1 CH 8. The somatic sensory systemSensitize mechanical stimuli (light, vibration, pressure, tension), pain and temp Ascending pathway (peripheral to brain) Cutaneous and subcutenous somatic sensory receptors Free nerve ending and specialized nerve ends Functional classification; Mechanoreceptor/nociceptor/thermoceptor Morphological classification; free nerve and encapsulated types Quality of stimulus; specific receptor and location of central target Quantity; rate of action poetntial adapt

2 The Major Classes of Somatic Sensory ReceptorsTable 8.1 .    The Major Classes of Somatic Sensory Receptors Receptor type Anatomical characteristics Associated axonsa (and diameters) Axonal conduction velocities Location Function Rate of adaptation Threshold of activation Free nerve endings Minimally specialized nerve endings C, Aδ 2–20 m/s All skin Pain, temperature, crude touch Slow High Meissner's corpuscles Encapsulated; between dermal papillae Aβ 6–12 μm Principally glabrous skin Touch, pressure (dynamic) Rapid Low Pacinian corpuscles Encapsulated; onionlike covering Subcutaneous tissue, interosseous membranes, viscera Deep pressure, vibration (dynamic) Merkel's disks Encapsulated; associated with peptide- releasing cells Aβ All skin, hair follicles Touch, pressure (static) Ruffini's corpuscles Encapsulated; oriented along stretch lines Stretching of skin Muscle spindles Highly specialized (see Figure 9.5 and Chapter 15) Ia and II Muscles Muscle length Both slow and rapid Golgi tendon organs Highly specialized (see Chapter 15) Ib Tendons Muscle tension Joint receptors Minimally specialized — Joints Joint position

3 Different adaptation by nerve fiberSlow adapting/tonic Rapid adapting/phasic Adaptation Different adaptation by nerve fiber

4 All of them-low threshold (sensitive receptor)Meissner carpuscle; 40% of afferent fiber, glabrous skin (low frequency-vibration) Pacinian corpuscle; fine surface texture, high-frequency vibration, rapid adapted. Located in Interosseous membrane (vibrations, transmitted to skeleton)- wings of bird, bills of goose, merkel’s disk slow adapting, highly inervated in palm (shape, edges, rough texture) and light pressure Ruffini’s corpuscle extension, slow adapting. Linked to special vesicle containing cells-> peptide release, interal stimuli

5 Two points discriminationDensity of receptor Receptive field large-receptive field  low discrimination In finger tip; receptive field 1-2mm Palm; 5-10 mm Practice, fatigue, stress also affect the sensitivity. Central nerve system (phatom limb)

6

7

8 Proprioceptor ; self receptorcontinuous information about position of limbs or body (visceral motor system) Low threshold mechanoceptor, intergrated into vestibular system, bloood pressure (in heart, and major vessels) Located in muscle spindle, golgi tendon

9 Active tactile explorationMuscle spindles few in large skeletal muscle, whereas extraocular and intrinsic muscle of head and neck show the rich-location For fine regulation and accurate control Muscle spindle is absent in middle ear Joint receptor limb position and joint movement Golgi tendon, collagen fiber Active tactile exploration Haptics ; mouse

10

11 Dorsal column-medial lemniscus pathway; mechano/propioceptionSpino-talamic (anterolateral )pathway-pain, temp 3 neuron Ventral posterior lateral nucleus (VPL)

12 Ophthalmic, maxillary, mandibular branchesPrinciple nucleus mechanosensory Spinal nucleus pain, Temp # ventral posterior medial (VPM) nucleus

13

14 Somatic sensory cortexbrodmann’s area 3b and 1 cuteous stimuli 3a proprioceptor 2 area tactile and proprioceptor

15 Somatotopic maps Functional reception is differentEx) 3b area sensing single finger, whereas multifingers stimulation are received by 1 and 2 area Area 1 sense direct skin stimulation; area 2 required the complx stimulation (shape)

16 CH 9 pain nociceptor ; cell body in dorsal root ganglia, afferent action potential Free nerve ending, slow conduction, unmyelinated Ad – 20m/s, myelinated axon; sensing mechanical and mechanothermo-stimuli C-fiber- 2m/s, unmyelinated; mechnical, thermal, chemical (polymodal) Ad-mechanosensitive, Ad-mechanothermal, polymodal

17 2 kinds of pain First pain-rapid and sharp Second pain-long lasting

18 Transduction of nociceptive signalsNociceptor sensing heat and capsaicin vanilloid receptor (VR-1 or TRPV1) in A and C fiber; it can sense 45oC and capsaicin Vanilloid like receptor (VRL-1 or TRPV2) ; 52oC but not capsaicin Transiet receptor potential (TRP) cnannel; voltage-gated potassium or cyclinc nuleotide channel Resting stage closed. Endovanilloids produced in injury tissue

19

20 Central pain pathway Spinal injury Dissociated sensory loss

21 Referred pain; visceral pain  cutaneous painAnginal pain (heart muscleupper chest, left arm)

22 Affective-motivational pain

23

24 Hyperalgesia sunburnSensitization Hyperalgesia sunburn Peripheral sensitization inflammatiory soup Product of damaged tissue  TRPV1 Prostagland GPCR cAMP; Figure 9.6 Central sensitization Sensitized pain neuron activated by mechanoreceptor Allodynia 무해자극통증 Windup NMDA receptor blocker LTP-like enhancement of postsynaptic potential long lasting eg) COX-2

25 Primary afferent fibersSubstances Released Following Tissue Damage Source: Modified from Fields, 1987. Substance Source Potassium Damaged cells Serotonin Platelets Bradykinin Plasma Histamine Mast cells Prostaglandins Leukotrienes Substance P Primary afferent fibers

26 Neuropathic pain Phantom limbs and phantom pain

27 Descending control of pain perceptionInterpretation of pain World war II Injury in battle vs injury in domestic life reward, benefit, curcumstance Topdown influence The placebo effect Physiological basis of pain modulation Midbrain pain relief produce (figure 9.7); periaqueductal gray of midebrain Gate theory of pain pain modulation ; broken bone Endo-opinoids

28