Modulation of bursting activity in molluscan neurons

In subesophageal ganglia of Helix pomatia there are cells which rhythmically show self-sustained membrane oscillations accompanied by bursts of repetitive impulses. Aim of the present research has been to show the possibility of a bursting activity also in neurons non-bursting at rest. Two microelectrodes were inserted into the cell for potential recording and sinusoidal current injection. Responses of silent and beating pacemaker cells to the injected sinusoidal currents are synchronized to the driving cycle and the train of repetitive action potentials is similar to that of bursting cells. The tested frequencies were between .01 and 1 Hz, the amplitudes of sinusoidal currents were up to 8 nA and the DC component between -1 and 8 nA. The instantaneous firing frequency and the number of action potentials during a burst depend on the amplitude and the frequency of alternating current. A model proposed to explain adaptation was modified to describe electrical responses to sinusoidal currents. By means of constant depolarizing currents, four parameters (the proportionality constant between the initial firing frequency and the stimulating current, the decay constant of the frequency, the inhibitory current from a single nerve impulse and the decay time constant of the inhibitory current) characterizing the neuron were determined and with these parameters the model well fits experimental data and it accounts also for the neuron different responses to successive stimulation cycles till at the steady-state discharge pattern