Nerve root map · Anatomy Atlas

C4

Dermatome: Top of the shoulder
Key muscle: Diaphragm (C3–C5, via the phrenic nerve)
Reflex: None

Irritation of the diaphragm or pericardium refers pain to the C3–C5 dermatomes of the shoulder and neck — e.g. the Kehr sign (left shoulder pain) with splenic rupture.

Study: Phrenic nerve, Diaphragm, Cervical plexus

C5

Dermatome: Lateral arm
Key muscle: Elbow flexion (biceps); shoulder abduction (deltoid)
Reflex: Biceps (C5–C6)

Upper trunk (C5–C6) injury is Erb palsy — e.g. shoulder dystocia — with the "waiter's tip" posture. Cervical roots exit above their vertebra, so a C4–C5 disc compresses C5.

Study: Cervical vertebrae, Brachial plexus, Deltoid, Biceps brachii

C6

Dermatome: Lateral forearm, thumb and index finger
Key muscle: Wrist extension
Reflex: Brachioradialis (C6); biceps (C5–C6)

A C5–C6 disc herniation compresses the C6 root.

Study: Cervical vertebrae, Intervertebral disc, Spinal nerves and roots

C7

Dermatome: Middle finger
Key muscle: Elbow extension (triceps)
Reflex: Triceps (C7)

A C6–C7 disc compresses C7 — the most common cervical radiculopathy.

Study: Cervical vertebrae, Triceps brachii

C8

Dermatome: Ring and little fingers, medial hand and distal forearm
Key muscle: Finger flexion (flexor digitorum profundus of the middle finger)
Reflex: None

Lower trunk (C8–T1) injury is Klumpke palsy (arm pulled upward, Pancoast tumor, cervical rib): claw hand, sometimes with Horner syndrome.

Study: Brachial plexus, Ulnar nerve

T1

Dermatome: Medial forearm
Key muscle: Little-finger abduction (intrinsic hand muscles)
Reflex: None

Klumpke palsy and Pancoast tumors involve C8–T1. If the T1 root is avulsed near the cord, sympathetic fibres to the head are interrupted, causing ipsilateral Horner syndrome.

Study: Brachial plexus

T4

Dermatome: Nipple
Key muscle: Intercostal muscles (each intercostal nerve supplies the muscles and a band of skin of its space)
Reflex: None

Herpes zoster reactivates in a dorsal root ganglion and causes a unilateral, dermatomal rash that doesn't cross the midline.

Study: Intercostal space, Breast, Spinal nerves and roots

T10

Dermatome: Umbilicus
Key muscle: Anterior abdominal wall muscles (thoracoabdominal nerves T7–T11)
Reflex: None

Early appendicitis pain is periumbilical: midgut visceral afferents enter the cord at about T10, the dermatome of the umbilicus.

Study: Anterior abdominal wall, Vermiform appendix, Intercostal space

L1

Dermatome: Inguinal region
Key muscle: Internal oblique and transversus abdominis (iliohypogastric and ilioinguinal nerves)
Reflex: Cremasteric (L1–L2)

The iliohypogastric and ilioinguinal nerves (L1) can be injured during hernia repair.

Study: Lumbar plexus, Inguinal canal

L2

Dermatome: Upper anterior thigh
Key muscle: Hip flexion (iliopsoas)
Reflex: Cremasteric (L1–L2)

Resisted hip flexion while sitting tests the L2–L3 myotome.

Study: Psoas major, Lumbar plexus

L3

Dermatome: Lower anterior thigh and knee
Key muscle: Knee extension (quadriceps)
Reflex: Patellar (L3–L4)

Carried mainly by the femoral nerve (L2–L4).

Study: Femoral nerve, Quadriceps femoris

L4

Dermatome: Medial leg and medial malleolus
Key muscle: Ankle dorsiflexion (tibialis anterior); knee extension
Reflex: Patellar (L4)

A typical posterolateral herniation compresses the root numbered for the lower vertebra, so L3–L4 → L4; a far-lateral (foraminal) L4–L5 herniation also compresses L4.

Study: Intervertebral disc, Spinal nerves and roots, Femoral nerve

L5

Dermatome: Lateral leg, dorsum of the foot and first web space
Key muscle: Great-toe extension (extensor hallucis longus); hip abduction; heel walking
Reflex: No reliable reflex

An L4–L5 posterolateral herniation compresses L5. Foot drop with weak inversion points to the L5 root; with normal inversion, to the common fibular nerve.

Study: Intervertebral disc, Common fibular nerve, Gluteus medius and minimus

S1

Dermatome: Lateral foot and sole
Key muscle: Plantarflexion and foot eversion; toe walking
Reflex: Achilles (S1)

An L5–S1 posterolateral herniation compresses S1.

Study: Intervertebral disc, Triceps surae and calcaneal tendon, Tibial nerve

S2–S4

Dermatome: Saddle area — perineum and perianal skin (S2–S5)
Key muscle: External anal and urethral sphincters (pudendal nerve); detrusor contraction (pelvic splanchnic nerves)
Reflex: Anal wink and bulbocavernosus (S2–S4)

Cauda equina syndrome — saddle anesthesia, urinary retention, reduced anal tone, bilateral sciatica — needs an emergency MRI. "S2, 3, 4 keep the poop off the floor."

Study: Cauda equina, Pudendal nerve, Pelvic autonomic nerves, Perineum