The Action Potential

I. Introduction

II. Action potentials allow a rapid and reliable transfer of signals over large distances.

A. Principles and significance

The action potential. A general scheme for the course of an action potential.(Reichert, Introduction to Neurobiology)

B. Some channels open and close in a voltage-dependent manner

C. Conformational changes in the voltage-dependent Na⁺ channel constitute the basis of the action potential.

A model of the voltage-sensitive Na+ channel. (A) At rest, the activation gate (AG) is closed, the inactivation gate (IG) is open. (B) Activation, both gates open. (C) Inactivation, activation gate open, inactivation gate closed. (D). Refractory period, both gates closed. (Dowling, Neurons and Networks)

D. A transient inward Na⁺> current depolarizes the neuron.

Self-amplifying cycle which leads to the rapid opening of many voltage-dependent Na⁺ channels on initiation of an action potential.

Time course of an action potential. The opening of a few Na⁺ channels leads to an initial depolarization. If this depolarization exceeds a threshold value then a rapid regenerative opening of many other Na⁺ channels follows, resulting in the depolarizing phase of an action potential. Since the Na⁺ channel spontaneously inactivates, the repolarization of the membrane occurs automatically. (Reichert, Introduction to Neurobiology)

 

Distribution of charge across an axonal membrane at and around the site of action potential generation. The influx of positive charge into the axon during the generation of an action potential leads to the depolarization of adjacent membrane. Subsequent action potential generation occurs across the membrane in front of the impulse, but not in back of it because of the refractory period. The action potential thus goes only in one direction. (Dowling, Neurons and Networks)

E. An outward K ⁺ current increases the repolarization rate of the action potential.

Effects of current (I) on axonal membrane. Hyperpolarizing and weak depolarizing currents evoke only passive responses in the membrane. Stronger; depolarizing currents evoke active responses. (Dowling, Neurons and Networks)

Refractory phases following an action potential (after Eckert) (Reichert, Introduction to Neurobiology)

F. Classic voltage clamp experiments provide a phenomenological description of ionic currents during the action potential.

With a voltage clamp, membrane potential of the axon (V _m) is maintained at the command voltage level by current (I) fed back into the axon. (Dowling, Neurons and Networks)

 

 

The elementary currents which flow through single ion channels as measured by the patch clamp technique. Schematic representation of the patch clamp recording technique. (after Neher and Sakmann) (Reichert, Introduction to Neurobiology)

 

Form of an action potential generated by voltage dependent Na⁺ and K⁺ channels. The variation of the membrane potential with time, as well as the number of open channels per mm² of membrane surface are shown for a given axon region. (Reichert, Introduction to Neurobiology)

 

 

Response of an axon membrane to a depolarizing voltage step under voltage clamp conditions. The total current is the ionic current that is measured under normal conditions. K ⁺ current is the ionic current that is measured when the extracellular Na ⁺ is replaced by choline or when Na ⁺ channels are blocked with tetrodotoxin (TTX). The Na ⁺ current can be seen when the K ⁺ current is blocked using tetraethylammonium (TEA). (Reichert, Introduction to Neurobiology)

The effects of drugs on the inward Na ⁺ and outward K ⁺ currrents. TTX blocks the inward Na ⁺ current. TEA blocks the outward K ⁺ current., and pronase eliminated the time-dependence of the Na ⁺current (best seen when TEA is also present.). (Dowling, Neurons and Networks).

G. Signal propagation occurs in millisecond pulses

H. The conduction velocity can be increased by myelination and localized concentrations of ion channels.

Exponential decay of membrane voltage (V) with axon length. The length constant (lambda), the distance from the source at which the voltage has declined by about two-thirds, is 1/e. With high R _m and/or low R _i , thje length constant is increased; with low R _m and/or high R _i , the length constant is decreased. (Dowling, Neurons and Networks)

The effect of myelination on the properties of the axonal membrane. Myelin increases R _m and decreases C _m . If the same amount of current is injected into a myelinated and unmyelinated axon, the voltage developed across the myelinated axon is greater because of the increased R _m . Furthermore, the time constant (Tau = RC) of the membrane is shortened relative to R _m because of the decreased C _m . Thus, an action potential is generated faster in the myelinated fiber. (Dowling, Neurons and Networks)

Theoretical relationship between fiber diameter and conduction velocity for myelinated and unmyelinated fibers (after Lembke, Morell, Norton and Rushton) (Reichert, Introduction to Neurobiology)

SSaltatory conduction down a myelinated axon. Action potentials are generated only at the nodes of Ranvier; between the nodes, there is passive spread of potential. (Dowling, Neurons and Networks)

 

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