Cells can have very complex geometries, and when they do the voltage can vary dramatically between different regions. If ionic current flows through a restricted part of a cell's membrane, then the membrane potential at that location will change rapidly, but the membrane potential at distant locations will change more slowly and the change will be smaller. Voltage changes spreading through a cell act as signals to change membrane properties and trigger cellular events such as exocytosis and muscle contraction. Electrical signaling allows the nervous system to control and organize behavior.

Electrical signals in cells fall into two general classes. If the membrane conductance is independent of voltage, then the spread of voltage is passive. This type of signal, also referred to as electrotonic, travels a limited distance. On the other hand, when voltage alters the membrane conductance, then a voltage signal can regenerate itself and propagate without decrement over unlimited distances. This chapter will examine passive electrical signaling and the following chapter will treat active propagation.

The study of passive voltage changes serves a number of purposes. (1) Some biologically important voltage changes spread passively; passive spread is especially important when voltage changes are small. (2) Passive voltage changes are of technical importance in the design and interpretation of electrophysiological experiments. (3) Passive signaling serves as a baseline from which one goes on to study active propagation.

The principles of passive signaling derive from the basic rules of electrical circuits. Voltage drives current through resistors.