Motor units

Skeletal muscle fibers associate with one another in motor units; this structuring is fundamental to increase the "control" of the muscle shortening otherwise limited to the level of the single fibrocells.

What is motor unity?

Muscles are innervated by pools (families) of motor neurons; comparing the number of nerve cells to that of muscle fibrocells, it turns out that the motor neurons are far inferior to the fibers to be stimulated. As a matter of logic, each motor neuron is responsible for the innervation of several fibrocells and, according to the definition of Sherrington, the association between nervous and muscular component constitutes the MOTOR UNIT.

Each fibrocellula of the motor unit has only one motor plate (neuromuscular junction, between the nerve and the fiber), which excludes the reciprocal interaction of the motor units. In light of this, and considering the fact that the motoneuron stimulus corresponds to a simultaneous contraction of all the fibers innervated by it, it is possible to state that: the motor unit is the minimum amount of muscle tissue that the nervous system can control : unity motor = functional unit of movement.

Growth and definition of motor units

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

Motor unit coexistence

Motor units do not contract simultaneously; they act asynchronously to prevent the phenomenon of fatigue and to guarantee contractile delivery over time.

Moreover, the muscle fibers of the individual motor units are NOT contiguous but are mixed with those of other functional groups (in themselves innervated) occupying a surrounding space of about 20-30% of the overall muscle volume.

The nature of motor units varies significantly both within the same muscle and between different tissues ; also the number of muscle fibers present in each single motor unit varies significantly and is much lower in muscles that are subject to "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 stressed motor units.

The various motor units differ from each other by:

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

Classification

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

Slow (lens - 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 force is reduced by 75%); they consist of white fibers (type IIB).
Fast resistant (fast and resistant - FR): they have intermediate characteristics between the two mentioned above, contraction times are short and high strength; at 2 'they retain more than 75% of the force delivered at the start. Usually made of white fibers (type IIA).

NB . It has been shown that each motor unit ONLY supplies fibers belonging to the same category and generally the motor unit / fiber type association is the one just mentioned.

The motor units determine the athletic characteristics of each subject; they can be modified in part with training and orientate (especially FRs) towards one metabolism rather than another, but essentially, their nature cannot be changed.

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