A myotome is defined as ‘a group of muscles innervated by a single spinal nerve root‘. They are clinically useful as they can determine if damage has occurred to the spinal cord, and at which level the damage has occurred. In this article we shall look at the embryonic origins of myotomes, their distribution in the adult and their clinical uses. Pro Feature - 3D Model You've Discovered a Pro Feature Access our 3D Model Library Explore, cut, dissect, annotate and manipulate our 3D models to visualise anatomy in a dynamic, interactive way. Learn More Origin of Myotomes Skeletal muscle development can be traced to the appearance of somites. By day 20 the trilaminar disc has formed and the mesoderm has differentiated into different areas. The area directly adjacent to the neural tube is known as the paraxial mesoderm. From day 20 onwards the paraxial mesoderm begins to differentiate further into segments known as somites. 44 pairs of somites are formed, however some of these regress until 31 pairs remain, corresponding to 31 pairs of spinal nerves in the adult. Somites are composed of a dorsal and ventral portion. The ventral portion forms the sclerotome, the precursor of the ribs and vertebral column. The dorsal portion consists of the dermomyotomes. As the embryo continues to develop the myotome proliferates and eventually develops into muscle. By TeachMeSeries Ltd (2025) Fig 1Somites adjacent to the neural tube. Distribution of Myotomes Most muscles in the upper and lower limbs receive innervation from more than one spinal nerve root. They are therefore comprised of multiple myotomes. For example, the biceps brachii muscle performs flexion at the elbow. It is innervated by the musculocutaneous nerve, which is derived from C5-7 nerve roots. All three of these spinal nerve roots can be said to be associated with elbow flexion. The table below details which movement is most associated with each myotome: Upper Limb Lower Limb C5 – Shoulder abduction C6 – Elbow flexion C7 – Elbow extension C8 – Finger flexion T1 – Finger abduction L2 – Hip flexion L3 – Knee extension L4 – Ankle dorsiflexion L5 – Great toe extension S1 – Ankle plantarflexion Clinical Relevance Assessing Spinal Cord Lesions In the assessment of a suspected spinal cord lesion, the clinician can test myotome function. This can help determine if there is spinal cord damage, and where the damage is located. Myotomes are tested in terms of power, and graded 1-5: 0 = total paralysis. 1 = palpable or visible contraction. 2 = active movement, full range of motion (ROM) with gravity eliminated. 3 = active movement, full ROM against gravity. 4 = active movement, full ROM against gravity and moderate resistance in a muscle specific position. 5 = (normal) active movement, full ROM against gravity and full resistance in a muscle specific position expected from an otherwise unimpaired person. By TeachMeSeries Ltd (2025) Fig 2The motor assessment component of the ASIA chart. Do you think you’re ready? Take the quiz below Pro Feature - Quiz Myotomes Question 1 of 3 Submitting... Skip Next Rate question: You scored 0% Skipped: 0/3 1800 More Questions Available Upgrade to TeachMeAnatomy Pro Challenge yourself with over 1800 multiple-choice questions to reinforce learning Learn More Print Article Rate This Article