Analysis of mechanism of conduction of nerve impulses during the relative refractory phase by a mathematical model of the giant squid axon

By use of the mathematical model of Hodgkin and Huxley we have investigated changes in membrane currents and in ionic permeability of the membrane during conduction of an action potential along a refractory axon. At the start of the relative refractory period, the action potential has a graded character, and is extinguished over a distance of a few millimeters from the site of stimulation (decremental conduction). These changes are brought about not only by the low value of the inactivation variable h and the high potassium conductivity g_K, but also by the inhibitory influence on the recovery process exerted by the spreading wave of depolarization. The graded peak can be propagated along the whole fiber in approximately 0.6 msec at 18.5° after the end of the absolute refractory phase. At this time the wave of the recovery process (increase of h and decrease of g_K) begins to oppose the wave of depolarization, with the result that at each subsequent point along the fiber h increases above and g_K decreases below the preceding value. Under these circumstances, conduction becomes incremental, and in order to evoke a propagated action potential all that is required is to induce a subthreshold local response at the point of stimulation.A. V. Vishnevskii Institute of Surgery, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 4, pp. 434–441, July–August, 1971.