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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins
Neuroscience: Exploring the Brain, 3e
Chapter 12: The Somatic Sensory System
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IntroductionIntroduction
• 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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TouchTouch
• 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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TouchTouch
• Mechanoreceptors (Cont’d)
– Pacinian corpuscles
– Ruffini's endings
– Meissner's corpuscles
– Merkel's disks
– Krause end bulbs
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TouchTouch
• Mechanoreceptors (Cont’d)
– Small and large receptive fields
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TouchTouch
• Mechanoreceptors (Cont’d)
– Receptors - receptive field size and adaptation rate
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TouchTouch• Mechanoreceptors (Cont’d)
– Two-point discrimination
• Receptive field density
• Receptive field size
• Computing power
• Special neural mechanisms
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TouchTouch• Primary Afferent Axons
– AC
– C fibers mediate pain and temperature
– Amediates touch sensations
mediates acute, early pain
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TouchTouch• The Spinal cord
– Spinal segments (30)- spinal nerves within 4 divisions of spinal cord.
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TouchTouch
• Spinal cord (Cont’d)
– Divisions of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn
• Myelinated A axons (touch-sensitive)
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TouchTouch
• Dorsal Column–Medial Lemniscal Pathway
– Touch and proprioception
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TouchTouch• The Trigeminal Touch Pathway
– Somatosensory information from face
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TouchTouch
• Somatosensory Cortex
– S1 = Area 3b
– Adjacent areas:
• Postcentral gyrus:
3a,1,2,
• Posterior Parietal
Cortex: 5,7
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TouchTouch• Somatosensory Cortex
– Cortical Somatotopy: Homunculus
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TouchTouch
• Somatosensory Cortex (Cont’d)
– S1: Rat“Barrel cortex” (vibrissae)
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TouchTouch
• Somatosensory Cortex (Cont’d)
– 3b and 1 – Two mirror image maps - Owl monkey
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TouchTouch
• Somatosensory Cortex (Cont’d)
– Cortical Map Plasticity
– Remove digits or overstimulate – examine somatotopy before and after
– Maps are dynamic
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TouchTouch
• 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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PainPain• 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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PainPain
• Nociception and the Transduction of Painful Stimuli (Cont’d)
– Types of Nociceptors
• Polymodal
• Mechanical
• Thermal
– Hyperalgeia
• Primary and secondary
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PainPain
• Primary Afferents and Spinal mechanisms
– First pain and second pain
– Referred pain: Angina
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PainPain• Ascending Pain Pathways
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PainPain
• 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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PainPain• Ascending Pain Pathways (Cont’d)
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PainPain• Pain Regulation
– Afferent Regulation
– Gate theory of pain - Melzack and Wall
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PainPain• Pain Regulation
– Descending Regulation ->
– The endogenous opiates
• Opioids and endomorphins
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TemperatureTemperature• Thermoreceptors
– “Hot” and “cold” receptors
– Varying sensitivities
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TemperatureTemperature• Thermoreceptors
– Hot and cold receptors
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TemperatureTemperature
• 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 RemarksConcluding 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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TouchTouch
• The Spinal cord
– Dermatomes- 1-to-1 correspondence with segments
– Shingles
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