Regulation of spike initiation and propagation in an Aplysia sensory neuron: gating-in via central depolarization.

Afferent transmission can be regulated (or gated) so that responses to peripheral stimuli are adjusted to make them appropriate for the ongoing phase of a motor program. Here, we characterize a gating mechanism that involves regulation of spike propagation in Aplysia mechanoafferent B21. B21 is striking in that afferent transmission to the motor neuron B8 does not occur when B21 is at resting membrane potential. Our data suggest that this results from the fact that spikes are not actively propagated to the lateral process of B21 (the primary contact with B8). When B21 is peripherally activated at its resting potential, electrotonic potentials in the lateral process are on average 11 mV. In contrast, mechanoafferent activity is transmitted to B8 when B21 is centrally depolarized via current injection. Our data suggest that central depolarization relieves propagation failure. Full-size spikes are recorded in the lateral process when B21 is depolarized and then peripherally activated. Moreover, changes in membrane potential in the lateral process affect spike amplitude, even when the somatic membrane potential is virtually unchanged. During motor programs, both the lateral process and the soma of B21 are phasically depolarized via synaptic input. These depolarizations are sufficient to convert subthreshold potentials to full-size spikes in the lateral process. Thus, our data strongly suggest that afferent transmission from B21 to B8 is, at least in part, regulated via synaptic control of spike initiation in the lateral process. Consequences of this control for compartmentalization in B21 are discussed, as are specific consequences for feeding behavior.