Standard Article

Motor Unit

  1. C.J. Heckman1,
  2. Roger M. Enoka2

Published Online: 1 OCT 2012

DOI: 10.1002/cphy.c100087

Comprehensive Physiology

Comprehensive Physiology

How to Cite

Heckman, C. and Enoka, R. M. 2012. Motor Unit. Comprehensive Physiology. 2:2629–2682.

Author Information

  1. 1

    Northwestern University, Evanston, Illinois

  2. 2

    University of Colorado, Boulder, Colorado

Publication History

  1. Published Online: 1 OCT 2012


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. © 2012 American Physiological Society. Compr Physiol 2:2629-2682, 2012.