Muscle force potentiation induced by active dendrites of spinal motoneuron during locomotor like movement

Abstract

Voltage-gated calcium channels generating persistent inward currents (Ca-PICs) in the dendrites of spinal motoneurons are actively involved for generation and control of normal movement [1]. However, the Ca-PIC effects on muscle force have been implied based solely on the input-output properties of the motoneurons. This study is focused on how the Ca-PIC activation influences the force output of motor units during locomotor-like movements. A model of motor unit was constructed combining two physiologically realistic models: one for the motoneuron including active dendrites and the other for the muscle unit reflecting movement dependent force production. The long-lasting step current with alternating excitatory and inhibitory pulses was injected at the soma of the motoneuron model and resulting force output was calculated from the muscle unit model while dynamically varying the muscle length. Force production without Ca-PIC activation was simulated by excluding the excitatory pulses from the current step protocol. Figure 1 shows the comparison of the force production with and without Ca-PIC activation during the movement. Ca-PIC induced force production was not apparent when the motoneuron transitioned into the stable firing state from the quiescent state by Ca-PIC activation (see the first half of the simulation in Figure 1). However, when the motoneuron switched from the low- to the high-frequency firing state by Ca-PIC activation, the force was increased by an average of almost 30% (see the second half of the simulation in Figure 1). The simulation results suggest that the Ca-PIC activation in the motoneuron dendrites may potentiate force production of the motor unit depending on the firing history of the motoneuron during the movement.