Abstract
Nerve terminal responses produced by stimulating the motor nerve were recorded extracellularly from the nerve endings of the frog sartorius muscle. A triphasic response occurred in the proximal areas of the nerve ending, beginning with a positive phase. Ionotophoretic application of tetrodotoxin, tetraethylammonium, and 4-aminopyridine indicated that the negative phase reflected inward sodium current and the third (positive) phase indicated outward potassium current. A late slow negative component was recorded using CaCl2-filled electrodes during perfusion of nerve-muscle preparations with a calcium-free solution containing tubocurarine. This component was dependent on the Ca2+ concentration present in the electrode, increasing when tetraethylammonium and 4-aminopyridine were added and disappearing under the effects of Co2+. Similar components were recorded using microelectrodes containing Sr2+ and Ba2+. It was deduced that the slow components in the response indicate currents passing through voltage-dependent calcium channels in the presynaptic membrane of the nerve ending. The time course of the calcium current is compared with that of transmitter release at the synapse.
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