Electrical cross-sectional imaging of human motor units in vivo

Stuart Maitland,Enrique Escobedo-Cousin,Ian Schofield,Anthony O'Neill,Stuart Baker,Roger Whittaker

doi:10.1016/j.clinph.2021.12.022

Abstract

ObjectivesIn many neuromuscular diseases, weakness results from a disruption in muscle fibres' arrangement within a motor unit. Limitations in current techniques mean that the spatial distribution of fibres in human motor units remains unknown. MethodsA flexible multi-channel electrode was developed and bonded to a clinical electromyography (EMG) needle. Muscle fibre action potentials were localised using a novel deconvolution method. This was tested using simulated data, and in recordings collected from the tibialis anterior muscle of healthy subjects. ResultsSimulated data indicated good localisation reliability across all sections of the electrode except the end sections. A corrected fibre density was estimated up to 1.4 fibres/mm2. Across five recordings from three individuals, between 4 and 14 motor units were detected. Between 1 and 20 muscle fibres were localised per motor unit within the electrode detection area, with up to 220 muscle fibres localised per recording, with overlapping motor unit territories. ConclusionsWe provide the first direct evidence that human motor units spatially overlap, as well as data related to the spatial arrangement of muscle fibres within a motor unit. SignificanceAs well as providing insights into normal human motor physiology, this technology could lead to faster and more accurate diagnosis in patients with neuromuscular diseases.

Highlights

The spatial distribution of muscle fibres within a motor unit is of fundamental interest in both healthy and diseased muscle
An increased error was seen near the ends of the electrode array, where fewer electrode potentials were within the recording radius of the fibre to allow accurate deconvolution
Across five recordings from three individuals, between 4 and 14 motor units were identified per recording

Summary

Introduction

The spatial distribution of muscle fibres within a motor unit is of fundamental interest in both healthy and diseased muscle. The only means of accurately demonstrating the spatial distribution of muscle fibres has been the glycogen depletion technique (Edstrom and Kugelberg, 1968). These complex experiments can only be performed in animals since they require the histological analysis of explanted muscle cross-sections exposed to highfrequency electrical stimulation for several minutes. These experiments demonstrate that individual muscle fibres within a motor unit are widely separated (Bodine et al, 1988; Bodine-Fowler et al, 1990), and that a single motor unit can cover up to 50% of the cross-sectional area of the muscle (Buchthal et al, 1959).

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