The alimentary canal has four layers. Which list below has them in correct order starting from the lumen?
A. Serosa, submucosa, mucosa, muscularis externa
B. Submucosa, mucosa, muscularis externa, serosa
C. Mucosa, submucosa, muscularis externa, serosa
D. Muscularis externa, mucosa, submucosa, serosa
The Correct Answer and Explanation is :
The correct answer is:
C. Mucosa, submucosa, muscularis externa, serosa
Explanation:
The alimentary canal, also known as the gastrointestinal (GI) tract, consists of several distinct layers. These layers, listed from the innermost (closest to the lumen) to the outermost, are organized as follows:
- Mucosa: This is the innermost layer, directly in contact with the contents of the lumen. The mucosa serves as a protective barrier, and it also facilitates absorption and secretion. It is composed of three sub-layers:
- Epithelium: This layer comes into direct contact with the food and is responsible for absorption and secretion.
- Lamina propria: A layer of connective tissue that supports the epithelium and houses blood vessels and immune cells.
- Muscularis mucosae: A thin layer of smooth muscle that helps move the mucosal layer and increase surface area.
- Submucosa: Located just outside the mucosa, the submucosa contains larger blood vessels, lymphatics, and nerves. It supports the mucosa and allows it to be flexible. The submucosa also contains glands in certain parts of the GI tract (such as the stomach) that secrete enzymes or mucus.
- Muscularis Externa: This layer is composed of smooth muscle and is responsible for the motility of the GI tract. It has two sub-layers:
- Inner circular layer: Constriction of this layer narrows the lumen.
- Outer longitudinal layer: Contraction of this layer shortens the length of the tract.
Together, these muscle layers allow for peristalsis, the wave-like contractions that propel food through the digestive system.
- Serosa (or Adventitia in certain regions): The serosa is the outermost layer, composed of connective tissue covered by a layer of epithelium. It provides protection and structural support. In certain regions of the GI tract, the serosa is replaced by adventitia, which does not have an epithelial layer but is still connective tissue that anchors the GI tract to surrounding structures.
This layered structure ensures the efficient functioning of the alimentary canal by providing protection, enabling movement, and facilitating absorption.