Principles of motor unit physiology evolve with advances in technology

Review article, 2016

Movements are generated by the coordinated activation of motor units. Recent technological advances have made it possible to identify the concurrent activity of several tens of motor units, in contrast with much smaller samples available in classic studies. We discuss how these advances in technology have enabled the development of a population perspective of how the central nervous system controls motor unit activity and thereby the forces exerted by muscles.
Movements are controlled by the coordinated activation of neuromuscular units that produce force: the motor units (28, 48). Each motor unit comprises a motoneuron and a muscle unit, where the latter refers to the muscle fibers innervated by the motoneuron. The nervous system produces movements by delivering synaptic inputs to motoneurons that innervate at least several muscles. Once activated, the motoneurons engage the muscle units in the involved muscles to produce both synergistic and antagonistic muscle forces.

To perform movements accurately, the neural drive to muscles (the ensemble output of motoneurons) transmitted by motoneurons from supraspinal centers and sensory receptors must be reliable. As a first approximation, motoneurons process synaptic inputs by functioning as integrate-and-fire systems (66), which means that motoneurons are activated when the time integral of the synaptic inputs causes a change in membrane potential that exceeds the voltage threshold of the motoneuron. The muscle force at which this occurs is known as the recruitment threshold of the motor unit. The rate at which motoneurons discharge action potentials is positively associated with the difference between the synaptic input received by the motoneuron and its voltage threshold. Modulation of discharge rate is known as rate coding (48).

Motor units transduce the neural activation signal into muscle forces, which means that the discharge characteristics of motor units contain information about the neural control signal. It is for this reason that methods were developed to record and decode the discharge characteristics of motor units with intramuscular electrodes (1, 28). One feature of such methods is the high selectivity of the recording, which ensures signal detection but limits the number of motor units that can be discriminated concurrently. Recent developments in electrode technology and biological signal processing have greatly reduced this limitation by making it possible to monitor the concurrent activity of many motor units (85). The concurrent recordings and computational modeling have enabled the development of a population perspective of how the nervous system controls movement. Several key findings indicate that classic concepts of motor unit function derived from recording the activity of only a few motor units need to be revised.

The aim of the current review is to describe the influence of recent advances in technology on our current understanding of how the nervous system controls motor unit activity and thereby the forces exerted by muscles.