How are motor units recruited?

Updated: Oct 16, 2019
  • Author: Friedhelm Sandbrink, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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As a general rule, motor units are recruited in order of their size. When the muscle is activated initially, the first motor units to fire are small in size and weak in the degree of tension they can generate. Starting with the smallest motor units, progressively larger units are recruited with increasing strength of muscle contraction. The result is an orderly addition of sequentially larger and stronger motor units resulting in a smooth increase in muscle strength. [2]

This orderly recruitment of sequentially larger motor units is referred to as the "Henneman size principle," or simply "size principle." [2, 3, 4] Recording from the ventral rootlets in cats and measuring the amplitudes of motor axon spikes, Henneman et al concluded that motor axon diameter, conduction velocity and, by further inference, motor neuron cell size all increase with functional threshold. [2]

There are exceptions to the size-ordered activation of motor units. Motor unit recruitment patterns vary for different movement tasks, depending on many factors, including the mechanical function of the muscle, sensory feedback, and central control. [3] After nerve injury, the relationship between motoneuron size and the number and size of muscle fibers that the motoneuron reinnervates is initially lost. [4] With time, however, a size-dependent branching of axons accounts for the rematching of motor neuron size and muscle unit size, and the size-ordered organization of motor units properties is restored. [4]

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