Digestive System Overview

Study Notes

Consists of the digestive tract (gastrointestinal (GI) tract or alimentary canal) and accessory digestive organs.
The digestive tract includes the oral cavity, esophagus, stomach, small and large intestines.
Accessory digestive organs include the salivary glands, liver, gallbladder, and pancreas.

Ingestion: introduction of food and liquid into the oral cavity.
Mastication: dividing solid food into digestible pieces.
Motility: muscular movements of materials through the tract.
Secretion: secretion of mucus, digestive enzymes, acidic and alkaline fluids, and bile.
Hormone release: for local control of motility and secretion.
Chemical digestion: enzymatic degradation of large macromolecules in food to smaller molecules and their subunits.
Absorption: of the small molecules and water into the blood and lymph.
Elimination: of indigestible, unabsorbed components of food.

The wall of the digestive tract has four main layers: mucosa, submucosa, muscularis, and serosa.

Composed of epithelial lining, lamina propria, and muscularis mucosae.
Epithelial lining: varies depending on the region of the digestive tract.
Lamina propria: made up of loose connective tissue rich in blood vessels, lymphatics, lymphocytes, and smooth muscle cells.
Muscularis mucosae: a thin layer of smooth muscle separating the mucosa from the submucosa.

Composed of inner circular and outer longitudinal smooth muscle cells.
Contains blood and lymph vessels.
Myenteric (Auerbach) nerve plexus: generates and coordinates contractions of the muscularis.

A thin sheet of loose connective tissue rich in blood vessels, lymphatics, and adipose tissue.
Covered with a simple squamous covering epithelium or mesothelium.

Enteric nervous system: extends from esophagus to anus and is grouped into two plexuses:
- Myenteric plexus (plexus of Auerbach): found between the longitudinal and circular layers of muscularis.
- Submucosal plexus (Meissner's plexus): found within the submucosa.
Autonomic nervous system:
- Sympathetic NS: decreases GI function, motility, and secretions.
- Parasympathetic NS: increases GI function, motility, and secretions.

Hirschsprung disease (congenital aganglionic megacolon): plexuses in the digestive tract's enteric nervous system are absent.
Chagas disease (trypanosomiasis, infection with the protozoan Trypanosoma cruzi): plexuses in the digestive tract's enteric nervous system are severely injured, disturbing digestive tract motility.

Lined with stratified squamous epithelium, keratinized or nonkeratinized depending on the region.
Keratin layer: protects the oral mucosa from damage during mastication.
Nonkeratinized squamous epithelium covers the soft palate, cheeks, floor of the mouth, and pharynx.

Has three zones:
- Inner zone: lining mucosa with a thick, nonkeratinized epithelium and many minor labial salivary glands.
- Outer zone: thin skin consisting of epidermal and dermal layers, sweat glands, and many hair follicles with sebaceous glands.
- Vermilion zone: covered by very thin keratinized stratified squamous epithelium, lacks salivary or sweat glands, and is kept moist with saliva from the tongue.

Separates the oral cavity from the nasal cavity.
Nasal side: both palates covered by pseudostratified ciliated columnar epithelium with goblet cells and mixed glands.
Oral side:
- Soft palate: non-keratinized stratified squamous epithelium.
- Hard palate: keratinized stratified squamous epithelium.
- Purely mucous glands.