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Myotomes

The spinal cord supplies origin to 31 pairs of spinal nerves. Each and every muscle in the body is supplied by one or more spinal nerves. Myotomes is said to be a portion of skeletal muscles which is innervated by a single spinal cord level. In simple words myotome is defined as a group of muscles which is innervated by single spinal nerve root. Myotome is a part of somatic nervous system and the somatic nervous system is a part of peripheral nervous system.

Myotomes

Myotomes

Spinal NerveActionMuscles
C1 & C1Neck FlexionRectus lateralis, Rectus capitis anterior, longus capitis, longus colli, longus cervicus, sternocleidomastoid
C3Neck Side FlexionLongus capitis, longus cervicus, trapezius, scalenus  medius
C4Shoulder ElevationDiaphragm, trapezius, levator scapula, scalenus anterior & medius
C5Shoulder AbductionRhomboid major & minor, deltoid, supraspinatus, infraspinatus, teres minor, biceps, scalene anterior & medius
C6Elbow Flexion; Wrist ExtensionSerratus anterior, latissiumus dorsi, subscapularis, teres major, pectoralis major (clavicular head) biceps brachii, coracobrachialis, brachioradialis, supinator, extensor carpi radialis longus, scalenus anterior, medius & posterior
C7Elbow Extension; Wrist FlexionSerratus 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
C8Thumb extension; Ulnar DeviationPectoralis 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  
T2-12Not testedThoracic nerves control muscles in the trunk and abdomen, and are generally not tested.
L1-2Hip FlexionPsoas, iliacus, sartorius, gracilis, pectineus, adductor longus, adductor brevis
L3Knee ExtensionQuadriceps, adductor longus, magnus & brevis.
L4Ankle DorsiflexionTibalis anterior, quadriceps,tensor fasciae late, adductor magnus, obturator externus, tibialis posterior
L5Toe ExtensionExtensor hallucis longus, extensor digitorum longus, gluteus medius & minimus, abturator internus, semimembranosus, semitendinosus, peroneus tertius, popliteus
S1Ankle Plantarflexion; Ankle Eversion; Hip Extension,; Knee FlexionGastrocnemius, soleus, gluteus maximus, obturator internus, piriformis, biceps femoris, semitendinosus, popliteus, peroneus longus & brevis, extensor digitorum brevis
S2Knee FlexionBiceps 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
S3No Myotome 
S4Anal WinkMuscles of the pelvic floor and bladder

Clinical Significance

Myotomes re much more complex to test then dermatomes, since each skeletal muscle is innervated by nerves derived from more than one spinal cord.

Testing movements at following joints can help in localizing injury to particular spinal cord level. For example:

  • Muscles which are responsible for the movement of the shoulder joint are innervated by spinal nerves from C5 and C6.
  • Muscles which are responsible for the movement of the elbow joint are innervated by spinal nerves from spinal cord levels C6 and C7.
  • Muscles of the hand are innervated primarily by spinal nerves from spinal cord levels C8 and T1 .

 

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