Question about muscularis layers in GI tract

Q. I am slightly confused about muscularis externa. Is muscularis externa only present in places where muscularis mucosa is not? For example, the oral cavity, upper esophagus, and anal canal. Is it possible for them both to be present? And muscularis is not always present as a layer in the GI tract, correct? When present does it typically present in the same layer that the muscularis mucosa would be?

A. Those are great questions! As you noted, there are two “muscularis” layers in the GI tract: the muscularis mucosa and the muscularis externa.

  • The muscularis mucosa is a thin little layer of smooth muscle that is part of the mucosa (which includes epithelium, lamina propria, and muscularis mucosa). It innervates the inner layers of the mucosa, and it also shows you nicely where the mucosa ends, and the submucosa begins.
  • The muscularis externa is a thick layer of muscle that provides the main structural support all along the GI tract. It sits between the submucosa and the serosa/adventitia.

Basically the GI tract is just a tube with four concentric layers, like this:

From inside to outside, there’s mucosa (epithelium, lamina propria, and muscularis mucosa), submucosa, muscularis externa, and serosa/adventitia.

All levels of the GI tract have this exact structure, except for:

  • the mouth (which I’m not allowed to discuss with you lol)
  • the anus (which is only slightly different…its mucosa is comprised of just epithelium, with no lamina propria or muscularis mucosa).

I think that pretty much covers it.  Having said all that, here are my direct answers (in blue)!

I am slightly confused about muscularis externa. Is muscularis externa only present in places where muscularis mucosa is not? For example, the oral cavity, upper esophagus, and anal canal. No. Both the muscularis mucosa and muscularis externa are present at every level of the GI tract except for mouth and anus (as mentioned above). The upper esophagus btw is not an exception – it has the same four-layered structure you see everywhere else. Is it possible for them both to be present? Yep! And muscularis is not always present as a layer in the GI tract, correct? No – both types of muscularis layers are pretty much always present. When present does it typically present in the same layer that the muscularis mucosa would be? No – the muscularis mucosa is always part of the mucosa (the innermost of the four layers), and the muscularis externa is always located between the submucosa and serosa/adventitia. 

Quick video on stomach vs. small intestine

Stomach and small intestine can be confusing when you’re first learning this stuff – they both have glands at the bottom and other structures (pits in the stomach, villi in the small intestine) on top, and it can be hard to visualize all of that in 3D.

So I made a short video explaining the difference. I think it’s easier to understand if you can see someone drawing on the pictures. I hope if some of you had the same problem that this will solve it!