Central pattern generator mediating swimming in Tritonia. II. Initiation, maintenance, and termination.

Publisher Website
Google Scholar
Abstract
The central pattern-generating network mediating swimming in the mollusk, T. diomedea, is believed to consist of at least 3 populations of cerebral interneurons designated C2, dorsal swim interneurons (DSI), and ventral swim interneurons (VSI). The bursts of DSI are superimposed on a large ramp depolarization (10-15 mV), which dissipates slowly over the course of a swim. Cycle period is inversely related to the amplitude of the ramp depolarization. A cycle begins when the DSI are released from inhibition by the VSI. The DSI excite the C2, leading to a period of coactive firing that is maintained by reciprocal excitation between C2 and DSI. The C2 and DSI bursts excite VSI with a delay. Inhibition from VSI terminates the DSI burst, which breaks the C2/DSI reciprocal excitation. Due to the loss of excitation from C2 and DSI, the VSI spike frequency declines, which releases the DSI from inhibition to complete a cycle. Transient sensory stimulation establishes the ramp depolarization and initiates a swim sequence. Oscillation is maintained as long as activity in DSI is sufficient to excite C2 on a cycle-by-cycle basis. A swim sequence terminates when the DSI spike frequency during the initial segment of a cycle is insufficient to depolarize C2 into a burst. A swim sequence always terminates on a weak DSI burst that corresponds to the final weak dorsal flexion at the end of a swim.