Myotomes, Spinal Nerve Roots, and Dermatomes

The body is divided from top to bottom into motor zones described as myotomes. The muscle movement of each myotome is controlled by motor nerves coming from the same motor portion of a spinal nerve root. This differs from a dermatome, which is a zone on the skin in which sensations of touch, pain, temperature, and position are modulated by the same sensory portion of a spinal nerve root.

Myotomes and dermatomes are mapped, and the location of sensory or motor deficits correspond to specific nerve roots. Based on your history and physical examination, your doctor or physical therapist can determine the specific nerve root(s) or spinal core level(s) that could be causing your problem.

Myotomes and dermatomes are part of the peripheral nervous system, and myotomes are part of the somatic (voluntary) nervous system, which is part of your peripheral nervous system. The peripheral and central nervous systems communicate with one another.

Man holding spine structure in his hand and demonstrating to patient.
Nazar Abbas Photography / Getty Images

Muscles and Nerves, Oh My!

Every muscle cell in your body functions based on nerve signals. In fact, your muscles need nerve signals to maintain their resting tone and stability. And without at least some communication from a nerve, muscles begin to decay.

Muscle-nerve communication occurs at the motor endplate, a portion of the muscle. The neuromuscular junction is a structure that includes a nerve cell, along with the muscle endplate.

Each nerve cell innervates (provides signals) several muscle fibers. A single nerve and its corresponding muscle fibers comprise a motor unit. Every fiber that is part of a motor unit contracts (shortens) to move when its respective nerve is fired. It's an all or nothing event. Motor units take turns firing, and this prevents them from becoming exhausted

A nerve cell can innervate as few as six to 10 muscle cells for fine, detailed actions such as finger or eye movements. Or a nerve cell can innervate hundreds of muscle cells for powerful actions, such as those carried out by the mid-back and arm muscles.

All this is signaling happens at the microscopic level. A many-fibered muscle contains an innumerable collection of motor units.

Myotomes: A Global Nerve-Muscle Perspective

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

During a physical exam, your doctor would consider the location of myotomes and dermatomes to identify the specific spinal nerve(s) that may underlie problems such as muscle weakness and sensory changes.

The chart below shows the actions produced by each nerve.

There is often a small overlap in myotome zones, where nerves will innervate the muscles in the zones where they are mapped, and may also innervate nearby muscles as well.

Spinal Level (Nerve) Action Muscles
C1 & C2 Neck Flexion Rectus lateralis, Rectus capitis anterior, longus capitis,
longus colli, longus cervicus,
sternocleidomastoid
C3 Neck Side Flexion Longus capitis, longus cervicus, trapezius, scalenus medius
C4 Shoulder Elevation Diaphragm, trapezius, levator scapula, scalenus anterior & medius
C5 Shoulder Abduction Rhomboid major & minor, deltoid, supraspinatus, infraspinatus, teres minor, biceps, scalene anterior & medius
C6 Elbow Flexion; Wrist Extension Serratus anterior,
latissiumus dorsi, subscapularis, teres major, pectoralis major (clavicular head) biceps brachii, coracobrachialis, brachioradialis, supinator, extensor carpi radialis longus, scalenus anterior, medius & posterior
C7 Elbow Extension; Wrist Flexion Serratus anterior, latissiumus dorsi, pectoralis major (sternal head), pectoralis minor, pronator teres, flexor carpi radialis, flexor digitorum superficialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, scalenus medius & posterior
C8 Thumb extension; Ulnar Deviation Pectoralis major (sternal head), pectoralis minor, triceps, flexor digitorum superficialis, flexor digitorum profundus, flexor pollicus longus, pronator quadratus, flexor carpi ulnaris, extensor pollicus longus, extensor pollicus brevis, extensor indicis, abductor pollicus brevis, flexor pollicus brevis, opponens pollicus, scalenus medius & posterior.
T1 Finger abduction Pectoralis major, pectoralis minor, triceps, flexor digitorum superficialis, flexor digitorum profundus, flexor pollicus longus, pronator quadratus, flexor carpi ulnaris, extensor pollicus longus, extensor pollicus brevis, extensor indicis, abductor pollicus brevis, flexor pollicus brevis, opponens pollicus Lumbricals, and Interossei
T2-12 Not tested Thoracic nerves control muscles in the trunk and abdomen, and are generally not tested.
L1-2 Hip Flexion Psoas, iliacus, sartorius, gracilis, pectineus, adductor longus, adductor brevis
L3 Knee Extension Quadriceps, adductor longus, magnus & brevis.
L4 Ankle Dorsiflexion Tibalis anterior, quadriceps,tensor fasciae late, adductor magnus, obturator externus, tibialis posterior
L5 Toe Extension Extensor hallucis longus, extensor digitorum longus, gluteus medius & minimus, abturator internus, semimembranosus, semitendinosus, peroneus tertius, popliteus
S1 Ankle Plantarflexion; Ankle Eversion; Hip Extension,; Knee Flexion Gastrocnemius, soleus, gluteus maximus, obturator internus, piriformis, biceps femoris, semitendinosus, popliteus, peroneus longus & brevis, extensor digitorum brevis
S2 Knee Flexion Biceps femoris, piriformis, soleus, gastrocnemius, flexor digitorum longus, flexor hallucis longus, Intrinsic foot muscles (except abductor hallcuis), flexor hallucis brevis, flexor digitorum brevis, extensor digitorum brevis
S3 No Myotome
S4 Anal Wink Muscles of the pelvic floor and bladder

The Myotome Dance

If you're the kind of person who learns by doing, check out one or more of the YouTube videos linked below. These were produced by physical therapy assistant school cohort groups (and one physiology class) from around the country. Because each video has both strong and weak points educationally, it's probably a good idea to look at all of them. Note: Most of the dances are done to a very fast beat. Move at the pace your body (particularly your neck) can handle safely. Most likely, this means going slower than the physical therapy students.

Overall, though, these dances may give you a good idea about what myotomes actually do, and why they are important. And the music is pretty good, too.

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  1. Stifani N. Motor neurons and the generation of spinal motor neuron diversity. Front Cell Neurosci. 2014;8:293. doi:10.3389/fncel.2014.00293

  2. Van Straaten MG, Cloud BA, Morrow MM, Ludewig PM, Zhao KD. Effectiveness of home exercise on pain, function, and strength of manual wheelchair users with spinal cord injury: a high-dose shoulder program with telerehabilitation. Arch Phys Med Rehabil. 2014;95(10):1810-1817.e2. doi:10.1016/j.apmr.2014.05.004

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