Ascending aorta

UPDATED: The ascending aorta is the first and most proximal portion of the aorta. About 5 cm. in length and 3 cm. in diameter at its origin, its proximal end begins at the superior aspect of the outflow tract of of the left ventricle, at the ventriculoaortic junction. 

The ascending aorta ends superiorly at an imaginary horizontal plane (blue dotted line) that passes through the sternal angle (of Louis), continuing distally with the aortic arch. This is an important anatomical landmark, as many surgeons use as the superior border of the ascending aorta an oblique plane that passes proximal to the brachiocephalic trunk (yellow dotted line). Although this landmark could be useful in surgery, it is not anatomically correct.

Since the sternal angle (of Louis) also indicates the superior border of the pericardial sac, it can be said that the ascending aorta is completely intrapericardial, and in surgery the pericardial sac should be the anatomical landmark used to separate the ascending aorta from the aortic arch.

From its point of origin at the ventriculoaortic junction, the aorta presents with a dilated region where the aortic valve is located. The aortic valve is one of the two  semilunar valves of the heart, and the dilation of this region is caused by the presence of the sinuses of Valsalva. This dilated bulbous segment is known as the aortic root.

The dilated, sinus portion, or aortic root segment of the ascending aorta continues superiorly with the tubular portion of the ascending aorta. The area of transition between these two components is marked by a sharp crease known as the sinotubular junction (STJ). The dilation of the aortic root is caused by the presence of the sinuses of Valsalva, named after Antonio Maria Valsalva (1666 - 1723).

The tubular portion of the ascending aorta ascends with an inclination anteriorly and to the right. The ascending aorta presents with a slight anterior bulge causing the transverse section of this aortic segment to be slightly oval.

Only two arteries arise from the ascending aorta, both usually at the aortic root segment, just inferior to the STJ. These are the right coronary artery and the left coronary artery. There are anatomical variations where only one, or up to five different coronary arteries have been described.

Image property of:CAA.Inc.. Artist:Dr. E. Miranda

Layers of the GI tract

The gastrointestinal (GI) tract is formed, with a few exceptions, by four concentric layers of tissue. These are, from deep to superficial, the mucosa, submucosa, muscular (or muscularis) and the serosa layers. This is the simplified version. The fact is that there are more sublayers.

The mucosa layer is characterized by the presence of intestinal villi, which in the stomach and small intestine contribute to absorption of the digested food. The mucosa has a thin layer of connective called the "lamina propia" and external to it a thin layer of smooth muscle, the muscularis mucosae.

The submucosa layer is formed by irregular connective tissue and contains on its most external region a plexus of nerves and neurons, the "submucosal plexus of Meissner", which provides parasympathetic innervation to glands and the muscularis mucosae.

The muscular layer, also known as the "muscularis" is composed of two sublayers of smooth muscle. The deep layer contains circular fibers and is known either as the "circular muscle layer" or the "muscularis interna", the superficial layer contains longitudinal smooth muscle fibers and is known as the "longitudinal muscle layer" or the muscularis externa. Between both muscle layers lies the "myenteric plexus of Auerbach", a layer of sympathetic and parasympathetic nerves and neurons that provides nerve supply to the muscular layer. The combined action of this plexus on the muscular layer is responsible for peristalsis.

The serosa layer is the outer or external layer and is formed by a layer of peritoneum. As such, this layer can also be called "visceral peritoneum".

There are variations from GI organ to GI organ in the arrangement, content, glands, thickness of the layers, etc. The most important differences can be found in the thoracic esophagus and most of the rectum which are devoid of a serosa layer, and in the stomach, where there is a third muscular layer, deep to the circular layer, called the "oblique layer" that contributes fibers to the lower esophageal sphincter found at the esophagogastric junction.

An important point to make is the presence of two interconnected ganglionated plexuses that are represented in the GI tract by the submucosal plexus of Meissner and the myenteric plexus of Auerbach which form the GI intrinsic autonomic nervous component . These two plexuses extend from the esophagus to the rectum and allow for the GI tract to operate almost independently from the extrinsic autonomic nervous system which moderates their activity. Ganglionated plexuses are present in organs that have rhythmic activity, such as peristalsis. Ganglionated plexuses are also present in the heart.

Sources:
1. "The bowel and beyond: the enteric nervous system in neurological disorders" Rao, M & Gershon, M. Nat Rev Gastroenterol Hepatol. 2016 Sep; 13(9): 517–528. 
2. "Advances in Enteric Neurobiology: The “Brain” in the Gut in Health and Disease" Kulkami, S et al. Journal of Neuroscience 31 October 2018, 38 (44) 9346-9354
3. "The Brain-Gut Connection" John Hopklins Health
4. "Think Twice: How the Gut's "Second Brain" Influences Mood and Well-Being" Hadhazy, B. Scientific American February 2010

Images property of:CAA.Inc.Artist:Dr. E. Miranda