purves, Table 9.2  Simulation of activity patterns in different mechanosensory afferents in the fingertip Purves, Figure 9.6 Studying somatosensation and medical pathology 15  Measuring the sense of touch in the laboratory www.youtube.com/watch?v=2ENakkiIZc0  Loss of touch and proprioception www.youtube.com/watch?v=kUsSU_MVYd8  Dermatomes: using deficits to map the location of spinal cord injury Purves, Clinical app. Where does somatosensory information go in the brain? 19  Pathway for low threshold mechanosensing Touch information goes to the Cortex Tertiary neurons (in the thalamus) Secondary sensory neurons (in the brainstem) Primary sensory neurons Purves, Figure 9.8  Dorsal column – medial lemniscus pathway - Nerve endings of mechanosensitive neurons in periphery - Cell bodies in the DRG - Central axon: - ascends in dorsal column to a nucleus in the medulla (brainstem) - where it makes an first excitatory synapse (glutamate) - Axons of medulla neurons (secondary somatosensory neurons) - cross the midline - then ascend in the brain to the thalamus (s-s thalamus: ventro-posterior -VP- complex) - there they make excitatory synapses (glutamate) - Axons of the thalamus - ascend to the cortex, L4 - there they make excitatory synapses (glutamate)  Proprioceptors in the musculoskeletal system Efferent fiber carries information from the CNS towards the periphery … an efferent fiber … an afferent fiber Purves, Figure 9.7  Proprioceptive pathways for the upper and lower body Proprioceptive information goes to the Cerebellum Purves, Figure 9.9 How is somatosensory information organized in the brain? 24  Somatosensory portions of the thalamus and their cortical targets in the postcentral gyrus Purves, Figure 9.10  Somatotopic order in the human primary somatosensory cortex Locations with "high spatial resolution“ (hands, face, lips, toes) are strongly represented areas in the "homunculus" Purves, Figure 9.11  Connections within the somatosensory cortex establish functional hierarchies Purves, Figure 9.12  Neurons in the primary somatosensory cortex form functionally distinct columns Spatially overlapping but functionally distinct columns Purves, Figure 9.13 How “plastic“ is somatosensory organization in the brain? 29  Functional changes in the somatosensory cortex following amputation of a digit Amputation causes neighboring regions to “invade neural territory“ Purves, Figure 9.14  Functional expansion of a cortical representation by a repetitive behavioral task Repeated use causes regions to expand and “invade“ as well Purves, Figure 9.15  Summary: Somatosensation - Important concepts and keywords What are the diverse kinds of information sensed by the Somatosensory system? How do mechanosensory channels work? How was the PIEZO channel discovered (leading to a Nobel Prize in 2021)? What are the 4 kinds of Touch receptor types and what distinguishes them? How are somatosensory receptive fields determined and where are they most precise? Where is tactile (touch) versus proprioceptive sensing relayed to in the higher brain? What is a somatotopic map? How is it organized in somatosensory cortex? What happens to a somatotopic organization after finger amputation or behavioral training? 32