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PRINCIPLES OF HUMAN PHYSIOLOGY 7 THIRD EDITION Cindy L. Stanfield | William J. Germann Nerve Cells and Electrical Signaling PowerPoint® Lecture Slides prepared by W.H. Preston, College of the Sequoias Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Chapter Outline I. Overview of the Nervous System II. Cells of the Nervous System III. Establishment of the Resting Membrane Potential IV. Electrical Signaling Through Changes in Membrane Potential V. Maintaining Neural Stability Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. I. Overview of the Nervous System Figure 7.1 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. II. Cells of the Nervous System • Neurons • Excitable cells • Glial cells • Copyright Support cells © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Components of a Neuron Figure 7.2 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Structural Classes of Neurons Figure 7.3 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Functional Classes of Neurons Figure 7.4 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. III. Establishment of the Resting Membrane Potential • Determining the equilibrium potentials for potassium and sodium ions • Resting membrane potential of neurons • Approximately -70 mV • Exists because more negative charges inside cell and more positive charges outside cell Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Electrical Potentials Table 7.1 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Membrane Potential of Neurons • Typical neuron • Permeable to potassium and sodium • 25 times more permeable to potassium • Ion distribution • Outside cell • Sodium and chloride • Inside cell • Potassium and organic anions Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Potential: Neuron • Chemical driving forces • K+ out • Na+ in Figure 7.8a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Potential: Neuron • Membrane more permeable to K+ • More K+ leaves cell than Na+ enters • Inside of cell becomes negative Figure 7.8b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Potential: Neuron • Electrical forces develop • Na+ into cell • K+ into cell • Due to electrical forces • K+ outflow slows • Na+ inflow speeds Figure 7.8c Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Potential: Neuron • Steady state develops • Inflow of Na+ is balanced by outflow of K+ • Resting membrane potential = -70mV Figure 7.8d Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Potential: Neuron • Sodium pump maintains the resting potential Figure 7.8e Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Membrane Potential The resting membrane potential is closer to the potassium equilibrium potential +60 mV Copyright ENa -70 mV Resting Vm -94 mV EK © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Forces Acting on Ions • If membrane potential is not at equilibrium for an ion, then the Copyright • Electrochemical force is not 0 • Net force acts to move ion across membrane in the direction that favors its being at equilibrium • Strength of the net force increases the further away the membrane potential is from the equilibrium potential © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Potential: Forces on K+ • Resting potential = -70mV • EK = -94mV • Vm is 24mV less negative than EK Copyright • Electrical force is into cell (lower) • Chemical force is out of cell (higher) • Net force is weak: K+ out of cell, but membrane is highly permeable to K+ © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Resting Potential: Forces on Na+ • Resting potential = -70mV • ENa = +60mV • Vm is 130mV less negative than ENa Copyright • Electrical force is into cell • Chemical force is also into cell • Net force is strong: Na+ into cell, but membrane has low permeability to Na+ © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. A Neuron at Rest • Small Na+ leak at rest (high force, low permeability) • Small K+ leak at rest (low force, high permeability) • Sodium pump returns Na+ and K+ to maintain gradients Figure 7.8e Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. IV. Electrical Signaling Through Changes in Membrane Potential • Describing changes in membrane potential • Graded potentials • Action potentials • Propagation of action potentials Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Membrane Potential Changes • Resting potential—reference point • Depolariation • Repolarization • Hyperpolarization Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Membrane Potential Changes Figure 7.9 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Types of Electrical Signals • Graded potentials • Small • Communicate over short distances • Action potentials Copyright • Large • Communicate over long distances © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Graded Potentials • Initiated by a stimulus • Small change in membrane potential • Magnitude varies (graded) Figure 7.10a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Graded Potentials • Some are depolarizing • Some are hyperpolarizing Figure 7.10b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Purpose of Graded Potentials • Graded potentials determine whether or not an action potential will occur • Threshold • Level of depolarization necessary to elicit action potential • Excitatory • Depolarization • Inhibitory • Hyperpolarization Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Graded Potentials • Spread by electrotonic conduction • Are decremental • Magnitude decays as it spreads Figure 7.11 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Graded Potentials Can Sum • Temporal summation • Same stimulus • Repeated close together in time • Spatial summation Copyright • Different stimuli • Overlap in time © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Temporal Summation Figure 7.12a–b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Spatial Summation Figure 7.12c Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Summation: Cancelling Effects Figure 7.12d Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Action Potentials • Excitable membranes have ability to generate action potentials • Action potential • Rapid large depolarization used for communication • In neurons • Copyright Action potentials travel along axons from cell body to axon terminal (or if afferent neuron, from receptor to terminal) © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Graded Versus Action Potentials Table 7.2 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Phases of an Action Potential • Depolarization • Repolarization • After-hyperpolarization Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Phases of an Action Potential Figure 7.13a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Phases of an Action Potential Figure 7.13b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Depolarization to Threshold • Graded potentials bring membrane to threshold • Threshold triggers • Rapid opening of sodium channels • Regenerative mechanism Copyright • Slow closing of sodium channels • Slow opening of potassium channels © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Voltage-Gated Sodium Channel • Two gates associated with channel • Activation gate • Voltage dependent • Opens at threshold and depolarization • Positive feedback • Inactivation gate • Voltage and time dependent • Close during depolarization • Open during depolarization Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Sodium Channel Gating Figure 7.14 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Sodium and Potassium Gating Threshold stimulus Depolarization of membrane Open sodium channels Positive feedback Net positive charge in cell (depolarization) Membrane sodium permeability Sodium flow into cell Delayed effect (1 msec) Sodium channel inactivation gates close Membrane sodium permeability Delayed effect (1 msec) Open potassium channels Membrane potassium permeability Potassium flow out of cell Sodium flow into cell Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Negative feedback Net positive charge in cell (repolarization) Figure 7.15 Sodium and Potassium Gating Summary Table 7.3 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Concept of Threshold Figure 7.16 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. All-or-None Principle • Threshold • Minimum depolarization necessary to induce the regenerative mechanism for the opening of sodium channels • Threshold depolarization action potential • Subthreshold depolarization no action potential • Suprathreshold depolarization action potential • Action potential from threshold and suprathreshold stimulus are same magnitude • Copyright 100 mV © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Refractory Period • Period of time following an action potential • Marked by decreased excitability • Types Copyright • Absolute • Relative © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Refractory Periods • Absolute Copyright • Spans all of depolarization and most of the repolarization phase • Second action potential cannot be generated • Sodium gates are inactivated © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Refractory Periods • Relative Copyright • Spans last part of repolarization phase and hyperpolarization • Second action potential can be generated—with a stronger stimulus • Some sodium gates closed, some inactivated © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Causes of Refractory Periods Figure 7.17a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Causes of Refractory Periods Figure 7.17b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Causes of Refractory Periods Figure 7.17c Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Consequences of Refractory Periods • All-or-none principle • Frequency coding • Unidirectional propagation of action potentials Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Frequency Coding Figure 7.18a Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Frequency Coding Figure 7.18b Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Conduction: Unmyelinated Extracellular fluid Axon hillock Unmyelinated axon Plasma membrane (a) Resting Site of original action potential + Extracellular fluid + + – + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – Intracellular fluid – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + + + + + + + + + + Extracellular fluid + + + + – + – – – – – – – + + + + Site A – + + + + – – – + – – – – + + + + + + + – – – – Site B – – – – + + + + + + + + + + + + – – – – – – – – – – – – – – – – + + + + + + + + Region of depolarization (b) Initiation Direction of action potential propagation + + + – + + + + + – – – – – – – Site A – – – – – – – – + + + + + + + + (c) Propagation – – – – + + + + Site B + + + + – – – – + + + + – – – – Site C – – – – + + + + + + + + – – – – – – – – + + + + RefractoryRegion of state depolarization + + + + + + + + – – – – – – – Site A – – – – – – – – + + + + + + + + – (d) Propagation continues Copyright + + + + – – – – Site B – – – – + + + + – – – – + + + + Site C + + + + – – – – + + + + – – – – Site D – – – – + + + + Region of Refractory Region of depolarization repolarization state (resting state) © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 7.19 Factors Affecting Propagation • Refractory period • Unidirectional • Axon diameter • Larger • Less resistance, faster • Smaller • More resistance, slower • Myelination Copyright • Saltatory conduction • Faster propagation © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Conduction: Myelinated Fibers Extracellular fl uid Axon hillock Myelinated axon Myelin sheath + Node of Ranvier + + + + + + + + + + + + + + – – – + + + + – + + + – – – – – – – – – – – – – – – – – – Intracellular – + + + – – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + + + + + + + + + + Extracellular + + + – – – fl uid – – – + + + fl uid Direction of action potential propagation + + + + + + + + + + + + + + + + + + + – – – + + + – – – – – – – – – – – ++ + – – – – – – – – – – – – – – – – – – – – – – ++ + – – – – – – – – – – – + + + + + + – – – + + + + + + + + + + + + + + + + Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. Figure 7.20 Conduction Velocity Comparisons Table 7.4 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings. V. Maintaining Neural Stability • Graded potentials and action potentials tend to dissipate Na+ and K+ concentration gradients • But only small percent of ions actually move • Na+ and K+ pump prevents dissipation Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.