Myotomes, Spinal Nerve Roots, and Dermatomes

The body is divided from top to bottom into motor zones described as myotomes. The muscle movement of each myotome is controlled by motor nerves coming from the same motor portion of a spinal nerve root. This differs from a dermatome, which is a zone on the skin in which sensations of touch, pain, temperature, and position are modulated by the same sensory portion of a spinal nerve root.

Myotomes and dermatomes are mapped, and the location of sensory or motor deficits correspond to specific nerve roots. Based on your history and physical examination, your healthcare provider or physical therapist can determine the specific nerve root(s) or spinal core level(s) that could be causing your problem.

Myotomes and dermatomes are part of the peripheral nervous system, and myotomes are part of the somatic (voluntary) nervous system, which is part of your peripheral nervous system. The peripheral and central nervous systems communicate with one another.

Muscles and Nerves, Oh My!

Every muscle cell in your body functions based on nerve signals. In fact, your muscles need nerve signals to maintain their resting tone and stability. And without at least some communication from a nerve, muscles begin to decay.

Muscle-nerve communication occurs at the motor endplate, a portion of the muscle. The neuromuscular junction is a structure that includes a nerve cell, along with the muscle endplate.

Each nerve cell innervates (provides signals) several muscle fibers. A single nerve and its corresponding muscle fibers comprise a motor unit. Every fiber that is part of a motor unit contracts (shortens) to move when its respective nerve is fired. It's an all or nothing event. Motor units take turns firing, and this prevents them from becoming exhausted

A nerve cell can innervate as few as six to 10 muscle cells for fine, detailed actions such as finger or eye movements. Or a nerve cell can innervate hundreds of muscle cells for powerful actions, such as those carried out by the mid-back and arm muscles.

Myotomes: A Global Nerve-Muscle Perspective

A myotome is the group of muscles on one side of the body that are innervated by one spinal nerve root.

During a physical exam, your healthcare provider would consider the location of myotomes and dermatomes to identify the specific spinal nerve(s) that may underlie problems such as muscle weakness and sensory changes.

The chart below shows the actions produced by each nerve.

There is often a small overlap in myotome zones, where nerves will innervate the muscles in the zones where they are mapped, and may also innervate nearby muscles as well.

The Myotome Dance

If you're the kind of person who learns by doing, check out one or more of the YouTube videos linked below. These were produced by physical therapy assistant school cohort groups (and one physiology class) from around the country. Because each video has both strong and weak points educationally, it's probably a good idea to look at all of them. Note: Most of the dances are done to a very fast beat. Move at the pace your body (particularly your neck) can handle safely. Most likely, this means going slower than the physical therapy students.

Overall, though, these dances may give you a good idea about what myotomes actually do, and why they are important. And the music is pretty good, too.

• Ladies in Blue Do the Myotome Dance
• Dr. Burke-Doe's Physiology Class Dances to Bon Jovi
• Austin Community College PTA Class of 2013 (With Costume!) (Note: This one is both dermatome and myotome.)