Motor unit

Motor unit refers to a single motor neuron and all of the muscle fibers it innervates. It is the smallest functional unit of the muscular system. The concept of the motor unit was first proposed by Charles Scott Sherrington in 1897.

Structure

A motor unit consists of a motor neuron, an axon, and the muscle fibers innervated by the axon. The motor neuron's cell body is located in the spinal cord and its axon extends through nerves to individual muscle fibers.

Function

The function of a motor unit is to facilitate muscle contraction. When a motor neuron is activated, all of the muscle fibers it innervates are stimulated to contract. The strength of the contraction can be varied by changing the frequency of activation of the motor unit.

Types of Motor Units

There are two main types of motor units: slow-twitch and fast-twitch. Slow-twitch muscle fibers (Type I) are more efficient at using oxygen to generate fuel for continuous, extended muscle contractions over a long period. They fire more slowly than fast-twitch fibers and can go for a long time before they fatigue. Fast-twitch muscle fibers (Type II) are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly.

Clinical Significance

Abnormalities in motor units can lead to a variety of neuromuscular disorders, including motor neuron diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA).

See Also

• Neuromuscular junction
• Muscle contraction
• Motor neuron