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 • Pacinian corpuscles• Ruffini's endings• Meissner's corpuscles• Merkel's disks• Krause end bulbs

Touch

• Mechanoreceptors (Cont’d)– Small and large receptive fields

Touch

• Mechanoreceptors (Cont’d)– Receptors - receptive field size and adaptation rate

Touch

• Mechanoreceptors (Cont’d)– Two-point

discrimination• Receptive field

density• Receptive field

size

Touch

• Primary Afferent Axons– AC– C fibers mediate pain and temperature– Amediates touch sensations

Touch

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

spinal cord

Touch

• The Spinal cord– Dermatomes- 1-to-1 correspondence with segments– Shingles

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)

Touch

• Somatosensory Cortex (Cont’d)– 3b and 1 –

Two mirror image maps - Owl monkey

Touch

• Somatosensory Cortex (Cont’d)– Cortical Map Plasticity– Remove digits or

overstimulate – examine somatotopy before and after

– Maps are dynamic

Touch

• Somatosensory Cortex (Cont’d) – 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 ( such as bradykinin, a vasodialator)• Histamine

Pain

• Nociception and the Transduction of Painful Stimuli – 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• 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