The cervical spine, or neck, is the uppermost section of the spinal column, comprising seven individual segments labeled C1 through C7. This bony structure supports the weight of the head while allowing for extensive movement and protecting the spinal cord within. The C5 and C6 segments, in particular, are central components within the neck that play a significant role in controlling movement and sensation in the upper limbs.
Anatomical Placement of C5 and C6
The C5 and C6 vertebrae are located in the mid-cervical region of the neck, positioned below C4 and above C7. Each one consists of a large, cylindrical body in the front, which bears weight and is separated from the next vertebra by an intervertebral disc. The disc located between the C5 and C6 vertebral bodies is known as the C5-C6 disc space.
The spinal cord runs through a canal formed by the stacked vertebrae, and at each level, nerve roots branch off to exit the spine. These nerve roots pass through small openings called the intervertebral foramen. The C5 nerve root exits the spinal canal through the foramen above the C5 vertebra, while the C6 nerve root exits above the C6 vertebra. Narrowing of the foramen, such as from disc material or bone spurs, can compress the nerve root as it exits.
Motor Function and Muscle Control
The C5 and C6 nerve roots contain motor fibers that govern specific movements and muscle contractions in the shoulder and arm. These nerve roots are primary contributors to the brachial plexus, the complex network of nerves that controls the entire upper extremity.
The C5 nerve root is largely responsible for initiating shoulder abduction. It provides motor control to the deltoid muscle, the large cap of the shoulder, and contributes to the function of the biceps brachii muscle. This nerve root also supplies the infraspinatus muscle, which helps rotate the shoulder joint outward.
The C6 nerve root continues the motor control of the upper limb, focusing on elbow flexion and wrist extension. It is involved in the flexing action of the biceps and the forearm muscle that helps rotate the forearm. C6 also innervates the extensor muscles of the wrist, allowing the hand to be angled backward at the wrist joint.
Sensory Function and Dermatomes
The C5 and C6 nerve roots carry sensory information back to the spinal cord and brain. The specific area of skin supplied by a single spinal nerve root is called a dermatome.
The C5 dermatome covers the skin over the shoulder cap and extends down the lateral, or outer, part of the upper arm, sometimes reaching as far as the elbow. The C6 dermatome follows a path down the lateral forearm and into the hand, specifically covering the thumb and the index finger.
Common Symptoms of C5 and C6 Nerve Impingement
When the C5 or C6 nerve root is compressed or irritated, a condition known as cervical radiculopathy occurs, which causes symptoms directly related to the nerve’s function. This irritation often stems from a herniated disc, where the inner material of the C5-C6 disc presses on the nerve, or from degenerative changes like bone spurs that narrow the foramen.
Impingement of the C5 nerve root typically presents as weakness when raising the arm. Patients may also notice a reduction in the initial strength of elbow bending. The sensory symptoms include pain or numbness that radiates into the C5 dermatome, affecting the shoulder and upper arm.
When the C6 nerve root is affected, the primary motor symptom is weakness in the wrist extensor muscles. Elbow flexion strength, controlled by the biceps, can also be reduced. Sensory symptoms follow the C6 dermatome, causing pain, tingling, or numbness that travels down the arm and into the thumb and index finger.
The pain associated with this nerve irritation is frequently described as sharp or burning and may worsen with certain neck movements, such as extending the neck. These neurological symptoms are usually felt only on the side of the body where the nerve root is compressed.