D R R A S H M I M IS H R A

INTRODUCTION &FUNCTIONAL ANATOMY OF GIT

INTRODUCTION:

Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed and used as nutrient by the body.

Most of the substances in the diet cannot be utilized as such. These substances must be broken into smaller particles so

that they can be broken into smaller particles and can be absorbed in to blood and distributed to various parts of the body for utilization.

The digestive system is responsible for these functions.

FUNCTIONS OF GIT SYSYTEM

1. Digestive functions. The major function of the gastrointestinal system is to transfer nutrients, minerals and water from external environment to the circulating body fluids for distribution to all the body tissues. This function is accomplished by following processes:

a. Ingestion of food. -It involves Placing the food into the mouth. -Most of the foodstuffs are taken into mouth as large particles mainly made of

carbohydrates, proteins and fats. -Chewing the food into smaller pieces is carried out with the help of teeth and jaw

muscles. This process is called mastication. - Lubrication and moistening of the food is done by the saliva. - Swallowing the food (deglutition). It refers to pushing the bolus of food from mouth

into the stomach. It is accomplished in three phases: oral phase, pharyngeal phase and oesophageal phase.

CONTD….

2. Digestion of food. It refers to the conversion of complex insoluble large organic molecules (food) into soluble, smaller and simpler molecules which can be easily absorbed. Digestion of food is accomplished with the help of hydrochloric acid and digestive juices

containing various enzymes.

3. Absorption of digested food. Absorption of food refers to the movement of digested molecules from the lumen of alimentary canal across its epithelial lining to the blood or lymph. The absorbed water, electrolytes and nutrients are carried away to the various tissues by the circulating blood.

CONTD…

4. Egestion i.e. excretion of unwanted undigested food by the alimentary canal in the form of faeces is called defaecation

NON-DIGESTIVE FUNCTIONS

The main non-digestive function of the gastrointestinal system is its role as an immune system.

The lymphoid tissue in the tonsils, adenoids and Peyer’s patches constitute an important part of body’s immune system.

These provide both the humoral and cellular immunity, which is especially effective against the micro-organisms trying to enter the body from the alimentary canal.

FUNCTIONAL ANATOMY OF THE DIGESTIVE SYSTEM:

Digestive system is made up of gastrointestinal tract(GI TRACT) OR alimentary canal and accessory organs, which help in the process of digestion and absorption.

GI tract is a tubular structure extending from the mouth up to the anus with a length of about 30 feet.

It opens to the external environment on both sides.

GI tract is formed by two types of organs:

1- PRIMARY DIGESTIVE ORGANS.

2- ACCESSORY DIGESTIVE ORGANS.

1- PRIMARY DIGESTIVE ORGANS:

These are the organs where actual digestion takes place. These organs are:

1- Mouth

2-Pharynx

3-Esophagus

4-Stomach

5-Small intestine

6-Large intestine

2- ACCESSORY DIGESTIVE ORGANS:

These are the organs which help the primary digestive organs in the process of digestion.

These organs are:

1- Teeth

2-Tongue

3- Salivary glands

4- Exocrine part of pancreas

5- Liver

6- Gallbladder

ORAL CAVITY

HISTOLOGY:

In general wall of 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:

It is the innermost layer of the wall of GI tract.

It is also called gastrointestinal mucosa or mucous membrane.

It faces the cavity of GI tract.

The mucosa has three layers of structures:

1- Epithelial lining which is in contact with content of GI

tract.

2- Lamina propria formed by connective tissue.

3- Muscularis mucosa formed by the smooth muscle fibers.

2- SUBMUCUS LAYER:

This is present in all part of GI tract except mouth and pharynx.

This layer contains loose collagen fiber, elastic fiber, reticular fibers and few cells of connective tissue.

Blood vessels, lymphatic vessels and nerve plexus are present in this layer.

3- MUSCULAR LAYER

This layer in lips, cheeks and wall of pharynx have skeletal muscle fiber.

The esophagus have both skeletal and smooth muscle fibers.

Wall of stomach and intestine is formed by smooth muscle fiber.

The smooth muscle fiber in stomach are arranged in three layers:

1- INNER OBLIQUE LAYER.

2-MIDDLE CIRCULAR LAYER

3- OUTER LONGITUDINAL LAYER.

The smooth muscle fiber in the intestine are arranged in two layers:

1- INNER CIRCULAR LAYER.

2- OUTER LONGITUDINAL LAYER.

4- serous or fibrous layer:

Outermost layer of the wall of GI tract is either serous or fibrous in nature.

The serous layer is formed by connective tissue and mesoepithelial cells.

It is also called serosa or serous membrane.

It covers stomach, small intestine and large intestine.

The fibrous layer is otherwise called fibrosa.

It is formed by connective tissue.

It covers pharynx and esophagus.

NERVE SUPPLY TO G.I.T

GIT receive two type of nerve supply

1. Intrinsic

1. extrinsic

Intrinsic nerve supply

It is present within the wall of GI tract from esophagus to anus

Nerve fibers are interconnected & form 2 major networks

1. Auerbach’s plexus

2. Meissner’s plexus

These nerve plexus contain cell bodies,processes of nerve cells & receptors

Receptors in the G.I.T are stretch receptors & chemoreceptors.

Intrinsic nerve supply is controlled by the extrinsic nerve supply.

AUERBACH’S PLEXUS

It is also known as myenteric nerve plexus.

Present b/ inner circular muscle layer & the outer longitudinal muscle layer

FUNCTION OF MYENTERIC PLEXUS

It regulate movements of G.I.T

Some fibers accelerate movement by secreting excitatory neurotansmitters such as Ach, serotonin, substance P

Some fiber inhibit by secreting inhibitory neurotransmitters :- vasoactive intestinal polypeptide(VIP),neurotensin

MEISSNER’S NERVE PLEXUS

Also known as submucus nerve plexus.

It is present b/w muscular layer & submucosal layer of G.I.T

The function of these plexus is to regulate secretory function of G.I.T

These nerve fibers cause constriction of blood vessels of GI tract.

EXTRINSIC NERVE SUPPLY

They control the enteric nervous system

They are controlled by ANS.

Both sympathethic & parasympathetic divisions of ANS innervate the GIT

Sympathetic nerve fibers

Preganglionic sympathetic nerve fibers to GI tract arise from lateral horns of spinal cord b/w 5th thoracic & second lumbar segments (T5-L2).

Funtions of sympathetic nerve fibers

Sympathetic nerve fibers inhibit the movement & decrease the secretions of GI tract by secreting the neurotransmitter noradrenaline.

It also cause constriction of sphincters.

Parasympathetic nerve fibers

It pass through the cranial & sacral nerves.

The pre & postganglionic parasympathetic nerve fibers to mouth & salivary glands pass through facial and glossopharyngeal nerves.

Nerves from esophagus, stomach, small intestine & upper part of the large intestine pass through vagus nerve.

The preganglionic nerve fibers to lower part of L.intestine arise from 2nd,3rd,4th sacral segments (S1,S2 & S3) of spinal cord & pass through pelvic nerve.

All preganglionic nerve fiber synapse with postganglionic nerve fibers in the myenteric & submucus plexus.

FUNCTIONS

Parasympathetic nerve fibers accelerate movements & increase the secretions of GIT

The neurotransmitter secreted by the parasympathetic nerve fibers is Ach.

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