The Digestive System

• The digestive system consists of the digestive tract, including the oral cavity, esophagus, stomach, small and large intestines, and anus.
• It also includes associated glands like the salivary glands, liver, and pancreas.
• The digestive system is also known as the gastrointestinal (GI) tract or alimentary canal.
• Its primary function is to obtain molecules from ingested food for the body's maintenance, growth, and energy needs.
• Digestion involves breaking down proteins, complex carbohydrates, nucleic acids, and fats into smaller molecules for absorption in the small intestine lining.
• The large intestine absorbs most water and electrolytes.
• The digestive tract's inner layer provides a protective barrier between the tract's contents and the body's internal environment.

General Structure of the Digestive Tract

• The GI tract has a common structural plan throughout its length.
• It is a hollow tube with a lumen and a wall made of four main layers: mucosa, submucosa, muscularis, and serosa.
• The mucosa consists of an epithelial lining, lamina propria, and muscularis mucosae.
• The lamina propria is loose connective tissue rich in blood vessels, lymphatics, lymphocytes, and smooth muscle cells, often with small glands.
• The muscularis mucosae is a thin smooth muscle layer separating the mucosa from the submucosa and allowing local movements.
• The mucosa is also called a mucous membrane.
• The submucosa contains denser connective tissue, larger blood and lymph vessels, and the submucosal (Meissner) plexus of autonomic nerves.
• It may also contain glands and lymphoid tissue.
• The muscularis (or muscularis externa) is composed of smooth muscle cells in two or more sublayers.
• The inner sublayer has circular fiber orientation, while the outer one is longitudinal.
• The connective tissue between the muscle sublayers contains blood and lymph vessels, as well as the myenteric (Auerbach) nerve plexus.
• Myenteric nerve plexus is composed of autonomic neurons aggregated into small ganglia interconnected by pre- and postganglionic nerve fibers.
• Together with the submucosal plexus, it forms the enteric nervous system that regulates contractions of the muscularis, to mix and propel the luminal contents forward, generated and coordinated by the myenteric plexus.
• The serosa is a thin layer of loose connective tissue rich in blood vessels, lymphatics, and adipose tissue, covered by simple squamous epithelium (mesothelium).

Additional Info

• In the abdominal cavity, the serosa is continuous with mesenteries, which support the intestines.
• Mesenteries are continuous with the peritoneum, which lines the abdominal cavity.
• In areas where the digestive tract is directly attached to adjacent structures (e.g., esophagus), the serosa is replaced by adventitia, a connective tissue layer lacking mesothelium.
• The mucosa and submucosa contain free immune cells and lymphoid nodules that constitute the MALT (mucosa-associated lymphoid tissue).
• The digestive tract contains thousands of microbial species, with the mucosa-associated immune defense system providing backup to the epithelial lining
• The lamina propria is rich in macrophages and lymphocytes, which produce IgA antibodies.
• IgA antibodies undergo transcytosis into the intestinal lumen, forming an IgA complex that provides protection against specific viral and bacterial pathogens.

Oral Cavity

• The oral cavity is lined with stratified squamous epithelium that can be keratinized, partially keratinized, or nonkeratinized.
• The keratinized cell layers resist abrasion and are best in the masticatory mucosa on the gingiva (gum) and hard palate.
• The lamina propria in these regions rests on the periosteum of underlying bone.
• Nonkeratinized squamous epithelium predominates in the lining mucosa of the soft palate, cheeks, floor of the mouth, and pharynx.
• The lining mucosa overlies a thick submucosa with many minor salivary glands that secrete continuously.
• Throughout the oral cavity, the epithelium contains transient antigen-presenting cells and has rich sensory innervation.
• The lips (labia) have a core of striated muscle, making them mobile for ingestion, speech, and communication.
• The lips have three covered surfaces: internal mucous, red vermilion zone, and outer surface.
• The internal mucous surface has lining mucosa with thick, nonkeratinized epithelium and minor labial salivary glands.
• The vermilion zone is covered by thin keratinized stratified squamous epithelium and is transitional between the oral mucosa and skin without salivary or sweat glands.

Tongue

• The tongue is a mass of striated muscle covered by mucosa, which manipulates ingested material during chewing and swallowing.
• Muscle fibers are oriented in all directions, allowing high mobility.
• Connective tissue between the small fascicles of muscle is penetrated by the lamina propria, which makes the mucous membrane strongly adherent to the muscular core.
• The lower surface of the tongue is smooth with typical lining mucosa.
• The dorsal surface is irregular, with hundreds of small protruding papillae on its anterior two-thirds and massed lingual tonsils on its posterior third.
• Papillary and tonsillar areas are separated by the sulcus terminalis.
• Lingual papillae are elevations of the mucous membrane that assume various forms and functions.
• There are four types of papillae: filiform, fungiform, foliate, and vallate.

Tongue Papillae Types

• Filiform papillae are numerous, have an elongated conical shape, are heavily keratinized, and provide a rough surface for food movement during chewing.
• Fungiform papillae are less numerous, lightly keratinized, and interspersed among filiform papillae, with mushroom-shaped vascularized and innervated cores of lamina propria.
• Foliate papillae consist of several parallel ridges on each side of the tongue, anterior to the sulcus terminalis; they are rudimentary in humans, especially older individuals.
• Vallate papillae are the largest, with diameters of 1 to 3 mm, aligned just in front of the terminal sulcus.
• Ducts of serous salivary (von Ebner) glands empty into the groove surrounding each vallate papilla, providing a continuous flow of fluid.
• Secretions from these salivary glands contain a lipase that prevents the formation of a hydrophobic film that would hinder gustation.
• Taste buds are ovoid structures within the stratified epithelium on the tongue's surface, which sample the general chemical composition of ingested material.
• Approximately 250 taste buds are present on each vallate papilla, as well as on fungiform and foliate papillae, and widely scattered elsewhere on the dorsal and lateral surfaces of the tongue.
• A taste bud has 50 to 100 cells, about half of which are elongated gustatory (taste) cells with a 7- to 10-day life span, as well as supportive cells, immature cells, and basal stem cells.
• The base of each bud rests on the basal lamina and is entered by afferent sensory axons that form synapses with the gustatory cells.
• At the apical ends of the gustatory cells, microvilli project toward a 2-µm-wide opening in the structure called the taste pore.
• Molecules (tastants) dissolved in saliva contact the microvilli through the pore and interact with cell surface taste receptors.

Pharynx

• The pharynx is a musculomembranous tube, some 12 to 15 cm in length, with openings into the nasal and oral cavities.
• It communicates laterally with the middle ear and contributes to both the respiratory and digestive tracts.
• The pharynx consists of three regions: nasopharynx, oropharynx, and laryngopharynx.
• The oral pharynx is lined by a stratified squamous non-keratinizing type of epithelium that lacks both muscularis mucosae and submucosa.
• The epithelium rests on a lamina propria with a thick layer of longitudinally oriented elastic fibers.
• The muscularis externa is composed of somewhat irregularly arranged skeletal muscle, the longitudinal and constrictor muscles of the pharynx.