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Understanding the Somatic Sensory System: Touch, Pain, and Temperature Pathways

This chapter delves into the intricacies of the somatic sensory system, exploring how it enables the body to feel touch, pain, and temperature. From the mechanisms of mechanoreceptors to the pathways of nociception, this chapter provides a comprehensive overview of the somatic sensory system. It also covers topics such as the organization of the somatosensory cortex, pain regulation, and the thermoregulation process.

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Understanding the Somatic Sensory System: Touch, Pain, and Temperature Pathways

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  1. Neuroscience: Exploring the Brain, 3e 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 (Cont’d) • 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 • Computing power • Special neural mechanisms

  8. Touch • Primary Afferent Axons • Aa, Ab, Ad, C • C fibers mediate pain and temperature • Abmediates touch sensations - Ad mediates acute, early pain

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

  10. Touch • Spinal cord (Cont’d) • Divisions of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn • Myelinated Ab axons (touch-sensitive)

  11. Touch • Dorsal Column–Medial Lemniscal Pathway • Touch and proprioception

  12. Touch • The Trigeminal Touch Pathway • Somatosensory information from face

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

  14. Touch • Somatosensory Cortex • Cortical Somatotopy: Homunculus

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

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

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

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

  19. 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

  20. Pain • Nociception and the Transduction of Painful Stimuli (Cont’d) • Types of Nociceptors • Polymodal • Mechanical • Thermal • Hyperalgeia • Primary and secondary

  21. Pain • Primary Afferents and Spinal mechanisms • First pain and second pain • Referred pain: Angina

  22. Pain • Ascending Pain Pathways

  23. 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

  24. Pain • Ascending Pain Pathways (Cont’d)

  25. Pain • Pain Regulation • Afferent Regulation • Gate theory of pain - Melzack and Wall

  26. Pain • Pain Regulation • Descending Regulation -> • The endogenous opiates • Opioids and endomorphins

  27. Temperature • Thermoreceptors • “Hot” and “cold” receptors • Varying sensitivities

  28. Temperature • Thermoreceptors • Hot and cold receptors

  29. Temperature • The Temperature Pathway • Organization of temperature pathway • Identical to pain pathway • Cold receptors coupled to Ad and C • Hot receptors coupled to C

  30. 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

  31. End of Presentation

  32. Touch • The Spinal cord • Dermatomes- 1-to-1 correspondence with segments • Shingles

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