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1. Describe the boundaries and contents of the posterior mediastinum.

The posterior mediastinum is the posterior portion of the 'inferior mediastinum.'

The posterior mediastinum is the area:

• Inferior to the plane of the sternal angle & T4/5 IV disc,

• Superior to the diaphragm (at T5-T12 levels),

• Posterior to the pericardium,

• Anterior to the vertebral column, &

• Medial to the pleurae.

Posterior Mediastinum Contents:

The major contents of the posterior mediastinum typically include:

Nerves:

• • Vagus nn. (CN X)

    • Esophageal plexus

    • Anterior vagal trunk

    • Posterior vagal trunk

  • Sympathetic trunks: T5-12 ganglia

    • Greater splanchnic nn.

    • Lesser splanchnic nn.

    • Least splanchnic nn.

Arteries:

• • Thoracic aorta

    • Parietal brs.

      • Posterior intercostal aa. (3-11)

      • Subcostal aa.

    • Visceral brs.

      • Bronchial aa.

      • Mediastinal aa.

      • Esophageal aa.

      • Pericardial aa.

      • Superior phrenic aa.

Veins:

• • Azygos v.

    • Hemi-azygos v.

    • Accessory hemi-azygos v.

Viscera:

• • Esophagus

Misc.:

• • Thoracic duct

2. Describe the course of the thoracic part of the aorta, organizing its branches according to pattern of dispersal and parietal and visceral targets.

The descending aorta runs from the arch of the aorta (near T3 vertebral level) to its bifurcation into the common iliac aa. (near L4). The descending aorta is divided into thoracic and abdominal parts by the diaphragm (near T12) delimiting the boundary between them. 

The thoracic aorta compliments and anastomose with branches of the internal thoracic a. (from the first part of subclavian a.) to supply blood to the wall of the thorax, and also supplies elements of the thoracic viscera; thus, the thoracic aorta has both parietal (serving the wall) and visceral branches.

Parietal branches:

• • Parietal brs. of the thoracic aorta typically exit the vessel posterolaterally, with posterior intercostal aa. typically servicing the 3rd - 11th intercostal spaces, and the subcostal aa. supplies the body wall below the 12th rib. Parietal brs. of the thoracic aorta typically anastomose with branches of the internal thoracic aa. (eg anterior intercostal brs. and musculophrenic brs.) to serve the thoracic wall.

Visceral branches:

• • Visceral brs. of the thoracic aorta typically exit the vessel anterolaterally to supply structures within the thorax, including the: bronchi, esophagus, elements of the mediastinum, pericardium, and the superior surface of the diaphragm.

Typically, there are three bronchial aa.:

• • two left bronchial aa., and

  • one right bronchial a., which may either originate from the right third posterior intercostal a., or from one of the two left bronchial aa.

The bronchial aa. supply the bronchi, tissues of the lung, bronchopulmonary (hilar) lymph nodes, the proximal portion of the esophagus, etc.

Distal to the bronchial aa. are:

• • mediastinal brs., which serve tracheobronchial lymph nodes and other tissues of the posterior mediastinum,

  • pericardial brs., which serve the posterior portion of the pericardium,

  • esophageal aa., which supply the remainder of the esophagus, and

  • superior phrenic aa., which serve the diaphragm.

3. Describe the azygos venous system, explaining the pattern of tributaries draining the left and right intercostal veins.

The azygos venous system is a series of unpaired (a- = not, zygos = yoke) veins into which the posterior intercostal, subcostal, and lumbar vv. of the thoracic and abdominal walls drain.

Intercostal neurovascular bundles consist of a vein, artery, and nerve (vein nearest to the rib: VAN). Anterior intercostal vv. drain into the internal thoracic vv., whereas posterior intercostal vv. drain into a variety of sources. See table below for details.

The azygos system of veins can exhibit a great deal of variation. The azygos v. typically runs along the right side of the vertebral column in this region, arching over the root of the R. lung to drain into the superior vena cava.

The hemi-azygos and accessory hemi-azygos vv. are inconstant. Both vessels may be present; however, each vessel or both vessels may be absent. The accessory hemi-azygos v. is the superior partner of the azygos v. (‘a goes first’). It typically runs along the left side of the vertebral column between T5-T8 and drains into the azygos v.

The hemi-azygos v. is the main tributary of the azygos v. and is often referred to as the inferior partner of the azygos v. It typically ascends as far as T9 and drains into the azygos v.

4. Describe the major tributaries (trunks) of the thoracic duct, and the course of the thoracic duct through the posterior mediastinum.

The thoracic duct is the major lymphatic vessel and duct of the body, receiving and conducting lymph from all but the right head & neck, right upper limb, and right half of the thorax. The thoracic duct receives lymph from the following sources:

• • Left jugular trunk,

  • Left subclavian trunk,

  • Left bronchomediastinal trunk

  • Intercostal trunks

  • Cisterna chyli (a variable, dilated confluence of the lumbar and visceral lymphatic trunks of the abdominopelvic region).

The pattern of lymphatic trunk drainage is inconstant, thus it is not unusual for trunks to independently enter the vicinity of the venous angles without first joining a duct.

The thoracic duct enters the thoracic cavity through the aortic hiatus (near T12), ascends lateral (right) to the esophagus, transitions from right-to-left between the esophagus and vertebral column (near T5), ascends lateral (left) to the esophagus, and wraps posteriorly behind the subclavian a. (root of neck) to enter the venous system in the vicinity of the left venous angle.

5. Describe the courses and major branches of the vagus nerves (CN X) in the thorax.

Recall that the vagus n. (CN X) exits the jugular foramen, then descends the neck, packaged in the carotid sheath, posterior to the internal jugular v. and common carotid a. At the root of the neck, the vagus n. (as well as superior and inferior cervical cardiac branches, which arise from the vagus in the carotid sheath) emerges from the carotid sheath as it enters the thoracic inlet (superior thoracic aperture).

Right vagus n.

• • The right vagus n. enters the thorax via the superior mediastinum, medial to the phrenic n., and anterior to the subclavian a. Once inside the thorax, the right recurrent laryngeal n. branches from the vagus n., and wraps posteriorly around the right subclavian a. before ascending out of the thorax to the larynx. The right vagus n. descends lateral (right) to the trachea, passing posterior to the root of the right lung. Posterior to the root of the lung, the right vagus supplies bronchial brs. (pre-ganglionic parasympathetic) to the right pulmonary plexus. The right vagus n. further descends the posterior mediastinum, providing branches to the esophageal plexus, while moving to the posterior aspect of the esophagus where it is joined with branches from the left vagus n. to become the posterior vagal trunk. The posterior vagal trunk exits the thorax through the esophageal hiatus (near T10), enters the abdominal cavity, and becomes the major supplier of parasympathetics to the foregut and midgut.

Left vagus n.

• • The left vagus n. enters the thorax via the superior mediastinum, medial to the phrenic n., and anterior to the subclavian a. Once inside the thorax, the left vagus n. descends anterior to the arch of the aorta, and spawns the left recurrent laryngeal n., which wraps posteriorly around the concavity of the arch in the vicinity of the ligamentum arteriosum before ascending out of the thorax to the larynx. The left vagus n. descends posterior to the root of the left lung, supplying bronchial brs. (pre-ganglionic parasympathetic) to the left pulmonary plexus. The left vagus n. further descends the posterior mediastinum, providing branches to the esophageal plexus, while moving to the anterior aspect of the esophagus where it is joined with branches from the right vagus n. to become the anterior vagal trunk. The anterior vagal trunk exits the thorax through the esophageal hiatus (near T10), enters the abdominal cavity, and becomes a minor supplier of parasympathetics to the foregut.

A mnemonic to help remember which vagus n. forms the bulk of which vagal trunk is LARP (left anterior, right posterior).

6. Describe the sympathetic trunks in the thorax. Detail the ganglia of origin, constituent fibers, and targets of the thoracic splanchnic nerves.

Recall that in the cervical region, sympathetic ganglia are condensed into superior, middle, and inferior cervical ganglia, and also that the inferior cervical ganglion frequently coalesces with the 1st thoracic ganglion (=cervicothoracic / stellate ganglion. Aside from the potential of the 1st thoracic ganglion joining the inferior cervical ganglion, thoracic sympathetic ganglia are present at each spinal level (T1/2-T12). This type of regularity is easily observed in the regions posterolateral to the posterior mediastinum, adjacent to the heads of the ribs.

If the T1 ganglion fuses with the inferior cervical ganglion to become the cervicothoracic (stellate) ganglion, thoracic ganglia are successively numbered (2, 3, 4…) inferiorly from the cervicothoracic ganglion. In humans, each thoracic ganglion connects with two successive spinal nerves. Connections between ganglia and spinal nerves take the form of white rami communicantes (nerves → ganglia; pre-ganglionic; myelinated) and gray rami communicantes (ganglia → nerves; post-ganglionic; unmyelinated).

Splanchnic nn. are aggregations of pre-synaptic sympathetic fibers that pass through the trunk without synapsing. These fibers ultimately synapse in pre-vertebral (pre-aortic) ganglia of the abdominal region.

The thoracic splanchnic nn. are aggregates of sympathetic preganglionic fibers to pre-aortic ganglia and plexuses. They include the:

• Greater thoracic splanchnic n.

  • T5-9/10

  • Targets: Celiac ganglia, Aorticorenal ganglion, Suprarenal (adrenal) gland

  • Always present

• Lesser thoracic splanchnic n.

  • T9-10

  • Targets: Aorticorenal ganglion

  • Typically present

• Least thoracic splanchnic n.

  • T12

  • Targets: Renal plexus

  • Rarely present

The pre-aortic ganglia and plexuses are mixed autonomic structures which serve the abdominal viscera and associated vasculature. These will be addressed in more detail in GI sessions to follow

7. Illustrate the parts of the esophagus, describe the pathway of the esophagus from the neck to the abdomen, and discuss how the esophagus is innervated.

The esophagus is a continuation of the laryngopharynx, connecting the pharynx to the stomach. The esophagus is tripartite, consisting of the following regional parts:

• Cervical part

  • Transition from laryngopharynx begins at the inferior margin of the cricoid cartilage (about C6)

  • Upper esophageal sphincter 

    • Cricopharyngeal part of inferior pharyngeal constrictor

    • In the laryngopharynx

  • Consists of striated muscle

    • Innervated by recurrent laryngeal nn.

  • Supplied by the inferior thyroid aa.

• Thoracic part

  • Enters superior mediastinum at root of neck (superior thoracic aperture)

    • Posterior to trachea, anterior to vertebral column

    • Supplied by bronchial aa.

    • Striated muscle

      • Recurrent laryngeal nn.

  • Transitions to posterior mediastinum at thoracic plane

    • Supplied by esophageal aa.

    • Smooth muscle

      • Esophageal plexus (autonomic)

  • Ends at the esophageal hiatus of the diaphragm (about T10)

• Abdominal part

  • Shortest part

  • Deviates left

  • Lower esophageal sphincter

    • Gastro-esophageal (cardiac) sphincter

  • Transitions into the cardiac region of the stomach

The esophagus is innervated by the recurrent laryngeal nn. (striated muscle portions) and the esophageal plexus (smooth muscle portions. The esophageal plexus is a mixed autonomic plexus, largely consisting of fibers from the vagal trunks.

Parasympathetic contributions to to esophageal plexus include:

• Vagus brs.

• Vagal trunks & brs. 

• Effects: Peristalsis

• Associated visceral afferent fibers: feedback/reflex

Sympathetic contributions to to esophageal plexus include:

• Preganglionics:

  • T4-6

• Postganglionics:

  • Middle & Inferior cervical ganglia brs.

    • Vasomotor brs. to upper esophagus via inferior thyroid perivascular plexuses

  • T4-6 & cardiac brs.

    • Vasomotor brs. to esophageal plexus

• Effects: Vasoconstriction

• Associated visceral afferent fibers: visceral pain