CorpusHuman Anatomy
NervousMuscle group

Myotomes

Also known as: myotome, segmental innervation of muscles, reflex roots

A myotome is the group of muscles innervated by a single spinal segment (ventral root). Testing a key movement for each root, together with the deep tendon reflexes, localises radiculopathies and cord lesions.

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Overview

Most limb muscles receive fibres from two or more adjacent segments, which are mixed in the plexuses. A single root lesion therefore weakens but rarely paralyses a muscle. Clinicians test a standardised key movement for each root, as in the ASIA examination, together with the deep tendon reflexes, each of which depends on one or two segments. Weakness plus reflex loss in one root's territory, with dermatomal sensory change, indicates a radiculopathy rather than a peripheral nerve lesion.

Parts & subdivisions

  1. C3–C5
    Diaphragm (phrenic nerve). Shrugging (C3–C4) also involves the accessory nerve (XI).
  2. C5
    Shoulder abduction (deltoid) and elbow flexion (biceps). Reflex: biceps (C5–C6).
  3. C6
    Wrist extension and elbow flexion. Reflex: brachioradialis (supinator) (C5–C6).
  4. C7
    Elbow extension (triceps), wrist flexion and finger extension. Reflex: triceps (C7, C6–C8).
  5. C8
    Finger flexion (flexor digitorum profundus, grip).
  6. T1
    Finger abduction and adduction (interossei, intrinsic hand muscles).
  7. L2
    Hip flexion (iliopsoas).
  8. L3
    Knee extension (quadriceps) and hip adduction. Reflex: patellar knee jerk (L3–L4).
  9. L4
    Ankle dorsiflexion (tibialis anterior) and foot inversion.
  10. L5
    Great toe extension (extensor hallucis longus), hip abduction (gluteus medius) and foot eversion. No reliable reflex; the medial hamstring jerk is variable.
  11. S1
    Ankle plantarflexion (gastrocnemius and soleus), foot eversion and hip extension (gluteus maximus). Reflex: ankle (Achilles) jerk (S1–S2).
  12. S2–S4
    Pelvic floor, the anal and urethral sphincters, and bladder (parasympathetic). Reflexes: anal wink and bulbocavernosus (S2–S4).
  13. Superficial reflexes
    Abdominal (T8–T12; lost in UMN lesions), cremasteric (L1–L2) and plantar (L5–S1; extensor response = Babinski).

Anatomy

Development

The myotome of each somite gives rise to the muscles supplied by that segment: epaxial muscles via the dorsal ramus and hypaxial and limb muscles via the ventral ramus.

Function

  • Provide the segmental motor map used to localise root and cord lesions.
  • Form the basis of the ASIA motor score in spinal cord injury.

Clinical significance

Reflex grading

0 = absent; 1+ = reduced (or present only with reinforcement, as in the Jendrassik manoeuvre); 2+ = normal; 3+ = brisk; 4+ = very brisk with clonus. Asymmetry matters more than absolute grade.

Mnemonics

Reflex roots

"S1, 2 buckle my shoe (ankle jerk); L3, 4 kick the door (knee jerk); C5, 6 pick up sticks (biceps, brachioradialis); C7, 8 lay them straight (triceps)."

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.