An analysis of the membrane potential along a clamped squid axon.

A partially depolarized squid axon membrane is assumed to have a quasi-steady state negative resistance, the membrane potential is clamped at one point, and a distribution of potential along the axon is obtained from the cable equation. Nominal experimental values of -2 ohm cm(2) for the membrane and 6 ohm cm(2) for the internal and external current electrodes and the axoplasm and sea water between them are used for illustration. The potential and current may be uniform for an axon and electrode length less than 1.2 mm. For a long axon the potential varies as the cosine of the distance within 0.8 mm of the control point. Beyond this the potential variation is exponential and the entire pattern is about 5 mm long. The average current density out to 0.3 mm from the control point is within 10 per cent of the potential clamp value. These distributions are stable for control amplifications of about unity and more.