The phrenic nerve is a mixed nerve that provides motor innervation to the diaphragm. Thus, the nerve is involved in respiration. Complete paralysis of the structure is life-threatening.
What is the phrenic nerve?
The plexus of nerves in the neck is titled by the technical term cervical plexus. The nerve structure contains both motor and sensory branches. The motor branches of the structure reach the ventral cervical muscles, the infrahyal muscles and the diaphragm. The phrenic nerve is one of the branches of the cervical plexus and arises from the cervical plexus. It is a motor peripheral nerve containing fibers from spinal cord segments C3 and C4. In addition, the nerve contains portions from spinal cord segment C5. In one fifth of all people, so-called accessory phrenici occur in the cervical region in addition to the main branch. The main branch of the phrenic nerve innervates the diaphragm in particular and is therefore also known as the phrenic nerve. The nerve is a mixed nerve, containing motor fibers as well as sensitive fibers. The diaphragm is the only area that the nerve supplies with motor fibers. The sensitive supply area of the nerve includes some internal organs.
Anatomy and structure
The phrenic nerve arises from the cervical plexus and passes between the subclavian artery and subclavian vein. The nerve tracts caudally on the scalenus anterior muscle and travels in the mediastinum on the ventral side of the pulmonary pedicle between the pleura parietalis and external pericardium to the diaphragm, whose caudal and cranial sides the nerve covers with its branches. Arriving at the diaphragm, the phrenic nerve gives off two collateral branches known as the rami phrenicoabdominalis dexter and phrenicoabdominalis sinister, which penetrate the diaphragm. In addition, the rami in the abdominal region innervate the peritoneum parietale below the diaphragm. From here, the ramus phrenicoabdominalis dexter traverses the foramen venae cavae. The ramus phrenicoabdominalis sinister passes through muscle fibers of the diaphragm. Thus, the phrenic nerve essentially passes through the thoracic aperture, pleural dome, pericardium, and diaphragm, with its branches penetrating into the abdomen to reach the liver, pancreas, and abdominal wall.
Function and Tasks
The phrenic nerve is a mixed nerve. Thus, it has different tasks. Motorically, it exclusively innervates the diaphragm and thus the central respiratory muscle. In this context, the nerve is essentially involved in respiratory movement. The diaphragm is subject to central nervous control by the respiratory center within the medulla oblongata and pons. The nerve cell assemblies in these brain areas control the motor root cells from the phrenic nerve, which are located in the cervical medulla. The phrenic nerve is thus involved in the involuntary control of the diaphragm by the respiratory center and in this way ensures the automatism of vital respiratory movements. The diaphragm has a dome shape. The contraction of the musculature by the phrenic nerve is automatic via the associated spinal cord segments. The individual segments project to the respiratory center. During phrenicus-mediated contraction of the diaphragm muscles, the thoracic cavity enlarges. Due to adhesions of the diaphragm and the pleura parietalis, the muscle contraction also leads to an increase in lung volume. The resulting negative pressure causes the person to inhale automatically. In addition, phrenic-mediated diaphragmatic contraction plays a role in abdominal compression, such as is relevant to the birth process. Apart from these functions, the phrenic nerve also has tasks in the field of sensory innervation. The sensitive supply areas of the nerve are the pleura parietalis in the form of the pars mediastinalis and diaphragmatica, the pericardium and the peritoneum, especially that of the liver or gallbladder and the entrance to the stomach. The phrenic nerve is present on both sides of the human body and thus performs the above tasks bilaterally.
Diseases
Like any other nerve, the phrenic nerve can be affected by irritation and paralysis. Confusion of the nerve, for example, may be the result of surgical opening of the pericardium. When parts of the sensitive supply areas are irritated, the resulting pain often radiates to the right shoulder. This phenomenon is known as the Eiselsberg phenomenon.Frequently, irritation of the nerve occurs in the course of a gaseous laparoscopy. Patients complain of chest and shoulder pain afterwards. The gas exerts pressure on the diaphragm area during the procedure and irritates the nerve in this way. Pleurisy in the sense of inflammation of the pleura can also result in nerve irritation and in this case is often associated with hiccups. Far more serious is the symptomatology of phrenic nerve palsy. In such a paralysis, the diaphragm becomes flaccid on one side. The abdominal organs are no longer pressed down by the diaphragm and push upwards when they become flaccid. The result is a diaphragmatic high, which causes more or less severe breathing difficulties. Unilateral diaphragmatic herniation is not life-threatening. Bilateral failure of the phrenic nerve can be fatal. The phrenic nerve is usually unaffected by phenomena such as paraplegia from the fifth cervical vertebra. Exclusively in cases of deformities up to the third cervical vertebra, massive and life-threatening disturbances of lung function are to be expected. Complete loss of the phrenic nerve is usually traumatic. Neurological diseases rarely lead to complete paralysis of the nerve. An exception is ALS disease, which gradually degrades the motor cranial nerve nuclei and leads to respiratory muscle culture failures as it progresses.
