The cervical spine is the region of the vertebral column that supports the neck. Eight nerves emerge from between the vertebrae of the neck, called cervical nerves. They are referred to as C1 through C8. Each cervical nerve leads to different parts of the body, supporting sensory, motor or organ function. When a nerve is damaged, the specific areas of the body that it serves can show symptoms.
The fourth and fifth cervical nerves supply motor function to the muscles of the shoulder and arm. When either of these nerves is compromised, muscle weakness can result. Specifically, the C4 nerve enervates the levator scapulae, trapezius and rhomboid muscles. Damage to the C4 nerve may result in difficulty in elevating the shoulders. The C5 nerve supplies the deltoid, supraspinatus, infraspinatus and biceps muscles. Damage to the C5 nerve can affect the ability to raise the arm or bend it at the elbow.
Pain, Numbness or Paresthesia
Each nerve detects sensory information from specific areas of skin, called dermatomes. When pain, numbness or paresthesia--commonly referred to as “pins and needles”--appears over a dermatome, it could signify damage to the corresponding nerve root. The C4 nerve receives sensory input from the dermatome that covers the shoulder, collarbone, and shoulder blade. Pain, numbness or paresthesia in these areas can signal damage to the C4 nerve.
The C5 nerve receives sensory input from the dermatome that is located over the deltoid muscle, as well as the outer, or lateral, part of the arm.
Applying a quick strike from a tendon hammer to a tendon is a technique used to test reflexes. This input activates sensory receptors within the tendon, which sends a message to the brain, and a motor response results, causing the muscle to contract. The C5 cervical nerve can affect the reflexes of the biceps and brachioradialis muscles.
Cervical nerves 4 and 5, with additional help from the third cervical nerve, supply the diaphragm muscle, which assists in breathing. Damage to these nerves, particularly C4, can disrupt motor function to the diaphragm and result in difficulty breathing. A paper published in 1988 in the "British Journal of Neurosurgery" describes a case in which damage to the cervical nerves caused partial paralysis of an opera singer’s diaphragm, which made singing difficult operatic passages impossible and threatened to end her career.