Muscles often consist of thousands of muscle fibers working together. These muscle fibers are stimulated and controlled by motor neurons and are organized into motor units. A motor unit consists of a single motor neuron and all the skeletal muscle fibers it innervates, working together with other motor units to contract a muscle. Typical muscles are controlled by around 100 large motor neurons. The bodies of these motor neurons are located in the spinal cord (or, in the case of cranial nerves, the brain stem). Motor neurons send long axons out through the ventral root of the spinal cord, which is a cluster of axons of motor neurons. The cell bodies of the axons lie in the gray tissue of the spinal cord. These axons travel in progressively smaller clusters of peripheral nerves, until they reach the appropriate muscle. There, motor neuron axons branch and innervate between numerous muscle fibers. The muscle fibers of a single motor unit are typically scattered across the muscle. In a mature muscle, each muscle fiber receives input from only one motor neuron.
The force that a motor unit can generate depends on how much force each muscle fiber can generate and how many muscle fibers are present in the unit. Motor neurons that control fast-twitch muscle fibers usually innervate many muscle fibers. This allows them to rapidly generate large forces. Motor neurons that control slow-twitch muscle fibers are often smaller and innervate fewer muscle fibers. Although this produces a smaller force, multiple small motor units allow for much finer motor control.
The number of motor units required for a task depends on how much muscle tension needs to be generated. Motor units consisting of slow-twitch muscle fibers are usually recruited for a task first. These fibers are more resistant to fatigue and allow greater control. If more muscle tension is required, more fast-twitch motor units will be recruited.