The spinal nerves are mixed nerves that originate from the spinal cord, forming the peripheral nervous system.

Each spinal nerve begins as an anterior (motor) and a posterior (sensory) nerve root. These roots arise from the spinal cord, and unite at the intervertebral foramina, forming a single spinal nerve.

The spinal nerve then leaves the vertebral canal via the intervertebral foramina, and then divides into two:

- Posterior rami – supplies nerve fibres to the synovial joints of the vertebral column, deep muscles of the back, and the overlying skin.

- Anterior rami – supplies nerve fibres to much of the remaining area of the body, both motor and sensory.

The nerve roots L2-S5 arise from the distal end of the spinal cord, forming a bundle of nerves known as the cauda equina. 

Actions of the spinal nerves

C1–C6: Neck flexors

C1–T1: Neck extensors

C3, C4, C5: Supply diaphragm (mostly C4)

C5, C6: Move shoulder, raise arm(deltoid); flex elbow (biceps)

C6: externally rotate (supinate) the arm

C6, C7: Extend elbow and wrist (triceps and wrist extensors); pronate wrist

C7, C8: Flex wrist; supply small muscles of the hand

T1–T6: Intercostals and trunk above the waist

T7–L1: Abdominal muscles

L1–L4: Flex thigh

L2, L3, L4: Adduct thigh; Extend leg at the knee (quadriceps femoris)

L4, L5, S1: abduct thigh; Flex leg at the knee (hamstrings); Dorsiflex foot (tibialis anterior); Extend toes

L5, S1, S2: Extend leg at the hip (gluteus maximus); Plantar flex foot and flex toes

Dermatomes

A dermatome is defined as ‘a strip of skin that is innervated by a single spinal nerve‘. They are of great diagnostic importance, as they allow the clinician to determine whether there is damage to the spinal cord, and to estimate the extent of a spinal injury if there is one present.