Motor pattern switching in the heartbeat pattern generator of the medicinal leech: membrane properties and lack of synaptic interaction in switch interneurons.

The motor program for heartbeat in the medicinal leech is produced by a central pattern generator that regularly switches between two alternative coordination states. A pair of switch heart interneurons reciprocally alternate between rhythmically active and inactive states to effect these switches. During spontaneous switches in the activity state of switch interneurons, there was no correlation between the duration of a particular activity state and beat period, indicating that the timing networks for the switch cycle and the beat cycle are relatively independent. Simultaneous recordings from two switch heart interneurons showed that a perturbation in the electrical activity of one does not influence switching of the other and that there is no synaptic interaction between them. Using voltage clamp, we characterized an L-like Ca2+ current (measured as Ba2+ currents), inactivating and non-inactivating K+ currents, a persistent Na+ current, and a hyperpolarization-activated inward current in switch interneurons. Dynamic clamp experiments show that "subtraction" of an artificial switch leak conductance (described previously by Gramoll et al. 1994) from a switch interneuron when it is in the inactive state causes it to display activity associated with the active state. We discuss how the switch leak conductance may interact with the intrinsic currents of switch interneurons to control their activity state.