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
| Spinal Nerve | Action | Muscles |
|---|---|---|
| C1 & C1 | 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 | Â | Â |
| 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 | Â |
| S4 | Anal Wink | Muscles 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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