Axon diameter and fluctuation in excitability.

A nerve fiber responds with an action potential in a fraction of all trials, if stimulated with identical electrical stimuli at threshold intensity and at a frequency such that the interval between the stimuli is larger than the recovery period (BLAIR and ERLANGER, 1932, 1933; PECHER, 1936). PECHER (1937, 1939) showed that this phenomenon is an inherent property of the nerve fiber. It is not due to variations in external circumstances, viz, stimulus intensity, hence the name, fluctuation in excitability. The sequence of these responses is a haphazard one; the probability of response has the same value each time and is independent of the preceding reactions (VERVEEN, 1960). The probability of response depends on stimulus intensity. PECHER (1939) suggested already the existence of a Gaussian relationship between response probability and stimulus intensity. In previous studies on nerve fibers of frog and of crag-fish (VERVEEN, 1960, 1961) it was shown that these relations approximate the Gaussian