Untitled

Abstract

We modeled a segmental oscillator of the timing network that paces the heartbeat of the leech. This model represents a network of six heart interneurons that comprise the basic rhythm-generating network within a single ganglion. This model builds on a previous two cell model (Nadim et al., 1995) by incorporating modifications of intrinsic and synaptic currents based on the results of a realistic waveform voltage-clamp study (Olsen and Calabrese, 1996). Due to these modifications, the new model behaves more similarly to the biological system than the previous model. For example, the slow-wave oscillation of membrane potential that underlies bursting is similar in form and amplitude to that of the biological system. Furthermore, the new model with its expanded architecture demonstrates how coordinating interneurons contribute to the oscillations within a single ganglion, in addition to their role of intersegmental coordination.

Bistability of bursting and silence regimes in a model of a leech heart interneuron

“…Bistability of bursting and silent regimes had been demonstrated previously [24], but the mechanism supporting it was not described. The model contained nine voltage dependent currents: fast calcium current-I CaF , slow calcium current-I CaS , fast sodium current-I N a , delayed rectifier-potassium current-I K1 , persistent potassium current-I K2 , fast transient potassium current-I KA , hyperpolarization activated cationic current-I h , persistent sodium current-I P , and leak current-I leak and was described by a system of 14 differential equations [23] (Appendix). The parameters of the canonical model were tuned to produce activity with a waveform close to the one experimentally observed [23,24].…”

“…1). All parameters including E leak were set to the canonical values, but g leak was set to 10.7 nS; the canonical value is 9.9 nS [23,24]. The initial conditions of voltage and gating variables were set so that the model initially exhibited silence.…”

“…The predictions of this work could be tested in further studies since synaptic interaction within this CPG can be blocked pharmacologically [21,22]. These neurons are particularly attractive for analysis in terms of the theory of dynamical sys-tems since their membrane ionic currents have been measured and well described by a canonical model using the HodgkinHuxley formalism [23]. The canonical model has proven itself as a powerful tool in predicting new phenomena [23,24].…”

“…These neurons are particularly attractive for analysis in terms of the theory of dynamical sys-tems since their membrane ionic currents have been measured and well described by a canonical model using the HodgkinHuxley formalism [23]. The canonical model has proven itself as a powerful tool in predicting new phenomena [23,24]. Depending on the value of the parameters, the model can exhibit different regimes including tonic spiking, bursting, silence, and subthreshold oscillations.…”

Identifying Crucial Parameter Correlations Maintaining Bursting Activity

“…For our study, we used the HCO computational model of Hill et al [29], which was successfully developed to replicate the electrical activity (rhythmic alternating bursting of mutually inhibitory neurons) of the oscillator (HN) interneurons of the leech heartbeat CPG under a variety of experimental conditions. This HCO model consists of a two reciprocally inhibitory model HN interneurons, represented as single isopotential electrical compartments with Hodgkin and Huxley [30] type intrinsic and synaptic membrane conductances.…”