Spinal nerves are the major nerves of the body. There are a total of 31 symmetrical pairs of spinal nerves that emerge from different segments of the spine. Each spinal nerve contains both sensory and motor nerve fibers. These relay motor (movement), sensory (sensation), and autonomic (involuntary functions) signals between the spinal cord and other parts of the body.
Spinal nerves can be impacted by a variety of medical conditions, resulting in pain, weakness, or decreased sensation. A pinched nerve, which occurs when there is pressure on or compression of a spinal nerve, is a common issue.
This article explores the anatomy of spinal nerves and their functions, as well as conditions that can impair spinal nerves and how they're treated.
Anatomy
The spine is made up of vertebrae (back bones) that protect and surround the spinal cord, which is a column of nerve tissue.
Spinal nerves branch out from the spinal cord. These are peripheral nerves, or those that run through other parts of the body and transmit message to and from the brain/spinal cord.
These nerves are located at the cervical (neck), thoracic (upper back), lumbar (lower back), sacral (sacrum, which forms part of the pelvis), and coccygeal (tailbone) levels.
Each pair of spinal nerves are dedicated to certain regions of the body.
Structure
The spinal nerves are relatively large nerves that are formed by the merging of two nerve roots: a sensory nerve root and a motor nerve root.
Sensory nerve roots emerge from the back of the spinal cord and the motor nerve roots from the front of the spinal cord. As they join, they form the spinal nerves on the sides of the spinal cord.
The spinal cord is composed of nerve cells that serve to relay messages between the brain and the peripheral nerves.
The spinal nerves receive sensory messages from tiny nerves located in areas such as the skin, internal organs, and bones. The spinal nerves send sensory messages to the sensory roots, then to sensory fibers in the posterior (back or dorsal) part of the spinal cord.
The motor roots receive nerve messages from the anterior (front or ventral) part of the spinal cord and send the nerve messages to the spinal nerves. These messages eventually make their way to small nerve branches that activate muscles in the arms, legs, and other areas of the body.
There are 31 pairs of spinal nerves:
- Eight cervical spinal nerves on each side of the spine called C1 through C8
- Twelve thoracic spinal nerves in each side of the body called T1 through T12
- Five lumbar spinal nerves on each side called L1 through L5
- Five sacral spinal nerves in each side called S1 through S5
- One coccygeal nerve on each side called Co1
Location
Spinal nerves are distributed about evenly along the spinal cord and spine. Each spinal nerve exits the spine by traveling through the foramen, which are openings at the right and left sides of the vertebrae.
The spinal nerves are formed within a few centimeters of the spine on each side. Some groups of spinal nerves merge with each other to form a large plexus, or network of interlacing nerves. Other spinal nerves divide into smaller branches without forming a plexus.
There are five main plexi formed by the spinal nerves:
- Cervical plexus: Composed of the merging of spinal nerves C1 through C5, these divide into smaller nerves that carry sensory messages and provide motor control to the muscles of the neck and shoulders.
- Brachial plexus: Formed by the merging of spinal nerves C5 through T1, this plexus branches into nerves that carry sensory messages and provide motor control to the muscles of the arm and upper back.
- Lumbar plexus: Spinal nerves L1 through L4 converge to form the lumbar plexus. This plexus splits into nerves that carry sensory messages and provide motor control to the muscles of the abdomen and leg.
- Sacral plexus: Spinal nerves L4 through S4 join together. They then branch out into nerves that carry sensory messages and provide motor control to the muscles of the legs.
- Coccygeal plexus: The merging of nerves S4 through Co1, this plexus supplies motor and sensory control of the genitalia and the muscles that control defecation.
Anatomic Variation
There are numerous described variants of spinal nerve anatomy, such as roots from multiple spinal nerves connecting, early splits in nerve branches, or missing branches. These are generally discovered during surgery for an injury to the spine, spinal cord, or spinal nerve, or testing done in preparation for such a procedure.
A 2017 study evaluating the spinal nerve anatomy of 33 deceased people identified spinal nerve plexus variants in 27.3% of them. This suggests that variation is not uncommon and that it doesn’t commonly produce noticeable problems.
Function
The spinal nerves have small sensory and motor branches. Each of the spinal nerves carries out functions that correspond to a certain region of the body. These are muscle movement, sensation, and autonomic functions (involuntary functions).
Because their function is so well understood, when a particular spinal nerve becomes impaired, the resulting deficit often pinpoints which spinal nerve or nerves are affected.
Motor
Motor messages to the spinal nerves originate in the brain. The motor strip (primary motor cortex) in the brain initiates a command for muscle control. This command is sent to the spine through nerve impulses and then travels through the motor root to the spinal nerve.
Motor stimulation is very specific. It may activate the whole spinal nerve or just one of its branches to stimulate a very small group of muscles, depending on the command from the brain.
Myotomes, groups of muscles supplied with nerves from a spinal nerve root, are areas of spinal nerve control distribution throughout the body.
Each physical movement requires one or more muscles, which is activated by one or more spinal nerve branches. For example, the biceps muscle is controlled by C6 and the triceps muscle is controlled by C7. Muscles can also be activated by signals passed down through combinations of multiple spinal nerves.
Autonomic
The autonomic, or involuntary, function of spinal nerves helps control the body’s internal organs, such as the bladder and the intestines.
There are fewer autonomic branches of the spinal nerves than there are the motor and sensory branches.
Sensory
The spinal nerves receive messages including touch, temperature, position, vibration, and pain from the small nerves in the skin, muscles, joints, and internal organs of the body.
Each spinal nerve corresponds to a skin region of the body, described as a dermatome. For example, sensation near the belly button is sent to T10; sensation from the hand is sent to C6, C7, and C8.
The sensory dermatomes do not match up perfectly with the motor myotomes.
Dermatomes
Associated Conditions
Spinal nerves can be affected by a number of conditions. These situations can cause pain,sensory changes, and/or weakness.
The diagnosis of a spinal nerve problem involves several steps. The first is a physical examination, which can identify impairment corresponding to a dermatome and/or myotome. Reflexes also correspond to spinal nerves, and they are usually diminished in these situations as well, further helping to identify which nerves are involved.
Nerve tests can help in identifying which spinal nerves are involved and how extensive the impairment is:
- Electromyography (EMG) uses needle electrodes inserted through the skin into muscles to measure electrical activity in muscle fibers.
- Nerve conduction studies (NCS) use shock-emitting electrodes placed on the skin directly over the nerve to measure nerve function.
The following are possible diagnoses.
Herniated Disc
Discs act as cushions or shock absorbers for the vertebrae. A herniated disc, also referred to as a slipped disc, is when part of the jelly-like material at the center of a disc leaks into the spinal canal.
A herniated disc occurs when the vertebral bones and their cartilage, ligaments, tendons, and muscles are disrupted, allowing the structures to fall out of place. This compresses the spinal cord and/or the spinal nerve. The first symptoms can include neck or back pain (often the low back) or tingling down the arm or leg.
A herniated disc can be a medical emergency because it can cause permanent damage to the spinal cord.
Treatment includes oral anti-inflammatory medications, physical therapy, injections of pain medication or anti-inflammatory medication, and possibly surgical repair and stabilization of the spine.
Types of Herniated Discs
Foramen Narrowing
The foraminal openings through which spinal nerves travels are not much larger than the nerves themselves. Inflammation and bone degeneration can compress a spinal nerve as it travels through the foramen, producing pain and tingling. This is often described as a pinched nerve.
Weight gain and swelling can cause or exacerbate a pinched nerve. During pregnancy, for example, many women experience the symptoms of a pinched nerve. This can resolve after weight loss or even with the redistribution of weight. Some pregnant women notice improvement of symptoms even before having their baby, and most have a complete resolution after giving birth.
There are a number of treatments for foraminal narrowing, including anti-inflammatory medication and physical therapy. Interventional procedures such as surgery are not usually necessary.
What Causes a Pinched Nerve?
Shingles
Shingles, also called herpes zoster, is reactivation of the virus that causes chickenpox. Shingles are characterized by a severely painful rash.
After you recover from a chickenpox infection, the virus remains in your body, in a nerve root. If it reactivates later in life, which usually happens because of a weak immune system, it causes pain and skin lesions in the region supplied by a nerve root or a whole spinal nerve.
Shingles rash typically follows a dermatome, which is why the rash tends to be confined to one "strip" of skin on one side of the body.
A case of shingles generally resolves on its own within three to five weeks, but early treatment with antiviral medications may help it heal slightly faster and limit the severity of pain.
There is a vaccine that can prevent shingles, however, that's recommended for adults 50 and older, and particularly those at increased risk for developing a reactivation of the virus.
Guillain-Barré Syndrome (GBS)
Guillain-Barrésyndrome (GBS), also called acute demyelinating polyneuropathy, causes weakness of the peripheral nerves. It can affect many spinal nerves at a time.
Typically, GBS initially causes tingling in the feet, followed by weakness in the feet and legs, which advances to weakness of the arms and chest muscles. It can eventually impair the muscles that control breathing. Respiratory support with a mechanical ventilator is sometimes necessary until the condition resolves.
This disease is caused by demyelination, which is a loss of the protective myelin (fatty layer) that surrounds each nerve. Once this myelin is lost, the nerves don’t function the way they should, resulting in muscle weakness. The myelin is eventually replaced and the nerves can function again, but medical support is necessary in the interim.
Another similar disease, chronic inflammatory demyelinating polyneuropathy (CIDP), is a recurring form of GBS in which the symptoms can occur every few months or years, with partial or complete recovery each time.
Medical care is needed to monitor breathing and oxygen levels for those with GBS or CIDP symptoms, with intensive care support given as needed.
GBS and CIDP can be treated with either intravenous immunoglobulin(IVIG), a therapy given through a vein that suppresses the immune system, or a procedure called plasma exchange that filters the blood.
Trauma
Spinal nerves can become injured in major traumatic accidents. Car accidents, falls, or blunt force (such as from a contact sport or intentional injury), for example, can cause swelling, stretching, or a tear in the cervical spinal nerves or the cervical plexus.
Heavy lifting, falling, and accidents may injure the lumbar spinal nerves or the lumbar plexus.
Rarely, the spinal nerves become injured during a medical procedure, especially during a major surgery that involves extensive cancer near the spine.
Traumatic injury of a spinal nerve requires therapy and/or surgery.
What Is Whiplash?
Neuropathy
Neuropathy is a disease of the peripheral nerves. CIDP and GBS are two types of neuropathy. Most neuropathies involve small nerve branches, but they can affect the spinal nerves as well.
Common causes of neuropathy include chronic heavy alcohol intake, diabetes, chemotherapy for cancer treatment, vitamin B12 deficiency, and neurotoxic chemicals.
Sometimes, nerves can recover their function, but often, nerve damage is permanent. Treatment is focused on identifying the cause to prevent further damage.
Spine Disease
A number of diseases that affect the spine do not directly damage the spinal nerves, but they may produce symptoms that correspond to them.
Multiple sclerosis (MS, a disorder in which the immune system attacks myelin), vitamin B12 deficiency, degeneration of the spinal cord, and inflammatory myelopathy (spinal cord compression) are examples of diseases that may cause dysfunction of one or more spinal nerves.
In these instances, spinal nerve function is impaired because nerve fibers in the nearby sections of the spine cease to send or receive messages to andfrom the spinal nerves.
Treatment of spine disease depends on the cause. Sometimes the spinal nerve function can fully or partially recover with treatments, such as medication.
Meningitis
Meningitis is an infection or inflammation of the meninges, which is the lining that encloses and protects the spinal cord. It can disrupt the function of one or more spinal nerves and is considered a medical emergency.
Meningitis causes fevers, fatigue, neck stiffness, and headaches. It can also cause neurological symptoms such as weakness and sensory loss.
With timely treatment, meningitis can often resolve without permanent damage to the spinal nerves.
Cancer
Cancer in or near the spine can infiltrate (invade) or compress the spinal nerves, causing dysfunction. This can produce pain, weakness, or sensory changes involving one or more spinal nerves.
Treatment includes surgical removal of cancer, radiation, or chemotherapy. Recovery varies depending on how extensive the spinal nerve involvement is.
Rehabilitation
Most of the time, spinal nerve impairment is treatable. Mild inflammation can usually be managed with anti-inflammatory medication, and pain can usually be lessened with over-the-counter pain relievers. Physical therapy and exercises can help alleviate pressure and improve posture and muscle tone, reducing pain.
However, pain can be persistent and severe, requiring more aggressive interventions, such as injections or surgery.
Nerve damage causing sensory loss or muscle weakness may be the result of extensive or longer-lasting injuries to the spinal nerves. The nerves are less likely to recover if they have been transected (cut). Physical therapy is generally recommended as a way to optimize function by strengthening muscles that are supplied by healthy nerves.
Surgical repair of spinal nerves is a highly sophisticated procedure with varied results, depending on the extent and duration of the damage. Monitoring of nerve function during spine surgery and spinal nerve surgery may be required to reduce the risk of complications.
Summary
There are 31 pairs of spinal nerves that branch out from the spinal cord. Each carries out functions that correspond to a certain region of the body,
Many spine-related diseases, viral infections, and traumatic injuries can affect spinal nerves and lead to pain, weakness, and/or loss of sensation.
Treatments for spinal nerve impairment depend on the cause, but a full or partial recovery is often possible.
