The spine, or vertebral column, provides support and flexibility for the body. The uppermost section is the cervical spine, or neck, which consists of seven individual bones labeled C1 through C7. These cervical vertebrae are smaller than those in the rest of the spine, allowing for the high degree of mobility found in the neck. The cervical spine protects the spinal cord as it descends from the brain, and each numbered vertebra plays a role in movement and nerve signaling. The C4 vertebra is the fourth segment in this column, positioned in the mid-cervical region.
Pinpointing the C4 Vertebra: Anatomical Location
The C4 vertebra is situated in the neck directly between the third cervical vertebra (C3) above it and the fifth cervical vertebra (C5) below it. This location is roughly halfway down the neck column. C4 is considered a “typical” cervical vertebra, possessing a smaller, broader body than the bones found lower in the spine.
Clinicians can approximate the location of C4 using external anatomical landmarks in the front of the neck. The C4 vertebral body is found at the level of the upper margin of the thyroid cartilage, commonly known as the Adam’s apple. The C4 level also corresponds to the point where the common carotid artery bifurcates into its internal and external branches.
Structurally, C4 shares features common to most cervical vertebrae, including a transverse foramen, a small opening that allows for the passage of the vertebral artery and vein. The spinous process, the bony projection at the back of the vertebra, is usually bifid or forked in C4. This bifid shape is typical for the mid-cervical region, distinguishing it from the single spinous process of the C7 vertebra at the base of the neck.
The Role of C4 in Neck Movement and Support
The C4 vertebra is a component of the cervical spine designed to maximize motion while supporting the skull. Its small vertebral body and the orientation of its paired articular facets allow for a wide range of neck movements. These facets are angled to facilitate gliding motions that contribute to flexion, extension, lateral bending, and rotation of the head.
The C4 segment contributes to maintaining the natural curvature of the neck, known as the cervical lordosis. This gentle, inward C-shape is important for absorbing shock and evenly distributing the weight of the head. The high mobility of the C4 segment means it is subject to mechanical stress, making it and the C5 vertebra prone to injury or degenerative changes.
The surrounding musculature, including the deep neck flexors and extensors, attaches to the bony projections of C4, stabilizing the segment during movement. Muscles like the longus colli are anchored here, helping control the anterior motion of the neck. The combined action of the bone’s structure and the attached muscles ensures the head can move freely without compromising the spinal cord.
C4 and the Central Nervous System: Nerve Function
The C4 spinal nerve root exits the spine above the C4 vertebra and plays a role in motor and sensory function. This nerve root is a major contributor to the cervical plexus, a network of nerves that supplies the muscles and skin of the neck, upper chest, and shoulder. Damage or compression to this nerve root can lead to specific symptoms in the areas it serves.
A primary function of the C4 nerve root is its contribution to the phrenic nerve, which is formed by nerve fibers from C3, C4, and C5. The phrenic nerve is the sole source of motor input to the diaphragm, the muscle responsible for breathing. This anatomical connection is often summarized by the mnemonic “C3, 4, and 5 keep the diaphragm alive,” highlighting that the C4 nerve is essential for respiration.
The C4 nerve root also has a distinct motor territory, or myotome, contributing motor fibers that control the levator scapulae muscle. This muscle is responsible for elevating the shoulder. Sensory fibers from the C4 nerve root supply a specific area of skin known as the dermatome, which includes the lower neck, the upper shoulder, and the skin over the collarbone. Therefore, clinical assessment of C4 nerve function involves testing for weakness in shoulder elevation and checking for altered sensation, such as numbness or tingling, across the upper shoulder area.
Injury to the C4 level of the spinal cord is serious because it can compromise the function of the phrenic nerve, potentially leading to paralysis of the diaphragm and the need for artificial respiratory support. A localized injury to the C4 nerve root, such as from a herniated disc, results in pain, weakness, and sensory changes in the shoulder and neck region. Understanding the location and function of the C4 vertebra and its corresponding nerve is essential for diagnosing and treating conditions affecting the mid-cervical spine.