Phrenic nerve
Encyclopedia
The phrenic nerve originates mainly from the 4th cervical nerve
, but also receives contributions from the 5th and 3rd cervical nerves (C3-C5) in humans.
The phrenic nerves contain motor, sensory, and sympathetic nerve fibers. These nerves provide the only motor supply to the diaphragm
as well as sensation to the central tendon. In the thorax, each phrenic nerve supplies the mediastinal pleura and pericardium.
The contribution of the C5 nerve to the phrenic nerve may be derived from an accessory phrenic nerve. Frequently, it is a branch of the nerve to the subclavius. If present, the accessory phrenic nerve lies lateral to the main nerve and descends posterior and sometimes anterior to the subclavian vein. The accessory phrenic nerve joins the phrenic nerve either in the root of the neck or in the thorax.
On the left; the phrenic nerve crosses anterior to the first part of the subclavian artery. On the right; it lies on the anterior scalene muscle and crosses anterior to the 2nd part of the subclavian artery. On both sides, the phrenic nerve runs posterior to the subclavian vein and anterior to the internal thoracic artery as it enters the thorax.
Found in the middle mediastinum, both phrenic nerves run from C3, C4 and C5 along the anterior scalene muscle
deep to the carotid sheath
.
On both sides the phrenic nerve runs posterior to the subclavian vein and posterior to the internal thoracic artery as it enters the thorax where it runs anterior to the root of the lung and into the pericardium between the fibrous and parietal layers.
Both of these nerves supply motor fibres to the diaphragm and sensory fibres to the fibrous pericardium
, mediastinal pleura
, and diaphragmatic peritoneum
.
The pericardiacophrenic arteries and veins travel with their respective phrenic nerves.
The contribution of the 5th cervical nerve may stem from an accessory phrenic nerve. Most often it is a branch of the nerve to the subclavius and may contain numerous phrenic nerve fibers. If the accessory phrenic nerve is present it lies lateral to the main nerve and descends posterior and occasionally inferior to the subclavian vein. The accessory phrenic nerve connects to the phrenic nerve in the thorax or the root of the neck.
arising from structures served by the phrenic nerve is often "referred
" to other somatic regions served by spinal nerves C3-C5. For example, a subphrenic abscess beneath the right diaphragm might cause a patient to feel pain in the right shoulder (Kehr's sign
).
Irritation of the phrenic nerve (or the tissues supplied by it) leads to the hiccough reflex. A hiccough is a spasmodic contraction of the diaphragm, which pulls air against the closed folds of the larynx
.
The phrenic nerve must be identified during thoracic surgery
and preserved. It passes anterior to the hilum
of the corresponding lung, and therefore can be identified easily. The right phrenic nerve may be crushed by the vena cava clamp during liver transplantation
. Severing the phrenic nerve, or a phrenectomy, will paralyse that half of the diaphragm. Diaphragm paralysis is best demonstrated by sonography. Breathing will be made more difficult but will continue provided the other nerve is intact.
Fortunately, the phrenic nerve arises from the neck (C3-C5) and innervates the diaphragm, which is much lower. Hence, patients who suffer spinal cord injuries below the neck are still able to breathe effectively, despite any paralysis of the lower limbs.
." - "Diagram of the cervical plexus."
Cervical nerves
The cervical nerves are the spinal nerves from the cervical vertebrae.Although there are seven cervical vertebrae , there are eight cervical nerves . All nerves except C8 emerge above their corresponding vertebrae, while the C8 nerve emerges below the C7 vertebra...
, but also receives contributions from the 5th and 3rd cervical nerves (C3-C5) in humans.
The phrenic nerves contain motor, sensory, and sympathetic nerve fibers. These nerves provide the only motor supply to the diaphragm
Thoracic diaphragm
In the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...
as well as sensation to the central tendon. In the thorax, each phrenic nerve supplies the mediastinal pleura and pericardium.
The contribution of the C5 nerve to the phrenic nerve may be derived from an accessory phrenic nerve. Frequently, it is a branch of the nerve to the subclavius. If present, the accessory phrenic nerve lies lateral to the main nerve and descends posterior and sometimes anterior to the subclavian vein. The accessory phrenic nerve joins the phrenic nerve either in the root of the neck or in the thorax.
Path
The phrenic nerve descends obliquely with the IJV across the anterior scalene, deep to the prevertebral layer of deep cervical fascia and the transverse cervical and suprascapular arteries.On the left; the phrenic nerve crosses anterior to the first part of the subclavian artery. On the right; it lies on the anterior scalene muscle and crosses anterior to the 2nd part of the subclavian artery. On both sides, the phrenic nerve runs posterior to the subclavian vein and anterior to the internal thoracic artery as it enters the thorax.
Found in the middle mediastinum, both phrenic nerves run from C3, C4 and C5 along the anterior scalene muscle
Scalene muscles
The scalene muscles are a group of three pairs of muscles in the lateral neck, namely the scalenus anterior, scalenus medius, and scalenus posterior.They are innervated by the spinal nerves C4-C6....
deep to the carotid sheath
Carotid sheath
The carotid sheath is an anatomical term for the fibrous connective tissue that surrounds the vascular compartment of the neck. It is part of the deep cervical fascia of the neck, below the superficial cervical fascia meaning the subcutaneous adipose tissue immediately beneath the skin.The deep...
.
- The right phrenic nerve passes over the brachiocephalic arteryBrachiocephalic arteryThe brachiocephalic artery is an artery of the mediastinum that supplies blood to the right arm and the head and neck....
, posterior to the subclavian veinSubclavian veinThe subclavian veins are two large veins, one on either side of the body. Their diameter is approximately that of the smallest finger.-Path:Each subclavian vein is a continuation of the axillary vein and runs from the outer border of the first rib to the medial border of anterior scalene muscle...
, and then crosses the root of the right lungLungThe lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
anteriorly and then leaves the thoraxThoraxThe thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
by passing through the vena cava hiatus opening in the diaphragm at the level of T8. The right phrenic nerve passes over the right atriumRight atriumThe right atrium is one of four chambers in the hearts of mammals and archosaurs...
. - The left phrenic nerve passes over the pericardium of the left ventricleLeft ventricleThe left ventricle is one of four chambers in the human heart. It receives oxygenated blood from the left atrium via the mitral valve, and pumps it into the aorta via the aortic valve.-Shape:...
and pierces the diaphragm separately.
On both sides the phrenic nerve runs posterior to the subclavian vein and posterior to the internal thoracic artery as it enters the thorax where it runs anterior to the root of the lung and into the pericardium between the fibrous and parietal layers.
Both of these nerves supply motor fibres to the diaphragm and sensory fibres to the fibrous pericardium
Pericardium
The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...
, mediastinal pleura
Mediastinal pleura
Different portions of the parietal pleura have received special names which indicate their position: thus, that portion which lines the inner surfaces of the ribs and Intercostales is the costal pleura; that clothing the convex surface of the diaphragm is the diaphragmatic pleura; that which rises...
, and diaphragmatic peritoneum
Peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
.
The pericardiacophrenic arteries and veins travel with their respective phrenic nerves.
The contribution of the 5th cervical nerve may stem from an accessory phrenic nerve. Most often it is a branch of the nerve to the subclavius and may contain numerous phrenic nerve fibers. If the accessory phrenic nerve is present it lies lateral to the main nerve and descends posterior and occasionally inferior to the subclavian vein. The accessory phrenic nerve connects to the phrenic nerve in the thorax or the root of the neck.
Clinical relevance
PainPain
Pain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...
arising from structures served by the phrenic nerve is often "referred
Referred pain
Referred pain is pain perceived at a location other than the site of the painful stimulus. An example is the case of ischemia brought on by a myocardial infarction , where pain is often felt in the neck, shoulders, and back rather than in the chest, the site of the injury...
" to other somatic regions served by spinal nerves C3-C5. For example, a subphrenic abscess beneath the right diaphragm might cause a patient to feel pain in the right shoulder (Kehr's sign
Kehr's sign
Kehr's sign is the occurrence of acute pain in the tip of the shoulder due to the presence of blood or other irritants in the peritoneal cavity when a person is lying down and the legs are elevated. Kehr's sign in the left shoulder is considered a classical symptom of a ruptured spleen...
).
Irritation of the phrenic nerve (or the tissues supplied by it) leads to the hiccough reflex. A hiccough is a spasmodic contraction of the diaphragm, which pulls air against the closed folds of the larynx
Larynx
The larynx , commonly called the voice box, is an organ in the neck of amphibians, reptiles and mammals involved in breathing, sound production, and protecting the trachea against food aspiration. It manipulates pitch and volume...
.
The phrenic nerve must be identified during thoracic surgery
Surgery
Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...
and preserved. It passes anterior to the hilum
Hilum of lung
Above and behind the cardiac impression is a triangular depression named the hilum, where the structures which form the root of the lung enter and leave the viscus. These include the pulmonary artery, superiormost on the left lung, the superior and inferior pulmonary veins, lymphatic vessels and...
of the corresponding lung, and therefore can be identified easily. The right phrenic nerve may be crushed by the vena cava clamp during liver transplantation
Liver transplantation
Liver transplantation or hepatic transplantation is the replacement of a diseased liver with a healthy liver allograft. The most commonly used technique is orthotopic transplantation, in which the native liver is removed and replaced by the donor organ in the same anatomic location as the original...
. Severing the phrenic nerve, or a phrenectomy, will paralyse that half of the diaphragm. Diaphragm paralysis is best demonstrated by sonography. Breathing will be made more difficult but will continue provided the other nerve is intact.
Fortunately, the phrenic nerve arises from the neck (C3-C5) and innervates the diaphragm, which is much lower. Hence, patients who suffer spinal cord injuries below the neck are still able to breathe effectively, despite any paralysis of the lower limbs.
External links
- "Left side of the mediastinumMediastinum
The mediastinum is a non-delineated group of structures in the thorax, surrounded by loose connective tissue. It is the central compartment of the thoracic cavity...
." - "Diagram of the cervical plexus."