FIG. 4.

Adjusting the synaptic scaling factor σ (see methods, Eq. 6) for inhibitory synaptic input in the ensemble model to produce an optimal duty cycle of the model motor neuron activity across segments and sides. Membrane potential traces from three pairs of model motor neurons, representative of the front, middle, and rear model motor neurons—left side peristaltic and right side synchronous—as σ was varied. Duty cycles of the model traces illustrated are indicated in the gray boxes. At a low value of σ (σ = 0.6) the model heart motor neurons in the middle segments [HE(7)–HE(13)], represented in the figure by HE(10) model motor neuron pair [one receiving peri(staltic) and one sync(hronous) inputs], approximate the values for duty cycle observed in the living system (Norris et al. 2007), whereas front and rear model motor neurons do not receive enough inhibition [represented by the HE(6) and HE(14) model motor neuron pairs]. The unity σ used in the standard model results in good matching of the duty cycles between the living system and the ensemble model for the HE(14) model motor neuron pair and for the HE(6) model motor neuron receiving synchronous input (sync). This value for σ, however, results in smaller duty cycles for the middle [HE(10)] model motor neuron pair. For the duty cycle of the HE(6) model heart motor neuron receiving peristaltic input (peri) to match measured values requires an increase in σ (σ = 1.4). This larger value resulted in poor matching of the duty cycles of the majority of the model motor neurons in the ensemble model. Table 1 lists the duty cycles for the living system and the standard model.