Motor unit

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A motor unit comprises a single motor neuron together with all of the muscle fibers innervated by it and thus represents the smallest functional unit for the control of voluntary and involuntary motor functions of a skeletal muscle .

The development of strength in a muscle is primarily determined by the number and size of the active (“recruited”) motor units. Adapted to the tasks of a muscle and the necessary force gradation, skeletal muscles differ both in the total number of their motor units and in the number of muscle fibers innervated per motor neuron:

  • Small motor units have around 100 to 300 muscle fibers and allow a fine gradation of strength (e.g. outer eye muscles and muscles of the fingers). Older data on smaller motor units with fewer than 25 muscle fibers (e.g. in the platysma , musculus obliquus superior , musculus opponens pollicis ) were partly collected from stillbirths and have not been proven in healthy people.
  • Large motor units comprise up to 2000 muscle fibers and are suitable for grading coarse strength (e.g. four-headed knee extensor muscles - quadriceps femoris muscle ).

The size of the innervating motor neuron corresponds roughly to the size of a motor unit. Small motor neurons, with a small cell soma and a relatively thin axon , supply a small number of muscle fibers, while larger motor neurons, with a large cell soma and a relatively thick axon, are responsible for a larger number of muscle fibers. The muscle fibers of individual motor units are not located next to each other, but are distributed in the muscle over a cross-sectional area of ​​up to 1 cm². The metabolism of the associated muscle fibers is adapted to the prevailing contraction process (slow or rapid twitch fiber types ) or the typical activation frequency of the respective motor neuron. In the case of muscle contraction of increasing strength, small motor units are initially used (recruited), and large motor neurons are alternately activated for greater tension (see also Henneman's principle ). For maximum power development of a motor unit, the neural pulse frequency is increased so that individual twitches superimpose and add up.

Determination procedure

In animals, the number of muscle fibers per motor unit is counted in specially pretreated histological sections. To do this, a single motor neuron or its nerve fiber is continuously excited so that the associated muscle fibers of the motor unit contract and use up their glycogen stores. Then a preparation with PAS staining (glycogen detection) is made and the amount of the lighter muscle fibers is counted in the cross section.

In humans, samples are taken from the muscle by means of a biopsy and examined. If the number of muscle fibers is recorded in the histological section and this is divided by the number of incoming myelinated nerve fibers minus the estimated number of nerve fibers for muscle spindles , the average size of motor units in this tissue sample results.

See also

Individual evidence

  1. Detlev Drenckhahn (Ed.): Anatomie Volume 1. Urban & Fisher, Munich 2008