A model for intersegmental coordination in the leech nerve cord

The neuronal circuits that generate swimming movements in the leech were simulated by a chain of coupled harmonic oscillators. Our model incorporates a gradient of rostrocaudally decreasing cycle periods along the oscillator chain, a finite conduction delay for coupling signals, and multiple coupling channels connecting each pair of oscillators. The interactions mediated by these channels are characterized by sinusoidal phase response curves. Investigations of this model were carried out with the aid of a digital computer and the results of a variety of manipulations were compared with data from analogous physiological experiments. The simulations reproduced many aspects of intersegmental coordination in the leech, including the findings that: 1) phase lags between adjacent ganglia are larger near the caudal than the rostral end of the leech nerve cord; 2) intersegmental phase lags increase as the number of ganglia in nerve cord preparations is reduced; 3) severing one of the paired lateral connective nerves can reverse the phase lag across the lesion and 4) blocking synaptic transmission in midganglia of the ventral nerve cord reduces phase lags across the block.