Excitation coupling in muscle.

How much mechanical activity follows a membrane depolarisation of known amplitude and duration? For the steady state case we have the result of Hodgkin (1958), significant in view of what follows, that contracture tension varies for low levels approximately as the sixth power of the depolarisation. To answer the more difficult question involving time, one might think that something like "voltage clamp" technique is necessary. I shew that this is not so. Brown and Sichel (1936) using single sartorius fibres found a graded tension response to increasing transverse electric field stimulation when the membrane was rendered non-propagating by partial depolarisation. One can now improve on their methods in several directions. First of all one can use the result of Nastuk and Hodgkin (1950), that regenerative membrane currents are carried by sodium, to obtain an electrically passive membrane independent of resting potential. Second one can use latency relaxation (Sandow, 1947; Abbott and Ritchie, 1951) as the earliest and an independent measure of activity. Finally by study of the threshold strength-duration relations in the propagating case one can determine the equivalent circuit of the membrane and its intra and extracellular environment, and thus determine the actual time course of membrane potential under a given externally applied field.