An open-source simulation package for heterogeneous neuromuscular systems

Abstract

A pool of motor units comprising various types of spinal motoneuron and muscle fiber determines the force genesis of a muscle for proper movements [1]. For realistic simulation of neuromuscular systems, automation of model construction and parallelization of computing environment have been demanded. Here, we present computer software that allows for the biophysically plausible, computationally efficient, and experimentally tractable modeling and simulation of the heterogeneous motor unit pool. The simulator for motor unit pools was developed by applying the bottom-up approach from subcellular mechanisms to system behaviors. The motor unit pool's heterogeneity was realized in a way that biophysically plausible cellular models of individual motoneurons and muscle fibers systematically reflected type-specific properties experimentally identified. This building-block approach also suited the demands for automated model construction, parallelized simulation, and hierarchical analysis linking subcellular activities to population behavior. Three modes of modeling and simulation are independently available under the simulation environment: 1) A heterogeneous pool of motoneurons can be automatically constructed and parallelly simulated under physiological input conditions, including intracellular, synaptic, and neuromodulatory inputs. 2) A heterogeneous pool of muscle fibers can be automatically modeled and parallelly simulated under physiological input conditions of electrical stimulation and length variation. 3) A heterogeneous pool of motor units can be automatically constructed and parallelly simulated under physiological input conditions applied to the motoneuron pool and muscle model. Representative simulation examples are presented for each mode to bring out the important features of the simulation package. The simulation software may provide a testbed to get insights into cellular mechanisms underlying motor control in normal states, pathogenesis underlying motor impairment, and design of assistive devices under abnormal states.