Question

Describe the structure and function of the layers and sub-layers of the alimentary canal. Include a description of the "enteric nervous system" (the major nerve plexuses associated with the digestive system).

Answer 1

LAYERS OF GASTROINTESTINAL TRACT-- In general, wall of the GI tract is formed by four layers which are from inside out :-

1.Mucus layer

2.Submucus layer

3.Muscular layer

4.Serous or fibrous layer.

1. MUCUS LAYER:-  Mucus layer is the innermost layer of the wall of GI tract. It is also called gastrointestinal mucosa or mucus membrane. It faces the cavity of GI tract. Mucosa has 3 layers of structures:-

i. Epithelial lining ii. Lamina propria iii. Muscularis mucosa.

*Epithelial Lining:- Epithelial lining is in contact with the contents of GI tract. The type of cells in this layer varies in different parts of GI tract. The inner surface of mouth, surface of tongue, inner surface of pharynx and esophagus have stratified squamous epithelial cells. However, mucus membrane lining the other parts such as stomach, small intestine and large intestine has columnar epithelial cells.

*Lamina Propria:- Lamina propria is formed by connective tissues, which contain fibro blasts, macrophages, lymphocytes and eosinophils.

*Muscularis Mucosa:- Muscularis mucosa layer consists of a thin layer of smooth muscle fibers. It is absent in mouth and pharynx. It is present from esophagus onwards.

2. SUBMUCUS LAYER:- Submucus layer is also present in all parts of GI tract, except the mouth and pharynx. It contains loose collagen fibers, elastic fibers, reticular fibers and few cells of connective tissue. Blood vessels, lymphatic vessels and nerve plexus are present in this layer.

3. MUSCULAR LAYER:- Muscular layer in lips, cheeks and wall of pharynx contains skeletal muscle fibers. The esophagus has both skeletal and smooth muscle fibers. Wall of the stomach and intestine is formed by smooth muscle fibers.

Smooth muscle fibers in stomach are arranged in three layers:- i). Inner oblique layer ii). Middle circular layer iii). Outer longitudinal layer.

Smooth muscle fibers in the intestine are arranged in two layers:- i). Inner circular layer ii). Outer longitudinal layer.

Auerbach nerve plexus is present in between the circular and longitudinal muscle fibers. The smooth muscle fibers present in inner circular layer of anal canal constitute internal anal sphincter. The external anal sphincter is formed by skeletal muscle fibers.

4).SEROUS OR FIBROUS LAYER:- Outermost layer of the wall of GI tract is either serous or fibrous in nature. The serous layer is also called serosa or serous membrane and it is formed by connective tissue and mesoepithelial cells. It covers stomach, small intestine and large intestine. The fibrous layer is otherwise called fibrosa and it is formed by connective tissue. It covers pharynx and esophagus.

****NERVE SUPPLY TO GASTROINTESTINAL TRACT :-GI tract has two types of nerve supply:-

I. Intrinsic nerve supply

II. Extrinsic nerve supply.

•INTRINSIC NERVE SUPPLY- – ENTERIC NERVOUS SYSTEM:- Intrinsic nerves to GI tract form the enteric nervous system that controls all the secretions and movements of GI tract. Enteric nervous system is present within the wall of GI tract from esophagus to anus. Nerve fibers of this system are interconnected and form two major networks called --

1. Auerbach plexus

2. Meissner plexus.

These nerve plexus contain nerve cell bodies, processes of nerve cells and the receptors. The receptors in the GI tract are stretch receptors and chemoreceptors. Enteric nervous system is controlled by extrinsic nerves.

1). Auerbach plexus:- Auerbach plexus is also known as myenteric nerve plexus. It is present in between the inner circular muscle layer and the outer longitudinal muscle layer .

•Functions of Auerbach plexus- Major function of this plexus is to regulate the movements of GI tract. Some nerve fibers of this plexus accelerate the movements by secreting the excitatory neurotransmitter substances like acetylcholine, serotonin and substance P. Other fibers of this plexus inhibit the GI motility by secreting the inhibitory neurotransmitters such as vasoactive intestinal polypeptide (VIP), neurotensin and enkephalin.

2). Meissner Nerve plexus:- . It is otherwise called submucus nerve plexus. It is situated in between the muscular layer and the submucosal layer of GI tract.

• Functions of Meissner plexus- is the regulation of secretory functions of GI tract. These nerve fibers cause constriction of blood vessels of GI tract.

•EXTRINSIC NERVE SUPPLY-  Extrinsic nerves that control the enteric nervous system are from autonomic nervous system. Both sympathetic and parasympathetic divisions of autonomic nervous system innervate the GI tract .