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  1. Transduction • Stimulus is changed into electrical signal • Different types of stimuli • mechanical deformation • chemical • change in temperature • electromagnetic

  2. Sensory systems • All sensory systems mediate 4 attributes of a stimulus no matter what type of sensation • modality • location • intensity • Timing (Duration)

  3. Classification of Nerve Fibers • Two systems in use • Erlanger’s • A • , , ,  • B • C • Lloyd’s • I, II, III, IV • Speed of conduction depends on fiber diameter and myelination • 1 micron = 1 meter/sec (unmyelinated) • Myelination increases conduction velocity 6 fold • 5 micron myelinated fiber would conduct at a speed of 30 M/sec

  4. Erlanger’s • Diameter (microns) Velocity (meters/sec) • A •  8-20 50-120 •  5-12 30-70 •  2-8 10-50 •  1-5 3-30 • B • 1-3 3-15 • C • < 1 <2 (unmyelinated) • This system is used in motor nerves and skin afferents; motor fibers are mostly A, A & skin afferents are mostly groups A, A, & C.

  5. Lloyd’s Diameter (microns) Velocity (meters/sec) • I 12-20 70-120 • II 4-12 24-70 • III 1-4 3-24 • IV <1 <2 (unmyelinated) • This system is used for afferents from receptors in muscle and spinal joints.

  6. Receptor Potential • Membrane potential of the receptor • A change in the receptor potential is associated with opening of ion (Na+) channels • Above threshold as the receptor potential becomes less negative the frequency of AP into the CNS increases

  7. Labeled Line Principle • Different modalities of sensation depend on the termination point in the CNS • type of sensation felt when a nerve fiber is stimulated (e.g. pain, touch, sight, sound) is determined by termination point in CNS • labeled line principle refers to the specificity of nerve fibers transmitting only one modality of sensation

  8. Adaptation • In response to a sustained stimulus a neuron will show a decreased firing rate over time • Fast vs. Slow slow IPS Applied + fast time

  9. Adaptation • Slow-provide continuous information (tonic)-relatively non adapting-respond to sustained stimulus • joint capsul • muscle spindle • Merkel’s discs • punctate receptive fields • Ruffini end organ’s (corpusles) • activated by stretching the skin

  10. Adaptation • Rapid (Fast) or phasic • react strongly when a change is taking place • respond to vibration • hair receptors 30-40 Hz • Pacinian corpuscles 250 Hz • Meissner’s corpuscles- 30-40 Hz • (Hz represents optimum stimulus rate)

  11. Mechanism of Adaptation • Membrane adaptation is thought to be due to entry of calcium ions during action potentials (AP) • Ca++ opens a K+ channel increasing permeability of the membrane for K+ taking membrane away from threshold • Side Note (SN) • Amount of Ca++ influx during depolarization is  to amount of neurotransmitter release

  12. Sensory innervation of Spinal joints • Tremendous amount of innervation with cervical joints the most heavily innervated • Four types of sensory receptors • Type I, II, III, IV

  13. Type I mechanoreceptors • Outer layers of joint capsul • fire at a degree proportional to joint movement or traction • low threshold • dynamic-fire with movement • slow adapting • tonic effects on lower motor neuron pools

  14. Type II Mechanoreceptors • Deeper layers of joint capsul • low threshold • rapidly adapting • completely inactive in immobilized joints • functions in joint movement monitoring • phasic effects on lower motor neuron pools

  15. Type III Mechanoreceptors • Recently found in spinal joints • very high threshold • slow adaptation • joint version of Golgi tendon organ

  16. Type IV receptors • Nociceptors • very high threshold • completely inactive in physiologic normal joint • activation with joint narrowing, increased capsul pressure, chemical irritation

  17. Mechanoreceptors • Information transmitted to the brain from mechanoreceptors in fingers allows us to: • feel the shape & texture of objects • play musical instruments • type on computer keyboards • palpate and perform adjustments • perform a multitude of tasks using our hands • Tactile information is fragmented by receptors & must be integrated by the brain

  18. Tactile information • The ability to recognize objects placed in the hand on the basis of touch alone is one of the most important complex functions of the somatosensory system. (Gardner & Kandel) • Tactile information obtained from palpation is crucial in the practice of chiropractic.

  19. Stereognosis • The ability to perceive form through touch • tests the ability of dorsal column-medial lemniscal system to transmit sensations from the hand • also tests ability of cognitive processes in the brain where integration occurs

  20. Receptors in skin • Most objects that we handle are larger than the receptive field of any receptor in the hand • These objects stimulate a large population of sensory nerve fibers • each of which scans a small portion of the object • Deconstruction occurs at the periphery • By analyzing which fibers have been stimulated the brain reconstructs the pattern

  21. Tactile • No single sensory axon or class of sensory axons signals all relevant information • Spatial properties are processed by populations of receptors that form many parallel pathways • CNS constructs a coherent image of an object from fragmented information conveyed in multiple pathways

  22. Sensory Modalities • Categories • Pressure receptors • Cold receptors • Warmth receptors • Nociceptors • Perception- • Wet = + of pressure and temperature receptors • Ticklishness = gentle + of pressure receptors • Itching = gentle + of nociceptors

  23. Microtexture • Humans can detect extremely fine textures • When such fine textures are stroked on the fingerpad skin, the fingerprint ridges vibrate and cause Pacinian Corpuscles to respond enabling the detection of the microtexture

  24. Tactile perception of shape • Major geometrical properties of shapes are well represented in the spatio-temporal responses of SA and RA afferent fiber populations (especially SA) • Depth of indentation &  in curvature of the skin surface are encoded by discharge rates of SAs • Velocity & rate of  in skin surface curvature are encoded by discharge rates of both SAs and RAs • SA = slow adapting • RA = rapidly adapting (MIT touch lab)

  25. Mechanoreceptors • Rapidly adapting cutaneous • Meissner’s corpuscles in glabrous (non hairy) skin • signals edges • Hair follicle receptors in hairy skin • Pacinian corpuscles in subcutaneous tissue • Slowly adapting cutaneous • Merkel’s discs have punctate receptive fields • senses curvature of an object’s surface • Ruffini end organs activated by stretching the skin • even at some distance away from receptor

  26. Receptors in the skin

  27. Somatosensory Cortex • Receives projections from the thalamus • Somatotopic organization (homoculus) • Each central neuron has a receptive field • size of cortical representation varies in different areas of skin • based on density of receptors

  28. Somatosensory Homoculus

  29. Lateral Inhibition • Where a 1st order neuron synapses it excites a 2nd order neuron as well as local interneurons that - neighboring 2nd order neurons • surround inhibition • A cell + more than average will have a larger effect on its neighbors than they will have on it • Found universally within the CNS • Enhances edges but does not improve acuity

  30. Somatosensory Cortex • Two major pathways • Dorsal column-medial lemniscal system • Most aspects of touch, proprioception • 1st order neurons synapse in the brain stem • Anterolateral system • Sensations of crude touch, nociception, temperature, tickle, itch and sexual sensations • 1st order neurons synapse in the dorsal horn of the spinal cord • 2nd order neurons of both pathways cross to other side and ascend, synapsing in the thalamus

  31. Somatosensory pathways • If 1st order neurons synapse in the spinal cord (anterolateral system), then 2nd order neurons cross over at the spinal cord level • If 1st order neurons synapse in the brain stem (dorsal column, medial lemniscal system), then 2nd order neurons cross over at the brain stem level • Clinical significance in spinal cord lesions • Hemisection of the cord • Loss of vibration/proprioception – ipsilateral • Analgesia - contralateral

  32. Somatosensory pathways

  33. Somatosensory Cortex (SSC) • Inputs to SSC are organized into columns by submodality • cortical neurons defined by receptive field & modality • some columns activated by rapidly adapting Messiner’s, others by slowly adapting Merkel’s, still others by Paccinian corp • most nerve cells are responsive to only one modality e.g. superficial tactile, deep pressure, temperature, nociception .

  34. Somatosensory cortex • Brodman area 3, 1, 2 (dominate input) • 3a-from muscle stretch receptors (spindles) • 3b-from cutaneous receptors • 2-from deep pressure receptors • 1-rapidly adapting cutaneous receptors • These four areas are extensively interconnected (serial & parallel processing) • Each of the 4 regions contains a complete map of the body surface

  35. Somatosensory Cortex • Detailed features of a stimulus are communicated to the brain • in early stages of cortical processing the dynamic properties of central neurons and receptors are similar (eg rapidly adapting cutaneous receptors connected to rapidly adapting 2nd and 3rd order neurons) • in the later stages of cortical processing the central nerve cells have complex feature detecting properties and integrate various sensory inputs

  36. Somatosensory Cortex • 3 different types of neurons in BM area 1,2 have complex feature detection capabilities • Motion sensitive neurons • respond well to movement in all directions but not selectively to movement in any one direction • Direction-sensitive neurons • respond much better to movement in one direction than in another • Orientation-sensitive neurons • respond best to movement along a specific axis

  37. Complex feature detection Response of a cortical neuron to sensory stimulus demonstrating complex feature detection properties

  38. Other Somatosensory Cortical Areas • Posterior parietal cortex (BM 5 & 7) • BM 5 integrates tactile information from mechanoreceptors in skin with proprioceptive inputs from underlying muscles & joints • BM 7 receives visual, tactile, proprioceptive inputs • intergrates stereognostic and visual information • Projects to motor areas of frontal lobe • sensory initiation & guidance of movement

  39. Secondary SSC (S-II) • Secondary somatic sensory cortex (S-II) • located in superior bank of the lateral fissure • projections from S-1 are required for function of S-II • projects to the insular cortex, which innervates regions of temporal lobe believed to be important in tactile memory

  40. Thermoreceptors • Sensitive to temperature of the skin • Two types • Cold • As skin is warmed, become inactive • Warmth • As skin is cooled, become inactive • Slow adapting • Discharge spontaneously under normal conditions • Active over a wide range of temperatures • Discharge phasically when skin temp. Δ rapidly

  41. Thermoreceptors At extremes of skin temps <100 C. > 500 C., thermoreceptors are inactive, but nociceptor activity quickly rises.

  42. Pain & Analgesia • Noxious Insults to Body stimulate Nociceptors • Nociceptors are activated by: • Mechanical Stimuli • Thermal Stimuli • Chemical Stimuli

  43. Sensations of Pain • Pricking • Burning • Aching • Stinging • Soreness

  44. Pain vs. Nociception • Nociception-reception of signals in CNS evoked by stimulation of specialized sensory receptors (nociceptors) that provide information about tissue damage • Pain-perception of adversive or unpleasant sensation that originates from a specific region of the body

  45. Perception of Pain • All perception involves an abstraction and elaboration of sensory inputs • highly subjective nature of pain is one the factors that makes it difficult to define and treat clinically

  46. Pain • Conspicuous sensory experience that warns of danger • Chronic pain is a massive economic problem- in US more than 2 million people are incapacitated by pain at any give time • Drives most chiropractic practices

  47. Nociceptors • Least differentiated of all sensory receptors • Can be sensitized by tissue damage • hyperalgesia • repeated heating • axon reflex may cause spread of hyperalgesia in periphery • sensitization of central nociceptor neurons as a result of sustained activation

  48. Sensitization of Nociceptors • Potassium from damaged cells-activation • Serotonin from platelets- activation • Bradykinin from plasma kininogen-activate • Histamine from mast cells-activation • Prostaglandins & leukotriens from arachidonic acid-damaged cells-sensitize • Substance P from the 1o afferent-sensitize

  49. Fast A delta fibers glutamate neospinothalamic mechanical, thermal good localization sharp, pricking Most terminate in VB complex of thalamus Slow C fibers * substance P/ glutamate 1o paleospinothalamic polymodal/chemical poor localization dull, burning, aching Diffuse termination; Reticular formation tectal area of mesen. Periaqueductal gray Nociceptive pathways

  50. *C-fibers • Secrete both substance P and glutamate • glutamate effects are transient and short acting • Fast component of C-fiber • Substance P is released more slowly and concentration builds over seconds-minutes responsible for longer lagging predominant effect (slow pain) • Synapse primarily in substantia gelatinosa (lamina II & III of spinal cord)