Inking in Aplysia californica. II. Central program for inking

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Abstract
The 3 motor cells that mediate inking in Aplysia have a high threshold and tend to discharge in a characteristic manner, consisting of a few spikes then a pause, followed by an accelerating burst in which the action potentials grow progressively larger in amplitude. This accelerating burst constitutes an essential aspect of the central program for inking. It occurs within a narrow range of stimulus intensities and correlates well with the steep input-output characteristic of inking behavior. The accelerating burst can occur without EPSP [excitatory postsynaptic potential] facilitation and is accompanied by a progressive increase in the input resistance of the motor cells. The increase in input resistance is voltage dependent and occurs in the absence of external Na+ and Ca2+, suggesting that it is due to the gradual inactivation of an early K+ conductance. A model is proposed of the burst mechanism involving depolarizing synaptic input and progressive inactivation of an early K+ current. The electrotonic coupling of the motor cells may contribute further to their burst tendency. During later phases of a burst, the spikes grow larger in amplitude and longer in duration thus increasing electrotonic transmission among the motor cells and providing a mechanism of positive feedback among them.