Structures in Posterior Mediastinum

Structures in the Posterior Mediastinum Esophagus Is a muscular tube (about 10 inches long) that is continuous with the pharynx in the neck and enters thorax behind the trachea. Has three constrictions: one at the level of the sixth cervical vertebra, where it begins; one at the crossing of the left main stem bronchus; and one at the tenth thoracic vertebra, where it pierces the diaphragm. The left atrium also presses against the anterior surface of the esophagus. Has a physiologic sphincter, which is the circular layer of smooth muscle at the gastroesophageal junction. By clinicians, this is called the inferior esophageal sphincter. Receives blood from three branches of the aorta (the inferior thyroid, bronchial , and esophageal arteries) and from the left gastric and inferior phrenic arteries. Blood Vessels and Lymphatic Vessels Thoracic aorta Begins at the level of the fourth thoracic vertebra. Descends on the left side of the vertebral column and then approaches the median plane to end in front of the vertebral column by passing through the aortic hiatus of the diaphragm. Gives rise to nine pairs of posterior intercostal arteries and one pair of subcostal arteries. The first two intercostal arteries arise from the highest intercostal arteries of the costocervical trunk. The posterior intercostal artery gives rise to a collateral branch, which runs along the upper border of the rib below the space. Also gives rise to pericardial, bronchial (one right and two left), esophageal, mediastinal, and superior phrenic branches. Azygos venous system Azygos (unpaired) vein Is formed by the union of the right ascending lumbar and right subcostal veins. Its lower end is connected to the IVC. Enters the thorax through the aortic opening of the diaphragm. Receives the right intercostal veins, the right superior intercostal vein , and the hemiazygos and accessory hemiazygos veins. Arches over the root of the right lung and empties into the SVC , of which it is the first tributary. Hemiazygos vein Is formed by the union of the left subcostal and ascending lumbar veins. Its lower end is connected to left renal vein. Ascends on the left side of the vertebral bodies behind the thoracic aorta, receiving the ninth, tenth, aeleventh posterior intercostal veins Accessory hemiazygos vein Begins at the fourth or fifth intercostal space; descends, receiving the fourth or fifth to eighth intercostal veins; turns to the right; passes behind the aorta; and terminates in the azygos vein. Superior intercostal vein Is formed by a union of the second, third, and fourth posterior intercostal veins and drains into the azygos vein on the right and the brachiocephalic vein on the left. Posterior intercostal veins The first intercostal vein on each side drains into the corresponding brachiocephalic vein. The second, third, and often the fourth intercostal veins join to form the superior intercostal vein. The rest of the veins drain into the azygos vein on the right and into the hemiazygos or accessory hemiazygos veins on the left. Lymphatics Thoracic duct Begins in the abdomen at the cisterna chyli , which is the dilated junction of the intestinal, lumbar, and descending intercostal trunks. Is usually beaded because of its numerous valves and may often be double or even triple. Drains the lower limbs, pelvis, abdomen, left thorax, left upper limb, and left side of the head and neck. Passes through the aortic opening of the diaphragm and ascends through the posterior mediastinum between the aorta and the azygos vein. Arches laterally over the apex of the left pleura and between the left carotid sheath in front and the vertebral artery behind, runs behind the left internal jugular vein, and then usually empties into the junction of the left internal jugular and subclavian veins. Right lymphatic duct Drains the right sides of the thorax, upper limb, head, and neck. Empties into the junction of the right internal jugular and subclavian veins. Autonomic Nervous System in the Thorax Is composed of motor, or efferent, nerves through which cardiac muscle, smooth muscle , and glands are innervated. Involves two neurons: preganglionic and postganglionic. It may include general visceral afferent (GVA) fibers because they run along with general visceral efferent (GVE) fibers . Consists of sympathetic (or thoracolumbar outflow) and parasympathetic (or craniosacral outflow) systems. Consists of cholinergic fibers (sympathetic preganglionic, parasympathetic preganglionic, and postganglionic) that use acetylcholine as the neurotransmitter and adrenergic fibers (sympathetic postganglionic) that use norepinephrine as the neurotransmitter (except those to sweat glands) Sympathetic nervous system Enables the body to cope with crises or emergencies and thus often is referred to as the fight or division Contains preganglionic cell bodies that are located in the lateral horn or intermediolateral cell column of the spinal cord segments between T1 and L2. Has preganglionic fibers that pass through the white rami communicantes and enter the sympathetic chain ganglion, where they synapse. Has postganglionic fibers that join each spinal nerve by way of the gray rami communicantes and supply the blood vessels, hair follicles (arrector pili muscles), and sweat glands. Increases the heart rate , dilates the bronchial lumen , and dilates the coronary arteries. Sympathetic trunk Is composed primarily of ascending and descending preganglionic sympathetic fibers and visceral afferent fibers, and contains the cell bodies of the postganglionic sympathetic (GVE) fibers. Descends in front of the neck of the ribs and the posterior intercostal vessels. Contains the cervicothoracic (or stellate) ganglion , which is formed by fusion of the inferior cervical ganglion with the first thoracic ganglion. Enters the abdomen through the crus of the diaphragm or behind the medial lumbocostal arch. Gives rise to cardiac, pulmonary, mediastinal, and splanchnic branches. Is connected to the thoracic spinal nerves by gray and white rami communicantes. Rami communicantes White rami communicantes Contain preganglionic sympathetic GVE (myelinated) fibers with cell bodies located in the lateral horn (intermediolateral cell column) of the spinal cord and GVA fibers with cell bodies located in the dorsal root ganglia. Are connected to the spinal nerves, limited to the spinal cord segments between T1 and L2. Gray rami communicantes Contain postganglionic sympathetic GVE (unmyelinated) fibers that supply the blood vessels, sweat glands, and arrector pili muscles of hair follicles. Are connected to every spinal nerve and contain fibers with cell bodies located in the sympathetic trunk. Thoracic splanchnic nerves Contain sympathetic preganglionic GVE fibers with cell bodies located in the lateral horn (intermediolateral cell column) of the spinal cord and GVA fibers with cell bodies located in the dorsal root ganglia. Greater splanchnic nerve Arises usually from the fifth through ninth thoracic sympathetic ganglia, perforates the crus of the diaphragm or occasionally passes through the aortic hiatus, and ends in the celiac ganglion. Lesser splanchnic nerve Is derived usually from the tenth and eleventh thoracic ganglia, pierces the crus of the diaphragm, and ends in the aorticorenal ganglion. Least splanchnic nerve Is derived usually from the twelfth thoracic ganglion, pierces the crus of the diaphragm, and ends in the ganglia of the renal plexus. Parasympathetic nervous system Promotes quiet and orderly processes of the body, thereby conserving energy. Is not as widely distributed over the entire body as sympathetic fibers; the body wall and extremities have no parasympathetic nerve supply. Has preganglionic fibers running in cranial nerves (CNs) III, VII, and IX that pass to cranial autonomic ganglia (i.e., the ciliary, submandibular, pterygopalatine, and otic ganglia), where they synapse with postganglionic neurons. Has preganglionic fibers in CN X and in pelvic splanchnic nerves that pass to terminal ganglia, where they synapse. Has parasympathetic fibers in the vagus nerve (CN X) that supply all of the thoracic and abdominal viscera, except the descending and sigmoid colons and other pelvic viscera. These structures are innervated by the pelvic splanchnic nerves (S2–S4). The vagus nerve contains the parasympathetic preganglionic fibers with cell bodies located in the medulla oblongata and the GVA fibers with cell bodies located in the inferior (nodose) ganglion. Decreases the heart rate, constricts bronchial lumen , and causes vasoconstriction of the coronary arteries. Right vagus nerve Gives rise to the right recurrent laryngeal nerve , which hooks around the right subclavian artery and ascends into the neck between the trachea and the esophagus. Crosses anterior to the right subclavian artery, runs posterior to the SVC, and descends at the right surface of the trachea and then posterior to the right main bronchus. Contributes to the cardiac, pulmonary, and esophageal plexuses. Forms the posterior vagal trunk (or gastric nerves) at the lower part of the esophagus and enters the abdomen through the esophageal hiatus. Left vagus nerve Enters the thorax between the left common carotid and subclavian arteries and behind the left brachiocephalic vein and descends on the arch of the aorta. Gives rise to the left recurrent laryngeal nerve , which hooks around the arch of the aorta to the left of the ligamentum arteriosum. It ascends through the superior mediastinum and the neck in a groove between the trachea and esophagus. Gives off the thoracic cardiac branches, breaks up into the pulmonary plexuses, continues into the esophageal plexus, and then forms the anterior vagal trunk. Organ-specific effects of autonomic activity Sympathetic stimulation increases heart rate, dilates coronary arteries, and thus increases blood flow through the vessels, supplying more oxygen and nutrients to the myocardium. Sympathetic nerve also produces bronchodilation and vasoconstriction of pulmonary vessels. Parasympathetic stimulation slows the heart rate, constricts the coronary arteries, produces bronchoconstriction (motor to smooth muscle) and vasodilation of the pulmonary vessels, and increases glandular secretion of the bronchial tree (secretomotor). ◄.....Go back to the Gross Anatomy homepage