The phrenic nerves arise from the spinal column at the height of the neck. The left and right phrenic nerve follow a different path, but both find their origin mainly in the (cervical nerve) C4, but also in the C3 and C5 vertebrae of the neck. The part originating from the C5 is the accessory phrenic nerve. From there, the right phrenic nerve passes the posterior side of the subclavian vein, the right root of the lung on the anterior side and the right atrium. The left phrenic nerve touches the pericardium at the left ventricle and pierces the diaphragm separately, on a lower level on the left side.
Its main functions are controlling the movement (contraction) of the diaphragm and sensation to the central tendon. The mediastinal pleura and the pericardium are in direct contact with the phrenic nerves in the thorax. For this purpose the phrenic nerves are a composition of motor, sensory and sympathetic nerve fibers.
As the phrenic nerve controls the movement of the diaphragm, it is essential for breathing. It is the nerve communicating the control over the tension of the diaphragm, allowing inhalation and exhalation but also the buildup of abdominal pressure necessary for excretion.
Irritation of the phrenic nerve can lead to spasmodic contractions of the diaphragm. Permanent damage to one of both phrenic nerves can result in the paralysis of half of the diaphragm making breathing more difficult. Breathing function however is preserved as long as one of the phrenic nerves is intact.