👅 Anatomy + Monogastrics

Digestion – Key Points

Definition
 Part of the process in which larger molecules are broken down into smaller component parts

Two Types of Digestion

Mechanical Digestion
  • Occurs through movements of the gastrointestinal (GI) tract
  • Physically breaks food into smaller pieces

Chemical Digestion
  • Involves chemical reactions that break molecular bonds
  • Produces molecules small enough to be absorbed through the intestinal membrane

Digestive System – Functions

1. Prehension
  • Grasping food using lips or teeth

2. Mastication
  • Mechanical grinding and breaking down of food (chewing)

3. Digestion
  • Chemical and physical breakdown of food

4. Absorption
  • Uptake of nutrients and water into the body

5. Elimination
  • Removal of waste products

Digestive System – Structure

Tubing
 Continuous passage that carries food through the body
 Includes mouth, esophagus, stomach, Intestines, and anus

Organs
 Salivary Glands: Produce saliva to moisten food and begin starch digestion
 Liver: Produces bile to help break down fats and processes nutrients from the blood
 Pancreas: Produces digestive enzymes and bicarbonate to aid chemical digestion

Digestive System – Components

Gastrointestinal Tract (GIT): Also known as digestive or alimentary tract
 Includes mouth, pharynx, esophagus, stomach, small and large intestine
 Considered a long tube from mouth to anus

Accessory Organs: Include tongue, salivary glands, liver, pancreas, gall bladder

Digestive System – Parts

Oral Cavity (Mouth): Is the entrance of the gastrointestinal tract
 Contains teeth, tongue, and other structures needed for food ingestion

Functions of the Mouth: Preparation of food for swallowing
 Prehension, mastication, insalivation
 Intraoral assessment of ingested material

Digestive System – Teeth

Teeth: Embedded in the upper maxilla bone and lower mandibula bone

Major Functions of the Teeth: Mastication or mechanical reduction in particle size
 Facilitate swallowing
 Increase surface area for chemical digestion by animal and microbial enzymes

Digestive System – Tongue

Tongue: Occupies the greater part of the oral cavity
 Extends caudally (toward the back of the mouth) into the oropharynx (the part of the throat behind the mouth that connects the oral cavity to the esophagus and larynx)

Structure of the Tongue:
 Extrinsic muscles: Anchor the tongue to surrounding structures like the jaw and hyoid bone, allowing movement
 Intrinsic muscles: Make up most of the tongue’s mass and allow it to change shape, such as curling, flattening, or lengthening

Digestive System – Tongue Regions (Apex)

Anatomical Regions of the Tongue: Divided into three regions

Apex: Is the tip of the tongue
 Freely movable to help manipulate food during chewing and swallowing
 Only caudally (toward the back) attached to the floor of the oral cavity, allowing flexibility
 Has 4 mucosa-covered surfaces: dorsal (top), 2 lateral borders (sides), and ventral surface (bottom)

Digestive System – Tongue Regions (Body)

Body of the Tongue: The middle portion of the tongue between the apex (tip) and the root (back)
 Ventrally anchored (attached on the underside) to the mandible by extrinsic lingual muscles, allowing movement and support
 Has 3 free surfaces that are not attached: dorsal (top) and 2 lateral borders (sides), which help manipulate food during chewing and swallowing

Digestive System – Tongue Regions (Root)

Root: The posterior (back) part of the tongue that connects to the throat
 Attached to and supported by the lingual process (a bony structure that anchors the tongue to the skull and hyoid apparatus)
 Only the dorsal surface (top) is free and covered by mucosa, while the rest is attached, providing stability during swallowing

Digestive System – Tongue Papillae

Papillae: Small projections on the surface of the tongue that have different functions depending on type
 Located on the Lingual Apex (LA), Lingual Body (LB), and Lingual Root (LR)

Functions of Papillae:
 Some have a mechanical function: Help with grooming and moving food toward the pharynx (throat) for swallowing
 Specialized papillae contain taste buds: Allow the animal to detect different taste sensations

Types of Papillae: Conical, Lenticular, Vallate

Digestive System – Tongue Papillae Details

Vallate Papillae: Large papillae located near the back of the tongue
 Often arranged in a V-shape on the back of the tongue
 Contain taste buds

Conical Papillae: Cone-shaped papillae mostly involved in mechanical functions
 Help move food toward the pharynx and assist in grooming

Gustatory Furrow: Groove surrounding vallate papillae
 Houses taste buds and channels dissolved food toward them for taste perception

Taste Buds: Sensory structures within specialized papillae
 Detect different taste sensations such as sweet, salty, sour, bitter, and umami

Digestive System – Salivary Glands

Salivary Glands: Deposit saliva into the oral cavity through ducts
 Extremely important in the digestion process

Composition of Saliva: Water, electrolytes, antibodies, glycoproteins, bicarbonate, and enzymes

Functions of Saliva:
 Starts the breakdown of starchy carbohydrates in the mouth (amylase is especially important in pigs)
 Lubrication: Moistens food to help swallowing
 Antibacterial Action: Helps control oral bacteria
 pH Regulation: Maintains optimal acidity for enzymes and oral health
 Digestion of Carbohydrates: Begins chemical digestion of starches in the mouth

Digestive System – Animals Lacking Salivary Amylase

Domestic Animals Without Salivary Amylase:
 Dogs: Cannot start starch digestion in the mouth
 Cats: Cannot start starch digestion in the mouth
 Ruminants: Rely on microbial fermentation in the stomach and intestines for starch digestion

Digestive System – Salivary Glands Types

Three Main Salivary Glands:

Mandibular Salivary Gland: Located near the lower jaw (mandible)

Parotid Salivary Gland: Located ventral (below) to the ear
 Produces mostly watery saliva and a large part of total saliva

Sublingual Salivary Gland: Located under the tongue

Digestive System – Pharynx (Throat)

Pharynx: Common passage for food and air
 Connects the nasal and oral cavities with the trachea (airway) and esophagus respectively
 During respiration, air is directed to the ventral (front) larynx
 During swallowing, food and water are directed to the dorsal (back) esophagus

Digestive System – Epiglottis

Epiglottis: Part of the laryngeal cartilage
 Covers the glottis (opening of the trachea) during swallowing
 Prevents food and liquids from being aspirated (inhaled into the lungs) into the trachea

Digestive System – Esophagus

Esophagus: Collapsible muscular tube that extends from the pharynx (throat) to the stomach
 Collapsible means the esophagus is normally closed and flattened when empty, but expands when food or liquid passes through
 Travels dorsal (behind) to the trachea until it reaches the thoracic cavity
 Transports food and liquids from the mouth to the stomach using coordinated muscular contractions called peristalsis

Digestive System – Esophagus Muscle Layers

Muscle Layers: Two layers of muscle
 Inner: Circular layer, wraps around esophagus to constrict and push food
 Outer: Longitudinal layer, runs along esophagus to shorten and move food

Muscle Type by Species:
 Skeletal muscle: Horse, cattle, dogs – allows voluntary control of swallowing
 Smooth muscle: Cats, primates – involuntary, controlled automatically

Digestive System – Peristalsis

Peristalsis: Rhythmic, involuntary waves of smooth muscle contraction that move food (ingesta) down through GI tract
 Allows digestion to occur by transporting food from esophagus to stomach

Digestive System – Esophagus Peristalsis Variations

Peristalsis: Most species have one-way peristalsis, moving food toward stomach
 Ruminants have two-way peristalsis: Food can move back toward the mouth (regurgitation)
 Regurgitation allows rechewing of food bolus to decrease particle size
 Improves digestion by rumen microbes, making nutrients easier to break down

Digestive System – Esophagus Sphincters and Layers

Sphincters: First and second sphincters encountered are:
 Upper esophageal sphincter – controls entry of food into esophagus
 Lower esophageal sphincter (cardiac sphincter) – controls entry of food into stomach

Muscle Layers:
 Inner circular layer – acts as sphincter when reaching stomach
 Outer longitudinal layer – helps shorten and move esophagus during peristalsis

Other Features:
 Peristalsis only moves food in coordinated waves
 Mucus-producing glands – lubricate esophagus and ease food passage

Digestive System – Esophagus Species Differences

Muscle Type by Species:
 Dogs and ruminants – Entire esophagus is skeletal muscle, allowing voluntary control of swallowing
 Horse – Cranial 2/3 is skeletal muscle, distal is 1/3 smooth muscle (allows partial voluntary control)
 Pigs – Cranial 1/3 is skeletal muscle, rest smooth muscle
 Primates and cats – Distal esophagus is smooth muscle, involuntary
 Birds and alligators – Entire esophagus is smooth muscle, involuntary

Digestive System – Esophagus Issues

Megesophagus and Obstructions:
 Fluoroscopy and barium swallow can be used to visualize esophagus function
 Horse choking – food stuck in equine esophagus, can block passage to stomach
 Barium swallow in a dog – highlights movement of food and identifies abnormalities like megaesophagus

Digestive System – Monogastric Animals

Monogastric Animals: Also known as non-ruminants
 Include dogs, pigs, horses
 Have a single-chambered stomach

Digestion in Monogastrics:
 Involves secretion of mucus, hydrochloric acid (HCl), and digestive enzymes
 No microbial fermentation occurs in the stomach

Digestive System – Stomach

Stomach: Located at the end of the esophagus
 Placed inside the abdominal cavity

Functions of the Stomach:
 Storage of ingested food
 Mechanical and chemical breakdown of food
  Decreases particle size
  Increases surface area for enzymes
  Makes chemical digestion easier

Digestive System – Stomach Regions

Stomach Tissues: Depending on species, stomach may have glandular, non-glandular, or both types of luminal tissues (epithelial layers that line the internal, open space (lumen) of tubular organs, glands, and ducts, such as the mammary ducts, intestines, and blood vessels)

Glandular Regions: Stomach can be portioned into three main glandular regions (fundus, body, pylorus) for secretion of acid and digestive enzymes

Composite Stomach: Pigs and horses have both glandular and non-glandular tissues
 Glandular region: Secretes acid and enzymes
 Non-glandular region: Mainly storage and mechanical processing of food

Digestive System – Cardia

Cardia: Part of the stomach that connects to the esophagus
 Glands produce thick layer of alkaline mucus
 Protects stomach lining from acidic gastric secretions

Digestive System – Fundus

Fundus: Region of the stomach where rugae (folds in the stomach lining) are more prominent
 Can expand as needed to accommodate food and gas
 Functions to store gas and food if needed
 Produces mucus to protect stomach lining from gastric acid

Digestive System – Pylorus

Pylorus: Connects the stomach to the duodenum
 Produces mucus that coats the stomach lining
 Protects stomach from acidic gastric secretions

Digestive System – Functions of the Simple Stomach

Functions:
 Storage and mechanical breakdown of ingested food
 Controls influx of food into the small intestine
 Secretes digestive juices to break chemical bonds in food
 Produces intrinsic factor, which facilitates Vitamin B12 absorption in the small intestine

Digestive System – Stomach Functions by Region and Species

Cardia: Produces alkaline mucus to protect stomach lining

Species Differences:
 Canine, porcine stomach – entirely glandular, secretes digestive acids and enzymes
 Equine stomach – fundus non-glandular (margo plicatus separates glandular and non-glandular regions)
 Bovine stomach – first three chambers (rumen, reticulum, omasum) non-glandular, primarily for storage and fermentation

Digestive System – Equine Stomach: Margo Plicatus

Margo Plicatus: Ridge that separates non-glandular fundus from glandular regions of equine stomach
 Non-glandular fundus: Mainly storage, no acid or enzyme secretion
 Glandular region: Secretes acid and digestive enzymes for chemical digestion

Images: Endoscopic view and stomach specimen showing margo plicatus

From Corrections Post:

Horses
 Large non-glandular area near the esophagus (dorsal fundus) → prone to ulcers
 Rest of the stomach (cardiac, mostly fundic, pyloric) is glandular

Pigs
 Very small non-glandular area near the esophagus
 Most of the stomach is glandular

Ruminants
 Abomasum is glandular
 Forestomachs (rumen, reticulum, omasum) are non-glandular

Digestive System – Gastric Pits (Dogs)

Gastric Pits: Found in cardia, fundus, and pylorus regions of the stomach

Cell Types and Functions:
 Parietal Cells: Produce hydrochloric acid (HCl) for chemical digestion and killing microbes
 Chief Cells: Produce enzyme precursor pepsinogen, which is converted to pepsin to break down proteins
 Mucous (Neck/Progenitor) Cells: Produce protective mucus to coat the stomach lining and prevent damage from acid

Digestive System – Pyloric Glands

Pyloric Glands: More branched than glands in body or fundus

Cell Types and Functions:
 Mucus-Secreting Cells: Produce protective mucus for stomach lining
 G Cells: Secrete gastrin, a hormone that enters bloodstream and stimulates acid secretion
 Other Cells: Include parietal and chief cells as already described in body/fundus

Digestive System – Autonomic Nervous System Actions

Autonomic Nervous System (ANS): Part of the nervous system that controls involuntary body functions

Sympathetic Nervous System: “Fight or flight” branch of ANS
 Prepares body for stress or activity
 Reduces digestion by slowing peristalsis and decreasing secretions

Parasympathetic Nervous System: “Rest and digest” branch of ANS
 Promotes relaxation and maintenance functions
 Increases digestion by enhancing peristalsis and stimulating secretions

Stomach:
 Sympathetic Stimulation: Reduces peristalsis, decreases gastric secretions
 Parasympathetic Stimulation: Increases peristalsis, increases gastric secretions

Small Intestine:
 Sympathetic Stimulation: Reduces peristalsis, decreases intestinal secretions
 Parasympathetic Stimulation: Increases peristalsis, increases intestinal secretions

Large Intestine:
 Sympathetic Stimulation: Reduces peristalsis, decreases intestinal secretions
 Parasympathetic Stimulation: Increases peristalsis, increases intestinal secretions

Liver:
 Sympathetic Stimulation: Increases conversion of glycogen to glucose, releases glucose into bloodstream
 Parasympathetic Stimulation: Not applicable

Digestive System – Stimulation of Secretions

Anticipation of Meal:
 Parasympathetic nervous system is stimulated
 Release of acetylcholine (Ach)
 Increases flow of saliva
 Triggers release of HCl, pepsinogen, and gastrin in stomach

Food Enters Stomach:
 Local reflexes and vagal reflexes stimulate more Ach release
 Gastrin returns via bloodstream to stimulate additional HCl release

Digestive System – Stomach Contractions and Emptying

Gastric Contractions:
 Distension (stretching) of stomach causes fundus and body to relax
 Distension also triggers peristalsis in pyloric region to mix and push food toward small intestine

Stomach Emptying:
 Controlled by balance between gastrin signaling peristalsis in pylorus and feedback from duodenum slowing emptying
 Gastrin increases smooth muscle contraction at pylorus
 Gastrin also relaxes pyloric sphincter to allow food to enter duodenum

Digestive System – Gastrointestinal Tract (Herbivores)

Gastrointestinal Tract: Anatomical differences exist between species due to eating habits

Herbivores: Eat plants
 Conversion of plant material into usable nutrients depends on fermentation in the gastrointestinal tract
 Examples: Cow, Horse – specialized stomach and intestines adapted for plant digestion