Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Neuroscience: Exploring the Brain, 3e Chapter 12: The Somatic Sensory System

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

Neuroscience: Exploring the Brain, 3e

Chapter 12: The Somatic Sensory System


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


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


TouchTouch

• Mechanoreceptors (Cont’d)

– Pacinian corpuscles

– Ruffini's endings

– Meissner's corpuscles

– Merkel's disks

– Krause end bulbs


TouchTouch


– Small and large receptive fields


TouchTouch


– Receptors - receptive field size and adaptation rate


TouchTouch• Mechanoreceptors (Cont’d)

– Two-point discrimination

• Receptive field density

• Receptive field size

• Computing power

• Special neural mechanisms


TouchTouch• Primary Afferent Axons

– AC

– C fibers mediate pain and temperature

– Amediates touch sensations

mediates acute, early pain


TouchTouch• The Spinal cord

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


TouchTouch

• Spinal cord (Cont’d)

– Divisions of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn

• Myelinated A axons (touch-sensitive)


TouchTouch

• Dorsal Column–Medial Lemniscal Pathway

– Touch and proprioception


TouchTouch• The Trigeminal Touch Pathway

– Somatosensory information from face


TouchTouch

• Somatosensory Cortex

– S1 = Area 3b

– Adjacent areas:

• Postcentral gyrus:

3a,1,2,

• Posterior Parietal

Cortex: 5,7


TouchTouch• Somatosensory Cortex

– Cortical Somatotopy: Homunculus


TouchTouch

• Somatosensory Cortex (Cont’d)

– S1: Rat“Barrel cortex” (vibrissae)


TouchTouch


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


TouchTouch


– Cortical Map Plasticity

– Remove digits or overstimulate – examine somatotopy before and after

– Maps are dynamic



TouchTouch


– The Posterior Parietal Cortex

• Involved in somatic sensation, visual stimuli, and movement planning

• Agnosia

• Astereoagnosia

• Neglect syndrome


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


PainPain

• Nociception and the Transduction of Painful Stimuli (Cont’d)

– Types of Nociceptors

• Polymodal

• Mechanical

• Thermal

– Hyperalgeia

• Primary and secondary


PainPain

• Primary Afferents and Spinal mechanisms

– First pain and second pain

– Referred pain: Angina


PainPain• Ascending Pain Pathways


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


PainPain• Ascending Pain Pathways (Cont’d)


PainPain• Pain Regulation

– Afferent Regulation

– Gate theory of pain - Melzack and Wall


PainPain• Pain Regulation

– Descending Regulation ->

– The endogenous opiates

• Opioids and endomorphins


TemperatureTemperature• Thermoreceptors

– “Hot” and “cold” receptors

– Varying sensitivities


TemperatureTemperature• Thermoreceptors

– Hot and cold receptors


TemperatureTemperature

• The Temperature Pathway

– Organization of temperature pathway

• Identical to pain pathway

– Cold receptors coupled to A and C

– Hot receptors coupled to C


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


End of Presentation


TouchTouch

• The Spinal cord

– Dermatomes- 1-to-1 correspondence with segments

– Shingles



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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Neuroscience: Exploring the Brain, 3e Chapter 12: The Somatic Sensory System