Motor Unit

Abstract

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Inhibiting persistent inward sodium currents prevents myotonia

“…In motor neurons, repetitive firing is triggered by PICs, which are carried by both Na + and Ca + channels 16, 17, 18, 19. A NaPIC has been described previously in skeletal muscle20; this NaPIC lacked fast inactivation and activated between −80 and −70mV, such that it would convert a steady depolarization from K + buildup into myotonia.…”

“…Importantly, it also implies that NaPIC has a role in normal muscle function. One role of PICs in neurons is to convert steady depolarizations caused by the asynchronous firing of many weak synaptic inputs into repetitive spiking 16, 17, 18, 19. However, because skeletal muscle fibers receive only one synaptic input, there is no need for a PIC to convert multiple asynchronous synaptic firings into repetitive muscle APs.…”

“…However, because skeletal muscle fibers receive only one synaptic input, there is no need for a PIC to convert multiple asynchronous synaptic firings into repetitive muscle APs. The normal function of NaPIC in muscle might be to improve the fidelity of neuromuscular transmission during periods of intense stimulation; in addition to enabling repetitive firing, PICs in neuronal dendrites serve as amplifiers for weak synaptic inputs 18, 32. During intense exertion, the neuromuscular junction is stimulated at rapid rates, resulting in depression of the endplate potential, such that synaptic transmission might fail 33.…”

“…In neurons, repetitive firing is maintained by persistent inward currents (PICs), which are activated by long depolarizations and mediated by Na + (Na + persistent inward current [NaPIC]) or Ca 2+ 16, 17, 18, 19. A NaPIC has also been described in wild‐type (WT) skeletal muscle 20.…”

Web-based neuromuscular simulator applied to the teaching of principles of neuroscience

“…During a given motor task, two fundamental mechanisms are involved to increase the force level (Heckman and Enoka, 2012): i) the recruitment of new motor units, and ii) previously recruited motor units increase their firing rate, called rate coding in motor control. Both mechanisms may be easily studied using the neuromuscular simulator presented in this work.…”