myotomen

Myotome (plural myotomes; in some languages rendered as myotomen) refers to a group of muscles innervated by a single spinal nerve root. The concept is used to describe the motor component of the segmented body plan formed during embryonic development by somites, which gives rise to both the spinal nerves and their target muscles.

Anatomy: Each spinal nerve root contributes to one or more myotomes. In common clinical practice, myotomes are mapped roughly from C5 to T1 in the upper limb and from L2 to S1 in the lower limb. The idea is to relate specific muscle actions to a nerve root, for example: C5–C6 areas control shoulder abduction and elbow flexion (deltoid and biceps); C6–C7 areas control wrist extension and elbow extension; C8–T1 areas control finger flexion and intrinsic hand movements; L2–L4 involve hip flexion and knee extension; L5 involves dorsiflexion and great toe movements; S1 governs plantar flexion. In practice there is overlap between neighboring myotomes and individual variation.

Development and clinical relevance: Myotomes arise from motor neurons in the ventral horn associated with the developing spinal cord segments. Clinically, myotome testing is used in neurological examinations to localize lesions affecting spinal nerve roots, such as radiculopathy. Testing involves resisted movements of representative muscles and comparing strength to contralateral sides.

Limitations: Because of overlap, compensatory strategies, and anatomic variation, myotome findings should be interpreted with sensory (dermatome) mapping and imaging studies, rather than in isolation.