Graded Potentials Sensory Receptor Potentials Sue Keirstead, Ph.D. Assistant Professor Dept. of Integrative Biology and Physiology Stem Cell Institute E-mail: [email protected] Tel: 612 626 2290 Class 5: Graded Potentials (sensory receptor potentials) 1. Draw a picture of a primary sensory (afferent) neuron, and label its components and their location in the body. Include the sensory receptor (dendrites), soma, and the peripheral and central branches of the axon. Use an arrow to indicate the direction of information flow. 2. Define a sensory receptor and recognize the difference between this and a plasma membrane receptor. 3. Define sensory transduction and explain the mechanism by which this process occurs (i.e. what type of channels mediate it, how are they activated, etc.) 4. Compare and contrast the properties of leak channels and mechanically-gated channels found on sensory receptors. 5. Define a sensory receptor potential and explain how it is generated. Explain how the size of a receptor potential can be increased by a larger stimulus. Right side of brain Left side of brain Cerebral cortex Brain Thalamus Primary sensory neuron Spinal cord Sensory receptor Copyright © 2016 by John Wiley & Sons, Inc. All Resting Membrane Potential (MP when only leak channels are open) Plasma Extracellular fluid Cytosol membrane Na+/K+ ATPase 3 Na+ Resting + + + + K concentration K electrical membrane Na concentration Na electrical gradient gradient potential gradient gradient (–70 mV) ATP 2 K+ ADP The membrane potential stabilizes around –70 mV IF there are more K leak channels than Na leak channels AND the NaK ATPase is actively maintaining the ion gradients Class 4, LO4: List the conditions that must exist and which proteins must be present in order for a cell to have a negative resting membrane potential (i.e. think about ion channels, carrier proteins, etc.) Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Depolarizing graded potential mV) ( Resting membrane potential Membrane potential potential Membrane Time in milliseconds (msec) Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Ion Channels • “Leak” channels – randomly open and close, so at any given time, there are some leak channels open – Determine resting membrane potential • Transduction channels – change a physical stimulus into an electrical event (i.e. change in membrane potential) – Mediate sensory receptor potentials – Size and duration of the receptor potential is determined by how many transduction channels are opened by the stimulus, and for how long (e.g. graded potential) Cerebral cortex Interneuron Thalamus Interneuron 4 Integration of sensory Stimulus input in the CNS 1 Stimulation Sensory Axon of sensory of sensory receptor neuron receptor Graded potential Action potentials 2 Transduction 3 Generation of of the stimulus action potentials Copyright © 2019 by John Wiley & Sons, Inc. All rights reserved. Sensory Transduction: conversion of one type of stimulus energy (e.g. pressure, temperature, etc.) to an electrical signal. Extracellular fluid Ca2+ Cation channel Cation channel Plasma Na+ closed open membrane Influx of Na+ and Ca2+ causes a depolarizing receptor potential Cytosol Mechanically-gated transduction channel Transduction of membrane deformation at rest (mechanical stimulus) into a receptor potential Sensory Receptors: have transduction channels that are opened by a specific type of stimulus (e.g. mechanical, thermal, chemical) LO3: Define sensory transduction and explain the mechanism by which this process occurs (i.e. what type of channels mediate it, how are they activated, etc.) Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Characteristics of a stimulus 1. Intensity 2. Modality: type of stimulus (pressure, temperature, touch, etc.) 3. Location Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Coding of stimulus intensity by the Central size of the receptor potential axon (CNS) - the larger the stimulus, more transduction Soma (dorsal root channels are open, larger receptor ganglion potential Information flow Peripheral axon (peripheral nerve) Action potentials Dendrites Sensory receptor Receptor potentials Stimulus Stimulus strength On Off Time Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Central Modality of a sensory receptor = the type of stimulus that optimally activates the receptor Examples: Pacinian corpuscle: Area deformed by a vibration Nerve ending stimulus (dendrite) Multilayered capsule Extracellular fluid Ca2+ Mechanically- Mechanically- Na+ Plasma gated cation gated cation membrane channel closed channel open Influx of Na+ and Ca2+ causes a depolarizing Cytosol receptor potential (a) A Pacinian corpuscle at rest (b) Transduction in a Pacinian corpuscle LO3: Define sensory transduction and explain the mechanism by which this process occurs (i.e. what type of channels mediate it, how are they activated, etc.) Copyright © 2019 by John Wiley & Sons, Inc. All rights reserved. Warm receptor Thermoreceptors have Cold receptor (free nerve endings) transduction channels that open (free nerve endings) when temperature increases or when temperature decreases. Extracellular Extracellular Ca2+ fluid + 2+ Na fluid Na+ Ca Warm Camphor Cold Menthol temperatures temperatures Plasma membrane Plasma membrane of warm receptor of cold receptor Sensory Sensory neuron neuron TRPV3 TRPM8 channel channel Influx of Na+ and Ca2+ Influx of Na+ and Ca2+ causes a depolarizing causes a depolarizing receptor potential receptor potential Cytosol Cytosol Copyright © 2019 by John Wiley & Sons, Inc. All rights reserved. Nociceptors have transduction Polymodal nociceptor channels that open when exposed to: (free nerve endings) - extreme heat - intense mechanical stimuli - chemicals released from cells that have been damaged ECF Na+ Extreme Ca2+ Capsaicin heat Plasma membrane of nociceptor Sensory TRPV1 neuron channel Influx of Na+ and Ca2+ causes a depolarizing ICF receptor potential Copyright © 2019 by John Wiley & Sons, Inc. All rights reserved. Ion Channels • “Leak” channels – randomly open and close, so at any given time, there are some leak channels open – Determine resting membrane potential • Transduction channels – change a physical stimulus into an electrical event (i.e. change in membrane potential) – Mediate sensory receptor potentials – Size and duration of the receptor potential is determined by how many transduction channels are opened by the stimulus, and for how long (e.g. graded potential) Sensory receptors that are peripheral endings of sensory neurons Encapsulated nerve endings Axon Dendrites Stimulus Triggers APs at first node/cluster of VG channels Receptor potential Action potentials Propagate into CNS propagated along length of axon Free nerve endings (dendrites) Axon Stimulus Triggers APs at first node/cluster of VG channels Triggers Receptor potential Action potentials Propagate into CNS Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Free nerve endings Merkel disc (itch, pain, temperature, (slowly adapting, touch & pressure) tickle) Meissner corpuscle Skin: (rapidly adapting, touch & Epidermis low frequency vibration) Dermis Ruffini corpuscle (slowly adapting, stretch & pressure) Hair root plexus (rapidly adapting, touch, i.e. movement of hair) Pacinian corpuscle (rapidly adapting, high Subcutaneous layer frequency vibration) What determines the modality of a sensory receptor? 1. The type of transduction channels 2. The structure within which the channels are located (e.g. Paccinian corpuscle) 3. The location of the receptor in the tissue Coding of stimulus location Receptive field of a somatic sensory neuron Skin Somatic sensory neuron (aka primary sensory neuron) Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Cerebral cortex Third-order neuron Thalamus Second-order neuron Brain stem nuclei Brain stem First-order neuron Spinal nerve Spinal cord Sensory receptors Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. .