Myotomes

Development

The rostrocaudal organisation of the spinal cord happens early in intrauterine development. During the third week of gestation, the notochord has developed and the mesoderm lateral to it has differentiated into three columns. The column running directly next to the notochord is the paraxial mesoderm. The paraxial mesoderm will then start to divide into cube-shaped, bilaterally paired segments called somites. Each of these segments corresponds to a division of the vertebral column and spinal cord in an adult. By the end of the fifth week of gestation, all pairs of somites are present as 38-40 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal segments. Each somite will differentiate into three different regions. The ventral portion will form the sclerotome while the dorsal portion forms the dermomyotome which later splits into the dermatome and myotome.

• sclerotome – forms the ribs and vertebrae
• dermatome – forms connective tissue and dermis of the skin
• myotome – forms the skeletal muscle of the neck, trunk, and limbs

What is a myotome?

A myotome is a group of muscles which are innervated by a single spinal nerve which has derived from the same embryological segment. In this way, it is described as the motor equivalent of a dermatome, which is an area of skin innervated by a single spinal nerve. Individual muscles can be innervated by more than one nerve or by a nerve which originates from multiple spinal nerves. Therefore, muscles can be made up of more than one myotome. 

For example, the quadriceps femoris muscle is innervated by the femoral nerve. The femoral nerve arises from the lumbar plexus and has its origins from L2-L4. Therefore the quadriceps femoris muscle is a part of the L2, L3, and L4 myotomes. The main function of the quadriceps femoris muscle is to extend the leg at the knee joint so this movement can be used to test L2-L4 myotomes. The femoral nerve also innervates the iliacus, pectineus, and sartorius muscles. Therefore all of these muscles, including the quadriceps femoris muscle, are part of the L2-L4 myotomes.

Check out our learning materials to learn more about the spinal nerves anatomy.

Myotome testing

Myotomes are tested by asking patients to perform different movements which are associated with different spinal nerves because individual muscles can be a part of multiple myotomes where myotomes are made up of multiple muscles which can perform different actions. Specifically, the clinician performs the tests with resisted isometric contractions, and the joint being tested should be at the resting position, or near it. Muscle contractions should be held for at least 5 seconds. This will help pinpoint if specific spinal nerves are lesioned, diseased, or injured if the patient is unable to perform these movements fully. 

Looking for a way to improve your learning process? Check out our 8 steps to create your own anatomy flashcards.

Like dermatomes, myotomes can overlap. So even though myotomes have been mapped to certain areas of the body, the mapping is not precise because of natural variation in where muscles receive their innervation. For example, the musculocutaneous nerve (C5-C7) innervates the muscles in the anterior compartment of the arm – coracobrachialis, biceps brachii, and brachialis muscles. If it is absent, these muscles can instead be innervated by branches from the median nerve (C5-T1).

Round up your knowledge with our materials about the body movements.