Subdivisions of the mediastinum
Superior mediastinum: located between the superior thoracic aperture and the plane between teh sternal angle and T4/T5 vertebrae (transverse thoracic plane)
Inferior mediastinum: located between the transverse thoracic plane and the diphragm (inferior thoracic aperture). The inferior mediastinum is further subdivided into:
What is the significance of the transverse thoracic plane?
The transverse thoracic plane is formed by the sternal angle and T4/T5. Its the division between the superior and inferior mediastinum.
The arch of aorta is above the plane and the pulmonary trunk is below. The trachea bifurcates and the azygos vein enters SVC at teh level of this plane.
Major structures in superior mediastinum
Thymus, brachiocephalic veins, SVC, arch of aorta and branches, trachea, oeseophagus and many other smaller structures.
The thoracic inlet (T1, R1 and manubrium) is transversed by teh trachea, oeseophagus, vesses of upper limba nd head and neck, various nerves, lymphatics, apex of lung and cervical pleura.
It is the continuation between superior mediastinum and root of the neck.
A small amount of CT (fat), sternopericardial ligament and inferior part of thymus.
Loosely connects the sternum to the fibrous pericardium.
Lymphoid tissue, large from birth to puberty. In adulthood mostly replace by adipose tissue. Can be restimulated with autoimmune diseases, endocrine disorders and cancer of the thymus.
Fibrous pericardium (fused to central tendon of the diaphragm and tunica adventitia at teh roots of the great vessels), parietal and visceral layers of the seorus pericardium, with paricardial cavity between.
Innervation and blood supply of pericardium
The fibrous and parietal layer of serous pericardium is supplied by the phrenic nerves (pain fibres) and pericardiacophrenic artery (from internal thoracic artery).
Oesophagus, descending aorta, azygos system of veins, thoracic duct, vagal trunks, sympathetics.
Coninuities between superior mediastinum and posterior mediastinum
Oeseophagus, aorta, numerous arteries, veins, nerves, lymphatics.
Location of oeseophagus.
In the superior mediastinum it is located between the rachea and vertebral bodies.
In the posterior mediastinum it is located between the heart and aorta.
Arteries in middle mediastinum
Arteries in superior mediastinum
- Arch of aorta and small branches (e.g. right bronchial a);
- Brachiocephalic trunk (divides into right common carotid and right subclavian arteries)
- Left common carotid artery
- Left subclavian artery
Arteries in posterior mediastinum
- Descending (thoracic) aorta
- Most posterior intercostal arteries
- Left bronchial arteries
- Oeseophageal arteries
- Superior phrenic artery
Superior mediastinum: superior vena cava, brachiocephalic veins (R, L) from union of internal jugular vein and subclavian vein.
Posterior mediastinum: azygos system of veins, IVC.
Azygos system of veins
Drain posterior intercostal vv and oesophagus and ascending lumbar veins.
Azygos vein and right superior intercostal vein (which joins azygos vein). Supreme intercostal v (or 1st intercostal vein) usually drains into the right brachiocephalic v.
Hemiazygos and accessory hemiazygos veins (to azygos vein) and left superior intercostal veina nd supreme intercostal usually drain to L brachiocephalic vein.
Enters thorax through aortic hiatus in diaphragm, where it continues from cisterna chyli (chyle cistern). It then ascends between the oeseophagus and vertebral bodies, between aorta and azygos vein (post mediastinum), it terminates in the left brachiocephalic vein or left subclavian vein (sup mediastinum).
Motor fibres to diaphragm.
Sensory to pericardium (except visceral), mediastinal pleura, central diaphragmatic pleura and central diaphragmatic peritoneum.
Path of the right phrenic nerve
The right phrenic nerve descends along the SVC, passing anterior to the root of the lung, and between mediastinal pleura and fibrous pericardium finally piercing the diaphragm.
Path of the left phrenic nerve
The left phrenic nerve descends along the arch and ascending aorta passing anterior to the root of the lung, and between mediastinal pleura and fibrous pericardium piercing the diaphragm (thus sensory to the inferior central portion o fthe diaphragm).
Vagus nerves and trunk
Parasympathetic to thoracic viscera and many abdominal viscers.
In the neck the vagus is located within the carotid sheath (with common carotid a, internal jugular v). The right vagus travels to right of trachea and the left vagus to left of arch of aorta.
The vagus nn descend posterior to the roots of lung with the left and right vagus splitting to form most of oesophageal plexus, which then reforms as the anteiror and posterior vagal trunks at the lower oesophagus, which enter abdomen with oeseophagus.
Recurrent laryngeal nerves
Branches of vagus nerve, the left loops aorund the arch of aorta just posterior to ligamentum arteriosum and the right loops aorund the subclavian artery. They ascend to larynx between trachea and oesophagus.
Where are sympathetic preganglionic neuron cell sbodies located?
In T1-L2 lateral horn of spinal cord. Axons exit via the ventral root __> spinal nerve --> white ramus communicans --> sympathetic trunk (paravertebral ganglia).
Some synapse in the trunk with the postganglionic axons re-joining the spinal nerves via gray ramus communcans, or travelling via thoracic cardiac and pulmonary nerves to heart and lungs.
Alternatively preganglionic acons may go through sympathetic ganglia without snaypsing, to form the thoracic splanchnic nerves (greater T5-9(10), lesser T10-11, least T12).
Thoracic splanchnic nerves
Sympathetic preganglionic axons which descend to abdominal cavity to prevertebral ganglia (e.g. coeliac ganglion), where they synapse (posthanglionic axons to abdominal viscera).
Thoracic splanchnic nerves
Sympathetic preganglionic axons which descend to abdominal cavity to prevertebral ganglia (e.g. coeliac ganglion), where they synapse (postganglionic axons to abdominal viscera).
Mix of parasympathetic and sympathetic and visceral sensory axons.