Gastrointestinal Physiology Overview

Specialized tubes within salivary glands that modify primary saliva, reabsorbing ions and adding bicarbonate, leading to a hypotonic, alkaline final product.

Contractile cells surrounding acini that help squeeze saliva out of the gland by contracting.

Isotonic fluid secreted by acinar cells, containing water, electrolytes, and enzymes.

Modified primary saliva that undergoes changes in ion content and pH in the ducts, becoming hypotonic and alkaline.

The parasympathetic nervous system stimulates salivary secretion through cranial nerves VII (facial) and IX (glossopharyngeal), leading to increased salivary flow and a watery saliva.

The primary functions of the gastrointestinal (GI) system are ingestion, digestion, and absorption of food. This process is essential for obtaining nutrients from our diet.

The GI system is a long tube extending from the mouth to the anus, divided into different segments with sphincters controlling the passage of food. Each segment performs specific functions, and the tube's wall has a similar basic structure across different segments.

The three accessory glands of the GI system are the salivary glands, pancreas, and liver. They are not directly part of the digestive tube but contribute their secretions, crucial for digestion and absorption.

Food is sequentially digested as it passes from one segment of the GI tract to the next, beginning with the mouth and ending with the small intestine. Digestion and absorption are completed by the time food leaves the small intestine.

The large intestine focuses on absorbing water and electrolytes, processing undigested material, and preparing waste for elimination as stool. It does not contribute to further digestion or absorption.

The three main functions of the GI system include nutrition, excretion, and defense. Nutrition is the primary function, encompassing ingestion, digestion, and absorption of food. Excretion involves eliminating undigested material and certain wastes. Defense is achieved through physical and chemical mechanisms, combating harmful substances.

Sphincters act as valves within the GI tract, controlling the movement of food from one segment to another. This helps regulate the digestive process and ensures that food is processed correctly in each segment.

The GI tube wall has four basic layers: mucosa, submucosa, muscularis propria, and serosa. Each layer has a specific function, contributing to the overall structure and function of the digestive system.

The gastrointestinal system functions in two main ways: (1) Secretory activities release substances essential for digestion, and (2) Motility activities move food along the digestive tract.

Neural regulation controls the gastrointestinal system through two main branches: (1) Intrinsic innervation (enteric nervous system) acts independently and utilizes neurotransmitters, and (2) Extrinsic innervation acts directly or indirectly via the enteric nervous system, involving parasympathetic and sympathetic nerves.

Humoral regulation involves chemical messengers (peptides) released from the gastrointestinal tract walls. These messengers can act as hormones, paracrines, or neurocrines.

Saliva is secreted by three pairs of salivary glands: Submandibular (60%, mixed), Parotid (25%, mainly serous), Sublingual (5%, mainly mucus).

Saliva performs many crucial functions: keeping oral tissues moist, protecting against irritants, bacteriostatic action, tooth protection, speech, mastication, swallowing, forensic medicine, taste, digestion, and buffering.

Mastication is the process of chewing food, breaking it down into smaller particles for easier swallowing and digestion.

Swallowing is a complex process that moves food from the mouth to the stomach. It involves multiple muscle contractions and coordination.

Acini are the grape-like structures within salivary glands, composed of acinar cells responsible for secreting saliva.

A wave of muscle contraction that pushes food (bolus) through the digestive tract. It starts in the pharynx and continues down the esophagus.

The initial wave of muscle contraction initiated by the swallowing center in the brain. It travels down the esophagus at a speed of 4 cm/s and takes 5-10 seconds.

The parasympathetic nervous system stimulates increased salivary gland activity, resulting in the production of more watery saliva rich in electrolytes and salivary amylase.

The sympathetic nervous system reduces salivary gland activity, producing thicker saliva with less water and electrolytes.

A wave of contraction that occurs if food gets stuck in the esophagus. It's initiated by the distension of the esophageal wall and is controlled by a reflex involving the vagus nerve.

A reflex controlled by the vagus nerve, where the same nerve carries both sensory and motor signals. This is how secondary peristalsis is triggered.

Myoepithelial cells contract in response to both sympathetic and parasympathetic stimulation, helping to push saliva out of the salivary glands.

The parasympathetic nervous system causes vasodilation (widening) of blood vessels in the salivary glands, increasing blood flow to the glands.

A muscular ring at the top of the esophagus that prevents air from entering the stomach. It's also known as the cricopharyngeal sphincter.

A muscular ring at the bottom of the esophagus that prevents stomach contents from backing up. It's also known as the gastroesophageal sphincter.

The sympathetic nervous system causes vasoconstriction (narrowing) of blood vessels in the salivary glands, decreasing blood flow to the glands.

This sphincter relaxes due to signals from the brain, food distension, and certain medications. It allows food to enter the stomach.

Salivary secretion is controlled by neural reflexes, which can be conditioned (learned) or non-conditioned (innate).

Factors that increase salivary secretion include taste (especially sour), smell, mastication, and conditioned reflexes.

Dehydration, fear, sleep, and severe mental effort can inhibit salivary secretion.

Respiration is inhibited and the larynx begins to rise. This helps to push the larynx out of the way of the food bolus and prevent it from entering the respiratory system.

The tongue continues to move backwards, pushing the food bolus against the epiglottis, which bends over the larynx to prevent food from entering the airway. However, the epiglottis isn't essential for swallowing.

The opening of the larynx is closed by contracting the muscles guarding it, and the vocal cords come together to block the passage of food.

The cricopharyngeal sphincter relaxes for about one second, allowing the food bolus to enter the esophagus, and then closes again to prevent reflux.

A wave of contraction produced by the superior constrictor muscle that pushes food down into the esophagus during the pharyngeal phase of swallowing.

The upper sphincter opens, allowing the food bolus to enter the esophagus. The sphincter then immediately closes to prevent air from entering the esophagus.

The palatopharyngeal folds are pulled towards the center of the throat to prevent large objects from passing and damaging the delicate esophageal epithelium.

Coordinated muscle contractions are essential for propelling the food bolus down the digestive tract while preventing food from entering the airway.