Dissector Answers - Posterior Mediastinum

Learning Objectives and Explanations:

1. Define the boundaries of the posterior mediastinum. (W&B 370-371, N231, TG4-14) 2. Describe the major contents of the posterior mediastinum and their relationships: (W&B 370, 408-416, N172, N192, N193, N194, N206, N207, N212, N232, N237, N238, N240, N260, TG4-35, TG4-35, TG4-36, TG4-36, TG4-37, TG4-37)

Once the thoracic aorta goes through the aortic hiatus in the diaphragm, its name changes to abdominal aorta.

Extra Explanation: The GI tract, including the esophagus, stomach, and intestine, originally develops midline. However, as other systems develop, two things happen. First, the trachea (tough little bugger) pushes the esophagus back against the vertebrae and slightly to the right. Below the sternal angle, after the trachea has bifurcated, the esophagus is "free" to do what it wants, which leads us to the second point. The stomach is pushed anterior and to the left by the growth of the liver. So, in its quest to find the stomach (and the esophageal hiatus at T10 level, which due to the dome-shape of the diaphragm is relatively anterior), the esophagus must move anterior and left.

Bottom line: the esophagus starts slightly right of median and pinned against the spine. It moves left and anteriorly as it descends.

As for the descending aorta, it starts left of midline and somewhat anterior to the vertebrae, due mainly to the way the arch develops (again, that trachea is hogging the midline space). After the bifurcation gets the trachea out of the way, the aorta moves midline. Furthermore, since the aortic hiatus is inferior to the esophageal hiatus (T12 level), it is also posterior to it, again since the diaphragm is a dome. So, in its quest to find the aortic hiatus, the descending aorta also moves posterior to lie on the vertebrae.

Bottom line: the descending aorta starts on the left and anterior to the trachea. It moves midline and posteriorly as it descends.

This set of cross-sections might help (no guarantee!):

3. Describe the organization of the thoracic sympathetic trunk, in addition to its visceral and splanchnic branches. (W&B 413-416, N158, N165, N160, N209, N240, N254, TG4-45, TG4-46, TG8-03, TG8-14)

The thoracic sympathetic trunk is the inferior continuation of the cervical sympathetic trunk, and after piercing the diaphragm, continues as the lumbar sympathetic trunk. The trunk is really just a highway for preganglionic sympathetic nerves (thoracolumbar) that need to reach levels superior or inferior to their level of origin in the spinal cord. The chain ganglia along the trunk are the sites of synapse between preganglionic neurons and postganglionic fibers (see box below).

A preganglionic (presynaptic) sympathetic neuron originates in the lateral horn of the spinal cord gray matter. It leaves the cord via the spinal nerve and enters the trunk via a white ramus communicans. The neuron can: a) immediately synapse in the ganglion, or b) travel up or down the trunk, subsequently synapsing in a ganglion superior or inferior to the level of origination of the neuron. In either case, the postganglionic (postsynaptic) neuron then leaves the ganglion via the gray ramus communicans to rejoin the spinal nerve for the purpose of distribution to its target, either via the ventral primary ramus or the dorsal primary ramus.

Besides the three paths mentioned above (up or down via the trunk, or out via the corresponding spinal nerve) there are two other paths that a sympathetic fiber can take. First, some enter visceral branches that directly innervate the smooth muscle, cardiac muscle, and glands of the thoracic viscera. There are contributions to the cardiac, pulmonary, esophageal, and aortic plexuses. Also, there are thoracic splanchnic nerves (Greek, splanchna = viscera), which are preganglionic fibers that travel into and synapse within the abdomen to provide sympathetic innervation for most of the abdominal viscera. (This can be a confusing point. The thoracic splanchnic nerves innervate abdominal viscera. Also, the label "splanchnic" refers to the innveration of viscera, not to a specific origin or make-up. You will see lumbar, pelvic, and sacral splanchnics that are not necessarily pre-ganglionic sympathetic nerves from the sympathetic chain.) These are listed here:

Nerve Level of Origin Site of Synapse
great thoracic splanchnic n. T5 - T9 celiac ganglion, superior mesenteric ganglion, suprarenal medulla
lesser thoracic splanchnic n. T10 - T11 aorticorenal ganglion
least thoracic splanchnic n. T12 renal plexus

Autonomic Nervous System

The autonomic nervous system, made of the parasympathetic nervous system and the sympathetic nervous system, controls all of the smooth muscle in our body, as well as performing some other specialized functions that will be presented in physiology. Smooth muscle is present throughout the body, in blood vessels and skin, but is most often considered in the context of thoracic, abdominal, and pelvic viscera.

In that it innervates muscle, causing contraction or relaxation, the ANS is similar to the system of skeletal muscle innervation. However, the structures of the two systems is different, hence the necessity to learn words like "synapse", and "ganglion". The nerves to skeletal muscle leave the CNS, travel to the muscle, and do their business at the neuromuscular junction. Sympathetic and parasympathetic nerves, however, take a detour along the way. In the ANS, there are two separate neurons that separate the spinal cord from the target, the preganglionic (presynaptic) and the postganglionic (postsynaptic) neurons. What do you suppose is in the middle of the two? Yep, a ganglion. What happens in a ganglion? A synapse.

Preganglionic sympathetic neurons will leave the spinal cord (in the thoracic and lumbar regions) and travel part-way to their target, either via the sympathetic trunk or a splanchnic nerve. Within that trunk or at the end of that splanchnic nerve, there is a ganglion within which the preganglionic sympathetic neuron will synapse with the postganglionic sympathetic neuron. The postganglionic sympathetic neuron will then travel to the target, usually smooth muscle of an arteriole, but also cardiac muscle, sweat glands, and errector pili muscles (small muscles in the skin that attach to hair follicles and can give you "goose bumps").

Preganglionic parasympathetic neurons usually leave the spinal cord (in the cranial and sacral regions) and travel directly to their target organ, sometimes piggybacking on splanchnic or other nerves. They will synapse in very small ganglia in or on the target organ. The postganglionic parasympathetic neurons, then, do not travel very far to reach their target tissue (usually smooth muscle or glands, but also cardiac muscle).

One more thing: You will come across abbreviations, either in the anatomy tables or some texts, like GVE, SVA, GSE, etc. This is old terminology that divided innervation up into general vs. special, visceral vs. somatic, and efferent vs. afferent. The current way of thinking, and the way that this course will be taught, divides innervation up functionally, as described above, with skeletal, sympathetic, and parasympathetic nerves, some of which are efferent and some of which are afferent.

Cultural enrichment: Check out these sections from the 1918 version of Gray's Anatomy of the Human Body! Some of the terms are (of course) out-of-date, but the illustrations are timeless.

The Trachea and Bronchi - The Lungs - The Pulmonary Veins - The Thoracic Duct - The Vagus Nerve - Surface Anatomy of the Thorax - Surface Markings of the Thorax

Questions and Answers:

4. Are there sympathetic branches to the lung? Along what do they distribute? (N209, N240, TG4-45, TG4-46, TG8-14)

Sympathetic fibers reach the lungs via the pulmonary plexuses, which are located along the roots of the lungs. Pulmonary plexuses are continuous with the cardiac plexus at the tracheal bifurcation. Additional sympathetics reach the pulmonary plexuses via the thoracic visceral nerves, which are branches from T1-T4/T5 sympathetic chain ganglia.

5. Where does the esophagus begin? Where does it pass into the abdomen? Where does it terminate? (N63, N194, N195, N232, N234, N235, TG4-45, TG4-46)

The laryngopharynx "becomes" the esophagus at the C6 level. The esophagus passes through the esophageal hiatus of the diaphragm, which is at the T10 level, to end in the cardiac portion of the stomach.

6. Consider the course, relations, constrictions of the esophagus. (N232, TG4-49, TG7-91)
For course and relations, see above. Constrictions of the esophagus are found at its beginning, at the tracheal bifurcation, and at the esophageal hiatus.
7. Describe the blood supply and venous drainage of the esophagus. Are there venous collaterals to stomach? (N238, TG4-49, TG7-91)

Two or three esophageal arteries branch from the descending aorta. Esophageal veins drain into the azygos system, which eventually reaches the superior vena cava. The esophageal tributaries of the left gastric vein drain the terminal esophagus. Since the gastric veins first drain into the portal system before going to the heart, this part of the esophagus is an important site of portal-caval (portal-systemic) anastomosis in cases of portal hypertension.

8. Do the right and left mediastinal pleurae come together? (N194, TG4-49)

Very low in the posterior mediastinum the esophagus sweeps forward, so there is potential for the right and left mediastinal parietal pleurae to touch one another posterior to esophagus and anterior to aorta. However, typically the anterior deviation of the esophagus is not sufficient to allow enough space for this contact of the pleurae.

9. Through what and at what level does the aorta enter the abdominal cavity? (N194, N195, TG4-37, TG4-38, TG4-39)

The descending thoracic aorta passes through the diaphragm at the aortic hiatus, a passageway between the two diaphragmatic crura, located at the T12 level.

10. What is the subcostal artery? (N264, TG4-49, TG7-91, TG4-39)

Below the 12th rib, there is no intercostal space or intercostal artery, so we call the segmental neurovascular structures subcostal.

11. Completely review the blood supply to an intercostal space. (N33, N191, N192, TG4-43)

Intercostal spaces in general are supplied by posterior and anterior intercostal arteries. Posterior intercostal arteries 3 through 11 are branches of the descending thoracic aorta. The first two posterior intercostal arteries are branches of the highest intercostal artery, which is a branch of the costocervical trunk from subclavian artery. Anterior intercostals are branches of the internal thoracic or the musculophrenic arteries.

12. What are the posterior branches of the posterior intercostal (segmental) arteries? What do they supply? (N171, N172, TG4-39, TG4-08)

The posterior branches of posterior intercostal arteries supply the deep and superficial back muscles, skin of the back, and the vertebral column. They have radicular branches that reach the spinal cord along the dorsal and ventral rootlets.

13. Observe the azygos venous system. If you have two primary veins, do they communicate with one another? How? Where? What is the pattern of venous drainage in your specimen? Are all the veins present? If not, where does the drainage go? (N238, TG4-38, TG4-39, TG4-40)

Hemiazygos and accessory hemiazygos usually cross the midline at T7, 8, or 9 to empty into the azygos vein. There are also connections between the various left-sided venous channels.

14. What vein drains the first intercostal space? Into what? What veins drain into the azygos system? (N238, TG4-38, TG4-39, TG4-40)

Both first posterior intercostal veins drain directly into their respective brachiocephalic veins. Bronchial and esophageal veins drain into the azygos system, the latter being a significant site of portal-caval anastomosis in cases of portal hypertension. (This is the second mention... could be important!)

15. How does the thoracic duct get into the thorax? At what level does it deviate to the left side? (N206, N266, TG4-43, TG4-44)

The thoracic duct enters the chest through the aortic hiatus along the right side of the aorta. It deviates to the left at the level of the sternal angle. (The trachea pushes the esophagus against the vertebral bodies which pushes thoracic duct to the left.)

16. Do you find posterior mediastinal lymph nodes or afferent lymph channels? (N239, TG4-43, TG4-44)

The thoracic duct is usually paralleled by posterior mediastinal nodes.

17. What are bronchomediastinal lymph trunks? (N208, TG4-43, TG4-44)

The lymph from the lungs and chest passes through the paratracheal nodes to form the bronchomediastinal lymph trunks. These drain to the thoracic duct on the left and the right lymphatic duct on the right.

18. Is the sympathetic trunk located within the posterior mediastinum? Does it change positions in different regions of the chest? (N N180, N254, TG4-45, TG4-46, TG8-14)

The sympathetic trunk lies on the heads of the ribs through most of the chest, so it is almost, but not quite, within the posterior mediastinum. It deviates anteromedially as it travels inferiorly to its exit through the diaphragm.

19. How many thoracic ganglia do you find? (N209, N240, TG4-45, TG4-46, TG8-14)

There are typically 12 thoracic ganglia, although the first may be fused with the inferior cervical ganglion to form a cervicothoracic (stellate) ganglion.

20. Identify white and gray rami communicans. What is their significance and distribution? What do they contain? (N164, N180, N209, N240, N254, TG4-45, TG4-46, TG8-14)

Since the preganglionic sympathetic neurons live within the spinal cord at T1 to L2 levels, these are the only levels where white rami communicans are found. Gray rami, on the other hand, are found at every level at which there are spinal nerves. Gray rami carry postganglionic sympathetic fibers back to the ventral primary rami, to be distributed along their branches (and also the branches of the dorsal primary rami).

21. Do you see thoracic visceral nerves to the aorta, esophagus, and trachea? What about to the cardiac and pulmonary plexuses? (N209, N240, TG4-45, TG4-46, TG8-03, TG8-14)

The first 4 or 5 thoracic ganglia (T1-4) give small visceral branches that pass anteroinferiorly to reach the cardiac, pulmonary, esophageal, and aortic plexuses, as well as the trachea.

22. Expose the greater (thoracic) splanchnic nerve. From what does it come? At what level? What types of fibers does it contain? To what does it distribute? (N209, TG4-45, TG4-46, TG8-03, TG8-14)

The greater thoracic splanchnic nerve is made by contributions from sympathetic chain ganglia at T5 to T9 (or T10) levels. These are preganglionic fibers that leave the chest to enter the abdomen. They synapse in the celiac ganglion and innervate the abdominal viscera that is supplied by the celiac trunk.