GSU NEUR 3000 - NEUR 3000 - Chapter 12 (32 pages)

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NEUR 3000 - Chapter 12



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NEUR 3000 - Chapter 12

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Pages:
32
School:
Georgia State University
Course:
Neur 3000 - Hon Principles of Neuroscience
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THE SOMATIC SENSORY SYSTEM NEUR 3000 Dr Joseph J Normandin SOMATOSENSATION 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 cells THE SKIN Skin has three layers Epidermis protective outer covering Dermis somatosensory receptors connective tissue vascular network Hypodermis anchor for muscles some somatosensory receptors THE SKIN TRANSDUCTION 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 TRANSDUCTION OF TOUCH TRANSDUCTION OF TOUCH TRANSDUCTION OF TOUCH All of these mechanoreceptors use large diameter myelinated A primary afferent axons SOMATOSENSATION 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 columns SOMATOSENSATION TOUCH DORSAL COLUMN FUNCTIONAL PATHWAY SOMATOSENSATION 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 PATHWAY PAIN 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 pathway PAIN 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 endings PAIN 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 channels PAIN TRANSDUCTION Nociception the transduction of painful stimuli results from many mechanisms Chemical nociceptors Express histamine receptors bradykinin receptors prostaglandin receptors TEMPERATURE TRANSDUCTION Thermoreception the transduction of thermal energy is based on the expression of the TRPs on thermoreceptors SOMATOSENSATION PAIN TEMP SPINOTHALAMIC FUNCTIONAL PATHWAY Specific pathways carry pain temperature information from the periphery to the brain SPECIAL PROCESSING IN THE PAIN FUNCTIONAL PATHWAY Some qualities of pain are processed before the brain A 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 SPECIAL PROCESSING IN THE PAIN FUNCTIONAL PATHWAY SPECIAL 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 pain SOMATOSENSATION 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 SOMATOSENSATION 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 degree SOMATOSENSATION TOUCH PAIN TEMP FUNCTIONAL PATHWAY SPINAL NERVE ARRANGEMENT Cells in primary somatosensory cortex S1 are arranged by the body surface plan Reflective of the density of body innervation Somatotopic map SOMATOSENSATION TOUCH PAIN TEMP FUNCTIONAL PATHWAY SOMATOSENSORY CORTEX SOMATOSENSATION TOUCH PAIN TEMP FUNCTIONAL PATHWAY SOMATOSENSORY CORTEX The receptive fields of primary somatosensory cortex cells can be changed by experience plasticity SOMATOSENSATION TOUCH PAIN TEMP FUNCTIONAL PATHWAY SOMATOSENSORY CORTEX SOMATOSENSATION TOUCH PAIN TEMP FUNCTIONAL PATHWAY SOMATOSENSORY CORTEX Somatosensory cortical neurons are selective for certain features Area 3 S1 neurons respond to any type of somatosensation Area 1 S1 neurons respond to texture Area 2 S1 neurons respond to size and shape Area 1 2 S1 respond to the direction of stimulation across the skin Area 5 7 Posterior parietal cortex activity is associated with SOMATOSENSATION TOUCH PAIN TEMP FUNCTIONAL PATHWAY SOMATOSENSORY CORTEX The emotional and cognitive aspects of pain are processed in the cortex The cingulate gyrus implicated in attention is active during pain perception It s more active if you are told the


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