The C6 nerve root is one of eight pairs of cervical spinal nerves emerging from the neck. It is a mixed nerve, containing both motor fibers (anterior root) that control muscle movement and sensory fibers (posterior root) that relay feeling from the skin. The C6 nerve root exits the spine between the C5 and C6 vertebrae and contributes to the brachial plexus, a network of nerves responsible for the function of the entire upper limb.
Muscles Controlled by C6
The C6 nerve root supplies motor function to muscles in the shoulder, upper arm, and forearm. Its primary motor function is elbow flexion and wrist extension. Damage to this single nerve root can affect multiple actions across the upper extremity.
Elbow Flexion
The biceps brachii muscle, which bends the elbow and rotates the forearm outward (supination), receives substantial innervation from C6, along with C5. The brachialis muscle, a powerful elbow flexor located deeper than the biceps, is also co-innervated by the C6 root. These two muscles work together to allow the arm to lift and pull objects toward the body.
Wrist Extension
The C6 root provides significant motor control to the muscles that extend the wrist. It supplies the extensor carpi radialis longus and brevis, which are the primary muscles used to lift the hand upward at the wrist. This extension is important because it allows the hand to be positioned effectively for grasping objects.
Other Contributions
C6 also contributes fibers to several other upper extremity muscles. These include the supinator muscle, which rotates the forearm, and the pronator teres, which rotates the forearm inward. Furthermore, C6 contributes to the innervation of the deltoid muscle (abduction) and several rotator cuff muscles, though C5 is generally the dominant root for shoulder movement.
The C6 Dermatome
A dermatome is a specific area of skin that receives sensory innervation from a single spinal nerve root. This sensory distribution is clinically useful because changes in sensation can help a clinician determine the precise level of nerve involvement in the spine.
The C6 dermatome follows a distinct sensory pathway down the arm. It begins high in the shoulder area and extends down the lateral, or thumb-side, aspect of the upper limb. It typically covers the skin over the lateral forearm. This distribution is often described as running along the radial side of the arm.
In the hand, the C6 dermatome supplies sensation to the thumb and the index finger. Any change in sensation, such as numbness or tingling, that follows this route can indicate a problem with the C6 nerve root.
Assessing C6 Function and Common Injuries
Medical professionals evaluate the integrity of the C6 nerve root by testing its motor, sensory, and reflex functions.
Reflex Testing
The primary deep tendon reflex associated with C6 is the biceps reflex, though the brachioradialis reflex is also significantly influenced by the C5 and C6 roots. A medical hammer is used to gently tap the tendon, which should elicit an involuntary muscle contraction.
When testing the biceps reflex, an absent or diminished response (hypo-reflexia) suggests a potential issue with the C5/C6 nerve roots or the associated reflex arc. Conversely, an exaggerated response (hyper-reflexia) may indicate a more extensive problem affecting the spinal cord itself, rather than just the nerve root. Motor strength is assessed by having the patient resist pressure during elbow flexion and wrist extension, which tests the muscles primarily innervated by C6.
Cervical Radiculopathy
The most common condition affecting the C6 nerve root is cervical radiculopathy, which is often caused by nerve root compression or irritation as it exits the spinal canal. This compression can result from a herniated disc, which pushes against the nerve root, or from foraminal stenosis, which is a narrowing of the bony opening (foramen) where the nerve exits. The C6 nerve root is frequently affected because the C5-C6 level of the cervical spine experiences substantial mobility.
Symptoms of C6 radiculopathy include:
- Weakness in the muscles it supplies, particularly elbow flexion and wrist extension.
- Pain that radiates down the arm.
- Numbness, tingling, or a burning sensation (paresthesia) that follows the C6 dermatome.
This sensory pattern affects the lateral forearm, thumb, and index finger. Recognizing this combined pattern of motor and sensory deficits is fundamental to diagnosing the level of nerve root injury.