Effects of Calcium Lack on Action Potential of Motor Axons

The effects of external calcium deprivation on certain characteristics of the action potential of the lobster motor axon have been studied. Upon exposure to calcium-free solution the spike amplitude is rapidly decreased within a few minutes and is followed by a slow linear decline. The rates of spike rise and fall are proportionally reduced more than the spike but follow similar time courses during calcium lack. Associated with these phenomena are the loss in the normal slow spike repolarization process, the development of a large and lengthy undershoot, and the appearance of a high degree of refractoriness. The mean increase in the refractory period is 525 per cent upon 10 minutes' exposure to calcium-free solution. These effects are completely reversible upon returning the axons to normal solution. These results are compared to similar effects of calcium deprivation on frog myelinated axons and squid and lobster giant axons recently observed by other workers. Increased refractoriness has been suggested as a possible explanation for the cessation of spontaneous firing seen during calcium deprivation (Adelman, 1956). I t is known that upon exposure to calcium-free Homarus solution, single lobster motor axons become spontaneously active, firing at frequencies as rapid as 100/see. Within seconds, the discharge frequency declines, and eventually the spike discharge ceases altogether, in spite of local potential activity of high frequency and amplitude. At this time, stimulation with long duration constant currents can elicit single responses but not repetitive firing. The failure of the spike process, despite what seem to be favorable conditions for excitation, is assumed to be a result of the axon being in a state of general refractoriness. That such a state could occur was indicated for the squid giant axon by Frankenhaeuser and Hodgkin (1957). Their work showed that in low or zero calcium a great tendency toward the refractory condition develops. Recently Dalton (1958) has found a tendency for spike "undershoot" (transient hyperpolarization following a spike) to develop in the "giant" axons of the lobster circumesophageal connectives upon exposure to low calcium. Such observations *This investigation was supported by research grant B-1314, National Institute of Neurological Diseases and Blindness, United States Public Health Service.