Modeling central pattern generators in sand crabs
Date
2003
Authors
Hodge, Alexander Fraser Cooper
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Abstract
Ordinary differential equation models based on those of Hodgkin and Huxley are presented in an effort to describe a part icular anomalous digging behaviour in t he anomuran decapod crust acean Emerita analoga. At the onset of a dig, the fourth (hindmost) right and left legs stroke together about half a cycle out of phase wit h the uropods (last abdominal appendages). During digging, however , the phase relationships between the fourth legs and uropods change and one leg ( either left or right) begins to lead the other. Two general approaches are taken here to model the central pattern generators (CP Gs) that coordinate each appendage, and these CPG models are coupled together in order to describe the full network. First, CPGs comprise cells, or small groups of cells described by Morris-Lecar equations. Simply coupling these cellular models together results in high dimensional systems of ordinary differential equations that can only be analysed numerically except in special symmetric cases. However , using similar equations to model each CPG permits averaging which reduces t he "important" dimension of t he system to three , namely a single phase variable for each appendage . So t he second approach taken here is to model each CPG as a single oscillator coupled to others by a phase response function, computed numerically from the cellular differential equations. The resulting three dimensional system is much easier to analyse and it is used to show that with minimal sensory input , a neural network consisting of three coupled oscillators i sufficient to account for the anomalous digging behaviour in Emerita.