Each spinal nerve forms where a sensory dorsal root and a motor ventral root join, then exits between two vertebrae. Each one supplies a strip of skin (dermatome) and a group of muscles (myotome).
Count and naming
There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal pair. Cervical nerves exit above the vertebra with the same number, except C8, and all the others exit below their vertebra.
Plexuses
In the neck and limbs, nerve roots mix in the cervical, brachial, lumbar and sacral plexuses. That is why one named limb nerve carries fibers from several roots.
Dermatome landmarks
Useful landmarks are C6 at the thumb, C7 at the middle finger, C8 at the little finger, T4 at the nipple, T10 at the umbilicus, L4 at the medial leg, L5 at the top of the foot and S1 at the lateral foot.
Root problems
A compressed root, often from a herniated disc, causes pain shooting along its dermatome, numbness in that strip, weakness of its muscles and a reduced reflex. Shingles also follows a single dermatome.
| Root | Skin landmark | Reflex |
|---|---|---|
| C5 | Lateral shoulder | Biceps |
| C6 | Thumb | Brachioradialis |
| C7 | Middle finger | Triceps |
| L4 | Medial lower leg | Knee (patellar) |
| S1 | Lateral foot and little toe | Ankle (Achilles) |
At the bedside
Testing sensation in each dermatome, strength in key muscles and the matching reflexes lets you name the level of a root or cord problem. In a spinal injury, mark the lowest normal level and recheck it over time.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).