Abstract
How can flexible phasing be generated by a central pattern generator (CPG)? To address this question, we have extended an existing model of the leech heartbeat CPG's timing network to construct a model of the CPG core and explore how appropriate phasing is set up by parameter variation. Within the CPG, the phasing among premotor interneurons switches regularly between two well defined states – synchronous and peristaltic. To reproduce experimentally observed phasing, we varied the strength of inhibitory synaptic and excitatory electrical input from the timing network to follower premotor interneurons. Neither inhibitory nor electrical input alone was sufficient to produce proper phasing on both sides, but instead a balance was required. Our model suggests that the different phasing of the two sides arises because the inhibitory synapses and electrical coupling oppose one another on one side (peristaltic) and reinforce one another on the other (synchronous). Our search of parameter space defined by the strength of inhibitory synaptic and excitatory electrical input strength led to a CPG model that well approximates the experimentally observed phase relations. The strength values derived from this analysis constitute model predictions that we tested by measurements made in the living system. Further, variation of the intrinsic properties of follower interneurons showed that they too systematically influence phasing. We conclude that a combination of inhibitory synaptic and excitatory electrical input interacting with neuronal intrinsic properties can flexibly generate a variety of phase relations so that almost any phasing is possible.
Highlights
Background to the current modelThe heartbeat central pattern generator (CPG) of medicinal leeches has been studied intensively for over two decades and has been characterized and modeled extensively
How can flexible phasing be generated by a central pattern generator (CPG)? To address this question, we have extended an existing model of the leech heartbeat CPG’s timing network to construct a model of the CPG core and explore how appropriate phasing is set up by parameter variation
We infer from these regular switches that there are no permanent asymmetries in the heartbeat CPG, i.e., in its synaptic connections or in the intrinsic properties of its components neurons
Summary
A role for compromise: synaptic inhibition and electrical coupling interact to control phasing in the leech heartbeat CPG. Each of the HN(3) and HN(4) interneuron pairs form reciprocal inhibitory connections with their respective contralateral homologs, forming half-center oscillators that pace activity in the circuit These oscillator interneurons (Figure 1A) and their reciprocal synaptic interactions have been biophysically characterized and a working model has been produced through several cycles of experimental testing and revision (Olsen and Calabrese, 1996; Hill et al, 2001). Periodic changes (∼20–40 times the heartbeat period) in the activity pattern (silent vs bursting) of the switch interneurons lead to periodic side-to-side changes in the coordination mode within the CPG by shifting the phase of middle premotor with respect to front premotor interneurons (Figure 1) (Norris et al, 2006) We infer from these regular switches that there are no permanent asymmetries in the heartbeat CPG, i.e., in its synaptic connections or in the intrinsic properties of its components neurons. We have identified two more pairs of heart interneurons in midbody ganglia 15 and 16; these neurons switch in time with the ipsilateral HN(6) and HN(7) interneurons but their connections from within the CPG are not currently well defined
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