Effective Stimulation Type and Waveform for Force Control of the Motor Unit System: Implications for Intraspinal Microstimulation.

Hojeong Kim,Youngchang Ju

doi:10.3389/fnins.2021.645984

Abstract

The input–output properties of spinal motoneurons and muscle fibers comprising motor units are highly non-linear. The goal of this study was to investigate the stimulation type (continuous versus discrete) and waveform (linear versus non-linear) controlling force production at the motor unit level under intraspinal microstimulation. We constructed a physiological model of the motor unit with computer software enabling virtual experiments on single motor units under a wide range of input conditions, including intracellular and synaptic stimulation of the motoneuron and variation in the muscle length under neuromodulatory inputs originating from the brainstem. Continuous current intensity and impulse current frequency waveforms were inversely estimated such that the motor unit could linearly develop and relax the muscle force within a broad range of contraction speeds and levels during isometric contraction at various muscle lengths. Under both continuous and discrete stimulation, the stimulation waveform non-linearity increased with increasing speed and level of force production and with decreasing muscle length. Only discrete stimulation could control force relaxation at all muscle lengths. In contrast, continuous stimulation could not control force relaxation at high contraction levels in shorter-than-optimal muscles due to persistent inward current saturation on the motoneuron dendrites. These results indicate that non-linear adjustment of the stimulation waveform is more effective in regard to varying the force profile and muscle length and that the discrete stimulation protocol is a more robust approach for designing stimulation patterns aimed at neural interfaces for precise movement control under pathological conditions.

Highlights

Recent advances in neural interface technology have allowed the direct modulation of nervous system functions by injecting currents into specific compartments of individual neurons (Holsheimer, 1998; Radivojevic et al, 2016)
The firing rate was enhanced at a current intensity higher than the recruitment threshold in the ascending stimulation phase, and firing was sustained below the recruitment threshold in the descending stimulation phase. This result was mainly attributed to the activation of persistent inward current (PIC)generating calcium channels at the dendrite underlying the nonlinear input–output relationship of the motoneuron
We further investigated the influence of the PIC channels responsible for the plateau potentials over the motoneuron dendrites on the stimulation waveforms required for force control

Summary

Introduction

Recent advances in neural interface technology have allowed the direct modulation of nervous system functions by injecting currents into specific compartments of individual neurons (Holsheimer, 1998; Radivojevic et al, 2016). The underlying mechanism of this non-linear input–output relationship has been suggested as the spatiotemporal interaction between action potential-producing membrane mechanisms at the soma and plateau potential-generating calcium channels in dendritic areas (i.e., 300–800 μm from the soma) (Kim et al, 2014). These dendritic calcium channels (presumably L-type Cav1.3 channels) are actively involved through monoaminergic neuromodulation due to the brainstem regarding normal motor behavior and through endogenous monoamines in regard to spinal cord injury (Heckmann et al, 2005)

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