HEARTBEAT CONTROL IN THE MEDICINAL LEECH - A MODEL SYSTEM FOR UNDERSTANDING THE ORIGIN, COORDINATION, AND MODULATION OF RHYTHMIC MOTOR PATTERNS

We have analyzed in detail the neuronal network that generates heartbe at in the leech. Reciprocally inhibitory pairs of heart interneurons f orm oscillators that pace the heartbeat rhythm. Other heart interneuro ns coordinate these oscillators. These coordinating interneurons, alon g with the oscillator interneurons, form an eight-cell timing oscillat or network for heartbeat. Still other interneurons, along with the osc illator interneurons, inhibit heart motor neurons, sculpting their act ivity into rhythmic bursts. Critical switch interneurons interface bet ween the oscillator interneurons and the other premotor interneurons t o produce two alternating coordination states of the motor neurons. Th e periods of the oscillator interneurons are modulated by endogenous R Famide neuropeptides. We have explored the ionic currents and graded a nd spike-mediated synaptic transmission that promote oscillation in th e oscillator interneurons and have incorporated these data into a cond uctance-based computer model. This model has been of considerable pred ictive value and has led to new insights into how reciprocally inhibit ory neurons produce oscillation. We are now in a strong position to ex pand this model upward, to encompass the entire heartbeat network, hor izontally, to elucidate the mechanisms of FMRFamide modulation, and do wnward, to incorporate cellular morphology. By studying the mechanisms of motor pattern formation in the leech, using modeling studies in co njunction with parallel physiological experiments, we can contribute t o a deeper understanding of how rhythmic motor acts are generated, coo rdinated, modulated, and reconfigured at the level of networks, cells, ionic currents, and synapses. (C) 1995 John Wiley and Sons, Inc.