ch 12 - the somatic sensory system
TRANSCRIPT
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Neuroscience: Exploring the Brain, 3e
Chapter 12: The Somatic Sensory System
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Introduction• Somatic Sensation
– Enables body to feel, ache, chill– Responsible for touch and pain– Somatic sensory system: Different from other
systems• Receptors: Broadly distributed• Responds to many kinds of stimuli
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Touch• Types and layers of skin
– Hairy and glabrous (hairless - e.g., palms)– Epidermis (outer) and dermis (inner)
• Functions of skin– Protective – Prevents evaporation of body fluids– Provides direct contact with world
• Mechanoreceptors– Most somatosensory receptors are mechanoreceptors
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Touch
• Mechanoreceptors (Cont’d)– Pacinian corpuscles– Ruffini's endings– Meissner's corpuscles – Merkel's disks– Krause end bulbs
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Touch• Mechanoreceptors (Cont’d)
– Small and large receptive fields
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Touch• Mechanoreceptors (Cont’d)
– Receptors - receptive field size and adaptation rate
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Touch• Mechanoreceptors (Cont’d)
– Two-point discrimination• Receptive field
density• Receptive field size• Computing power• Special neural
mechanisms
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Touch• Primary Afferent Axons
– AC– C fibers mediate pain and temperature– Amediates touch sensationsmediates acute, early pain
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Touch• The Spinal cord
– Spinal segments (30)- spinal nerves within 4 divisions of spinal cord.
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Touch• Spinal cord (Cont’d)
– Divisions of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn• Myelinated A axons
(touch-sensitive)
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Touch• Dorsal Column–Medial Lemniscal Pathway
– Touch and proprioception
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Touch• The Trigeminal Touch Pathway
– Somatosensory information from face
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Touch• Somatosensory Cortex
– S1 = Area 3b– Adjacent areas:
• Postcentral gyrus: 3a,1,2,• Posterior Parietal
Cortex: 5,7
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Touch• Somatosensory Cortex
– Cortical Somatotopy: Homunculus
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Touch• Somatosensory Cortex (Cont’d)
– S1: Rat“Barrel cortex” (vibrissae)
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Touch• Somatosensory Cortex (Cont’d)
– 3b and 1 – Two mirror image maps - Owl monkey
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Touch• Somatosensory Cortex (Cont’d)
– Cortical Map Plasticity– Remove digits or overstimulate – examine
somatotopy before and after– Maps are dynamic
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Touch• Somatosensory Cortex (Cont’d)
– The Posterior Parietal Cortex• Involved in somatic sensation, visual stimuli, and
movement planning• Agnosia• Astereoagnosia • Neglect syndrome
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Pain• Nociceptors• Pain and nociception
– Pain - feeling of sore, aching, throbbing – Nociception - sensory process, provides signals that
trigger pain• Nociceptors: Transduction of Pain
– Mechanically gated ion channels opened by:• Strong mechanical stimulation, temperature extremes,
oxygen deprivation, chemicals– Damaged cells release substances that open ion channels
• Proteases (-> bradykinin), STP, K+ ion channels• Histamine
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Pain• Nociception and the
Transduction of Painful Stimuli (Cont’d)
– Types of Nociceptors• Polymodal• Mechanical• Thermal
– Hyperalgeia• Primary and secondary
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Pain• Primary Afferents and Spinal mechanisms
– First pain and second pain– Referred pain: Angina
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Pain• Ascending Pain Pathways
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Pain• Ascending Pain Pathways
– Touch and pain systems segregated • Nerve endings in the skin• Diameter of axons• Connections in spinal cord
• Touch – Ascends Ipsilaterally• Pain – Ascends Contralaterally
– Brown-Séquard Syndrome– Trigeminal Pain Pathway
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Pain• Ascending Pain Pathways (Cont’d)
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Pain• Pain Regulation
– Afferent Regulation– Gate theory of pain - Melzack and Wall
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Pain• Pain Regulation
– Descending Regulation ->
– The endogenous opiates• Opioids and
endomorphins
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Temperature• Thermoreceptors
– “Hot” and “cold” receptors– Varying sensitivities
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Temperature• Thermoreceptors
– Hot and cold receptors
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Temperature• The Temperature Pathway
– Organization of temperature pathway• Identical to pain pathway
– Cold receptors coupled to A and C– Hot receptors coupled to C
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Concluding Remarks• Sensory systems exhibit similar organization and function • Somatic sensory information segregated within the spinal
cord and cerebral cortex – Parallel processing of information
• Perception of object involves the seamless coordination of somatic sensory information
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End of Presentation
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Touch• The Spinal cord
– Dermatomes- 1-to-1 correspondence with segments– Shingles
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