Repetitive Firing in Molluscan Giant Neurones

Abstract

ABSTRACT The repetitive responses of molluscan neurones to steady injected outward current have been examined using intracellular microelectrodes. Voltage-clamp experiments were also performed to investigate the participation of transport mechanisms for potassium current in neurones during firing. The maximum rate of rise of action potentials usually decreases during repetitive firing. Simultaneously an increasing duration of the falling phase of spikes is observed. The peak level of spikes in most cases is relatively stable if external Ca2+ concentration exceeds 3−4 mM. The peak level of action potentials markedly decreases during firing in solutions with low Ca2+ concentration. A similar effect is observed if the Ca2+ ions in normal Ringer solution are completely replaced by Mg2+ ions. With increasing external Ca2+ concentration the maximum rate of rise of action potentials becomes more stable during repetitive firing. With increasing external Ca2+ concentration the steady-state inactivation of fast transient potassium channels is removed. Increase in Ca2+ concentration (in range from 10–40 mM) is roughly equivalent to a hyperpolarization of 7–8 mV. Experiments with TEA show that in some neurones of Limnea stagnalis the last transient potassium channels participate in carrying potassium current during the falling phase of the action potentials at the beginning of repetitive firing. The role of potassium inactivation as well as of Ca2+ ions in mechanisms which stabilize the peak level of action potentials during repetitive firing is discussed.