Medical Physiology, 3rd Edition

CHAPTER 7. Electrical Excitability and Action Potentials

Edward G. Moczydlowski

Cellular communication in the nervous system is based on electrical and chemical signaling events that are mediated by ion channels. Certain types of cells, including neurons and myocytes, have a remarkable property called electrical excitability. In cells with this property, depolarization of the membrane above a certain threshold voltage triggers a spontaneous all-or-none response called an action potential. This action potential is a transient, regenerative electrical impulse in which the membrane potential (Vm) rapidly rises to a peak that is ~100 mV more positive than the normal, negative resting voltage (Vrest). Such signals, also called spikes, can propagate for long distances along nerve or muscle fibers. Conduction of action potentials allows information from sensory organs to be transmitted along afferent nerves leading to the brain. Conversely, the brain exerts voluntary and involuntary control over muscles and other effector organs by efferent nerves leading away from it.

In the first part of this chapter, we examine the biophysical and molecular basis of action potentials and the mechanisms that underlie their genesis and propagation. The second part deals with the structure and function of voltage-gated ion channel proteins. Finally, we examine the conduction properties of neurons—called cable properties—and how they determine the spread of action potentials along the axon.

Mechanisms of Nerve and Muscle Action Potentials

Physiology of Voltage-Gated Channels and Their Relatives

Propagation of Action Potentials