The Anatomy of Spinal Nerves

In This Article

Spinal nerves are the major nerves of the body. A total of 31 pairs of spinal nerves control motor, sensory, and other functions. These nerves are located at the cervical, thoracic, lumbar, sacral, and coccygeal levels.

Spinal nerves can be impacted by a variety of medical problems, resulting in pain, weakness, or decreased sensation. A pinched nerve occurs when there is pressure or compression of a spinal nerve, and it is the most common spinal nerve disorder.

Anatomy

The spinal nerves are peripheral nerves that transmit messages between the spinal cord and the rest of the body, including muscles, skin, and internal organs. Each spinal nerve is dedicated to certain regions of the body.

Structure

The spinal nerves are relatively large nerves that are formed by the merging of a sensory nerve root and a motor nerve root. These nerve roots emerge directly from the spinal cord—sensory nerve roots 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, and eventually to small nerve branches that activate muscles in the arms, legs, and other areas of the body.

There are 31 pairs of spinal nerves including:

  • 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, Co1

Location

Spinal nerves are distributed approximately evenly along the spinal cord and spine. The spine is a column of vertebral bones that protects and surrounds the spinal cord. Each spinal nerve exits the spine by traveling through the foramen, which are openings at the right and left sides of the vertebral bones of the spine.

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. Some spinal nerves divide into smaller branches, without forming a plexus.

A plexus is a group of nerves that combine with each other. There are five main plexi formed by the spinal nerves:

  • Cervical Plexus: Composed of the merging of spinal nerves C1 through 5, 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, and then branch out into nerves that carry sensory messages and provide motor control to the muscles of the legs.
  • Coccygeal Plexus: Composed of 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, but these are generally discovered during pre-operative testing or during surgery for an injury to the spine, spinal cord, or spinal nerve. A 2017 study evaluating the spinal nerve anatomy of 33 cadavers (deceased people) identified spinal nerve plexus variants in 27.3 percent of them. This suggests that variation is not uncommon, but 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 (control of internal organs).

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 (homunculus) 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, and 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.

The distribution of spinal nerve control throughout the body is described as a myotome. Each physical movement requires one or more muscles, which is activated by a branch of a spinal nerve. For example, the biceps muscle is controlled by C6 and the triceps muscle is controlled by C7.

Autonomic

The autonomic function of spinal nerves mediates 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 and sensation from the hand is sent to C6, C7, and 8. The sensory dermatomes do not match up perfectly with the motor myotomes.

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.

Electromyography (EMG) and nerve conduction study (NCV) can measure nerve function. These tests help in identifying which spinal nerves are involved and how extensive the impairment is.

Conditions affecting spinal nerves include the following.

Herniated Disc

A herniated disc, also referred to as a slipped disc, occurs when the structure of the vertebral bones and their cartilage, ligaments, tendons, and muscles are disrupted—allowing the vertebral structures to fall out of place, compressing the spinal cord and/or the spinal nerve. Usually, the first symptoms include neck pain 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, therapy, injections of pain medication or anti-inflammatory medication, and possibly surgical repair and stabilization of the spine.

Foramen Narrowing
The foraminal openings through which spinal nerves travels are not much larger than the nerves themselves. Inflammation and bony 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 women notice improvement of symptoms even before having their baby, and most have a complete resolution after the baby is born.

There are a number of treatments for foraminal narrowing, including anti-inflammatory medication and physical therapy. Interventional procedures such as surgery or injections are not usually necessary.

Shingles

A very common condition, shingles is reactivation of the virus that causes chicken pox, herpes zoster. Shingles are characterized by severe pain and are sometimes accompanied by a rash. If you have ever had a chickenpox infection, the virus remains in your body, in a nerve root, after recovery from the illness. When it reactivates—usually due to a weak immune system— it causes pain and skin lesions in the region supplied by a nerve root or a whole spinal nerve.

A case of shingles generally resolves on its own and medications don’t typically hasten recovery.

There is an immunization that can prevent shingles, however, and it may be recommended if you are susceptible to developing a reactivation of the virus.

Guillan Barre Syndrome (GBS)

GBS, also called acute demyelinating polyneuropathy, causes weakness of the peripheral nerves, and 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 usually 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 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.

GBS and CIDP can be treated with steroids and immunotherapy. Medical care is needed to monitor breathing and oxygen levels, with intensive care support as needed.

Trauma

Spinal nerves can become injured in major traumatic accidents. Whiplash injuries, falls, or neck trauma due to blunt force (such as in contact sports or intentional injury) 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 an interventional procedure, especially during a major surgery that involves extensive cancer near the spine. Traumatic injury of a spinal nerve requires therapy and/or surgery.

Polyneuropathy

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, vitamin B12 deficiency, and neurotoxic chemicals.

Sometimes, nerves can recover their function, but often, nerve damage is permanent and 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 specific spinal nerves. Multiple sclerosis (MS), vitamin B12 deficiency, subacute combined degeneration of the spinal cord, and inflammatory myelopathy are examples of spine disease that may cause dysfunction of one or more spinal nerves. In these instances, the spinal nerve function is impaired because the nerve fibers in the nearby sections of the spine cease to send or receive messages to and from the spinal nerves.

Treatment of spine disease depends on the cause. With some of these conditions, such as MS, the spinal nerve function can completely or partially recover with medication.

Meningitis

An infection or inflammation of the meninges, which is the lining that encloses and protects the spinal cord (underneath the spine), can disrupt the function of one or more spinal nerves. Meningitis causes fevers, fatigue, and headaches, and can cause neurological symptoms such as weakness and sensory loss. Usually, with timely treatment, meningitis resolves 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 managed with over-the-counter pain medication. Physical therapy and exercises can help alleviate pressure and improve posture and muscle tone, reducing pain.

However, pain can be 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. Spine surgery and spinal nerve surgery may require intraoperative monitoring of nerve function.

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