The physiological problem

We start by describing in some detail the physiological problem to be studied. An impulse that, say, carries a command from the brain to a muscle travels along a sequence of neurons that can be portrayed roughly as in the sketch in Fig. 2.1. When the impulse arrives at the dendrites on the left-hand side of the neuron, the stimuli given the dendrites are integrated at the cell body to form a nerve impulse. The nerve impulse travels along the axon to the branches of axons on the right-hand side of the neuron. The impulse then jumps to another set of dendrites and the process just described is repeated. Neurons vary considerably in size. The sciatic nerve of the giraffe contains an axon that may be several meters in length; many other axons are much shorter. The diameter of the squid axon, which has been the subject of many experimental studies, is ~ 0.5 mm but it can be as much as 1 mm. Its length is several centimeters.

The process by which an impulse travels along a sequence of neurons is quite complicated. For example, when the impulse jumps from one set of dendrites to another, both chemical and electrical processes, which are not well understood even today, play an important role. The subject of Hodgkin and Huxley's work is the process by which the impulse travels along the axon in the giant axon of the squid.