Digestion Introduction .
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| Gastrointestinal Tract Landmark . |
[1]. Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed and used as nutrients by the body.
[2]. 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 absorbed into blood and distributed to various parts of the body for utilization.
[3]. Digestive system is responsible for these functions. Digestive process is accomplished by mechanical and enzymatic breakdown of food into simpler chemical compounds.
[4]. A normal young healthy adult consumes about 1 kg of solid diet and about 1 to 2 liter of liquid diet every day.
[5]. All these food materials are subjected to digestive process, before being absorbed into blood and distributed to the tissues of the body.
[6]. Digestive system plays the major role in the digestion and absorption of food substances.
Functions of Digestive System .
The functions of digestive system include:
[1] . Ingestion or consumption of food substances .
[2] . Breaking them into small particles .
[3] . Transport of small particles to different areas of the digestive tract .
[4] . Secretion of necessary enzymes and other substances for digestion .
[5] . Digestion of the food particles .
[6] . Absorption of the digestive products (nutrients) .
[7] . Removal of unwanted substances from the body.
Functional Anatomy of Digestive System .
[1]. 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 .
[2]. GI tract is a tubular structure extending from the mouth up to anus, with a length of about 30 feet .
[3]. It opens to the external environment on both ends.
[4]. GI tract is formed by two types of organs:
1. Primary digestive organs.
2. Accessory digestive organs.
1. Primary Digestive Organs .
[1]. Primary digestive organs are the organs where actual digestion takes place.
[2]. Primary digestive organs are:
1. Mouth
2. Pharynx
3. Esophagus
4. Stomach
5. Small intestine
6. Large intestine.
2. Accessory Digestive Organs / Secondary Digestive Organs .
[1]. Accessory digestive organs are those which help primary digestive organs in the process of digestion.
[2]. Accessory digestive organs are:
1. Teeth
2. Tongue
3. Salivary glands
4. Exocrine part of pancreas
5. Liver
6. Gallbladder.
Gastrointestinal Tract Wall Layer .
In general, wall of the GI tract is formed by four layers which are from inside to outside :
1. Mucus layer
2. Sub-mucus layer
3. Muscular layer
4. Serous or fibrous layer.
1. Mucus layer
[1]. Mucus layer is the innermost layer of the wall of GI tract.
[2]. It is also called gastrointestinal mucosa or mucus membrane.
[3]. It faces the cavity of GI tract.
[4]. Mucosa has three layer of structures:
1. Epithelial lining
2. Lamina propria
3. Muscularis mucosa.
(1).Epithelial Lining .
[1]. Epithelial lining is in contact with the contents of GI tract.
[2]. The type of cells in this layer varies in different parts of GI tract.
[3]. The inner surface of mouth, surface of tongue, inner surface of pharynx and esophagus have stratified squamous epithelial cells.
[4]. However, mucus membrane lining the other parts such as stomach, small intestine and large intestine has columnar epithelial cells.
(2).Lamina Propria .
Lamina propria is formed by connective tissues, which contain fibro blasts, macrophages, lymphocytes and eosinophils.
(3).Muscularis Mucosa .
[1]. Muscularis mucosa layer consists of a thin layer of smooth muscle fibers.
[2]. It is absent in mouth and pharynx. It is present from esophagus onwards.
2. Sub-mucus layer
[1]. Sub-mucus layer is also present in all parts of GI tract, except the mouth and pharynx.
[2]. It contains loose collagen fibers, elastic fibers, reticular fibers and few cells of connective tissue.
[3]. Blood vessels, lymphatic vessels and nerve plexus are present in this layer.
3. Muscular layer .
[1]. Muscular layer in lips, cheeks and wall of pharynx contains skeletal muscle fibers.
[2]. The esophagus has both skeletal and smooth muscle fibers.
[3]. Wall of the stomach and intestine is formed by smooth muscle fibers.
[4]. Smooth muscle fibers in stomach are arranged in three layers:
i. Inner oblique layer
ii. Middle circular layer
iii. Outer longitudinal layer.
[5]. Smooth muscle fibers in the intestine are arranged in two layers:
i. Inner circular layer
ii. Outer longitudinal layer.
[6]. Auerbach nerve plexus is present in between the circular and longitudinal muscle fibers.
[7]. The smooth muscle fibers present in inner circular layer of anal canal constitute internal anal sphincter.
[8]. The external anal sphincter is formed by skeletal muscle fibers.
4. Serous or Fibrous Layer .
[1]. Outermost layer of the wall of GI tract is either serous or fibrous in nature.
[2]. The serous layer is also called serosa or serous membrane and it is formed by connective tissue and mesoepithelial cells.
[3]. It covers stomach, small intestine and large intestine.
[4]. The fibrous layer is otherwise called fibrosa and it is formed by connective tissue.
[5]. It covers pharynx and esophagus.
Nerve Supply to Gastrointestinal Tract .
GI tract has two types of nerve supply:
1. Intrinsic nerve supply
2.. Extrinsic nerve supply.
1. Intrinsic Nerve Supply-Enteric Nervous System .
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| Structure of intestinal wall with intrinsic nerve plexus . |
[1]. Intrinsic nerves to GI tract form the enteric nervous system that controls all the secretions and movements of GI tract.
[2]. Enteric nervous system is present within the wall of GI tract from esophagus to anus.
[3]. Nerve fibers of this system are interconnected and form two major networks are
1. Auerbach plexus
2. Meissner plexus.
[4]. These nerve plexus contain nerve cell bodies, processes of nerve cells and the receptors.
[5]. The receptors in the GI tract are stretch receptors and chemoreceptors.
[6]. 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 .
[1]. Major function of this plexus is to regulate the movements of GI tract.
[2]. Some nerve fibers of this plexus accelerate the movements by secreting the excitatory neurotransmitter substances like acetylcholine, serotonin and substance P.
[3]. 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 .
Meissner plexus is otherwise called sub-mucus nerve plexus. It is situated in between the muscular layer and submucosal layer of GI tract.
Functions of Meissner plexus .
Function of Meissner plexus is the regulation of secretory functions of GI tract. These nerve fibers cause constriction of blood vessels of GI tract.
2. Extrinsic Nerve Supply .
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| Extrinsic nerve supply to GI tract. |
[1]. Extrinsic nerves that control the enteric nervous system are from autonomic nervous system.
[2]. Both sympathetic and parasympathetic divisions of autonomic nervous system innervate the GI tract .
Sympathetic Nerve Fibers .
[1]. Preganglionic sympathetic nerve fibers to GI tract arise from lateral horns of spinal cord between fifth thoracic and second lumbar segments (T5 to L2).
[2]. From here, the fibers leave the spinal cord, pass through the ganglia of sympathetic chain without having any synapse and then terminate in the celiac and mesenteric ganglia.
[3]. The postganglionic fibers from these ganglia are distributed throughout the GI tract.
Functions of sympathetic nerve fibers .
Sympathetic nerve fibers inhibit the movements and decrease the secretions of GI tract by secreting the neurotransmitter noradrenaline. It also causes constriction of sphincters.
Parasympathetic Nerve Fibers .
[1]. Parasympathetic nerve fibers to GI tract pass through some of the cranial nerves and sacral nerves .
[2]. The preganglionic and postganglionic parasympathetic nerve fibers to mouth and salivary glands pass through facial and glossopharyngeal nerves.
[3]. Preganglionic parasympathetic nerve fibers to esophagus, stomach, small intestine and upper part of large intestine pass through vagus nerve.
[4]. Preganglionic nerve fibers to lower part of large intestine arise from second, third and fourth sacral segments (S2, S3 and S4) of spinal cord and pass through pelvic nerve.
[5]. All these preganglionic parasympathetic nerve fibers synapse with the postganglionic nerve cells in the myenteric and sub-mucus plexus.
Functions of parasympathetic nerve fibers .
[1]. Parasympathetic nerve fibers accelerate the movements and increase the secretions of GI tract.
[2]. The neurotransmitter secreted by the parasympathetic nerve fibers is acetylcholine (Ach).
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