A motor unit consists of one alpha motor neuron together with all the muscle fibers it stimulates. Since the human body contains, on average, 250,000,000 muscle cells and approximately 420,000 motor neurons, a motor unit will generally consist of a single motor neuron paired with many muscle fibers. In strength training, the early strength gains seen by novices are often not gains in size or number of muscle fibers, but activation of motor units that had been previously dormant.
The motor neuron is a specialized type of nervous cell that runs between the central nervous system and the muscles. Neurons typically consist of a cell body — the axon — and the dendrites. If a neuron were to be seen as a tree, the axon would be analogous to the trunk and the dendrites to the branches. Neurons found within the brain normally have relatively short axons, but neurons that are part of a motor unit — because they must connect to the muscles of the body — have elongated axons that run through the spinal cord, and out to the associated muscle fibers. Each muscle fiber is connected to a particular dendrite, and it is through the dendrites that messages are relayed between the central nervous system and the muscle fiber.
Muscle fibers are elongated cells, specialized to carry out the functions of the specific muscles of which they are a part. This is true of the cardiac muscles of the heart, the smooth muscles that make up the lining of many internal organs, and skeletal muscles. Only skeletal muscles, however, are under conscious control. The size and shape of the muscle fiber is dependent upon its function, with the smooth muscle cells being flattened and tile-like; skeletal muscle cells, long and rope-like; and cardiac muscle cells having some properties of the other two.
A single muscle usually consists of a number of motor units working together, known as the motor pool. When the central nervous system requires that a muscle contract, an electrical signal is sent along the motor neuron, stimulating the muscle fibers to contract. Normally, each contraction is followed by a brief period of relaxation of the muscle fibers, and this pattern repeats in a wave-like pattern, known as a twitch. Skeletal muscle fibers can be divided into slow twitch and fast twitch fibers, depending on the length of time required for contraction and relaxation to occur. Slow twitch fibers are associated with endurance, while fast twitch muscle fibers are associated with power.
Individuals may have a preponderance of one type of muscle fiber or the other, or a combination of the two. All the muscle fibers within a motor unit will be of a single type, meaning either fast twitch or slow twitch. This may include up to 1,000 muscle fibers, as in the large quadriceps muscles of the thigh, or fewer than ten, as seen in motor units requiring a high degree of precision, such as the muscles that control eye movement.
Upon contraction, the smallest motor unit, that is, the one associated with the fewest muscle fibers, is the normally the first activated. As the contraction progresses, larger motor units are brought into play. Efficient muscle contraction depends on the motor units within a muscle working effectively together. Regular physical training makes this kind of coordination easier.
Occasionally, a motor unit will receive a series of rapid contractile stimulations in such quick succession that the motor pool has no time to enter the relaxation phase of each twitch. When this occurs, it can build up to a state of maximal contraction, known as tetanic contraction. Significantly stronger than a natural twitch, tetanic contraction can result from a number of causes, such as illness or an adverse drug reaction. One of the more well-known reasons for this phenomenon is associated with tetanus infections.