1. Chapter 12 The Somatic Sensory System

2. 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

3. 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

4. Touch Mechanoreceptors Pacinian corpuscles Ruffini's endings Meissner's corpuscles Merkel's disks Krause end bulbs

5. Touch Mechanoreceptors (Cont’d) –Small and large receptive fields

6. Touch Mechanoreceptors (Cont’d) –Receptors - receptive field size and adaptation rate

7. Touch Mechanoreceptors (Cont’d) –Two-point discrimination Receptive field density Receptive field size

11. Touch Primary Afferent Axons –A  C –C fibers mediate pain and temperature –A  mediates touch sensations

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

13. Touch The Spinal cord –Dermatomes- 1-to-1 correspondence with segments –Shingles

16. Touch Spinal cord (Cont’d) –Divisions of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn Myelinated A  axons (touch- sensitive)

17. Touch Dorsal Column– Medial Lemniscal Pathway –Touch and proprioception

18. Touch The Trigeminal Touch Pathway –Somatosensory information from face

19. Touch Somatosensory Cortex –S1 = Area 3b –Adjacent areas: Postcentral gyrus: 3a,1,2, Posterior Parietal Cortex: 5,7

20. Touch Somatosensory Cortex –Cortical Somatotopy: Homunculus

21. Touch Somatosensory Cortex (Cont’d) –S1: Rat“Barrel cortex” (vibrissae)

22. Touch Somatosensory Cortex (Cont’d) –3b and 1 – Two mirror image maps - Owl monkey

23. Touch Somatosensory Cortex (Cont’d) –Cortical Map Plasticity –Remove digits or overstimulate – examine somatotopy before and after –Maps are dynamic

24. Touch Somatosensory Cortex (Cont’d) –The Posterior Parietal Cortex Involved in somatic sensation, visual stimuli, and movement planning Agnosia Astereoagnosia Neglect syndrome

25. 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

26. Pain Nociception and the Transduction of Painful Stimuli –Types of Nociceptors Polymodal Mechanical Thermal –Hyperalgeia Primary and secondary

27. Pain Primary Afferents and Spinal mechanisms –First pain and second pain –Referred pain: Angina

28. Pain Pain Regulation –Afferent Regulation –Gate theory of pain - Melzack and Wall

29. Pain Pain Regulation –Descending Regulation -> –The endogenous opiates Opioids and endomorphins

30. Temperature Thermoreceptors –“Hot” and “cold” receptors –Varying sensitivities

31. Temperature Thermoreceptors –Hot and cold receptors