phrenic nerve

The phrenic nerve is a nerve arising from the cervical, or neck region, of the spine that supplies movement to the diaphragm and some sensation to the chest and upper abdomen. The body contains a left and a right phrenic nerve which follow different paths, though they both begin in the C3, C4, and C5vertebrae of the neck. Of principle importance is the phrenic nerve’s role in causing the diaphragm to contract, a crucial step in the respiratory process.

The thoracic diaphragm is a dome-shaped sheet of muscle that pushes up beneath the lungs to control the contraction and expansion during respiration, or breathing. During an inhale, the diaphragm contracts to expand the lungs and allow air to fill the space. During an exhale, the diaphragm relaxes and expands against the lungs, causing the lungs to contract and push out the used air. The diaphragm may also exert pressure on the abdominal cavity, helping out process such as the excretion of vomit, feces, or urine. The motor fibers in the phrenic nerves signal to the diaphragm when to contract and relax so that these vital processes can take place.

Once leaving the cervical spine, the right phrenic nerve makes its way underneath the scalene muscles of the neck and passes under the clavicle between the subclavean vein on its top and the brachiocephalic vein on its bottom. It then traverses the root of the right lung, a body of vessels and tissue that connects the lung to the heart and trachea. The right phrenic nerve continues traveling towards the feet until it hits the caval opening, a hiatus, or hole, in the diaphragm at the level of the eight thoracic vertebra.

The left phrenic nerve also travels down along the scalene muscle in the neck. It then passes between the subclavean vein and the subclavean artery into the thoracic cavity. It continues to travel downward over the pericardium covering the left ventricle of the heart and then enters the diaphragm. Both the left and right phrenic nerves are accompanied by the left and right pericardiacophrenic nerves, respectively.

The sensory fibers of the phrenic nerve innervate and supply sensation to the pericardium, a sac that holds the heart, and the mediastinal parietal and diaphragmatic pleura, membrane layers that line the thoracic cavity and the diaphragm. The phrenic nerve also supplies sensation to the peritoneum, a membrane layer that lines the abdominal cavity.

Damage to the phrenic nerve, whether due to trauma in the cervical spine, a surgical accident, problems in the surrounding tissue, or another source of trauma can make breathing difficult or impossible. Usually, if one phrenic nerve is left in tact, the patient will retain his ability to breathe, though it will be more labored. Irritation of the phrenicnerve may cause a hiccup reflex, in which the diaphragm suddenly contracts. Irritation may also cause referred pain the tip of the shoulder blade, a symptom known as Kehr’s sign. Kehr’s sign is often a symptom of a ruptured spleen, or an abscess in the diaphragm or surrounding tissues.

The overall shape of the heart is that of a three-sided pyramid located in the middle mediastinum .When viewed from the heart’s apex, the three sides of the ventricular mass are readily apparent . Two of the edges are named. The acute margin lies inferiorly and describes a sharp anglebetween the sternocostal and diaphragmatic surfaces. The obtuse margin lies superiorly and is much more diffuse. The posterior margin is unnamed but is also diffuse in its transition.

One-third of the cardiac mass lies to the right of the midline and two-thirds to the left. The long axis of the heart is oriented from the left epigastrium to the right shoulder. The short axis, which corresponds to the plane of the atrioventricular groove, is oblique and is oriented closer to the vertical than to the horizontal plane

Anteriorly, the heart is covered by the sternum and the costalcartilages of the third, fourth, and fifth ribs. The lungs contactthe lateral surfaces of the heart, whereas the heart abuts ontothe pulmonary hila posteriorly. The right lung overlies the right surface of the heart and reaches to the midline. In contrast, the left lung retracts from the midline in the area of the cardiacnotch. The heart has an extensive diaphragmatic surface inferiorly. Posteriorly, the heart lies on the esophagus and the tracheal bifurcation and bronchi that extend into the lung. The sternum lies anteriorly and provides rigid protection to the heart during blunt trauma and is aided by the cushioning effects of the lungs.

The Pericardium and Its Reflections

The heart lies within the pericardium, which is attached to the walls of the great vessels and to the diaphragm. The pericardiumcan be visualized best as a bag into which the heart has beenplaced apex first. The inner layer, in direct contact with theheart, is the visceral epicardium, which encases the heart andextends several centimeters back onto the walls of the greatvessels. The outer layer forms the parietal pericardium, whichlines the inner surface of the tough fibrous pericardial sack. A thin film of lubricating fluid lies within the pericardial cavity between the two serous layers. Two identifiable recesses lie within the pericardium and are lined by the serous layer. The first is the transverse sinus, which is delineated anteriorly by the posterior surface of the aorta and pulmonary trunk andposteriorly by the anterior surface of the interatrial groove. The second is the oblique sinus, a cul-de-sac located behind the left atrium, delineated by serous pericardial reflections from the pulmonary veins and the inferior vena cava.

Mediastinal Nerves and Their Relationships to the Heart

The vagus and phrenic nerves descend through the mediastinumin close relationship to the heart .They enter through the thoracic inlet, with the phrenic nerve located anteriorly on the surface of the anterior scalene muscle and lying just posterior to the internal thoracic artery (internal mammary artery) at the thoracic inlet. In this position, the phrenic nerve is vulnerable to injury during dissection and preparation of the internal thoracic artery for use in coronary arterial bypass grafting. On the right side, the phrenic nerve courses on the lateral surface of the superior vena cava, again in harm’s way during dissection for venous cannulation for cardiopulmonary bypass. The nerve then descends anterior to the pulmonary hilum before reflecting onto the right diaphragm, where it branches to provide its innervation. In the presence of a left-sided superior vena cava, the left phrenic nerve is applied directly to its lateral surface. The nerve passes anterior to the pulmonary hilum and eventually branches on the surface of the diaphragm. The vagus nerves enter the thorax posterior to the phrenic nerves and course along the carotid arteries. On the right side, the vagus gives off the recurrent laryngeal nerve that passes around the right subclavian artery before ascending out of the thoracic cavity. The right vagus nerve continues posterior to the pulmonary hilum, gives off branches of the right pulmonary plexus, and exits the thorax along the esophagus. On the left, the vagus nerve crosses the aortic arch, where it gives off the recurrent laryngeal branch. The recurrent nerve passes around the arterial ligament before ascending in the tracheoesophageal groove. The vagus nerve continues posterior to the pulmonary hilum, gives rise to the left pulmonary plexus, and then continues inferiorly out of the thorax along the esophagus. A delicate nerve trunk known as thesubclavian loop carries fibers from the stellate ganglion to the eye and head. This branch is located adjacent to the subclavian arteries bilaterally. Excessive dissection of the subclavian artery during shunt procedures may injure these nerve roots and cause Horner syndrome.

The heart beats continuously based on the unique features of its component cells. A cardiac cycle begins when spontaneousdepolarization of a pacemaker cell initiates an action potential;this electrical activity is transmitted to atrial muscle cells, which contract, and to the conduction system, which transmits the electrical activity to the ventricle. Activation depends on components of the cell membrane and cell that induce andmaintain the ion currents that maintain and promote electricalactivation.

Similar to most excitable cells in the body, the activity of cells in the heart is triggered by an action potential. An action potentialis a cyclic activation of a cell consisting of a rapid change in the membrane potential (the electrical gradient across the cell membrane) and subsequent return to a resting membrane potential. This process depends on a selectively permeable cell membrane and proteins that actively and passively direct ion passage across the cell membrane. . The myocyte action potential is characterized by a rapid initial depolarization mediated by fast channels (sodium channels) and then a plateau phase mediated by slow channels (calcium channels). Further details of this process are introduced as their components are described.

The Sarcolemma

The cardiac cell is surrounded by a membrane (plasmalemma, or more specific to a muscle cell, sarcolemma). The structural components of the sarcolemma allow for the origination and then conduction of an electrical signal through the heart with subsequent initiation of the excitation-contraction coupling process. This leads to depolarization of atrial myocytes and, with an appropriate delay, depolarization of ventricular myocytes. The sarcolemma also participates in the regulation of excitation, contraction, and intracellular metabolism in response to neuronal and chemical stimulation. Each of these functions will be considered, with emphasis on the features of the cardiac sarcolemma that differ from the plasmalemma of other cells.

The Phospholipid Bilayer

A phospholipid bilayer provides a barrier between the extracellularcompartment and the intracellular compartment, or cytosol. It is only two molecules thick, consists of phospholipids and cholesterol aligned so that the lipid, or hydrophobic, portion of the molecule is on the inside of the membrane, and the hydrophilic portion of the molecule is on the outside . The sarcolemma which is a phospholipid bilayer, provides a fluid barrier that is particularly impermeable to diffusion of ions. Small lipid-soluble molecules such as oxygen and carbon dioxide diffuse easily through the membrane. The water molecule, although insoluble in the membrane, is small enough that it diffuses easily through the membrane (or through pores in the membrane). Other, slightly larger molecules (e.g., sodium, chloride, potassium, and calcium) cannot diffuse easily through the lipid bilayer and require specialized channels for transport.