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Somatosensory Systems Somatosensory Systems Sue Keirstead, Ph.D. Assistant Professor Dept. of Integrative Biology and Physiology Stem Cell Institute E-mail: [email protected] Tel: 612 626 2290 Class 8: Somatosensory System – Learning Objectives 1. Describe the 3 main types of somatic sensations: 1. tactile: light touch, deep pressure, vibration, cold, hot, etc., 2. pain, 3. Proprioception. 2. List the types of sensory receptors that are found in the skin (Figure 9.11). 3. Describe the two different modality-specific ascending somatosensory pathways and note which modalities are carried in each (Figure 9.10 and 9.13). 4. Describe how it is possible for us to differentiate between stimuli of different modalities in the same body part (i.e. fingertip). Consider this at the level of 1) the sensory receptors and 2) the neurons onto which they synapse in the ascending sensory systems. 5. Explain how one might determine the location of a spinal cord injury based on the modality of sensation that is lost and the region of the body (both the side of the body and body part) where sensation is lost (Figure 9.18). 6. Describe how incoming sensory inputs from primary sensory axons can be modified at the level of the spinal cord and relate this to the mechanism of action of some common pain medications (Figure 9-18). 7. Describe the homunculus and explain the significance of the size of the region of the somatosensory cortex devoted to a particular body part. Cerebral cortex Interneuron Thalamus Interneuron 4 Integration of sensory Stimulus input in the CNS 1 Stimulation Sensory Axon of sensory of sensory receptor neuron receptor Graded potential Action potentials 2 Transduction 3 Generation of of the stimulus action potentials Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Attributes of a stimulus 1. Intensity 2. Duration 3. Modality: type of stimulus • tactile: pressure, temperature, touch, vibration, tickle, etc. • Pain, itch • proprioception: muscle length, velocity of stretch, muscle tension 4. Location Coding of stimulus duration (a) Slowly adapting receptor (b) Rapidly adapting receptor Actionpotentials Actionpotentials in sensory in neuron in sensory neuronin On response Off response Receptorpotential Receptorpotential Stimulusstrength Stimulusstrength On Off On Off Time Time Free nerve endings Merkel disc (itch, pain, temperature, (slowly adapting, touch & pressure) tickle) Meissner corpuscle Skin: (rapidly adapting, touch & Epidermis low frequency vibration) Dermis Ruffini corpuscle (slowly adapting, stretch & pressure) Hair root plexus (rapidly adapting, touch, i.e. movement of hair) Pacinian corpuscle (rapidly adapting, high Subcutaneous layer frequency vibration) Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Pacinian corpuscle: Area deformed by a vibration Nerve ending stimulus Multilayered capsule Extracellular fluid Ca2+ Cation channel Cation channel Plasma Na+ closed membrane open Influx of Na+ and Ca2+ causes a depolarizing receptor potential Cytosol (a) A pacinian corpuscle at rest (b) Transduction in a pacinian corpuscle Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Pacinian corpuscle = Rapidly Adapting Receptor - receptor includes connective tissue structure that cushions the end of the axon - the end of the axon contains mechanically- gated transduction channels - the rapid adaptation is due to the From “Principles of Neural Science” by Kandel and Schwartz connective tissue cushioning which redistributes the pressure among the layers of the cushion - if the connective tissue is stripped away the receptor becomes slowly adapting Modality of Sensory Receptors ⁃ type of stimulus that optimally activates the receptor ⁃ Pacinian corpuscle modality is vibration ⁃ determined by the type of channels on the receptor membrane (mechanosensitive) plus the overall structure and location of the receptor in the tissue. From “Exploring the Brain” by Bear Receptor potentials are graded potentials that can sum over time and space. 9 From http://michaeldmann.net/mann4.html, Michael D. Mann, Ph.D. Proprioception – awareness of the position of your body. Gamma motor neuron to Sensory axons intrafusal muscle fibers Alpha motor neuron to extrafusal muscle fibers Muscle spindle capsule Sensory axon Secondary (flower-spray) Tendon ending Primary (annulospiral) ending Nuclear bag fiber Intrafusal muscle Nuclear chain fiber fibers Extrafusal muscle fibers Tendon organ capsule Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Somatosensory cortex Third-order neuron Thalamus (thalamus) Dorsal column nuclei Brain stem Second-order neuron (secondary sensory neuron) First-order neuron (primary sensory neuron) Modality-specific pathways Spinal nerve • Touch • Pressure • Vibration • Proprioception Spinal cord Sensory receptors LO 4 Describe how it is possible for us to differentiate between stimuli of different modalities in the same body part … Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. The size and density of receptive fields influences resolution Convergence of primary sensory axons onto a single secondary sensory neuron will reduce resolution. Caliper Receptive field Skin surface Somatic sensory neurons Postsynaptic To higher neuron parts of the brain One point of touch perceived The size and density of receptive fields influences resolution (precise localization) Caliper Receptive field Skin surface Two points of touch perceived Somatic because each stimulus activates a sensory neurons separate sensory pathway To higher parts of the Postsynaptic brain (i.e. thalamus) neuron Sensory neurons with overlapping receptive fields Stimulus Skin surface Receptive field Somatic sensory neuron Higher frequency of Lower frequency of action potentials action potentials Skin Action potentials Sensory neurons Inhibitory (Primary interneurons sensory neurons) – – – – – Postsynaptic neurons (secondary sensory neurons) Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Primary somatosensory cortex (SSC) Homunculus The size of the region in the SSC that receives information about a Leg specific body part is proportional Foot Shoulder Toes to the density of innervation of Genitals the body part (not to the size of the body part). Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. The “withrdrawal reflex” pathway is activated by a nociceptor First-order Nociceptor neuron Interneuron Skin Spinal cord Motor neuron Skeletal muscle Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Primary somatosensory cortex Limbic Third-order system neuron Thalamus Hypothalamus Reticular formation First-order Nociceptor neuron Skin Second-order neuron Second- Substance P order neuron Spinal cord First-order neuron Release of substance P Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Anterolateral (spinothalamic) pathway RIGHT SIDE LEFT SIDE OF BODY OF BODY Primary somatosensory cortex Third-order neuron Thalamus Midbrain Medulla Second-order neuron First-order neuron SPINOTHALAMIC TRACT Modality-specific Receptors for Spinal nerve pathways pain, temperature, itch, and tickle Spinal cord Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Dorsal column pathway Anterolateral (spinothalamic) pathway RIGHT SIDE LEFT SIDE RIGHT SIDE LEFT SIDE OF BODY OF BODY OF BODY OF BODY Primary somatosensory cortex Third-order Third-order neuron neuron Thalamus Thalamus Midbrain Midbrain Second-order Dorsal neuron Column Nuclei Medulla Medulla First-order neuron Second-order neuron First-order neuron DORSAL COLUMN SPINOTHALAMIC Receptors for Receptors for TRACT touch, Spinal pain, temperature, pressure, nerve itch, and tickle vibration, and Spinal cord Spinal cord proprioception LO 5: Explain how one might determine the location of a spinal cord injury based on the modality of sensation that is lost and the region of the body (both the side of the body and body part) where sensation is lost (Figure 9.18). Convergence of somatic and visceral first-order neurons on the same second-order neuron First-order neurons To brain Skin Second- order neurons Visceral organ (heart) Spinal cord Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Distribution of referred pain Lung and Liver and diaphragm Liver and gallbladder gallbladder Heart Stomach Pancreas Liver and Gallbladder gallbladder Small intestine Stomach Ovary Ovary Kidney Kidney Colon Urinary Appendix bladder Ureter Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Incoming pain signals can be modified by descending axons. Periaqueductal gray matter Midbrain Nucleus raphe magnus Medulla First-order neuron To brain Nociceptor Spinal cord Second-order neuron Pre- and post-synaptic Neuron from inhibition by nucleus raphe descending opioid magnus pathways Inhibitory interneuron First-order Endogenous sensory opioid neuron Opioid receptor Second-order neuron Inhibitory postsynaptic Substance P potential Substance P release is reduced or blocked LO 6. Describe how incoming sensory inputs from primary sensory axons can be modified at the level of the spinal cord ... Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Perception involves many areas of the brain, including association cortices, limbic system, hippocampus (memory), etc. PRIMARY SOMATOSENSORY CORTEX (postcentral gyrus) SOMATOSENSORY ASSOCIATION AREA COMMON INTEGRATIVE BROCA’S AREA AREA WERNICKE’S AREA VISUAL ASSOCIATION AREA PRIMARY VISUAL CORTEX PRIMARY AUDITORY CORTEX AUDITORY ASSOCIATION AREA GUSTATORY CORTEX Insula OLFACTORY CORTEX opyright © 2016 by John Wiley & Sons, Inc. All rights reserved. .
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