Spinal Nerve Cross-Section

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Spinal nerves are the peripheral nerves that connect the spinal cord to the diverse tissues of the body. Thirty-one pairs of spinal nerves extend from the spinal cord in the neck and back to supply the skin, muscles, and organs of the neck, trunk, and limbs.

Anatomy

The 31 pairs of spinal nerves are named for the region of the vertebral column from which they exit the spinal cord. There are 8 pairs of cervical nerves in the neck; 12 pairs of thoracic nerves in the upper back; 5 pairs of lumbar nerves in the lower back; 5 pairs of sacral nerves; and 1 pair of coccygeal nerves in the pelvis.

The 8 spinal nerves in the cervical region are named C1 through C8 from superior to inferior. Each spinal nerve exits the spine through the intervertebral foramen, between the skull and a vertebra or between two vertebrae. The C1 spinal nerve exits the spinal cord between the skull and the C1 vertebra, while the C2 nerve exits between the C1 and C2 vertebrae. This pattern repeats until the C8 spinal nerve exits inferior to the C7 vertebra at the base of the neck and superior to the T1 vertebra in the thorax.

In the thoracic and lumbar regions, there is a one-to-one correlation between the vertebrae and the spinal nerves. Each spinal nerve exits the spinal cord at the intervertebral foramen just inferior to the vertebra of the same name. So, for example, the T10 spinal nerve exits from the intervertebral foramen just inferior to the T10 vertebra and superior to the T11 vertebra.

The sacral region features 5 spinal nerves that exit the vertebral canal at the 5 vertebral foramina of the sacrum. The sacrum begins as 5 separate vertebrae that fuse to form a single bone, but each segment retains its own pair of foramina for the spinal nerves. The final pair of spinal nerves, the coccygeal nerve, exits from the inferior end of the sacrum and passes posterior to the coccyx.

While the cervical and thoracic spinal nerves extend from the spinal cord nearly horizontally, the lower spinal nerves extend in increasingly sharper angles. The reason for this anomaly is the spinal cord, which grows very slowly compared to the other tissues of the trunk and in adulthood ends in the lumbar region. The spinal nerves must then descend through the lumbar and sacral portions of the vertebral canal to reach their destinations in the lower back, pelvis, and legs.

Histology

Spinal nerves are classified as mixed nerves because they contain both afferent (sensory) and efferent (motor) neurons. Afferent neurons conduct information from sensory receptors found throughout the body to the central nervous system. Efferent neurons run in the opposite direction to provide signals from the central nervous system to the effectors (muscular and glandular tissue) of the body's organs.

Each spinal nerve begins at the spinal cord with its anterior and posterior roots. Much like the roots of a plant, the spinal nerve roots spread penetrate into the spinal cord. The anterior roots contain the axons of somatic and autonomic motor neurons that begin in the spinal cord and extend to the effectors. The posterior roots contain sensory neurons whose dendrites connect to sensory receptors in the body's tissues. The cell bodies of sensory neurons are found in masses of nervous tissue within the roots known as posterior root ganglia. From the posterior root ganglia the axons of sensory neurons continue through the roots to the spinal cord.

A thin layer of connective tissue known as the endoneurium individually wraps each neuron in a spinal nerve. Endoneurium provides protection to the neurons and helps to insulate the neuron within its own sheath. Neurons are bundled into small groups known as fascicles, which are in turn wrapped in a layer of connective tissue known as perineurium. Many fascicles and the blood vessels that support their neurons are bundled together to form the entire spinal nerve. The exterior of the spinal nerve is wrapped in yet another protective layer of connective tissue known as the epineurium.

Physiology

Neurons within the spinal nerves function by transmitting electrochemical signals known as action potentials along their axons and dendrites. Spinal nerves facilitate the transmission of action potentials by providing blood flow to neurons and by providing protection and insulation in the layers of connective tissue wrapping the neurons. Neurons require energy in the form of ATP along with many ions to transmit action potentials through the body. Blood vessels within spinal nerves provide these raw materials and remove wastes from the neurons to permit proper signal transmission. Neurons also require insulation to maintain their structural integrity and to increase the speed of action potential transmission. Spinal nerves provide insulation in the form of multiple layers of connective tissue wrapping the neurons and help to protect neurons by bundling them together into a stronger, more coherent structure.