Generality
The phrenic nerve is the bilateral mixed nerve, which has the task of innervating the diaphragm muscle.
The diaphragm is the breathing muscle par excellence.
The phrenic nerve originates at the level of the neck, from the anterior branches of the spinal roots C3, C4 and C5. Then, heading downwards (to be precise towards the diaphragm), it passes near the subclavian artery, the subclavian vein, the lungs and the heart.
The course of the phrenic nerve in the right half of the human body is slightly different from the course of the left phrenic nerve.
The phrenic nerve can be the protagonist of a medical condition known as diaphragmatic paralysis.
Phrenic nerve and its branches. Image from the teachmeanatomy.info website
Short review of what a nerve is
To fully understand what a nerve is, it is necessary to start from the concept of neuron .
Neurons represent the functional units of the nervous system . Their task is to generate, exchange and transmit all those (nervous) signals that allow muscle movement, sensory perceptions, reflex responses and so on.
Typically, one neuron consists of three parts:
- The so-called body, where the cell nucleus resides.
- Dendrites, which are equivalent to receiving antennae for nerve signals from other neurons or receptors located in the periphery.
- Axons, which are cellular extensions having the function of spreading the nervous signal. The axon covered with myelin (myelin sheath) is also called nerve fiber.
A bundle of axons forms a nerve .
Nerves can carry information in three ways:
- From the central nervous system (CNS) to the periphery . The nerves with this property are called efferent. The efferent nerves control the movement of the muscles, so I am in charge of the motor sphere.
- From the periphery to the CNS . Nerves with this capacity are called afferent. Nervous patients report to the CNS what they detected in the periphery, therefore they cover a sensitive (or sensory) function.
- From the SNC to the periphery and vice versa . Nerves with this double capacity are called mixed. Mixed nerves have a dual function: motor and sensory.
What is the phrenic nerve?
The phrenic nerve is the bilateral mixed nerve, which, among its various functions, has the important task of innervating the diaphragm muscle.
The bundles of axons that compose it derive, in part, from the brachial plexus and, in part, from the cervical plexus . The brachial plexus and the cervical plexus are two important even reticular formations of spinal nerves, having the function of innervating, respectively, the upper limbs (from the shoulder to the hand) and the neck-trunk section.
In anatomical language, the term "even" indicates that a given element - be it a bone, a blood vessel or a nervous structure - is present both on the right and on the left half of the human body
DIAPHRAGM: POSITION AND FUNCTION
The diaphragm is that laminar-shaped muscle that resides on the lower edge of the rib cage and separates the thoracic cavity from the abdominal cavity.
In addition to keeping the organs of the chest separate from the organs of the abdomen, this particular laminar muscle plays a fundamental role during the breathing process:
- In the inhalation phase, it contracts, pushing the abdominal organs downwards and inducing the lifting of the ribs closest to him. This expands the volume of the chest cavity and allows the lungs to take in the necessary air.
- In the exhalation phase, it is released, allowing the abdominal organs to ascend (NB: this also occurs thanks to the support of the abdominal muscles) and to the lower ribs to return to the normal position.
At this stage, the thoracic volume is clearly reduced.
Anatomy
Each phrenic nerve derives mainly from the anterior branch of the fourth cervical spinal root ( root C4 ) and, to a lesser extent, from the anterior branches of the third and fifth spinal root ( root C3 and stem C5 ).
The anterior branches of the C3 and C4 roots belong to the cervical plexus, while the anterior branch of the C5 root is part of the brachial plexus.
Returning to the phrenic nerve, the latter begins its course in the neck, exactly on the lateral edge of the anterior scalene muscle . From here, it continues downwards (ie towards the chest) and obliquely, bypassing the anterior scalene muscle anteriorly and remaining below the so-called prevertebral membrane (or fascia ).
At this point, the trend of the left phrenic nerve and the trend of the right phrenic nerve varies with each other. Indeed:
- The left phrenic nerve passes, anteriorly, to the first part of the subclavian artery and to the brachiocephalic artery and, posteriorly, to the subclavian vein. Then, it enters the thorax through the so-called upper thoracic opening, bypasses the aortic arch and the vagus nerve and continues in the direction of the diaphragm, passing over the apex of the left lung and traveling along the pericardial portion enveloping the left ventricle.
The path of the left phrenic nerve ends on the left half of the diaphragm.
- The right phrenic nerve leads anterior to the second tract of the subclavian artery and posteriorly to the subclavian vein. Then, it enters the thorax, through the upper thoracic opening, and continues in the direction of the diaphragm, passing over the apex of the right lung and traveling over the portion of pericardium enveloping the right atrium.
The course of the right phrenic nerve ends at the level of the right half of the diaphragm.
BRANCHE OF THE BRIDAL NERVE
At the end of his journey, both the left phrenic nerve and the right phrenic nerve gives rise to three main branches, very simply called: anterior branch, lateral branch and posterior branch.
vascularization
The flow of oxygen-rich blood to the phrenic nerve depends on the pericardiophrenic artery . The pericardiophrenic artery is a branch of the internal thoracic artery .
CHANGES
In some individuals, the origin or course of the phrenic nerve may undergo variations, compared to the picture above. For example, it is possible that:
- the right phrenic nerve and / or the left phrenic nerve run anteriorly to the subclavian vein;
- The phrenic nerve descends in the direction of the chest, remaining on the lateral edge of the anterior scalene muscle;
- The scalene nerve pierces the anterior scalene muscle;
- The phrenic nerve presents an accessory nerve, called an accessory phrenic nerve. Typically, the accessory phrenic nerve descends posteriorly to the subclavian vein and joins the phrenic nerve approximately at the level of the thorax;
Moreover, it is equally possible that:
- The phrenic nerve receives further nerve branches from the brachial or cephalic plexuses;
- The phrenic nerve sends some branches to innervate the subclavian muscle.
Function
The phrenic nerve includes axon bundles with motor functions and axon bundles with sensory functions. Besides, it's a mixed nerve.
MOTOR FUNCTIONS OF THE BRAKE NERVE
As already stated, the phrenic nerve is responsible for the motor control of the diaphragm, which is the main breathing muscle .
Therefore, adequate and efficient breathing depends on the proper functioning of the phrenic nerve.
SENSITIVE FUNCTIONS OF THE BRAKE NERVE
Through his sensory axons, the phrenic nerve innervates the mediastinal pleura, the central part of the diaphragmatic pleura, the central part of the diaphragmatic peritoneum and the pericardium . It should be remembered that nerves with sensitive functions transmit information from the periphery - hence from the areas just mentioned - to the central nervous system .
Clinic and pathologies
The phrenic nerve can be the victim of inflammation or damage .
Phrenic nerve inflammation is responsible for episodes of hiccups, while damage to it can lead to the appearance of a medical condition known as diaphragmatic paralysis .
HICCUP
The hiccups is an unexpected, involuntary and spasmodic contraction of the diaphragm, which is expressed in an inspiration followed by the sudden and noisy closing of the glottis.
Among the causes that can inflame the phrenic nerve and provoke, afterwards, hiccups, include:
- The presence of a tumor or cyst on the neck, which causes a compression phenomenon on the phrenic nerve;
- A condition of goiter such that a compression phenomenon occurs on the phrenic nerve;
- The presence of gastroesophageal reflux;
- The presence of a severe sore throat (pharyngitis) or a strong laryngitis.
DIAGRAMATIC PARALYSIS
Phrenic nerve damage, which leads to diaphragmatic paralysis, may be the consequence of:
- A mechanical trauma, occurring for example during a surgical procedure;
- A compression, due for example to the presence of a tumor in the thoracic cavity;
- A myopathy, resulting for example from a condition of myasthenia gravis;
- A neuropathy, resulting for example from a condition of diabetes (diabetic neuropathy).
Diaphragmatic paralysis is responsible for a paradoxical movement on the part of the diaphragm. In other words, the diaphragm rises, during inhalation, and lowers, during expiration (that is, it does the opposite of what it usually does).
The treatment of diaphragmatic paralysis involves a causal therapy (therefore a remedy for what causes damage to the phrenic nerve) and a symptomatic therapy.
