Digestive System Overview

Digestive System Overview

• The alimentary canal, or gastrointestinal (GI) tract, digests and absorbs food.
• The alimentary canal includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory digestive organs include teeth, tongue, gallbladder, salivary glands, liver, and pancreas.

Digestive Process

• There are six essential activities: ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.
• Ingestion involves taking food into the digestive tract.
• Propulsion includes swallowing and peristalsis (waves of muscle contraction and relaxation).
• Mechanical digestion involves chewing, mixing, and churning food.
• Chemical digestion is the catabolic breakdown of food by enzymes.
• Absorption involves moving nutrients from the GI tract to the blood or lymph.
• Defecation is the elimination of indigestible solid wastes.

Enteric Nervous System

• A specific nervous system of the GI tract.
• Made of about 100 million neurons along the gut wall, starting from the esophagus to the anus.
• Divided into two plexuses:
  • Myenteric plexus: controls GI tract movement.
  • Submucosal plexus: regulates GI secretions.

General Organization of the Gastrointestinal Tract

• The GI tract has four layers: mucosa, submucosa, muscularis propria, and serosa/adventitia.
• The mucosa consists of epithelium, lamina propria, and muscularis mucosa.
• The submucosa contains Meissner's (submucosal) plexus.
• The muscularis propria contains Auerbach's (myenteric) plexus, circular muscle, and longitudinal muscle.
• Serosa/adventitia is the outermost layer.

Salivation

• Saliva is secreted by serous and mucous cells in salivary glands.
• Saliva is mostly water (97-99.5%), hypo-osmotic, and slightly acidic.
• It contains electrolytes (Na+, K+, Cl-, PO42-, HCO3-), digestive enzymes (salivary amylase), proteins (mucin, lysozyme, defensins, IgA), and metabolic wastes (urea, uric acid).
• The salivary glands are structured into.
  • Extrinsic glands: parotid, submandibular, and sublingual.
  • Intrinsic glands (buccal glands): scattered around oral mucosa.
• The roles of saliva are: moistening, dissolving food chemicals, aiding in bolus formation, cleansing the mouth, and breaking down starch.

Nervous Regulation of Salivary Secretion

• Sympathetic regulation originates from T1 and T2, passing through the superior cervical ganglion and terminating at the salivary glands.
• Sympathetic stimulation produces trophic (concentrated) saliva rich in mucus and enzymes, and vasoconstriction.
• Parasympathetic regulation involves the Facial (VII) and Glossopharyngeal (IX) nerves activating the submandibular and sublingual glands and the parotid gland.
• This increases salivary secretions. These are further regulated by conditioned and unconditioned reflexes.

Taste

• Taste buds are primarily located on the tongue, embedded in papillae of the tongue.
• Taste sensations include sweet, sour, salty, and bitter.
• Mechanisms of taste transduction involve cranial nerves VII and IX carrying impulses to the solitary nucleus of the medulla, then to the thalamus, before reaching the gustatory cortex (taste) and limbic system (taste intensity).

Swallowing (Deglutition)

• Deglutition is the act of transporting food from the buccal cavity to the stomach.
• It involves three stages:
  • Buccal stage: tongue pushes bolus against soft palate and back of mouth triggering reflex.
  • Pharyngeal stage: upper esophageal sphincter relaxes while epiglottis closes to keep swallowed material out of the airways.
  • Esophageal stage: Food moves downward aided by gravity and peristalsis.
• Swallowing is initiated by food in the mouth, where it is masticated and lubricated.
• The bolus is pushed into the pharynx.
• Stimulation of mucosal mechanoreceptors in the pharynx activates afferent nerves, through the glossopharyngeal and vagus nerves to the swallowing center.
• Then, efferent motor nerves run through the vagus to control the pharyngeal muscles.
• The swallowing center coordinates swallowing muscles.

This quiz covers the key components of the digestive system, including the alimentary canal and accessory organs. It also explores the essential activities involved in digestion, such as ingestion, absorption, and defecation. Test your understanding of how the digestive process works and the role of the enteric nervous system.