A crucial aspect of nervous system control of skeletal muscles is the role of motor units. Every skeletal muscle fiber must be innervated by the axon terminal of a motor neuron in order to contract. Each muscle fiber is innervated by only one motor neuron. The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit. The size of a motor unit is variable depending on the nature of the muscle.
A small motor unit is arranged where a single motor neuron supplies a small number of muscle fibers in a muscle. Small motor units permit very fine motor control of the muscle for grasping, texting, and typing. A large motor unit is an arrangement where a single motor neuron supplies a large number of muscle fibers in a muscle. Large motor units are concerned with simple movements, such as powerfully extending the knee joint.
The best example is the large motor units of the thigh muscles or back muscles, where a single motor neuron will supply thousands of muscle fibers in a muscle. There is a wide range of motor units within many skeletal muscles which gives the nervous system a wide range of control over the muscles. The small motor units in the muscle will have smaller, lower threshold motor neurons that are more excitable, firing first to their skeletal muscle fibers, which also tend to be the smallest. Activation of these smaller motor units results in a relatively small degree of tension or contractile strength generated in the muscle.
As more strength is needed, larger motor units with bigger, higher threshold motor neurons are enlisted to activate larger muscle fibers. This increasing activation of motor units produces an increase in muscle tension known as recruitment. As more motor neuron units are recruited, the muscle contraction grows progressively stronger. In some muscles, the largest motor units may generate a contractile force of 50 times more than the smaller motor units in the muscle.
This allows a feather to be picked up using the biceps brachii arm muscles with minimal force and a heavy weight to be lifted by the same muscle by recruiting larger motor units. When necessary, the maximum number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle. But this cannot last for very long because of the energy required to sustain a muscle contraction. To prevent complete muscle fatigue, Motor units are generally not all simultaneously active, but instead some motor units rest while others are active, which allows for a longer muscle contraction.
The nervous system uses recruitment as a mechanism to effectively utilize skeletal muscles.