The cervical spine is the uppermost part of the spinal column, consisting of seven individual bones or vertebrae labeled C1 through C7. This structure serves the dual purpose of supporting the head, which typically weighs between 10 and 13 pounds, and providing a flexible, protective pathway for the delicate spinal cord. The cervical vertebrae facilitate the wide range of motion necessary for head and neck movements, including rotation, flexion, and extension. The fifth (C5) and sixth (C6) vertebrae are frequently discussed due to their location in a highly mobile segment and the significant nerve roots that exit at their level.
Anatomical Placement of C5 and C6
The C5 and C6 vertebrae are situated in the lower portion of the neck, known as the subaxial spine, positioned directly beneath C4 and above C7. Both C5 and C6 feature a vertebral body, which is the main weight-bearing structure, and a posterior arch that protects the spinal cord. A characteristic anatomical feature is the presence of a transverse foramen, a small opening on each side that provides a route for the vertebral artery as it travels toward the brain.
Separating the vertebral bodies is the intervertebral disc, designated C5-C6 for the disc located between these two vertebrae. The movement segment formed by C5, C6, and the disc between them is one of the most mechanically stressed areas in the cervical spine, making it highly susceptible to wear. Nerve roots exit the spinal canal through openings called neural foramina, with the C5 nerve root exiting above the C5 vertebra and the C6 nerve root exiting above the C6 vertebra.
The C5 and C6 vertebrae are part of the spinal segment that maintains the natural curvature of the neck. The bony projections on the sides of the vertebral bodies, called uncinate processes, form joints that stabilize the spine and limit side-to-side bending. These segments also possess a bifid, or split, spinous process at the back, which serves as an attachment point for neck muscles and ligaments.
Functional Roles of C5 and C6
The C5 and C6 spinal nerve roots play a substantial role in motor and sensory function in the upper body, as they are major contributors to the brachial plexus, the network of nerves that controls the arm. The specific muscles and skin areas they supply are categorized into myotomes (motor) and dermatomes (sensory). A precise understanding of these functional territories is important because a disruption at these levels can immediately affect the corresponding movements and sensations.
The C5 nerve root’s primary motor function, or myotome, is associated with shoulder abduction, the ability to lift the arm away from the body, largely controlling the deltoid muscle. It also contributes significantly to the initial bending of the elbow (elbow flexion), working with the biceps muscle. The sensory function of C5 covers the skin over the lateral aspect of the shoulder and the upper outer part of the arm, extending down toward the elbow.
The C6 nerve root shares the responsibility for elbow flexion with C5, contributing to the strength of the biceps muscle when the arm is bent. Its distinct motor function is controlling the wrist extensor muscles, which allow a person to bend the hand backward at the wrist. The sensory distribution of the C6 nerve root provides sensation to the radial side of the forearm, extending into the thumb and the index finger.
Both C5 and C6 nerve roots are involved in a number of other muscle groups, as most muscles receive innervation from multiple spinal levels. These listed movements are the most dominant and testable functions. For instance, C5 and C6 fibers combine with C7 to form the musculocutaneous nerve, which is the main nerve for the biceps and other elbow flexors. This interplay ensures that movement is smooth and coordinated across the shoulder and elbow joints.
Common Issues and Related Symptoms
Because the C5-C6 motion segment is subjected to frequent movement and load-bearing, it is the most common site for degenerative changes in the cervical spine. Conditions such as cervical disc herniation, where the inner material of the disc bulges outward, are frequently observed at this level. This bulging material can press against the exiting nerve root, leading to cervical radiculopathy, commonly known as a pinched nerve.
Another common issue is the formation of bone spurs, or osteophytes, which develop as a result of wear and tear on the joints, particularly the uncovertebral joints. These bony growths can narrow the space of the neural foramen, physically constricting the nerve roots as they exit the spine. This narrowing, known as spinal stenosis, is a progressive process that can cause persistent irritation of the C5 or C6 nerve.
When the C5 nerve root is compressed, the symptoms often manifest as weakness in the key muscles it supplies, such as difficulty lifting the arm due to deltoid weakness. Pain or a dull ache may be felt across the shoulder and upper arm, but significant numbness or tingling (paresthesia) is typically not a dominant feature in isolated C5 issues.
If the C6 nerve root is affected, the resulting symptoms are typically more pronounced in the forearm and hand. A person may experience weakness in bending the elbow or extending the wrist, and they might report a distinct pattern of pain, tingling, or numbness that travels down the arm. This sensory change often follows the C6 dermatome, radiating specifically into the thumb and the index finger.