The Motor Unit
Muscle contraction takes place thanks to the activation of motor neurons that are found in the spinal cord. Each motor neuron synapses with numerous fibers within the muscle. Since there are many more fibers than motor neurons, motor axons are branched in order to contact with many different fibers and to get a wide distribution. This reduces the possibility that damages in one or more neurons can modify muscle functionality. The motor neuron and all the fibers that make contact with it are known as the motor unit.
The quantity of fibers that the motor neuron contacts depend on the size of the motor neuron. Thus small motor neurons innervate few muscle fibers and they form small motor units that generate small forces. By contrast, large motor neurons innervate larger motor units and are more powerful.
The characteristics of the fibers determine the speed of muscle contraction. Small muscle fibers that contract slowly and generate small forces are called slow motor units. These are important in activities that require a sustained contraction in time and low intensity (maintenance of posture). The larger muscle fibers are capable of generating large forces rapidly, but for a short time; they are called fast fatigable motor units. They are often important for short efforts like jumping or running. There are medium-sized motor units known as motor units fast fatigue resistant that are not as fast but resist fatigue over time.