The term generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body.
Humans have pairs of spinal nerves, each roughly corresponding to a segment of the spinal column.
The spinal nerves are part of the peripheral nervous system .
Structure of the spinal nerves
Each spinal nerve is formed by the combination of nerve fibers from the dorsal and ventral roots of the spinal cord.
The dorsal roots carry afferent sensory axons, while the ventral roots carry efferent motor axons.
The spinal nerve emerges from the spinal column through an opening called the intervertebral foramen between adjacent vertebrae.
This is true for all spinal nerves except for the first pair of spinal nerves, which emerge between the occipital bone and the first vertebra. Therefore, the cervical nerves are numbered by the vertebra below, except C8, which exists below C7 and above T1.
The thoracic, lumbar, and sacral nerves are numbered by the anterior vertebra.
The S1 vertebra (also known as L6) or a sacralized L5 vertebra, the nerves are usually still counted down to L5 and the next nerve is S1. Thirty-one pairs of nerves exit the spinal cord through openings in the spinal column.
There are four main areas of the spine: the cervical spine or neck, the thoracic spine or chest, the lumbar spine or lower back, and the sacrum or coccyx. Below the sacrum, a small bone called the coccyx marks the end of the spine.
The nerves leaving the spinal cord are named according to their corresponding region of the spinal column. Spinal nerves are peripheral nerves that arise from each side of the spinal cord.
There are 31 pairs of spinal nerves, and they represent the spinal part of the peripheral nervous system.
The spinal nerves include the following structures:
- Trunk of the spinal nerve.
- Spinal (sensory) ganglion.
- Anterior branch of the spinal nerve.
- Posterior branch of the spinal nerve.
- Communicating branch of the spinal nerve.
- Meningeal branch of the spinal nerve.
The spinal nerves can be grouped according to the corresponding segments of the spinal cord and vertebrae:
- Cervical nerves (8 pairs).
- Thoracic nerves (12 pairs).
- Lumbar nerves (5 pairs).
- Nervians Foot (equals 5).
- Coccygeal nerves (1 pair).
The anterior branch of the spinal nerves and their branches form major nerve networks called nerve plexuses, which include:
- Cervical plexus.
- Brachial plexus.
- Lumbar plexus.
- Sacred plexus.
- Anococcygeal plexus.
However, the anterior branches of most thoracic spinal nerves do not form a plexus, but are called intercostal nerves and run along the intercostal spaces.
Functions of the spinal nerves
Outside the spinal column, the nerve divides into branches. The dorsal branch contains nerves that serve the dorsal portions of the trunk, carrying visceral motor, somatic motor, and somatic sensory information to and from the skin and back muscles (epaxial muscles).
The ventral branch contains nerves that serve the remaining ventral parts of the trunk and the upper and lower limbs (hypaxial muscles), which carry a visceral motor, somatic motor and sensory and information to and from the surface of the ventrolateral body, structures in the wall of the body and the extremities.
The meningeal branches (recurrent meningeal or sinuvertebral nerves) branch from the spinal nerve and re-enter the intervertebral foramen to serve the ligaments, dura, blood vessels, intervertebral discs, facet joints, and periosteum of the vertebrae.
The communicating branch contains autonomic nerves that serve visceral, motor, and sensory functions to and from the visceral organs.
The functions can be classified into:
The posterior, or dorsal, roots of the spinal nerves carry sensory information from the areas that serve the brain.
The area of the skin from which a particular nerve receives sensory input is known as the “dermatome” of that nerve.
For example, the skin on the shoulders sends sensory information to the fourth and fifth cervical nerves, also known as C4 and C5. The C7 nerve detects sensation in the middle finger.
The little fingers send sensory information to the first sacral nerve. Dermatome-specific pain or numbness could be a symptom of damage to its corresponding nerves.
The anterior or ventral roots of the spinal nerves carry motor impulses from the brain to the muscles, telling them to move. Damage to a nerve can cause weakness, or even paralysis, in the muscles it serves.
The sciatic nerve exits the spine through an opening in the pelvis called the sciatic foramen .
Sciatic nerve damage usually causes pain in the back of the legs and can also cause weakness in the flexor muscles of the leg.
The thoracic nerves supply motor function to the muscles of the abdomen and chest, as well as specific muscles in the hand that allow an individual to spread the fingers.
The cervical nerves serve the muscles of the arm and hand.
The spinal nerves also regulate the proper functioning of the internal organs.
Dysfunction in the autonomic nervous system can lead to life-threatening conditions.
Autonomic dysreflexia has symptoms such as sweating, severe headaches, a tingling sensation in the face and neck, patches on the skin around the neck, and goose bumps.
If left untreated, an individual with autonomic dysreflexia is vulnerable to the possibility of a fatal stroke .