Dermatomes by nerve root

What is a dermatome?

A dermatome is the area of skin whose sensation reaches the spinal cord through a single spinal nerve root. Each of the 29 roots from C2 to S5 has one; C1 usually has no sensory root. The face is not a spinal territory: the trigeminal nerve supplies it in three divisions, V1 to V3.

Each root below opens its own page; the Dermatome and Myotome Map Explorer lights them on a body map.

Cervical dermatomes

• C2 The back of the scalp, behind the ear and the angle of the jaw
• C3 The neck, front and back, down to the collarbones
• C4 The tops of the shoulders and a cape across the upper chest and back
• C5 The outer upper arm, over the deltoid, down to the elbow
• C6 The outer forearm, the thumb and the index finger
• C7 The middle finger, and a strip down the back of the forearm
• C8 The ring and little fingers, the inner edge of the hand and the inner forearm

Thoracic dermatomes

• T1 The inner side of the arm and elbow, and the top of the inner forearm
• T2 The armpit, the inner upper arm and a band across the upper chest and back
• T3 A band round the chest at the third rib
• T4 A band round the chest at the nipples
• T5 A band round the chest just below the nipples
• T6 A band round the trunk at the lower end of the breastbone
• T7 A band round the trunk along the lower ribs
• T8 A band round the trunk halfway from the breastbone to the umbilicus
• T9 A band round the trunk just above the umbilicus
• T10 A band round the trunk at the umbilicus
• T11 A band round the trunk below the umbilicus
• T12 The lower abdomen above the groin crease, and the flank above the hip

Lumbar dermatomes

• L1 The groin, just below the inguinal ligament, and the upper buttock
• L2 The front and outer thigh, running down and in
• L3 The lower front of the thigh and the inner knee
• L4 The inner shin, the inner ankle and the inner edge of the foot
• L5 The outer shin, the top of the foot and the big toe

Sacral dermatomes

• S1 The outer edge and sole of the foot, the heel and the lower back of the calf
• S2 A strip down the back of the thigh and calf, through the back of the knee
• S3 A ring round the anus on the inner buttocks, at the fold under the buttock
• S4 The skin round the anus, with S5
• S5 The skin round the anus, with S4

Trigeminal divisions of the face

• V1 The forehead and scalp to the crown, the upper eyelid and the bridge and tip of the nose
• V2 The lower eyelid, the cheek, the side of the nose and the upper lip
• V3 The lower lip, the chin, the jaw except its angle, the temple and the front of the ear

Which disc compresses which root

In the neck each root leaves above the vertebra of its number, so the C5/C6 disc meets C6, and C8, which has no vertebra of its own, leaves at C7/T1. In the lower back each root leaves below its vertebra, above the disc, so a typical L4/L5 herniation catches L5 on its way down while a far-lateral one catches the exiting L4.

The 10 key movements ISNCSCI grades

ISNCSCI grades one key movement for each root from C5 to T1 and L2 to S1, 10 in all, each chosen because that root mainly supplies it and it can be graded at the bedside.

The loop a tendon reflex runs is in the Reflex Arc Simulator, and the Muscles Worked by Exercise 3D Explorer colors a whole body by root in its root mode.

Sources. Key sensory points and key movements: International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), American Spinal Injury Association, 2019 revision. Muscles: Aids to the Examination of the Peripheral Nervous System, Medical Research Council and the Guarantors of Brain, 5th ed. (2010), cross-checked with Kendall et al. (2005). Reflex roots: Walker (1990), Clinical Methods, 3rd ed., Table 72.1, and StatPearls, “Physiology, Deep Tendon Reflexes”; the main root as the MSD Manual’s “How to Assess Reflexes” names it. Overlap: Sherrington (1893), Phil Trans R Soc Lond B 184:641-763.

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