Myotomes, spinal nerve roots, and dermatomes


From top to bottom, the body is divided into motor zones called myotomes. The movement of the muscles of each myotome is controlled by motor nerves that emanate from the same motor part of the spinal nerve root . This is in contrast to a dermatome, which is an area of the skin in which sensations of touch, pain, temperature, and position are modulated by the same sensory portion of the spinal nerve root.

Myotomes and dermatomes are mapped, and the location of sensory or motor deficits corresponds to specific nerve roots. Based on your history and physical exam, your doctor or physical therapist can determine the specific nerve roots or levels of the spinal cord that may be causing your problem.

Myotomes and dermatomes are part of your peripheral nervous system , while myotomes are part of your somatic (voluntary) nervous system, which is part of your peripheral nervous system. The peripheral and central nervous systems interact with each other.

Photos by Nazar Abbas / Getty Images

Muscles and nerves, oh my!

Every muscle cell in your body works on the basis of nerve signals. In fact, your muscles need nerve signals to keep them calm and stable. And without any connection to the nerve, the muscles begin to break down.

The connection between the muscle and the nerve occurs at the endplate, which is part of the muscle. The neuromuscular junction is the structure that includes the nerve cell together with the end plate of the muscle.

Each nerve cell innervates (transmits signals) several muscle fibers . A nerve and its corresponding muscle fibers form a motor unit . Each fiber that is part of a motor unit is compressed (shortened) to move when the corresponding nerve is involved. This is an all or nothing event. The motor units start up one by one and this prevents them from running out.

One nerve cell can innervate six to ten muscle cells for subtle and detailed actions, such as moving the fingers or the eyes. Or one nerve cell can innervate hundreds of muscle cells for powerful actions, such as those performed by the muscles of the mid-back and arms.

All of this signaling occurs at the microscopic level. The multifibular muscle contains innumerable motor units.

Myotomes: a global neuromuscular perspective

A myotome is a group of muscles on one side of the body that is innervated by a spinal nerve root.

During your physical exam, your healthcare provider will look at the location of myotomes and dermatomes to identify specific spinal nerves that may be underlying problems, such as muscle weakness and sensory changes.

The following table shows the actions performed by each nerve.

There is often a slight overlap of the areas of the myotome, where the nerves will innervate the muscles in the areas where they are located and can also innervate the nearby muscles.

Spinal (nerve) level Action Muscle
C1 and C2 Neck flexion Lateral rectus muscle, rectus anterior muscle of the head, longus muscle of the head,
longus colli , longus cervicus,
C3 Bending the neck to the side Long head muscle, long neck muscle, trapezius, middle scalene muscle
C4 Shoulder lift Diaphragm, trapezius, levator scapulae , scalene anterior and median muscle
C5 Shoulder abduction Large and small rhomboid muscles, deltoid, supraspinatus, infraosseous, small round, biceps, anterior and middle scalene
C6 Elbow flexion; Wrist extension Serratus anterior muscle
latissimus dorsi , subscapularis muscle, large round muscle, pectoralis major (clavicular head), biceps brachii, coracobrachii, brachioradialis, instep support, extensor radialis of the wrist, scalene anterior, medial and posterior muscle
C7 Elbow extension; Wrist flexion Serratus anterior, latissimus dorsi, pectoralis major (head of sternum), pectoralis minor, pronator teres, flexor radialis of the wrist, superficial flexor of the finger, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digiti minimi, minimal extensor digits, middle and posterior extensor digits
C8 Thumb extension; Ulnar deviation Pectoralis major (sternum), pectoralis minor, triceps, superficial flexor of the finger, deep flexor of the finger, long flexor, pronator quadratus, ulnar flexor of the wrist, extensor pollicis longus, extensor pollicis great, Indian extensor of the thumb of the arms, flexor thumb flexor, opposing thumb flexor, median and posterior scalene.
T1 Finger abduction Pectoralis major, pectoralis minor, triceps, superficial flexor of the fingers, deep flexor of the fingers, long flexor of the thumb, pronator quadratus, ulnar flexor of the wrist, extensor pollicis longus, extensor pollicis great, extensor index of the thumb, the thumb abductor, the flexor muscle of the thumb, the thumb and the interosseus
T2-12 Not tested The pectoral nerves control the muscles of the torso and abdomen and are generally not examined.
L1-2 Hip flexion Psoas, iliac, sartorius, gracilis, pectineus, adductor longus, adductor short
L3 Knee extension Quadriceps muscle, long adductor muscle, large and short muscles.
L4 Ankle dorsiflexion Tibialis anterior muscle, quadriceps muscle, tensor tardis fascia, adductor magnus muscle, external obturator muscle, tibialis posterior muscle
L5 Toe extension The extensor pollicis longus, the extensor digitorum longus, the gluteal medius and minimus muscles, the internal segment of the muscle, the semimembranosus muscle, the semitendinosus muscle, the tertiary peroneal muscle, the popliteal muscle.
S1 Plantar flexion of the ankle; Eversion of the ankle; Hip extension; Knee flexion Gastrocnemius, soleus muscle, gluteus maximus muscle, internal obturator muscle, piriformis muscle, biceps femoris, semitendinosus muscle, popliteus muscle, peroneus longus and peroneus brevis, short finger extensor
S2 Knee flexion Biceps femoris, piriformis muscle, soleus muscle, gastrocnemius muscle, long flexor of the toes, long flexor of the big toe, internal muscles of the foot (except the abductor big toe), short flexor of the big toe, short flexor of the toe big toes, short extensor digits
S3 Without myotome
S4 Anal wink Pelvic floor and bladder muscles

Myotome dance

If you are one of the learners by doing, check out one or more of the YouTube videos linked below. They were trained by cohort groups of school physical therapist assistants (and a physiology class) from across the country. Since each video has strengths and weaknesses in terms of education, they are probably all worth watching. Note: Most of the dances are done at a very fast pace. Move at a speed that your body (especially your neck) can safely handle. This most likely means walking more slowly than physical therapist students.

However, in general, these dances can give you a good idea of what myotomes actually do and why they are important. And the music is good too.

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