Slide 1SomatosensationThe SkinThe SkinTransduction of TouchTransduction of TouchTransduction of TouchTransduction of TouchSomatosensation: Touch: dorsal column Functional PAthwaySomatosensation: Touch: dorsal column Functional PAthwaySomatosensation: Touch: dorsal column functional pathwaySomatosensation: Touch: dorsal column functional pathwayPain & TemperaturePain & Temperature TransductionPain TransductionPain TransductionPain TransductionTemperature TransductionSomatosensation: Pain & Temp: Spinothalamic Functional pathwaySpecial processing in the Pain functional pathwaySpecial processing in the Pain functional pathwaySpecial processing in the Pain functional pathwaySpecial processing in the Pain functional pathwaySlide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32THE SOMATIC SENSORY SYSTEMNEUR 3000Dr. Joseph J. NormandinSOMATOSENSATION•“Touch” is many different sensations•Pressure•Vibration•Temperature•Pain•What else?•Collectively we call this somatosensation (body sense)•Somatosensation, like all senses, begins with transduction at receptor cellsTHE SKIN•Skin has three layers•Epidermis: protective outer covering•Dermis: somatosensory receptors, connective tissue, vascular network•Hypodermis: anchor for muscles, some somatosensory receptorsTHE SKINTRANSDUCTION OF TOUCH•Touch is transduced by four different mechanoreceptors•Pacinian corpuscles•Vibration around 200-300 Hz•Meissner’s corpuscles•Vibrations around 50hz•Sense of “texture”•Merkel’s discs•Points and edges•Ruffini’s endings•Stretch•The receptive fields of each mechanoreceptor and their response to mechanical energy gives us these specificitiesTRANSDUCTION OF TOUCHTRANSDUCTION OF TOUCHTRANSDUCTION OF TOUCH•All of these mechanoreceptors use large-diameter, myelinated Aβ primary afferent axonsSOMATOSENSATION: TOUCH:DORSAL COLUMN FUNCTIONAL PATHWAY•Axons from mechanoreceptors travel through the spinal cord to the brain•Mechanoreceptor axons combine to form sensory nerves•Sensory nerves combine and enter the spinal cord through the dorsal root•Cell bodies of these neurons make up the dorsal root ganglion•Bundles of axons continue to ascend the spinal cord forming the dorsal columnsSOMATOSENSATION: TOUCH:DORSAL COLUMN FUNCTIONAL PATHWAYSOMATOSENSATION: TOUCH:DORSAL COLUMN FUNCTIONAL PATHWAY•Axons from somatosensory neurons travel through the spinal cord to the brain•In the brainstem, dorsal root ganglion neurons synapse with dorsal column nuclei in the medulla•These medullary neurons project to ventral posterior nucleus of the thalamus (VP) on the other side of the brain•VP neurons send projections to primary somatosensory cortex (S1)SOMATOSENSATION: TOUCH:DORSAL COLUMN FUNCTIONAL PATHWAYPAIN & TEMPERATURE•Pain is an unpleasant experience normally associated with tissue damage•Pain evolved to help us avoid injury•Our sense of temperature helps us to understand environmental conditions•Evolved to help us maintain optimal body temperature•What do you do when you are cold?PAIN & TEMPERATURE TRANSDUCTION•Painful stimuli are transduced by somatosensory receptor cells called nociceptors•Free nerve endings can express particular membrane proteins responsible for transducing painful stimuli•Found everywhere in the body with the exception of brain tissue•Thermal energy is transduced by somatosensory receptor cells called thermoreceptors•Free nerve endings can express particular membrane proteins responsible for transducing thermal energy•Both pain and temperature are processed by the same functional pathwayPAIN TRANSDUCTION•Nociception (the transduction of painful stimuli) results from many mechanisms•Mechanical nociceptors•Express ion channels that can transduce mechanical pain•The receptor protein for mechanical pain has not be clearly defined•A recently isolated gene, SCN9A, from a family that doesn’t experience mechanical pain, encodes a sodium channel expressed on free nerve endingsPAIN TRANSDUCTION•Nociception (the transduction of painful stimuli) results from many mechanisms•Thermal nociceptors•Express TRPV1, TRPV2, TRPV3, TRPA1•Transient receptor potential (TRP) family of temperature-gated cation channelsPAIN TRANSDUCTION•Nociception (the transduction of painful stimuli) results from many mechanisms•Chemical nociceptors •Express histamine receptors, bradykinin receptors, & prostaglandin receptorsTEMPERATURE TRANSDUCTION•Thermoreception (the transduction of thermal energy) is based on the expression of the TRPs on thermoreceptorsSOMATOSENSATION: PAIN & TEMP:SPINOTHALAMIC FUNCTIONAL PATHWAY•Specific pathways carry pain & temperature information from the periphery to the brainSPECIAL PROCESSING IN THE PAIN FUNCTIONAL PATHWAY•Some “qualities” of pain are processed before the brain•Aδ fibers are large and myelinated = fast transmission•Quick sharp pain•TRP2 very high temperature receptor proteins use these fibers and synapse with motor neurons to from a reflex to move appendages away from intense heat•The sharp pain of heat as we touch something hot•C fibers are small and unmyelinated = slow transmission•Dull, constant or throbbing pain•TRPV1 high temperature receptor proteins use these fibers•The throbbing pain of heat that occurs just after the sharp painSPECIAL PROCESSING IN THE PAIN FUNCTIONAL PATHWAYSPECIAL PROCESSING IN THE PAIN FUNCTIONAL PATHWAY•Some “qualities” of pain are processed before the brain•Glutamate and Substance P are released by sensory neurons in the spinal cord•Glutamate release occurs with mild pain•Substance P released with more intense pain (hyperalgesia)SPECIAL PROCESSING IN THE PAIN FUNCTIONAL PATHWAY•The brain can modulate pain via an endogenous pain “gating” system•We can take advantage of this system to exogenously modulate painSOMATOSENSATION: TOUCH/PAIN/TEMP:FUNCTIONAL PATHWAY: SPINAL NERVE ARRANGEMENT•The axons that form each dorsal root come from specific parts of the skin•The patch of skin innervated by a particular spinal root is a dermatomeSOMATOSENSATION: TOUCH/PAIN/TEMP:FUNCTIONAL PATHWAY: SPINAL NERVE ARRANGEMENT•The axons that form each dorsal root come from specific parts of the skin•The patch of skin innervated by a particular spinal root is a dermatome•Dermatomes can overlap by a small degreeSOMATOSENSATION: TOUCH/PAIN/TEMP:FUNCTIONAL PATHWAY: SPINAL NERVE ARRANGEMENT•Cells in