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13 Receptor Potential - eceptor Potentials PSL302Y Lecture...

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Unformatted text preview: eceptor Potentials PSL302Y: Lecture 12, by Prof. MacKay! October 13, 2010 Wed., Oct.13, 2010 Receptor Potential - Most of the info we process in nervous system is derived from envmt; requires specialized receptor cells that interact w/ envmt and translate signals into action chemical ligand potentials that can travel to the brain - Concept of receptor potential analogous to post-synaptic potential, but instead of receptor transmitter we have external stimuli interacting with specialized receptor cells - Generally results in depolarization of sensory receptors upon receiving specific energy adenylyl (e.g. Light, heat, pressure, etc.) cyclase - EXCEPTION: photoreceptors hyperpolarize ATP Receptor Protein - Embedded in sensory cell membrane GT P cAMP AMP - G protein shape when specific energy received (i.e. thermal, light, chemical, Changes Receptor Potentials mechanical) sta ge of a m p lific a tion - Either chemical ligand 1) Cation channel opens = ionotropic receptor (rapid) receptor 2) Enzyme activated via G-protein coupling = metabotropic receptor - G-protein and enzyme activations amplify * weak signals = advantage of slow signaling G protein * * October 13, 2010 * adenylyl cyclase GT P cAMP AMP * ATP * sta ge of a m p lific a tion I.e. Olfactory receptor -Metabotropic -Odorant binds to cilium (buried in mucus) membrane's G proteincoupled receptor -> adenylyl cyclase = lots of cAMP (amplified) -Open cAMP-gated cation channel = influx of Ca++ -Depolarizing current produced Receptor Potentials October 13, 2010 2 Example: taste receptor cell on tongue - Pseudocilia sticking out into mucus: receptor proteins embedded in postsynaptic membrane - No axon: instead forms a synapse w/ secondary neuron w/ axon that carries signal into NS - Receptor cell makes synaptic cxn w/ axon terminal, not a cell body - Found in periphery of domain in senses - Receptor potential created by ligand 2 PSL302Y: Lecture 12, by Prof. MacKay! Wed., Oct.13, 2010 - Depolarizing current spread passively thru receptor cell - No a.p.: depolarizing current sufficient to open voltage-gated Ca++ channels, trigger exocytosis: transmitter diffuses across cleft to post-synaptic cell = a.p. Example: Mechanoreceptors in skin/muscle tissue detecting mechanical pressure - Opens receptor proteins: opens ion channels directly - Depolarizing membrane potential: must travel b/c receptor protein not located in axon terminal (act as receptor surface) - Depolarizing current travel to first excitable portion of axon membrane (before myelinated): high density Na+ channels -> summation of receptor potentials at excitable tissue = generate a.p. in same cell -> a.p. travels to NS Transmission of signal - 2 categories of sensory cell transmission 1) Sensory cell generates an action potential (by itself) at spike-generating zone October 13, 2010 2) Sensory cell releases vesicles when depolarized -> impulses (a.p.'s) generated in post-synaptic neuron by summation of receptor potentials Case 1 (right): summation point at branch in sensory axon - Sensory terminal where receptor potentials are generated -Depolarizing current passively spread (simultaneous conduction) to excitable membrane (action potential trigger zone) -Depends on how large receptor potentials are -> threshold potential = generate a.p. that travels down axon Case 2 (left): receptor cells are synaptically joined to post-synaptic axon - Sensory axon generates EPSPs -> summation of EPSPs -> threshold = a.p. Receptor Potentials October 13, 2010 eceptor Potentials Receptor Adaptation - Slowly-adapting: receptor potential sustained for duration of stimulus - Significant depolarization during stimulus duration Receptor Adaptation - Receptor responds tonically throughout stimulus application; Slowly adapting : receptor potential Slowly-adapting distorted: strongest at beginning - Rapidly-adapting: receptor potential elicited by change in sustained for duration of stimulus stimulus energy, decays to zero when stimulus is constant Rapidly-adapting : receptor potential - Depolarization disappears quickly elicited by change in stimulus energy, - Responds to stimulus changes, not constant decays to zero when stimulus is constant - Decrease: drop stimulus to get hyperpolarization; change potential in opposite direction, i.e. Thermo-receptors Habituation Repeated stimuli (identical) in succession elicit progressively weaker responses stimulus PSL302Y: Lecture 12, by Prof. MacKay! Wed., Habituation Oct.13, 2010 Repeated stimuli (identical) in succession elicit - Stimulus can be anything: perfectly square wave progressively weaker responses Habituation Receptor Potentials stimulus - Describes the response to repeated stimuli (identical) in succession = elicit progressively weaker responses - Rec-pot habituates in time to stimulus - I.e. Olfactory senses: eventually don't Coding of stimulus intensity receptor potential notice smell in room (desensitized Greater stimulus intensity greater receptor receptors) October 13, 2010 depolarization (graded potential) more transmitter released, and/or higher action Coding of stimulus intensity potential frequency - Want to know what causes the activity in the receptor impulse frequency is limited by refractory cells period - Easiest to code the intensity of the stimulus: intensity, receptor potential as by stimulus =increases recruit higher - Called "graded potentials" = channels opened stimulus intensity depolarizing threshold sensory neurons current = transmitters released at excitable membrane = a.p. freqcy - The a.p. freqcy ultimately conveys signal to CNS - Impulse freqcy in axon is limited by refractory period: max impulses is 1000/sec - Max impulses = 1000/sec; more realistically = 200-300 impulses/sec - Easy way to get around limit: diff sets of Receptor Potentials receptor cells, w/ low and high thresholds - Low intensity stimulus = recruit lowthreshold sensory axons (A group) Receptor Coding of modality A - stimulus strength = a.p. freqcy to ceiling Class of stimulus (light, heat, etc.) is coded (light heat etc ) by type of receptor; `labeled line' B Receptor code - Ceiling reached = low-threshold sensory within a modality, variety of stimulus axon can't signal anymore (saturated) qualities (e.g. colors) coded by a very restricted number of receptor types - Recruit high-threshold sensory axons = specific stimulus is coded by ratio of take over signaling activity across the population of receptors - Have to look over whole set of sensory axons to decode stimulus intensity - Thus: as stimulus intensity , recruit threshold sensory neurons Coding of modality Population Code - Class of stimulus (light, heat, mechanical, etc.) is coded Receptor B Receptor C by type of receptor Receptor A - Specific receptors dedicated to diff stimuli - Signal always interpreted as spec stimuli by CNS - I.e. artificially stimulate light receptors in eye `Labelled line' code approach Sensory Space - W/i a modality (stimulus), variety of stimulus qualities (e.g. colours) coded by a very restricted # receptor types - We don't have specific receptors for specific stimulus qualities - Receptors respond to stimulus -> diff combos of responses = diff qualities - Specific stimulus is coded by ratio of activity across the population of receptors October 13, 2010 6 7 PSL302Y: Lecture 12, by Prof. MacKay! elicited in a given receptor neuron each sensory neuron has its own r.f. e.g. the r.f. for a cutaneous sensory neuron is the skin territory in which adequate y q stimulation elicits a response. Wed., Oct.13, 2010 Receptive field (r.f.) Receptor Potentials - Spatial territory over which receptor cells are responding - I.e. R.f.s for cutaneous sensory neurons in fingertips smaller than r.f.s in trunk of body - I.e. Photoreceptor & assoc neuron have specific Axon reflex spatial field (patch on retina) in visual field - Carried thru-out sensory system, not just w/ Impulse initiated at one branch point receptor cells but all cells relaying signals to brainantidromically conducted to other branch points - Each sensory neuron has its own r.f. in nociceptors substance P is released - E.g. R.f. for cutaneous sensory neuron = skin causes vasodilation (red flare) ( ) territory in which adequate stimulations elicit response - Modality space w/i which a response is elicited in a given receptor neuron - Size of r.f. defined by amt of receptor branching - Very profuse branching in specific areas = hot spots in r.f.s = more sensitive Axon reflex - Important physiologically only in one class of cutaneous receptors = nociceptors - PAIN (high threshold) receptors: stimulus so intense it causes pain - Nociceptor in skin, diagram: - Injury = depolarizing potential initiated in sensory terminal - Generate a.p. at branch point -> travel down into CNS (orthodromically) - Impulse initiated at one branch pt is antidromically conducted to other branch points - At sensory branches: a.p. travels backward (antidromic conduction) to other sensory terminals adjacent to activated one - Bwd conduction can occur in any sensory axon, but usually doesn't do anything in low-sensory axons; only in nociceptors does it cause substance P release - In these nociceptors, peptide substance P is released: active in ECF - Causes vasodilation (red flare: red region around injury) - Swelling of skin cells = release histamine -> inflammation - Sensory cells in peripheral system: this info has to project into CNS for processing - Essential diffces btwn PNS and CNS Central vs. Peripheral N.S. - CNS is mechanically protected by bone (skull + vertebrae) - CNS is chemically protected by blood-brain barrier (protects CNS from toxins in blood) - PNS: all axons + somata (cell bodies) not protected by bone or by blood-brain barrier - Expose to potential toxins in bloodstream; thus see effects in neurotoxins in PNS first October 13, 2010 8 9 PSL302Y: Lecture 12, by Prof. MacKay! Wed., Oct.13, 2010 - Sensory axons project from skin and muscle tissue = carry signals into CNS - Axons act as receptor surfaces - Generate receptor potentials -> a.p.'s at branch points = conveyed to synaptic cxns w/ CNS - Sensory axons near CNS (spinal cord): - Axons project into spinal cord via dorsal roots (bundle of sensory axons which connect to spinal cord in specific segments) - Axons leave spinal cord via ventral roots - Don't need cell body: all you need is the axon = cell bodies sit on the sideline - Ganglions: Group of cell bodies of neurons in PNS (sit outside of CNS) - Nucleus: Group of cell bodies of neurons in CNS - Cell bodies of efferent axons in grey matter of CNS, not in ganglia October 13, 2010 10 Receptor Potentials Direction of signaling for labeling axons: - Afferent: PNS -> CNS (all sensory axons are afferent) via dorsal roots - Efferent: CNS -> PNS motor signals to muscle via ventral roots Table: Main Diffces in Labeling Btwn PNS and CNS -Nerve: or root (outside of spinal cord) -Contain both afferents and efferents Structure PNS CNS -Can get nerves that are only afferent or axon bundle nerve tract only efferent, but usually a nerve has a group of somata ganglion nucleus mixture of afferent and efferent neurons myelin Schwann cell oligodendrocyte -Myelination via glial cells ...
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