myotome

DEFINITION: A myotome is all the muscle fibers supplied by the motor axons of a single spinal nerve root (ventral root). It derives embryologically from the myotome portion of somites, which gives rise to skeletal muscle of the trunk and limbs (epaxial myotome -> deep back muscles via dorsal primary rami; hypaxial myotome -> body wall and limb muscles via ventral primary rami). Most muscles receive input from more than one root, so each myotome overlaps with its neighbors, and loss of a single root usually causes weakness rather than complete paralysis.

DERMATOME vs MYOTOME vs SCLEROTOME: Dermatome = skin area supplied by sensory fibers of one dorsal root. Myotome = muscle supplied by one ventral root. Sclerotome = bone/cartilage from somite.

KEY MYOTOMES (upper limb): - C1-C2: neck flexion/extension - C3-C4: neck lateral flexion, shoulder elevation (C3,4,5 keeps the diaphragm alive, via the phrenic nerve) - C5: shoulder abduction (deltoid), also elbow flexion (biceps, C5-C6) - C6: elbow flexion and wrist extension - C7: elbow extension (triceps) and wrist flexion - C8: finger flexion (long finger flexors) - T1: finger abduction/adduction (interossei)

KEY MYOTOMES (lower limb): - L1-L2: hip flexion (iliopsoas) - L3: knee extension (quadriceps) - L4: ankle dorsiflexion (tibialis anterior); also knee extension - L5: great toe extension (extensor hallucis longus); also hip abduction - S1: ankle plantarflexion (gastrocnemius/soleus), hip extension - S2-S3 (S2-S4): bladder/anal sphincter (S2-S4 via pudendal nerve), toe flexion

Reflex roots to remember: S1-S2 ankle jerk ("buckle my shoe"), L3-L4 patellar ("kick the door"), C5-C6 biceps ("pick up sticks"), C7-C8 triceps ("lay them straight").

CLINICAL SIGNIFICANCE: - Radiculopathy (e.g., disc herniation): weakness in a myotomal pattern plus dermatomal sensory loss and diminished reflex. Lumbar disc herniation typically compresses the traversing (lower) root: L4-L5 disc -> L5 root (foot drop, weak great toe extension); L5-S1 disc -> S1 root (weak plantarflexion, absent ankle reflex). Cervical: C5-C6 disc -> C6 root; C6-C7 disc -> C7 root (most common cervical radiculopathy). - Spinal cord injury: ASIA/ISNCSCI classification uses 10 key myotomes (C5, C6, C7, C8, T1, L2, L3, L4, L5, S1) and 28 dermatomes to determine the motor and sensory level. - LMN lesion of a root: weakness, atrophy, fasciculations, hyporeflexia in the myotomal distribution (contrast with UMN lesions: spasticity, hyperreflexia, Babinski). - Differentiating root vs peripheral nerve lesion: root lesion -> myotomal/dermatomal pattern; peripheral nerve lesion (e.g., common peroneal nerve -> foot drop with weakness of dorsiflexion and eversion but sparing inversion/L5 hip abduction) -> distribution of that nerve. Foot drop with preserved inversion = peroneal neuropathy; weak inversion and hip abductors = L5 radiculopathy. - Poliomyelitis, ALS and spinal muscular atrophy affect anterior horn cells and can produce segmental myotomal weakness. - Brachial plexus: Erb palsy (C5-C6) -> "waiter's tip" (adducted, internally rotated arm, extended elbow, pronated forearm); Klumpke (C8-T1) -> claw hand, ± Horner syndrome.

HIGH-YIELD TIP: Segmental level of the diaphragm is C3, C4, C5 ("keeps the diaphragm alive"). Lesions above C3-C5 cause respiratory failure.

TESTING: Grade strength with MRC scale 0-5, comparing sides, using the key muscle for each level.