Separation of sodium and potassium ion carrier systems in crustacean motor axon

¹ Physiology Department, College of Medicine, University of Florida, Gainesville, Florida

When a microscopic area (100 µ length) of the lobster single motor axon is subjected to isotonic potassium solution and the membrane potential maintained at resting level by an applied voltage, a slow prolonged depolarization may be elicited by adequate cathodal stimuli. This response is due to activity of the potassium carrier system. If some sodium chloride is included in the high potassium solution, both spike and prolonged depolarization may be initiated as a spike-dip-plateau. With normal amount of sodium in excess potassium solution the dip is absent, but the membrane resistance shows a remarkable increase at the juncture of the spike and the plateau. Chronaxie values of the spike are 0.4–0.5 msec, of the slow response 2.1–2.2 msec. In potassium-rich media containing sodium two separate inward currents are recorded during a voltage clamp, an early spikelike current, and a late prolonged one. It is concluded that two separate ion carrier systems are activated in a nerve membrane during excitation, one specific to sodium ions, the other to potassium ions, each system possessing quite different and independent characteristics from the other.