The cervical spine (C-spine) is the uppermost section of the vertebral column, consisting of seven bones (C1 through C7). The C3 vertebra is the third bone from the top, located approximately at the level of the jaw. It is considered the first “typical” cervical vertebra, distinguishing it from the specialized C1 and C2 bones above it. The C-spine supports the weight of the head and provides a protective bony tunnel for the spinal cord and its branching nerve roots.
C3’s Role in Neck Movement and Sensation
The third cervical nerve root (C3) emerges from the spinal cord at this level, carrying both motor and sensory fibers. Motor fibers from C3 contribute to moving the head and neck, innervating muscles responsible for lateral flexion and rotation.
The C3 nerve root provides motor control to the deep neck extensors and contributes innervation to the levator scapulae muscle, which helps elevate the shoulder blade. C3 fibers also run to the upper trapezius muscle, primarily aiding in proprioception—sensing the position and movement of the head.
On the sensory side, C3 is responsible for collecting feeling from a dermatome covering the side and back of the upper neck, extending toward the back of the head. This sensory area provides sensation to the skin of the upper neck and ear region. Damage or irritation to the C3 nerve root can manifest as pain, numbness, or tingling in this distribution.
Controlling the Diaphragm and Respiration
The most recognized function associated with the C3 level is its direct control over breathing. The C3 spinal segment, along with C4 and C5, collectively forms the origin of the Phrenic nerve. This nerve is the sole source of motor innervation for the diaphragm, the primary muscle of respiration.
The Phrenic nerve pathway begins high in the cervical spine and travels downward to the diaphragm. The diaphragm is a large, dome-shaped muscle that must contract and flatten to create a vacuum in the chest, pulling air into the lungs during inspiration. Without the rhythmic signals sent by the Phrenic nerve, the diaphragm cannot perform this function.
Although C3, C4, and C5 all contribute fibers, the C4 segment provides the largest portion, with C3 and C5 offering necessary contributions. The saying, “C3, 4, and 5 keep the diaphragm alive,” is a common way to remember this multi-segmental origin. Since the nerve arises from this high cervical region, any injury to the spinal cord at or above the C3 level will disrupt the signal pathway. This disruption means the brain’s commands to the diaphragm are cut off, leading to a complete loss of independent breathing.
Clinical Impact of C3 Damage
Damage to the spinal cord at the C3 level results in a condition known as high-level spinal cord injury (SCI), leading to profound and extensive neurological deficits. Injuries at this level result in tetraplegia, historically referred to as quadriplegia, which is the functional loss of movement and sensation in all four limbs and the torso. This paralysis extends down from the neck, leaving only the ability to move the head and neck muscles that are innervated by the C1 and C2 nerves.
The loss of function is comprehensive because the injury occurs so high up, affecting nearly all descending motor commands and ascending sensory signals. A complete C3 injury eliminates voluntary control over the arms, hands, trunk, and legs. The most immediate and life-threatening consequence, however, remains the respiratory failure, as the Phrenic nerve pathway is compromised.
Individuals with a complete C3 injury often require a mechanical ventilator for continuous respiratory support, as their diaphragm is paralyzed. Beyond breathing and movement, C3 injuries also severely impact automatic bodily processes, including temperature regulation and bowel and bladder control. Damage at C3 results in the maximum degree of functional loss and need for assistance with all daily activities.