Introduction to 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 formed by two types of organs-
- Primary digestive organs.
- Accessory digestive organs.
Primary Digestive Organs-
Primary digestive organs are the organs where actual digestion takes place. Primary digestive organs are-
- Small intestine
- Large intestine
Organs Accessory digestive organs are those which help primary digestive organs in the process of digestion. Accessory digestive organs are-
- Salivary glands
- Exocrine part of pancreas
Wall of Gastrointestinal tract-
Wall of the GI tract is formed by four layers which are from inside out-
- Mucus layer
- Submucus layer
- Muscular layer
- Serous or fibrous layer.
Mucus layer is the innermost layer of the wall of GI tract. It is also called gastrointestinal mucosa or mucus membrane.
Mucosa has three layers of structures-
- Epithelial lining- 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 cell
- 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.
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.
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.
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.
Nerves supply to Gastrointestinal tract-
Two types of nerve supply
- Intrinsic nerve supply- 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-
- Auerbach plexus
- Meissner plexus.
- 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-
- Sympathetic Nerve Fibers Preganglionic sympathetic nerve fibers to GI tract arise from lateral horns of spinal cord between fifth thoracic and second lumbar segments (T5 to L2)
- Parasympathetic nerve fibers to GI tract pass through some of the cranial nerves and sacral nerves. The preganglionic and postganglionic parasympathetic nerve fibers to mouth and salivary glands pass through facial and glossopharyngeal nerves.