A MODEL OF THE HYDROSTATIC SKELETON OF THE LEECH

A mathematical model of the hydrostatic skeleton of the leech has been developed to predict the shape of and internal pressure within the an imal in response to a given pattern of motor neuron activity in differ ent behaviors. The model incorporates experimental data on: the dimens ions of the animal at behavioral extremes, the passive properties of t he tissues, the active length-tension behavior of the muscles in respo nse to neural activation, the relations between firing frequencies and forces developed by the muscles. The model is based on three general assumptions: (i) the cross-sectional geometry of each segment is ellip tical, (ii) the volume of each segment remains constant during movemen t, (iii) the shape of the animal reflects dimensions that minimize the total potential energy. Presently the model is implemented to simulat e the vermiform elongation of the leech, predicting the shape and the pressure changes during behavior. The results are in good agreement wi th the experimental measurements. The pattern of motor neuronal activi ty was determined by the known intersegmental travel time and estimate d delay time between relaxation of the longitudinal muscles and the ac tivation of the circular muscles. The anesthetized state of the leech was taken as the reference state for the model in which the active and passive stresses are zero. (C) 1996 Academic Press Limited