The radicular nerves, often called nerve roots, are the initial pathways for all nerve signals traveling between the body and the central nervous system. These nerves emerge directly from the spinal cord within the vertebral column. They represent the starting point for the entire peripheral nervous system, extending into the limbs, torso, and head. Understanding these roots is important because they dictate both sensation and movement for specific areas of the body.
The Anatomical Foundation of Radicular Nerves
The spine is composed of 33 stacked vertebrae, forming a protective canal for the spinal cord. At nearly every level, a pair of radicular nerves branches off the spinal cord, one left and one right. They exit through small openings between the vertebrae known as intervertebral foramina. Each nerve is a mixed nerve, carrying both sensory and motor information.
The mixed spinal nerve is formed by two separate components: the dorsal root and the ventral root. The dorsal root, positioned toward the back, transmits sensory information from the body to the spinal cord. Conversely, the ventral root, located toward the front, carries motor commands out to the body’s muscles. These two roots merge as they exit the spine, forming the single spinal nerve that continues to branch out to the periphery.
These radicular nerves are organized into four main groups corresponding to the regions of the spine. There are eight pairs of Cervical nerves (C1-C8), twelve Thoracic pairs (T1-T12), five Lumbar pairs (L1-L5), five Sacral pairs (S1-S5), and one coccygeal nerve. Nerves C1 through C7 exit above their corresponding vertebrae, while C8 exits between the C7 and T1 vertebrae. All remaining nerves, from T1 downward, exit below their corresponding vertebrae. After exiting the spinal column, these roots combine and reorganize to form the major peripheral nerves, such as the sciatic nerve.
Mapping Sensory and Motor Function (Dermatomes and Myotomes)
The function of each radicular nerve group is mapped across the body using two concepts: dermatomes for sensation and myotomes for movement. A dermatome is an area of skin supplied by sensory nerve fibers from a single spinal nerve root. A myotome is a group of muscles that receives movement signals from a single spinal nerve root.
Cervical Radicular Nerves (C1-C8)
The eight pairs of Cervical nerve roots control the neck, shoulders, arms, and hands. The C5 root is responsible for sensation over the shoulder and the motor function of shoulder abduction (lifting the arm out to the side). The C6 root supplies sensation to the thumb and index finger, and its motor function includes bending the elbow and extending the wrist.
Movement in the triceps muscle, which straightens the elbow, is governed by the C7 root, which also provides sensation to the middle finger. Gripping strength and the ability to spread the fingers are controlled by the C8 and T1 roots. The C3, C4, and C5 roots collectively provide motor innervation to the diaphragm, making them involved in breathing.
Thoracic Radicular Nerves (T1-T12)
The twelve pairs of Thoracic nerves innervate the torso, providing sensation and motor control across the chest and abdomen. Symptoms related to these nerves are less common than in other regions, but they follow a distinct band-like pattern around the body. For instance, the T4 dermatome is located near the nipples, and the T10 dermatome aligns with the navel.
The motor function of the thoracic roots supplies the intercostal muscles between the ribs and the abdominal muscles, aiding breathing and trunk stability. Because the sensory bands are specific, irritation of a thoracic nerve root can cause pain that mimics a problem with an internal organ, such as a heart issue or gallbladder pain, due to referred sensation.
Lumbar and Sacral Radicular Nerves (L1-S5)
The Lumbar and Sacral groups control the hips, legs, and feet, and provide control for bowel and bladder function. The L2 and L3 nerve roots primarily supply sensation to the front of the thigh, enabling hip flexion and knee extension. The L4 root provides sensation to the inner side of the lower leg and foot. Its motor function is ankle dorsiflexion, the ability to lift the foot upward.
The L5 root supplies sensation to the top of the foot and controls the extension of the big toe, a movement tested to assess motor integrity. The S1 root runs down the back of the leg to the heel, a path often involved in lower back pain symptoms. Motor control for the S1 root is ankle plantar flexion, the action of pointing the toes downward. The S2 through S5 roots control the motor and sensory function of the pelvic floor, including bowel and bladder function.
When Radicular Nerves Are Compromised (Radiculopathy)
When a radicular nerve root becomes irritated, compressed, or damaged, the resulting condition is known as radiculopathy. This compression prevents the nerve from properly transmitting signals, leading to a predictable pattern of symptoms that follow the affected dermatome and myotome. Radiculopathy is most commonly seen in the cervical (neck) and lumbar (lower back) regions of the spine.
Nerve root compression is usually related to structural changes within the spine that narrow the space where the nerve exits. A herniated disc is one of the most common causes, where the inner material of a spinal disc pushes out and presses directly on the adjacent nerve root. Other causes include spinal stenosis (a general narrowing of the bony canal) and the formation of bone spurs, or osteophytes (bony overgrowths on the vertebrae).
Symptoms of radiculopathy include pain that travels away from the spine along the nerve root path, known as radicular pain. Sensory deficits manifest as numbness, tingling, or a “pins and needles” sensation (paresthesia) in the specific dermatome. Motor deficits present as muscle weakness, a reduced ability to perform the movement controlled by the myotome, or a loss of reflexes. Lumbar and sacral radiculopathy is often called “sciatica” when the pain radiates down the leg. Cervical radiculopathy is frequently described as a “pinched nerve,” causing pain and weakness that travels down the arm and into the hand.