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 9, we saw how an action potential arises from sequential changes in the membrane’s permeability to Na+ and K+ ions and how the membrane’s passive properties influence the speed at which the action potential is conducted. In this chapter, we describe in detail the voltage-gated ion channels that are critical for generating and propagating action potentials and consider how these channels are responsible for important features of a neuron’s electrical excitability.
Action potentials have four properties important for neuronal signaling. First, they can be initiated only when the cell membrane voltage reaches a threshold. As we saw in Chapter 9, 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, the membrane voltage will eventually reach a threshold, typically at around −50 mV, at which an action potential can be generated (see Figure 9–2C). 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 followed by a refractory period. For a brief time after an action potential is generated, the neuron’s ability to fire a second action potential is suppressed. The refractory period limits the frequency at which a nerve can fire action potentials, and thus limits the information-carrying capacity of the axon.
These four properties of the action potential—initiation threshold, all-or-none amplitude, conduction without decrement, and refractory period—are unusual for biological processes, which typically respond in a graded fashion to changes in the environment. Biologists were puzzled by these properties for almost 100 years after the action potential was first recorded in the mid-1800s. Finally, in the late 1940s and early 1950s, studies of the membrane properties of the giant axon of the squid by Alan Hodgkin, Andrew Huxley, and Bernard Katz provided the first quantitative insight into the mechanisms underlying the action potential.