Motor Units

Skeletal muscle fibers associate with each other in motor units; this structuring is essential to increase the "control" of the muscle shortening otherwise limited to the level of the single fiber cells.

What is the motor unit?

The muscles are innervated by pool (families) of motor neurons; comparing the number of the nerve cells to that of the muscle fiber cells it emerges that the motor neurons are far inferior to the fibers to be stimulated. Logically, each motor neuron is responsible for the innervation of several fiber cells and, according to the definition of Sherrington, the association between the nervous and muscular components constitutes the MOTOR "UNIT".
Each fiber cell of the motor unit is equipped with only one motor plate (neuromuscular junction, between the nerve and the fiber), which excludes the reciprocal interaction of motor units. In light of this, and considering the fact that the stimulation of the motor neuron corresponds to a simultaneous contraction of all the fibers innervated by it, it is possible to state that: the motor unit is the smallest amount of muscle tissue that the nervous system can control: motor unit = functional unit of movement.

Growth and definition of motor units

During development, the muscle fibers are not yet fully equipped with motor plates, therefore, they are not yet well innervated. The motor neurons, which will later complete the functional unit of movement, grow longitudinally following the stimulus of a trophic factor released by the muscle fibers, but what leaves you "speechless" is the efficiency with which the body completes the motor system; analyzing the number of growing motor neurons shows that their number is higher than that generally observed in adults. This happens because, in order to ensure the rapid completion of the various motor units, the body allows the development of numerous motor neurons which, however, , all except the definitive one, will undergo apoptosis (cell death). The chosen motor neuron is the first to reach the fibers which only at this point interrupt the secretion of the trophic factor. As already mentioned, several neurons CANNOT coexist in the motor unit; in the case of growth, this can only occur for a limited period after which the excess nerve cells will be eliminated.

Coexistence of motor units

The motor units do not contract simultaneously; they act asynchronously to prevent the phenomenon of fatigue and guarantee contractile delivery over time.
Furthermore, the muscle fibers of the individual motor units are NOT contiguous but mix with those of other functional groups (innervated per se) occupying a surrounding space of about 20-30% of the total muscle volume.
The nature of the motor units varies significantly either within the same muscle that between the different fabrics; the number of muscle fibers present in each single motor unit also varies considerably and is much lower in the muscles responsible for "delicate or precision" movements (eye, hand, etc.). In practice, the smaller the number of fibers innervated by the single motor unit, the greater the central control by increasing or decreasing the number of motor units solicited.
The various motor units differ from each other in:

Contraction time (period necessary for the development of maximum strength).
Force developed at the peak of the simple shock.
Maximum strength developed during a tetanus (the tetanus corresponds to numerous action potentials released in a very short time).
Loss of contraction force following a tetanus.

Classification

Based on the aforementioned parameters, the motor units are divided into three main classes:

Slow (slow - S): little force following a single shock or a tetanic shock and slow contraction times (> 50 milliseconds - ms); they usually contain red fibers (type I).
Fast fatigable (fast and fatigable - FF): they boast maximum speed and strength but with little resistance (after 2 "the strength is reduced by 75%); they are made up of white fibers (type IIB).
Fast resistant (fast and resistant - FR): they have intermediate characteristics between the two mentioned above, the contraction times are short and the strength is high; at 2 "they maintain more than 75% of the force delivered at the start. Usually made up of white fibers (type IIA).

NB. It has been shown that each motor unit innervates ONLY fibers belonging to the same category and generally the association motor unit / type of fiber is the one just mentioned.
The motor units determine the athletic characteristics of each subject; they can be modified in part with training and orient themselves (especially the RF) towards one metabolism rather than another, but essentially, their nature cannot be changed.

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