DIGESTIVE SYSTEM

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DIGESTIVE SYSTEM
The digestive system consists of the digestive tract (alimentary canal) and some accessory digestive organs. Its function is to prepare food for absorption.

DIGESTIVE PROCESSES
The activity of the digestive system can be grouped into the following:
1. INGESTION:- this is the process by which food is taken into the digestive tract via the mouth.

2. PROPULSION:- this the process by which food moves  along the alimentary tract and it includes swallowing and peristalsis process.

3. DIGESTION:- this involves 2 processes: mechanical and chemical digestion
Mechanical digestion:- this involves mechanical breakdown of food by chewing, mixing of food with saliva by the tongue, churning food in the stomach and segmentation in the small intestine.
Chemical digestion:- this involves series of catabolic steps in which complex food molecules are broken down to their chemical building blocks by enzymes secreted into the lumen of the alimentary canal. Chemical digestion begins in the mouth and is essentially completed in the small intestine.

4. ABSORPTION:- this is the process by which digested end products of food passes from the lumen of the digestive tract through the mucosal cells (by active or passive transport) into the blood or lymph for circulation round the body.

5. ELIMINATION:- this is the process by which indigestible substances are removed from the body through the anal canal as feces.

STRUCTURE OF THE DIGESTIVE SYSTEM
Digestive system can be divided into the digestive tract (gastrointestinal tract) and accessory digestive organs.
 The digestive tract (alimentary canal) is a continuous, muscular tube that winds through the body and extends from the mouth to the anus. It is about 9m long.
Its organs include: mouth, pharynx, esophagus, stomach, small intestine and large intestine.
The accessory digestive organs include: teeth, tongue, gall bladder, salivary glands, liver and pancreas.

LAYERS OF THE DIGESTIVE TRACT
Generally, the wall of the digestive tract is formed by 4 principal layers (tunics) with a consistent pattern from the esophagus down to the anal canal with modifications at appropriate levels based on the specific roles they performed.

➡️ Mucosa layer
➡️ Submucosal layer
➡️ Muscularis external
➡️ Serosal (adventitial) layer

MUCOSA
The mucosa also known as mucous membrane is the innermost layer of the alimentary canals. It comprises of 3 sub layers:
• Epithelial lining
• Lamina propria
• Muscularis mucosa

Epithelial lining:- this is in contact with contents of GIT. The cells type varies in different parts of the tract. The inner surface of the mouth, tongue, pharynx and esophagus have stratified squamous epithelium but that of the stomach, small intestine and large intestine is lined by simple columnar epithelium rich in mucus secreting cells.

The main functions of the epithelial lining of the digestive tract are to provide a selectively permeable barrier between the contents of the tract and the tissues of the body, to facilitate the transport and digestion of food, to promote the absorption of the products of this digestion, and to produce hormones that affects the activity of the digestive system. Cells in this layer produce mucus for lubrication and protection.

Lamina propria:- lies immediately below the epithelium, formed by loose areolar connective tissue. Its capillaries nourish the epithelium and absorb digested nutrients. And its mucosa-associated lymphoid tissue (MALT) helps to defend (protects) against viral and bacterial invasion which have free access to the digestive tract.

Muscularis mucosa:- it is usually absent in the mouth and pharynx, it consist of thin inner circular and an outer longitudinal layer of smooth muscle cells separating the mucosa from the submucosa.
 
SUBMUCOSA
This is composed of dense connective tissue with many blood and lymph vessels and a submucosal (meissner’s) nerve plexus. It may also contain glands and lymphold tissue.

MUSCULARIS EXTERNAL
The lips, cheeks and wall of pharynx have skeletal muscle fibres. The esophagus has both skeletal and smooth muscles. Walls of stomach and intestine have only smooth muscle. This layer is responsible for segmentation and peristalsis. The muscle fibres of the outer layer are arranged longitudinally while the inner layer muscles are arranged circularly. Between these 2 layers are blood vessels, lymph vessels and a network of sympathetic and parasympathetic nerves called the myenteric or Auerbach’s plexus. Contraction and relaxation of these muscles occurs in waves (peristalsis) which push contents of GIT onwards and also mix food with digestive juices. In several places along the tract, the circular layer thickens, forming sphincters that act as valves to prevent backflow and control food passage from one organ to the next.

SEROUS LAYER (ADVENTITIAL)
This is the outermost protective layer of the wall of the GI Tract. It is a thin layer of areolar loose connective tissue rich in blood and lymph vessels and adipose tissue, and a simple squamous covering epithelium (mesothelium). In the abdominal cavity, the serosal is continuous with the mesenteries (thin membranes covered by mesothelium on both sides), which support the intestines and with the peritoneum (a serous membranes that lines the cavity wall. In places where the digestive organ is bound to other organs or structures, however, the serosa is replaced by a thick adventitia, consisting of connective tissue, containing vessels and nerves, without the mesothelium.

 NERVE SUPPLY
➡️  Intrinsic nerve supply
➡️ Extrinsic nerve supply

INTRINSIC NERVE SUPPLY
➡️ Myenteric plexus (Auerbach’s plexus)
➡️ Submucosal (meissner’s) plexus
Myenteric plexus controls movement of the GIT, some fibres of this plexus accelerates movement of the tract by secreting excitatory neurotransmitters while some inhibits movements by secreting inhibitory neurotransmitters.
Submucosa plexus regulates the secretory functions of the GI tract and cause vasoconstriction.

EXTRINSIC NERVE SUPPLY
Extrinsic nerve supply of the GIT is from the sympathetic and parasympathetic division of ANS.

Sympathetic supply is from T5- L2 segments of the spinal cord. They inhibit movement of the GIT and decrease its secretion.

Parasympathetic supply is from cranial and spinal nerves.

Parasympathetic supply to mouth and salivary gland is through facial and glossopharyngeal nerve, those to the stomach, small intestine and upper part of large intestine is through the vagus nerve while those to the lower part of the large intestine arise from S2-S4 segment of spinal cord.

The function of the parasympathetic innervations is to accelerate the secretory activities of glands and increase GIT motility.

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