About Motor Units

In short, motor units are what allows our body to move. They are activated and contract once they have been signaled by the body to do so and, like a well-oiled machine, one motor unit will signal another motor unit within that muscle, and so on until all of these contractions cause the muscle to move. Though motor units have a few functioning parts and can vary by muscle function and size, they are without a doubt a vital part of our bodies.

Function

A motor unit is made up of a motor neuron and the muscle fibers that belong to a single muscle. When a motor unit is called upon, all the fibers within it contract. In order to make a muscle move, the fibers within all the motor units of that muscle will group together and contract. This larger group of motor units is commonly referred to as a motor unit pool.

Size

Depending upon the muscle, motor units can vary in size. Their size depends on the number of fibers that make up each unit. Naturally, larger muscles typically have more fibers and larger motor units. For example, one of the strongest muscles of the body is the thigh. It is also one of the muscles with the most muscle fibers.

Features

The muscle fibers that make up a motor unit can be found throughout that body part''s specific muscle. These fibers aren't typically side by side but can reside in close proximity to other fibers. Stronger or larger muscles tend to have these fibers spread throughout their landscape. Smaller muscles may have less fibers that cover less of the muscle's area. Motor units are found throughout the entire body, however. They are present wherever a muscle is present--which, in the human body, is virtually everywhere.

Time Frame

Motor units can decrease as a muscle becomes weak due to age, trauma, disease or other injuries. This happens as the motor neurons are progressively reduced, which also reduces the number of motor units that make up a muscle. In the neuromuscular disease myopathy, motor units can also be damaged as motor units become smaller while more motor fibers begin to die. The actual number of motor units remains fairly steady, however, until and unless myopathy advances to a very serious state.

Size

The amount of fibers found in the motor unit depend on the size and use of the muscle. Larger muscles that have less precise movements are made up of more fibers than smaller muscles with very specific movements. For example, a motor unit inside the eye will usually contain two to three fibers, with only three to five motor units residing in the entire eye. The calf muscle, however, can have between 1,000 and 2,000 motor fibers per motor unit.