Non-uniform distribution of currents and voltages along the axon

In the last chapter we assumed that the membrane potential of excitable cells was at all times uniform. In nature this is generally not the case and, in an axon for example, the potential (and the membrane current) are different for each instant in time, along its length. If measurements are made with microelectrodes positioned at equal intervals along an axon (Figure 8.1a), and if a constant current is injected into the middle of an axon (at x = 0), we find that after a steady-state has been obtained the membrane potential decays with distance (Figure 8.1b). The voltage drop between any two consecutive electrodes is also seen to decay (Figure 8.1c). Since the resistance (per unit length) of axoplasm is constant, this means that the current that flows through any given cross-section of the axoplasm must also decay with distance. If the membrane properties are uniform (with distance) then we can conclude that the current that flows across the membrane must also fall with distance. This is pictorially represented in Figure 8.2 where the spaces between the current lines become wider at greater distances from the current source.

Three-dimensional analysis of current distribution along the axon

A better representation of the current distribution along an axon is given in Figure 8.3, which shows the current pattern described in Figure 8.2 that corresponds to only a small wedge-shaped segment of the axon.