A myotome is a group of muscles innervated by the ventral root a single spinal nerve. This term is based on the combination of two Ancient Greek roots; “myo-” meaning “muscle”, and “tome”, a “cutting” or “thin segment”.
Like spinal nerves, myotomes are organised into segments because they share a common origin. However, myotome testing can be difficult since individual muscles can be innervated by more than one nerve and by nerves that originate from different spinal cord levels. It is generally done by checking a patient's ability to perform specific actions and checking for muscle weakness.
|Develop from somites
|Through precise movements (resisted isometric contraction) and tendon reflexes
|Intervertebral disc herniation
This article will discuss the development, testing, and function of myotomes.
- What is a myotome?
- Myotome testing
- Clinical relations
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.
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.
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.
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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.
Cervical and thoracic myotomes
The cervical and thoracic myotomes (C1-T12) are tested with the patient in a seated position. These are tested with movements of the neck and upper limb through the shoulder, elbow, wrist, metacarpophalangeal, and interphalangeal joints. Because the accessory nerve (CN XI) also innervates muscles of the neck, these can also be tested with some of the same movements.
|Flexion of the neck
|C3 and CN XI
|Lateral flexion of the neck
|C4 and CN XI
|Elevation of the shoulder
|Abduction, lateral rotation, and flexion of the arm at the shoulder joint
|Flexion of the arm at the shoulder joint
|Supination at the shoulder joint
|Extension of the forearm at the elbow joint
Flexion and extension of the hand at the wrist joint
|Medial rotation, adduction, and extension of the arm at the shoulder joint
|Pronation at the shoulder joint
Flexion and extension of the digits of the hand at the metacarpophalangeal and interphalangeal joints
|Extension of the thumb at the metacarpophalangeal joint
Ulnar flexion at the wrist joint
|Abduction of finger III metacarpophalangeal joints
Adduction of finger II, III, IV at the metacarpophalangeal joints
|Generally not tested. These nerves innervate the muscles of the thoracic and abdominal wall.
This type of testing relies on the patient being conscious and able. If the patient is unconscious, myotomes can be tested with tendon reflexes instead:
Lumbar and sacral myotomes
|Flexion and medial (internal) rotation of the thigh at the hip joint
|Adduction of the thigh at the hip joint
|Lateral (external) rotation of the thigh at the hip joint
|Extension of the leg at the knee joint
|Extension of the thigh at the hip joint
Inversion of the foot at the intertarsal joints
|Dorsiflexion of the foot at the ankle joint
|Abduction of the thigh at the hip joint
Eversion of the foot at the intertarsal joints
|Flexion of the leg at the knee joint
|Extension of the toes at the metatarsophalangeal joints
|Plantarflexion of the foot at the ankle joint
|Adduction of the toes at the metatarsophalangeal joints
|Anal reflexive contraction of the external anal sphincter
|Rectal and/or bladder dysfunction
Like cervical myotomes, lumbar and sacral myotomes can be tested with tendon reflexes if the patient is unconscious:
- L3-L4 – patellar ligament (tendon) tap
- S1-S2 – calcaneal tendon tap
Now that you've come this far, take the last step and test your knowledge on the spinal nerves and spinal column by taking the quiz below!
Muscle weakness and paralysis can arise from damage or compression to the spinal nerve that innervates it. This can happen with intervertebral disc herniations or spinal cord lesions.
When components of the intervertebral disc become displaced outside of the intervertebral disc space, this is an intervertebral disc herniation. Typically, the nucleus pulposus is the main component which herniates, but other materials of the intervertebral disc such as cartilage, bone, and annular tissue can herniate as well. Because the intervertebral disc is very close to spinal nerves, herniation can compress spinal nerves and result in myelopathy. The spinal cord level of myelopathy can then be determined through myotome testing.
Learn more about herniated disc risk factors, types, signs, symptoms, treatment, and complications with our article about herniated discs.
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