Graded Potentials Sensory Receptor Potentials

Sue Keirstead, Ph.D. Assistant Professor Dept. of Integrative Biology and Physiology Stem Cell Institute

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

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.)

Resting membrane potential Membrane potential potential Membrane

Time in milliseconds (msec)

• “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

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

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

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

• “Leak” channels – randomly open and close, so at any given time, there are some leak channels open – Determine resting membrane potential

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)

Third-order neuron

Thalamus

Second-order neuron Brain stem nuclei

Brain stem First-order neuron

Spinal nerve

Spinal cord

Sensory receptors