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Defining a Motor Unit

A motor unit consists of one motor neuron and all of the muscle fibers that it innervates. The motor neuron extends from the central nervous system and branches to form synapses with a number of muscle fibers after the nerve axon enters the muscle. A single motor neuron can innervate multiple muscle fibers, but there is no overlap in the innervations of the muscle fibers by different motor neurons.  One motor neuron can innervate hundreds or thousands of muscle fibers in a muscle group.  There can be any number of muscle fibers in a muscle and any number of motor neurons innervating these muscle fibers. The number of muscle fibers and motor neurons can vary from one muscle group to the next, subject to the size and the function of the muscle.

One of the important things to note when understanding motor units is that when a motor neuron is activate appropriately, it leads to activation of all the muscle fibers in the motor unit. This causes a twitch throughout the complete motor unit. A twitch is a brief all or nothing contraction of all the muscle fibers in the motor unit.  This means that the motor unit is either in a relaxed state or a state of full contraction. Understanding the all or nothing becomes important when determining the appropriate stimulus to be applied to achieve the desired muscle activation to provide the performance results. Because each muscle is made up of a large number of motor units, muscular contractions of the whole muscle are controlled by the body activating a different number of motor units.  Individuals who have participated in progressive training usually develop better muscle efficiency. Muscle efficiency along with coordination are what helps an athlete put a basketball through the net instead of hard off the back iron or to kick a soccer ball into the corner of the goal instead of into the 5th row of the stands.

Typically, motor neurons and muscle fibers are placed into three size categories.  These categories are small, intermediate, and large.  Smaller muscle fibers are innervated by smaller motor neurons. Larger muscle fibers are innervated by larger motor neurons. Small motor neurons are easier to excite leading to the motor unit contraction, but due to the small size of the muscle fiber, there is not much force generated with these motor units.  Although they are not equipped to generate high amounts of force, they are designed to generate ATP for longer durations of time and this leads to slow fatigue.  Larger motor units have a higher threshold (need to more substantial stimulus) for excitement, but they are able to generate more force due to the larger size of the muscle fiber in the motor unit.  By contrast to the smaller muscle fibers, larger muscle fibers generate more force, but are subject to rapid fatigue.

 

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