Ch12

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Ch12

  1. 1. THE SOMATO-SENSORY SYSTEM
  2. 2. SOMATIC SENSATION Enables body to feel, ache, chill. Responsible for feeling of touch and pain Different from other systems because receptors are widely distributed throughout all the body and responds to different kinds of stimuliTypes and layers of skin Hairy and glabrous (hairless) Epidermis (outer) and dermis (inner)Functions of skin Protective function Prevents evaporation of body fluids Provides direct contact with worldMechanoreceptors Most somatosensory receptors are mechanoreceptors. Pacinian corpuscles Ruffinis endings Meissners corpuscles Merkels disks Krause end bulbs
  3. 3. TOUCH RECEPTORS
  4. 4. TOUCH RECEPTORSTwo-point discrimination varies across the body surface (Importance of fingertips overelbow). Difference in density of receptors, size of receptive fields, brain tissue devolved inprocessing the information Big toe sole calf back lip forearm thumb Index finger
  5. 5. PRIMARY AFFERENT AXONS white matter Gray matter Dorsal root Big toe Dorsal root ganglion Dorsal root ganglion cell receptor Dorsal Spinal root nerve lip Primary Afferent Axons Aα, Aβ, Aδ, C C fibers mediate pain and temperature A β mediates touch sensations
  6. 6. PRIMARY AFFERENT AXONS
  7. 7. THE SPINAL CORDDivided in spinal segments (30)- spinal nerves within 4 divisionsDermatomes (area of the skin innervate by the R and L dorsal roots of a singlespinal segment) have 1-to-1 correspondence with segments
  8. 8. THE SPINAL CORDDivision of spinal gray matter: Dorsal horn; Intermediate zone; Ventralhorn Myelinated Aβ axons (touch-sensitive) mainly synapses in the dorsal horn with the second order sensory neurons
  9. 9. ASCENDING PATHWAYS Dorsal Column–Medial Lemniscal Pathway The Trigeminal Touch Pathway Touch information ascends through dorsal Trigeminal nerves column, dorsal nuclei, medial lemniscus, Cranial nerves and ventral posterior nucleus to primary somatosensory cortex S1 S1 dorsal column nuclei VPN trigeminal nucleus VPNdorsal column Medial lemniscus From face
  10. 10. SOMATOSENSORY CORTEXPrimary is area 3b Receives dense input from VP nucleus of the thalamus Lesions impair somatic sensations Electrical stimulation evokes sensory experiencesArea 3a receive information fromvestibular systemArea 1 receive information from 3b andcode for textureArea 2 receive information from 3b andcode for size and shapeOther areas Posterior Parietal Cortex (5,7)
  11. 11. SOMATOSENSORY CORTEXCortical Somatotopy (Homunculus)
  12. 12. SOMATOSENSORY CORTEXCortical Map PlasticityRemove digits or overstimulate – examinesomatotopy before and afterShowed reorganization of cortical maps
  13. 13. SOMATOSENSORY CORTEXThe Posterior Parietal Cortex Involved in somatic sensation, visual stimuli, and movement planning Lesion has been associated to: Agnosia, Astereoagnosia and Neglect syndrome
  14. 14. PAINPain - feeling associated to nociceptionNociception - sensory process, provides signals that trigger painNociceptors: Transduction of PainBradykinin , Mast cell activation: Release of histamineTypes of Nociceptors: Polymodal, Mechanical, Thermal and ChemicalHyperalgesia: highersensitivity to pain in tissuealready damagedPrimary occurs in thedamaged tissues andsecondary hyperalgesia inthe surroundingsBradykinin, prostaglandins,and substance P(secondary hyperalgesia)
  15. 15. PAINPrimary Afferents First pain mediated by fast axons and second pain by slower C fibers Spinal mechanisms brain Dorsal root Ventral root
  16. 16. PAIN ASCENDING PATHWAYSMain differences between touch and pain pathway Nerve endings in the skin Spinothalamic Pain Pathway Diameter of axons Connections in spinal cord Touch – Ascends Ipsilaterally Pain – Ascends Contralaterally Two pathways: 1) Spinothalamic Pain Pathway 2) The Trigeminal Pain Pathway
  17. 17. PAIN ASCENDING PATHWAYS
  18. 18. REGULATION OF PAINAfferent Regulation: gate theory of pain Dorsal horn To dorsal column To spinothalamic tract
  19. 19. REGULATION OF PAINDescending pain control pathway. Use of serotoninStimulation of the PAG cause deep analgesia The endogenuos opiates Opioids and endomorphins Primary auditory cortex Secondary auditory cortex
  20. 20. TEMPERATUREThermoreceptors “Hot” and “cold” receptors. Varying sensitivitiesThe Temperature Pathway Identical to pain pathway Cold receptors coupled to Aδ and C Hot receptors coupled to C

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