7: Propagated Signaling: The Action Potential

Introduction

• The Action Potential Is Generated by the Flow of Ions Through Voltage-Gated Channels

  • Sodium and Potassium Currents Through Voltage-Gated Channels Are Recorded with the Voltage Clamp

  • Voltage-Gated Sodium and Potassium Conductances Are Calculated from Their Currents

  • The Action Potential Can Be Reconstructed from the Properties of Sodium and Potassium Channels

• Variations in the Properties of Voltage-Gated Ion Channels Expand the Signaling Capabilities of Neurons

  • The Nervous System Expresses a Rich Variety of Voltage-Gated Ion Channels

  • Gating of Voltage-Sensitive Ion Channels Can Be Influenced by Various Cytoplasmic Factors

  • Excitability Properties Vary Between Regions of the Neuron

  • Excitability Properties Vary Between Types of Neurons

• The Mechanisms of Voltage-Gating and Ion Permeation Have Been Inferred from Electrophysiological Measurements

  • Voltage-Gated Sodium Channels Open and Close in Response to Redistribution of Charges Within the Channel

  • Voltage-Gated Sodium Channels Select for Sodium on the Basis of Size, Charge, and Energy of Hydration of the Ion

• Voltage-Gated Potassium, Sodium, and Calcium Channels Stem from a Common Ancestor and Have Similar Structures

  • X-Ray Crystallographic Analysis of Voltage-Gated Channel Structures Provides Insight into Voltage-Gating

  • The Diversity of Voltage-Gated Channel Types Is Generated by Several Genetic Mechanisms

• An Overall View

Nerve cells are able to carry electrical signals over long distances because the long-distance signal, the action potential, is continually regenerated and thus does not attenuate as it moves down the axon. In Chapter 6 we saw how an action potential arises from sequential changes in the membrane's permeability to Na⁺ and K⁺ ions. We also considered how the membrane's passive properties influence the speed at which action potentials are conducted. In this chapter we focus on the voltage-gated ion channels that are critical for generating and propagating action potentials and consider how these channels are responsible for many important features of a neuron's electrical excitability.

Action potentials have four properties important for neuronal signaling. First, they have a threshold for initiation. As we saw in Chapter 6, in many nerve cells the membrane behaves as a simple resistor in response to small hyperpolarizing or depolarizing current steps. The membrane voltage changes in a graded manner as a function of the size of the current step according to Ohm's law, ΔV = ΔI · R (in terms of conductance, ΔV = ΔI/G). However, as the size of the depolarizing current increases, eventually a threshold voltage is reached, typically at around −50 mV, at which point an action potential is generated (see Figure 6–2C). Second, the action potential is an all-or-none event. The size and shape of an action potential initiated by a large depolarizing current is the same as that of an action potential evoked by a current that just surpasses the threshold.¹ Third, the action potential is conducted without decrement. It has a self-regenerative feature that keeps the amplitude constant, even when it is conducted over great distances. Fourth, the action potential is ...