Phrenic Nerves

The phrenic nerves are a pair of crucial nerves that originate in the neck (C3-C5 levels of the spinal cord) and run down to the diaphragm, which is the main muscle involved in respiration^[1]. The term "phrenic" is derived from the Greek 'phren', meaning 'diaphragm'.

The primary function of the phrenic nerves is motor control of the diaphragm. When these nerves fire, they stimulate the diaphragm to contract, causing it to descend within the thoracic cavity and thus expand the lungs, allowing for inhalation. When the diaphragm relaxes, it moves back to its resting position, enabling exhalation^[3]. Without the phrenic nerves, voluntary control over breathing would not be possible.

In addition to this essential motor function, the phrenic nerves also have sensory functions. They carry sensory information from the central part of the diaphragm, as well as from the mediastinal pleura and pericardium (the lining around the heart), back to the central nervous system. This sensory pathway is why irritation to the diaphragm or structures around the heart can sometimes be perceived in areas where the phrenic nerves originate, such as the neck or shoulder - a phenomenon known as referred pain^[4].

The right and left phrenic nerves are somewhat different in their courses. The right phrenic nerve is usually shorter and runs a relatively straight course to the diaphragm, while the left phrenic nerve typically has a longer and more oblique course. Despite these anatomical differences, their functional roles are the same. Damage to or dysfunction of one or both of the phrenic nerves can cause difficulties with breathing, and in severe cases may necessitate mechanical ventilation^[6].