What Are Myotomes?

Myotomes are groups of muscles that receive signals from a single spinal nerve root. If your healthcare provider thinks you might have nerve damage, they may do myotome testing as a part of a complete neurological examination. 

Myotomes are mapped, and the location of problems 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.

This article discusses myotomes, what they are, and how they are tested.

How Myotomes Work

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.

The motor nerve roots are responsible for muscle movement. They branch out from the different parts of the spinal cord. A myotome is the group of muscles on one side of the body that receive signals from one nerve root.

Each nerve cell innervates (provides signals to) 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. 

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 vs. Dermatomes

Dermatomes are areas of skin that are supplied by a single sensory nerve root. The main difference between a dermatome and a myotome is that dermatomes control sensation while myotomes control movement.

Myotomes List

These are the main spinal levels associated with each movement, but movements are usually controlled by more than one level.

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.

Myotome Testing

Myotome testing helps identify muscle weakness in certain muscle groups. This information can help your healthcare identify the specific spinal nerve(s) that may be causing muscle weakness.

During myotome testing, the examiner will ask you to perform certain movements. They will use resistance to determine the strength of each muscle group. For example, they will ask you to extend your wrist while pressing against the motion. 

Example: Herniated Disc

A herniated disc occurs when the soft center of a spinal disc leaks out through a tear. The herniated disc can press on the nerve root, causing pain, numbness, and/or weakness. Herniated discs are a common cause of radiculopathy, which is the medical term for injury or damage to the nerve root.

Myotome testing can help your healthcare provider identify where the herniated disc is. If the injury is in the C5 nerve root, for example, you will have difficulty resisting downward pressure when raising your arm and shoulder. 

The myotome will also be given a grade of 0 to 5, with 0 indicating total paralysis and 5 indicating normal movement.

Summary

Myotomes are groups of muscles that receive signals from a specific spinal nerve root. If you have damage in one of these nerve roots, you may have weakness in the associated muscle group.

During myotome testing, you'll be asked to perform certain movements while the examiner provides resistance. The results of this test can help your healthcare provider identify the nerve root where the injury occurred and the severity of the problem.