MYOTOMES: CLINICAL EXAMINATION AND ANATOMICAL DISTRIBUTION - ANATOMY AND PHYSIOLOGY

A myotome is a set of muscle fibers innervated by a segmental, spinal, or spinal root or nerve. The motor axons of each segmental root or nerve innervate several muscles, and almost all muscles are innervated by more than one segmental nerve, and thus by an equivalent number of spinal segments.

In vertebrates, the dermatomes of the skin, the myotomes of skeletal muscle, and the sclerotomes of the vertebrae have a common embryological origin, the somites. These derive from the mesoderm and develop on each side and along the neural tube.

Somitos in embryonic development (Source: Homme en Noir Via Wikimedia commons)

delineation of the myotomes was relatively easier than that of the other segments derived from somites, such as sclerotomes and dermatomes.

This is due to the fact that the injury to a segmental root or nerve causes the immediate loss of function of the skeletal muscle innervated by said nerve and, consequently, the loss of the corresponding motility, which is easily detected in the neurological examination.

Muscle weakness, paralysis or absence of contraction, and alteration of the tendon reflexes are signs that allow evaluating the different myotomes of the musculoskeletal system.

Clinical examination

The evaluation of myotomes is generally used by some clinicians, by neurologists, orthopedists and, above all, by physical therapists.

In the neurological examination, detailed tests of each myotome allow to evaluate the integrity of the motor system associated with each of the examined myotomes. These tests examine isometric contractions under resistance and bone tendon reflexes.

The absence of any of the examined functions corresponding to a particular myotome allows the lesion to be located in the medullary segment or in the segmental root or nerve corresponding to the examined myotome.

On some occasions, when examining a particular myotome, there is not a total loss of function, but rather a weakness in the muscle contraction of the muscle group or groups corresponding to the myotome examined.

In these cases, the lesion can be located in the segmental nerve and one of the most frequent causes is root compression due to herniation of the intervertebral disc. The affected myotome allows to locate the intervertebral disc that is compressing the root.

Related roots

The roots related to the muscular functions of myotomes corresponding to the upper and lower extremities are shown below.

Spinal Root C1 and C2 → Muscles that flex and extend the neck

Spinal Root C3 → Muscles that flex the neck laterally

Spinal Root C4 → Muscles that elevate the Shoulder

Spinal root C5 → Muscles that produce shoulder abduction

C6 spinal root → Elbow flexor and wrist extensor muscles

Spinal root C7 → Elbow extensors and wrist flexors

Spinal root C8 → Extensor muscles of the fingers of the hand

Spinal root T1 → Muscles that abduct the thumb

Spinal root L2 → Muscles that flex the hip

Spinal root L3 → Muscles that produce knee extension

Spinal root L4 → Muscles responsible for dorsiflexion of the ankle

Spinal root L5 → Extensor muscles of the toes

Spinal root S1 → Muscles that produce plantar flexion of the ankle

Spinal root S5 → Knee flexor muscles

When examining muscle functions, the examiner exerts resistance against the action of the corresponding muscle. For example, for the right lateral flexion of the head, the examiner exerts force against this movement and in this way the myotome corresponding to the C3 root is evaluated.

Anatomical distribution

To describe the anatomical distribution of myotomes, although there are many variants, the distribution of the peripheral nerves, the medullary root of origin, as well as the related muscles are described below. Osteotendinous reflexes and related roots are also included.

Representative diagram of a spinal nerve (Source »Jmarchn via Wikimedia Commons)

Peripheral nerve and roots of origin

Axillary → C5 and C6

Supraclavicular → C3 and C4

Suprascapular → C5 and C6

Thoracic (long) → C5, C6 and C7

Musculocutaneous → C5, C6 and C7

Medial cutaneous forearm → C8 and T1

Lateral cutaneous of the forearm → C5 and C6

Posterior cutaneous of the forearm → C5, C6, C7 and C8

Radial → C5, C6, C7, C8 and T1

Medium → C6, C7, C8 and T1

Ulnar → C8 and T1

Pudendo → S2, S3 and S4

Lateral thigh cutaneous → L2 and L3

Medial cutaneous thigh → L2 and L3

Intermediate cutaneous thigh → L2 and L3

Posterior cutaneous of the thigh → S1, S2 and S3

Femoral → L2, L3 and L4

Shutter → L2, L3 and L4

Sciatic → L4, L5, S1, S2 and S3

Tibial → L4, L5, S1, S2 and S3

Common peroneal → L4, L5, S1 and S2

Superficial peroneum → L4, L5 and S1

Deep peroneal → L4, L5, S1 and S2

Lateral cutaneous leg → L4, L5, S1 and S2

Saphene → L3 and L4

Sural → S1 and S2

Medial plantar → L4 and L5

Plantar Lateral → S1 and S2

Here are each nerve root and its corresponding muscle:

C2 → Longus Colli, sternocleidomastoid and rectum capitis

C3 → Trapezius and splenius capitis

C4 → Trapezius and levator scapulae

C5 → Supraspinatus, infraspinatus, deltoid and biceps

C6 → Biceps, supinator, wrist extensors

C7 → Triceps and wrist flexors

C8 → Ulnar deviator, extensor pollicis, and adductor pollicis

L2 → Psoas, adductor hip

L3 → Psoas and quadriceps

L4 → Tibialis anterior, extensor hallucis

L5 → Extensor hallucis, peroneals, gluteus medius and ankle dorsiflexors

S1 → Glutes, peroneals and plantar flexors

S2 → Glutes and plantar flexors

S4 → Bladder and recti

Myotomes or segmental innervation of skeletal muscles are related to osteotendinous reflexes and their evaluation allows evaluating the integrity of the motor and sensory pathways, as well as the corresponding spinal segments.

Cutaneous-abdominal reflexes

- Aquilian reflex → S1 and S2

- Patellar Reflex → L2, L3 and L4

- Lower abdominal-cutaneous → T10-T12

- Middle abdominal-cutaneous → T8 and T9

- Upper cutaneous-abdominal → T6 and T7

Bicipital Reflex → C5, C6

Tricipital Reflex → C6, C7, C8

Radial Reflex → C5, C6 and C7

Some integrated examples including the root, muscles, function, and innervation of various myotomes are:

C5 → Biceps → Elbow flexion → Bicipital → Musculo-cutaneous

C7 → Triceps Brachii → Elbow Extension → Tricipital → Radial

L3 → Quadriceps crural → Knee extension → Patellar → Femoral

References

Gallardo, J. (2008). The segmental sensory innervation. Dermatomes, myotomes and sclerotomes. Rev. Chil. Anesthesia, 37, 26-38.
Lynn, M., & Epler, M. (2002). Fundamentals of musculoskeletal evaluation techniques. Principles of muscle evaluation techniques. 1st. edition. Madrid: Ed. Paidotribo, 20-34.
Magee, DJ (2013). Orthopedic physical assessment. Elsevier Health Sciences.
Marino, RJ, Barros, T., Biering-Sorensen, F., Burns, SP, Donovan, WH, Graves, DE,… & Priebe, M. (2003). International standards for neurological classification of spinal cord injury. The journal of spinal cord medicine, 26 (sup1), S50-S56.
Shultz, SJ, Houglum, PA, & Perrin, DH (2015). Examination of musculoskeletal injuries. Human Kinetics.

MYOTOMES: CLINICAL EXAMINATION AND ANATOMICAL DISTRIBUTION - ANATOMY AND PHYSIOLOGY - 2025