Repetitive Firing in Molluscan Giant Neurones

The somata of molluscan neurones respond with long trains of spikes to steady injected outward current. During such repetitive firing the maximum rate of rise of action potentials usually decreases as a result of inactivation of the transport mechanism for inward current. Significant inactivation of this transport system does not often change the peak level of the action potential (Magura, 1967). According to the investigations of Connor & Stevens (1971 a, c) the neurone's behaviour in the interval between spikes depends upon a potassium-conductance mechanism which is different from that in the squid giant axons. This mechanism represents the fast transient potassium channels which are on several criteria operationally distinct from channels of delayed rectification (Connor & Stevens, 19716; Neher, 1971; Gola, 1972). The purpose of this work was to perform an analysis of neuronal behaviour during the train of spikes. We shall conclude that during repetitive firing one may observe the inactivation of transport systems for inward current and for potassium current. The potassium inactivation is an important factor in stabilization of the peak level of action potentials during repetitive firing. It is also concluded that Ca ions in the external solution play an important role in the mechanisms which stabilize the peak level of spikes. In some neurones of Limnea stagnalis the fast transient potassium channels participate in carrying potassium current during the falling phase of the action potential at the beginning of firing.