Nerve Cells and Electrical Signaling

Chapter Outline Overview of the Nervous System Cells of the Nervous System Establishment of the Resting Membrane Potential Electrical Signaling Through Changes in Membrane Potential 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 Support cells Copyright © 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 ENa -94 mV EK -70 mV Resting Vm Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Forces Acting on Ions If membrane potential is not at equilibrium for an ion, then the 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 Copyright © 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 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+ Copyright © 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 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+ Copyright © 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 Large Communicate over long distances Copyright © 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 Different stimuli Overlap in time Copyright © 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 Action potentials travel along axons from cell body to axon terminal (or if afferent neuron, from receptor to terminal) Copyright © 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 Slow closing of sodium channels Slow opening of potassium channels Copyright © 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 Delayed effect (1 msec) Open potassium channels Membrane sodium permeability Sodium flow into cell Potassium flow out of cell Net positive charge in cell (depolarization) (repolarization) Positive feedback Negative Threshold stimulus Depolarization of membrane Open sodium channels Sodium channel inactivation gates close potassium Figure 7.15 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

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 supra-threshold stimulus are same magnitude 100 mV Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Refractory Period Period of time following an action potential Marked by decreased excitability Types Absolute Relative Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Refractory Periods Absolute Spans all of depolarization and most of the repolarization phase Second action potential cannot be generated Sodium gates are inactivated Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Refractory Periods Relative Spans last part of repolarization phase and hyperpolarization Second action potential can be generated—with a stronger stimulus Some sodium gates closed, some inactivated Copyright © 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 Unmyelinated axon + Extracellular fluid Resting Plasma membrane Intracellular fluid – Initiation Site of original action potential Site A Region of depolarization Site B Axon hillock Direction of action potential propagation Propagation Refractory state Site C repolarization (resting state) Site D continues (a) (b) (c) (d) Figure 7.19 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Factors Affecting Propagation Refractory period Unidirectional Axon diameter Larger Less resistance, faster Smaller More resistance, slower Myelination Saltatory conduction Faster propagation Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Conduction: Myelinated Fibers + Myelinated axon Myelin sheath Node of Ranvier Extracellular fl uid Direction of action potential propagation Intracellular – Axon hillock Figure 7.20 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

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.