Myotomes - Podcast Version 0:00 / 0:00 1x 0.25x 0.5x 0.75x 1x 1.25x 1.5x 1.75x 2x A myotome is defined as a group of muscles innervated by a single spinal nerve root. They are useful in identifying whether damage has occurred to the spinal cord or a nerve root, and determining the level of that injury. This article will explore the embryological origins of myotomes, their distribution in the adult, and their clinical relevance. 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 Each myotome originates from an early embryological structure known as a somite. These are paired blocks of mesoderm that give rise to skeletal muscle, bone, and dermis. The development of skeletal muscle can be traced to the formation of these somites. By approximately day 20, the trilaminar disc has formed, and the mesoderm has differentiated into several regions. The area immediately adjacent to the neural tube is known as the paraxial mesoderm. From around day 20, the paraxial mesoderm begins to segment into paired blocks called somites. 44 pairs of somites initially form, but several regress to leave 31 pairs (these correspond to the 31 pairs of spinal nerves in the adult). 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. Each somite differentiates into two main components: Ventromedial sclerotome – gives rise to the vertebrae and ribs. Dorsolateral dermomyotome – forms the dermis and skeletal muscle. As development continues, the myotome portion of each somite proliferates and differentiates into skeletal muscle fibres. By TeachMeSeries Ltd (2026) 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 (2026) 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 Frequent questions What is a myotome? A myotome is a group of muscles that are innervated by a single spinal nerve root. It plays a crucial role in diagnosing spinal cord or nerve root injuries by indicating the level of damage. How do myotomes originate during development? Myotomes originate from somites, which are paired blocks of mesoderm formed during early embryonic development. Each somite differentiates into components that give rise to skeletal muscle, vertebrae, and dermis. How are myotomes distributed in the body? Most muscles in the upper and lower limbs are innervated by multiple spinal nerve roots, meaning they consist of several myotomes. For instance, the biceps brachii muscle, responsible for elbow flexion, is innervated by the C5 to C7 nerve roots. What is the clinical significance of myotomes? Myotomes are clinically significant as they allow clinicians to assess spinal cord lesions by testing muscle power associated with specific myotomes. This assessment helps determine the presence and location of spinal cord damage. How is myotome function graded during assessment? Myotome function is graded on a scale from 0 to 5, where 0 indicates total paralysis and 5 represents normal muscle strength with full range of motion against resistance. This grading system aids in evaluating the severity of muscle impairment. Rate This Article