Myotomes

Myotomes are groups of muscles that are innervated by a single spinal nerve root. They reflect the segmental organization of the nervous system, which develops from somites that differentiate into dermatomes, myotomes, and sclerotomes. Each spinal nerve root contributes to one or more myotomes, with patterns that are relatively consistent in the cervical and lumbosacral regions but can show individual variation.

In clinical neurology, myotomes are used to assess motor function by testing movements associated with specific nerve roots. Commonly evaluated examples include movements linked to the cervical roots (such as shoulder abduction and elbow flexion for C5, wrist extension for C6, elbow or finger movements for C7–C8) and the lumbosacral roots (such as hip flexion or knee extension for L2–L4, dorsiflexion or toe movements for L5, and plantar flexion for S1–S2). There is substantial overlap between neighboring myotomes, and testing relies on resisted movements to grade strength.

Myotome testing aids localization of nerve root pathology, helping to distinguish radiculopathy from peripheral nerve disease when used in conjunction with sensory dermatome testing, reflex assessment, and other diagnostic tools. Limitations include anatomical variation, overlap between myotomes, and atypical innervation patterns. In practice, myotomes are part of a broader neurological examination, complemented by imaging, electrodiagnostic studies, and the clinical context to determine the site and mechanism of impairment.