The nervous system is divided into two systems. The Central Nervous System which is the brain and the spinal cord, and the Peripheral Nervous System which is the 12 cranial nerves, 31 pairs of spinal nerves and ganglia. The Peripheral Nervous System provides communication between the brain and spinal cord and the body’s muscles, glands and sensory receptors.
The spinal cord is the continuation of the brainstem, it lies protected within the vertebral column of the spine. A spinal nerve is any of the 31 pairs of nerves that arise from the spinal cord. The spinal nerves are named according to where it emerges and passes through the bones in the spinal vertebrae. As you can see in the image to the left, the nerve roots (in red) come off the spinal cord and out to the rest of the body between the vertebra. Most of the spinal nerves exit at the vertebrae that protects it, however, the C1 nerve exits above the C1 vertebra and the sacral and coccygeal nerves exit at these bones.
The 31 pairs of spinal nerves are:
- 8 cervical (C1-C8) nerves emerge from the cervical spine; cervical means of the neck (there are 8 cervical nerves, but only 7 cervical vertebra)
- 12 thoracic (T1-T12) nerves emerge from the thoracic spine; thoracic means of the chest
- 5 lumbar (L1-L5) nerves emerge from the lumbar spine; lumbar means from the lower back region
- 5 sacral (S1-S5) nerves emerge from the sacral bone; sacral means of the sacrum, the bony plate at the base of the vertebral column
- 1 coccygeal nerve emerge from the coccygeal bone; coccygeal means of the coccyx, the tailbone
Each spinal nerve is attached to the spinal cord by two roots: a dorsal (back, posterior) sensory root and a ventral (front, anterior) motor root . The fibers of the sensory root carry sensory impulses to the spinal cord—pain, temperature, touch and position sense (proprioception)—from tendons, joints and body surfaces. The motor roots carry impulses from the spinal cord.The nerves carry messages to and from the skin of specific body regions called dermatomes.
Dermatome Map of Upper Limb (Shoulder, Arm, and Hand)
The spinal nerves exit the spinal cord and pass through the intervertebral foramen, then divides into four branches, or networks, called plexuses.
- Cervical Plexus – the cervical plexus supply the neck and shoulders
- Brachial Plexus – the brachial plexus supplies the arm and upper back
- Lumbar Plexus – lumbar plexus supplies the abdomen and leg muscles
- Sacral Plexus – the sacral plexus provides nerves to the back of the thigh, most of the lower leg, the entire foot in part of the pelvis.
Relation of Spinal Nerves to Vertebrae