14.11: Motor Units
The motor unit is a fundamental component of the neuromuscular system and plays a crucial role in coordinating muscle contractions. It consists of a somatic motor neuron, which connects and controls multiple skeletal muscle fibers, forming a single functional segment. The axon of the motor neuron branches out and establishes synaptic connections known as neuromuscular junctions with individual muscle fibers within the motor unit.
Motor units come in different sizes, with smaller units consisting of only a few muscle fibers and larger units comprising a more significant number of fibers. This size variation reflects the specific requirements of different muscles and movements. For instance, the muscles responsible for producing the voice in the larynx have only 2 or 3 muscle fibers per motor unit. In comparison, the extraocular muscles range from 10 to 20 muscle fibers per motor unit. In contrast, larger motor units can control weight-bearing muscles that do not necessitate the same level of precision and comprise 2,000 to 3,000 muscle fibers in some motor units.
The motor units have fibers interspersed with each other for even distribution of contractile forces within a muscle. When the central nervous system initiates muscle movement, it activates multiple motor units within the relevant muscle groups. For example, when moving an object, signals are relayed to numerous motor units in the arm muscles. Initially, a few small motor units are recruited to initiate contractions. If the object being moved is light, like a book, the contractions generated by the small motor units are sufficient. However, larger motor units are recruited to provide the necessary strength when dealing with a heavier object, like a heavy box. This distributed use of motor units allows for more precise control over contraction force and helps protect muscles from premature fatigue. By selectively engaging motor units based on the task, the body can optimize its muscular efforts, ensuring both efficiency and effectiveness in movement execution.
