SECOND YEAR PHYSIOLOGY: GASTRO-INTESTINAL TRACT

GASTRO-INTESTINAL TRACT

The Gastro-Intestinal Tract (GIT) consists of hollow muscular tube starting from the oral cavity, where food enters the mouth, continuing through the pharynx, oesophagus, stomach and intestines to the rectum and anus, where undigested food in the form of wastes is expelled. There are various accessory organs that assist the tract by secreting enzymes to help breakdown of food into its component nutrients. Thus, the salivary glands, liver, pancreas and gall bladder have important functions in the digestive system. Food is propelled along the length of the GIT by peristaltic movements of the muscular walls.

WALLS OF GIT

GIT is formed by four layers:

1.       Mucus Layer

2.       Submucous Layer

3.       Muscularis propria

4.       Serosa/Adventitia 

 

Innermost layer of GI tract further composed up of three layers:

1.       Epithelial Lining

2.       Lamina Propria 

3.       Muscularis Mucosa

Mucosa:

·         The epithelium is the most exposed part of mucosa and it is lined with goblet cells in most parts that secret mucus to lubricate the food.

·         Underlying the epithelium is lamina propria that contains myofibroblasts, blood vessels, nerves, and several different immune cells as well as muscularis mucosa.

Submucosa:

·         The submucosa contains nerves including the Meissner’s Plexus, blood vessels, elastic fibres with collagen.

Muscularis:

·         Comprises layers of circular and longitudinal smooth muscles.

·         In between the two layers lies the Myenteric Plexus (Auerbach Plexus)

Serosa/Adventitia:

·         Made of loose connective tissue and coated in mucus so as to prevent any friction damage by rubbing against some other tissue.

COMPONENTS OF GIT

Primary components:

·         Mouth

·         Pharynx 

·         Esophagus

·         Stomach

·         Small Intestine

·         Large Intestine

Accessory Components:

·         Teeth

·         Tongue

·         Salivary glands

·         Exocrine part of pancreas 

·         Liver

·         Gallbladder

SALIVARY & ACESSORY GLANDS OF GIT

The accessory glands of digestive system consist from Salivary glands, Gall Bladder, Pancreas and Liver. The saliva is secreted by three pairs of major salivary gland and some minor salivary glands.

Major Salivary Glands

·         Parotid Gland

·         Submaxillary or submandibular gland 

·         Sublingual gland

Minor Salivary Glands

·         Lingual mucous glands

·         Lingual serous glands

·         Buccal glands

·         Labial glands

·         Palatal gland

NERVE SUPPLY TO GIT

GIT has two types of nerve supply:

1.     Intrinsic

2.     Extrinsic

      Intrinsic Nerve Supply:

·       Intrinsic nerves to GIT from the Enteric Nervous System that controls all secretions and movements of GIT.

·       Nerve fibers are interconnected and they form two major networks:

Auerbach’s Plexus

Meissner’s Plexus

Extrinsic Nerve Supply:

·         Extrinsic Nerves that control the enteric nervous system are from autonomic nervous system.

·         Both sympathetic and parasympathetic nerve fibers innervate GI tract.

·        Sympathetic nerve fibers arise from lateral horn of spinal cord between T5 to L2. From here the fibers pass and terminate in Celiac and mesenteric ganglia.

·         Postganglionic fibers from these ganglia are distributed throughout the GI tract.

·         Parasympathetic nerve fibers to GI tract pass through some cranial nerves (e.g. Vagus Nerve) and sacral nerves.

HORMONES OF GIT

Gut produces different hormones which are mentioned below:

GIT HORMONES:

Hormone	Major actions
Gastrin	↑Acid and pepsin secretion from gastric mucosa; ↑ECL proliferation; ↓cholangiocyte HCO3− secretion
CCK	↑Pancreatic enzyme secretion and gallbladder contraction
Secretin	↑Pancreatic bicarbonate and H2O secretion; ↑cholangiocyte HCO3− secretion
Motilin	↑Intestinal motility
Enteroglucagon	↑Enterocyte proliferation
Somatostatin	↓Gastric acid secretion and gastric motor activity; ↓cholangiocyte HCO3− secretion
Gastric inhibitory peptide	↑Glucose-stimulated insulin secretion and ↓gastric acid secretion
Pancreatic polypeptide	↓Pancreatic bicarbonate secretion
Peptide YY	↓Gastric motor activity and acid secretion

GIT ACTIVE NEUROPEPTIDES:

Neuropeptide	Major actions
Bombesin (gastrin-releasing peptide)	↑ Gastrin release
Substance P	↑Smooth muscle contraction mediates inflammatory response
Vasoactive intestinal peptide	↑Smooth muscle relaxation and pancreatic bicarbonate secretion
Calcitonin gene-related peptide	↑Somatostatin release and smooth muscle contraction; regulates cholangiocyte proliferation