The spinal column is composed of bones called vertebrae, and the section running through the neck is the cervical spine (C-spine). This region is made up of seven individual vertebrae, labeled C1 through C7, which support the head and allow for a wide range of motion. The C4 and C5 are two specific bones in this series, located in the middle portion of the neck. Understanding the location and function of these segments is important because they are frequently involved in neck pain, injury, and nerve-related issues. This article will focus on how to locate the C4 and C5 vertebrae and explain the roles they play in protecting the spinal cord and controlling upper body movement.
Pinpointing the Location in the Neck
The cervical spine begins beneath the base of the skull, where the C1 vertebra articulates with the head. It extends downward to C7, which connects to the first thoracic vertebra (T1), marking the transition into the upper back. The C4 and C5 vertebrae are positioned roughly in the center of this seven-bone segment, resting in the middle third of the neck.
Finding the exact location of these vertebrae externally is challenging because they are shielded by muscle and soft tissue. Clinicians often use palpable landmarks to estimate their position. For example, the C4 vertebra generally aligns with the superior border of the thyroid cartilage, commonly known as the Adam’s apple.
The C4 is situated directly below C3 and immediately above C5. The intervertebral disc between the C4 and C5 vertebral bodies is designated as the C4/C5 disc. While the bony prominences (spinous processes) are used for palpation, those of C4 and C5 are less distinct than the large bump of C7, making precise identification difficult for a layperson.
Structure of the C4 and C5 Vertebrae
The C4 and C5 vertebrae are classified as “typical” cervical vertebrae, sharing a general structure distinct from the C1 and C2 segments. Like all vertebrae, they feature a thick, cylindrical vertebral body at the front, which is the primary weight-bearing component. These bodies are smaller and broader than those in the lower back, reflecting the lighter load they support.
Behind the vertebral body is the protective vertebral arch, formed by two pedicles and two laminae, which encircle the central opening called the vertebral foramen. This ring of bone forms the canal that safely houses the spinal cord. The spinous process projects backward from the arch; in the C4 and C5, this process is typically short and often splits into two points, known as bifid.
The intervertebral disc between C4 and C5 connects the two vertebral bodies, acting as a fibrocartilaginous cushion. This disc is comprised of a tough, fibrous outer ring (annulus fibrosus), which contains a soft, gel-like inner core (nucleus pulposus). This structure permits slight movement and functions as a shock absorber against daily compressive forces.
Functional Significance of the C4/C5 Segment
The C4/C5 segment is important for supporting the head and connecting to major nerve pathways that control the upper body. The spinal nerve that exits the spinal canal between the C4 and C5 vertebrae is the C5 nerve root. This nerve root carries motor and sensory signals to specific muscles and areas of the skin.
The C5 nerve root primarily controls the motor function of the shoulder and upper arm. It is a primary controller for the deltoid muscle, allowing for the abduction of the arm. It also contributes significantly to the power of the biceps muscle, which is responsible for flexing the elbow.
The C4 and C5 segments also have a role in breathing. The C3, C4, and C5 nerve roots contribute fibers that form the phrenic nerve, which supplies the diaphragm, the muscle responsible for respiration. An injury at the C4/C5 level can sometimes compromise the ability to breathe effectively.
A common issue at this level, such as a herniated C4/C5 disc, can compress the C5 nerve root as it exits the spine. This compression can lead to symptoms that follow the nerve’s path, typically causing weakness in the deltoid and biceps muscles and a loss of sensation in the upper arm down to the elbow. The C4/C5 segment also allows for a substantial amount of the neck’s overall range of motion, including forward and backward tilting, rotation, and side bending.