ch 12 - the somatic sensory system

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

Neuroscience: Exploring the Brain, 3e

Chapter 12: The Somatic Sensory System


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


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


Touch

• Mechanoreceptors (Cont’d)– Pacinian corpuscles– Ruffini's endings– Meissner's corpuscles – Merkel's disks– Krause end bulbs


Touch• Mechanoreceptors (Cont’d)

– Small and large receptive fields



– Receptors - receptive field size and adaptation rate



– Two-point discrimination• Receptive field

density• Receptive field size• Computing power• Special neural

mechanisms


Touch• Primary Afferent Axons

– AC– C fibers mediate pain and temperature– Amediates touch sensationsmediates acute, early pain


Touch• The Spinal cord

– Spinal segments (30)- spinal nerves within 4 divisions of spinal cord.


Touch• Spinal cord (Cont’d)

– Divisions of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn• Myelinated A axons

(touch-sensitive)


Touch• Dorsal Column–Medial Lemniscal Pathway

– Touch and proprioception


Touch• The Trigeminal Touch Pathway

– Somatosensory information from face


Touch• Somatosensory Cortex

– S1 = Area 3b– Adjacent areas:

• Postcentral gyrus: 3a,1,2,• Posterior Parietal

Cortex: 5,7


Touch• Somatosensory Cortex

– Cortical Somatotopy: Homunculus


Touch• Somatosensory Cortex (Cont’d)

– S1: Rat“Barrel cortex” (vibrissae)



– 3b and 1 – Two mirror image maps - Owl monkey



– Cortical Map Plasticity– Remove digits or overstimulate – examine

somatotopy before and after– Maps are dynamic



– The Posterior Parietal Cortex• Involved in somatic sensation, visual stimuli, and

movement planning• Agnosia• Astereoagnosia • Neglect syndrome


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


Pain• Nociception and the

Transduction of Painful Stimuli (Cont’d)

– Types of Nociceptors• Polymodal• Mechanical• Thermal

– Hyperalgeia• Primary and secondary


Pain• Primary Afferents and Spinal mechanisms

– First pain and second pain– Referred pain: Angina


Pain• Ascending Pain Pathways


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


Pain• Ascending Pain Pathways (Cont’d)


Pain• Pain Regulation

– Afferent Regulation– Gate theory of pain - Melzack and Wall


Pain• Pain Regulation

– Descending Regulation ->

– The endogenous opiates• Opioids and

endomorphins


Temperature• Thermoreceptors

– “Hot” and “cold” receptors– Varying sensitivities


Temperature• Thermoreceptors

– Hot and cold receptors


Temperature• The Temperature Pathway

– Organization of temperature pathway• Identical to pain pathway

– Cold receptors coupled to A and C– Hot receptors coupled to C


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


End of Presentation


Touch• The Spinal cord

– Dermatomes- 1-to-1 correspondence with segments– Shingles

ch 12 - the somatic sensory system

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