Retrograde invasion of lobster stretch receptor somata in control of firing rate and extra spike patterning.

1. Extra spikes may be interleaved in the otherwise rhythmic discharge pattern of the lobster stretch receptor neuron, about 2 ms after an expected spike. A constant input to the neuron is maintained by injecting current intrasomatically. The axon recovers its excitability while the retrograde invasion of the soma and dendrites is still in progress, which provide electrotonic currents to reexcite the axon. 2. While extra spikes in the axon often arise from a prolonged somatic (dendritic?) depolarization, they may also arise from a delayed retrograde invasion of the soma. 3. Failure of retrograde invasion may cause a sudden jump in the rate of rhythmic discharge, demonstrating the role of the soma-dendritic afterhyperpolarization in the regulation of rhythmic firing rate. 4. The history of repetitive firing is often important. Because extra spikes often first appear during a decline in firing rate, turning on and then off, an additional current may sometimes activate the extra spike mode, thus doubling the resting firing rate in a metastable manner. Another mestastable state is associated with failure of retrograde invasion. 5. Extra spikes augment the high end of the frequency-current curve in some receptor neurons; in other cases, the extra spikes are seen only at low rhythmic firing rates, dropping out as current reaches intermediate values to create a paradoxical negative-sensitivity region (decline in total spikes per second with increasing current). 6. The results suggest that both the extent and the speed of active retrograde invasion of the soma and dendrites are likely candidates for pathophysiological mechanisms, since they may control whether extra spikes are generated.