Entrainment of a bursting neuron--II. Atypical cases

Entrainment to external rhythms was analyzed in atypical RPA-1 neurons of Helix pomatia L. stimulated orthodromically. Neurons with long-lasting bursts were tested only with supraharmonic frequencies. All cycle-stimulus relations described for typical oscillators did occur, with particularities due to the frequency of stimulation and the strong fluctuations of cycle parameters. Activity in pairs or triplets was refractory to entrainment. Instead, rhythmical pulses may transform the bimodal activity into a monomodal one. Continuously active neurons and silent ones were also tested. Fragmentation into bursts and long-period bimodal activity could be evoked respectively, suggesting the presence of a latent bimodal pacemaker behind these activities.