Motor Unit of Skeletal Muscle

Unlike some types of smooth muscle (single-unit type; ^ p. 70) and cardiac muscle fibers, which pass electric stimuli to each other through gap junctions or nexus (^ A; p. 16f.), skeletal muscle fibers are not stimulated by adjacent muscle fibers, but by motor neurons. In fact, muscle paralysis occurs if the nerve is severed.

One motor neuron together with all muscle fibers innervated by it is called a motor unit (MU). Muscle fibers belonging to a single motor unit can be distributed over large portions (1 cm2) of the muscle cross-sectional area. To supply its muscle fibers, a motor neuron splits into collaterals with terminal branches (^ p. 42). A given motor neuron may supply only 25 muscle fibers (mimetic muscle) or well over 1000 (temporal muscle).

Two types of skeletal muscle fibers can be distinguished: S - slow-twitch fibers (type 1) and F - fast-twitch fibers (type 2), including two subtypes, FR (2 A) and FF (2 B). Since each motor unit contains only one type of fiber, this classification also applies to the motor unit.

Slow-twitch fibers are the least fatigable and are therefore equipped for sustained performance. They have high densities of capillaries and mitochondria and high concentrations of fat droplets (high-energy substrate reserves) and the red pigment myoglobin (short-term O2 storage). They are also rich in oxidative enzymes (^ p. 72). Fast-twitch fibers are mainly responsible for brief and rapid contractions. They are quickly fatigued (FF > FR) and are rich in glycogen (FF > FR) but contain little myoglobin (FF^FR).

The fiber type distribution of a muscle depends on the muscle type. Motor units of the S type predominate in "red" muscles such as the soleus muscle, which helps to maintain the body in an upright position, whereas the Ftype predominates in "white" muscles such as the gastrocnemius muscle, which is involved in running activity. Each fiber type can also be converted to the other type. If, for example, the prolonged activation of fast-twitch fibers leads to a chronic increase in the cytosolic Ca2+ concentration, fast-twitch fibers will be converted to slow-twitch fibers and vice versa.

Graded muscle activity is possible because a variable number of motor units can be recruited as needed. The more motor units a muscle has, the more finely graded its contractions. Contractions are much finer in the external eye muscles, for example, which have around 2000 motor units, than in the lumbri-cal muscles, which have only around 100 motor units. The larger the number of motor units recruited, the stronger the contraction. The number and type of motor units recruited depends on the type of movement involved (fine or coarse movement, intermittent or persistent contraction, reflex activity, voluntary or involuntary movement, etc.). In addition, the strength of each motor unit can be increased by increasing the frequency of neuronal impulses, as in the tetanization of skeletal muscle (^ p. 67 A).

¡g Plate 2.10 Muscle types, motor unit

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