Muscle function and neuromuscular transformation in leech swimming

Animal locomotion is achieved by rhythmic muscle contraction and relaxation cycles, which are controlled by motoneuron ensembles that convey commands from central oscillators. The goal of this research is to understand muscle function and neuromuscular transformation in animal locomotion, and to establish mathematical models that predict the dynamic tension in muscle caused by motor commands. We proposed that muscle tensions expressed in behaving animals reflect three distinct factors: (1) phasic information provided by motoneuron commands, (2) body length resulting from interactions with adjacent muscle, and (3) information concerning the external physical environment. The investigation employed swimming behavior in medicinal leeches as a model system.;We explored the passive properties of leech muscle. Passive length-tension relationships were incorporated into a systems model comprising three nonlinear springs, two in series with two nonlinear dashpots, which describes well the tension statics and dynamics. We found that a drastically simplified model, a single nonlinear spring represented by an exponential function, describes well the passive property of muscle during rhythmic movement such as is characteristic of swimming leeches.;The neuronal control of swimming is mediated by motoneurons. Impulse frequencies of swim-related motoneurons were described accurately by adaptation dynamics and a threshold nonlinearity.;To explore the neuromuscular transformation in the context of leech swimming behavior, realistic patterns in motoneuron activity expressed as membrane potential oscillation, as well as sinusoidal length changes in body length were exploited. Active muscle tension was modeled as a product of a length-dependent exponential function and a frequency-dependent factor that is described by a second order transfer function with a time delay and a nonlinear static function. The model describes well the complex transformation from motoneuron impulse rates to muscle contraction strengths. A complete systems model for the neuromuscular system receiving input signals from central oscillators was built to predict tensions generated in swimming leeches and to capture the phase-dependence of tension development on motoneuron activity and body wall length.;Serotonin was applied to mimic the neurohormonal conditions in swimming leeches and its effects were investigated and captured in our systems model that describes tension development in leech swimming.