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.

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.1 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.2

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.

• Ladies in Blue Do the Myotome Dance
• Dr. Burke-Doe's Physiology Class Dances to Bon Jovi
• Austin Community College PTA Class of 2013 (With Costume!) (Note: This one is both dermatome and myotome.)

Dermatome

A dermatome is an area of skin supplied by a single spinal nerve.

If you imagine the human body as a map, each dermatome represents the area of skin supplied with sensation by a specific nerve root.

It is important to bear in mind that the dermatomes of the head are supplied by branches V1, V2 and V3 of the trigeminal nerve.

When assessing sensation, areas close to dermatomal boundaries should be avoided to minimise the risk of misinterpretation. The lists below describe locations that can be used to assess the dermatomes of the head, upper limb, torso and lower limbs.

Dermatomes of the head

Trigeminal nerve (CN V)

• V1: ophthalmic branch – the lateral aspect of the forehead
• V2: maxillary branch – the cheek
• V3: mandibular branch – the lower jaw (avoid the angle of the mandible as it is supplied by C2/C3)

Other

• C2: 1-2 cm lateral to the occipital protuberance
• C3: the supraclavicular fossa in the midclavicular line.

Dermatomes of the upper limb

• C4: over the acromioclavicular joint.
• C5: the lateral aspect of the lower edge of the deltoid muscle (known as the “regimental badge”).
• C6: the palmar side of the thumb.
• C7: the palmar side of the middle finger.
• C8: the palmar side of the little finger.
• T1: the medial aspect antecubital fossa, proximal to the medial epicondyle of the humerus.

Dermatomes of the trunk

• T2: the apex of the axilla.
• T3: the intersection of the midclavicular line and third intercostal space.
• T4: the intersection of the midclavicular line and the fourth intercostal space at the level of the nipples.
• T5: the intersection of the midclavicular line and the fifth intercostal space, horizontally located midway between the level of the nipples and the level of the xiphoid process.
• T6: the intersection of the midclavicular line and the horizontal level of the xiphoid process.
• T7: the intersection of the midclavicular line and the horizontal level at one quarter the distance between the level of the xiphoid process and the level of the umbilicus.
• T8: the intersection of the midclavicular line and the horizontal level at one half the distance between the level of the xiphoid process and the level of the umbilicus.
• T9: the intersection of the midclavicular line and the horizontal level at three-quarters of the distance between the level of the xiphoid process and the level of the umbilicus.
• T10: the intersection of the midclavicular line, at the horizontal level of the umbilicus.
• T11: the intersection of the midclavicular line, at the horizontal level midway between the level

Dermatomes of the lower limb

• L1: the inguinal region and the very top of the medial thigh.
• L2: the middle and lateral aspect of the anterior thigh.
• L3: the medial epicondyle of the femur.
• L4: the medial malleolus.
• L5: the dorsum of the foot at the third metatarsophalangeal joint.
• S1: the lateral aspect of the calcaneus.
• S2: at the midpoint of the popliteal fossa.
• S3: at the horizontal gluteal crease (the horizontal crease formed by the inferior aspect of the buttocks and the posterior upper thigh).
• S4/5: the perianal area.

Dermatological map of whole body

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Plexuses

We can classify groups of nerves into plexuses:

• Cervical plexus (C1 – C4): innervates the diaphragm, shoulders and neck.
• Brachial plexus (C5 – T1): innervates the upper limbs.
• Lumbosacral plexus (L2 – S1): innervates the lower extremities.

To learn how to assess sensation as part of a neurological examination, see our upper and lower neurological examination guides.