Digestive System 1

What are the functions of the digestive system?

• Prehension

• Mastication

• Chemical digestion of food

• Absorption of nutrients and water

• Elimination of wastes

Prehension

grasping of food with the lips or teeth

Mastication

The mechanical grinding and breaking down of food (chewing)

Monogastric animals

Have simple, single stomach

Foregut Fermenters (Ruminants)

Have multiple mixing and fermentation compartments in addition to the stomach

Hindgut Fermenters

Enlarged cecum and colon used for fermentation

What parts make up the general structure of the wall of the digestive tract?

• Mucosa

• Submucosa

• Muscle layer (Muscularis Externa)

• Serosa

Mucosa

Lining of GI tract; made of epithelium and loose connective tissue

Submucosa

Dense connective tissue; may contain glands

Muscle layer (Muscularis Externa)

• External to the submucosa

• Two layers

  • Circular muscle layer

  • Longitudinal muscle layer

What is the muscularis external stimulated by?

Stretch

Serosa

Outermost layer; thin, tough connective tissue

How is epithelial tissue classified?

1. Shape of the cells - Squamous, cuboidal, columnar

2. Number of layers of cells - Simple or stratified

3. Presence of surface specializations - Cilia, etc

Describe stratified squamous epithelium and where it is found in the body:

• Multilayered and thick

• Lines the mouth, esophagus,

Describe simple columnar epithelium and where it is found in the body:

• single layer of tall rectangular cells

• Runs from the junction of the esophagus and stomach through the intestines to junction of rectum and anus

• Nutrients are absorbed through this thin epithelium

How is smooth muscle stimulated?

By stretch

Where is striated (skeletal) muscle found in the GI tract?

• mouth

• pharynx

• cranial portion of esophagus

• external anal sphincter

What does striated muscle in the GI tract allow?

Voluntary control of:

• chewing

• mixing saliva with food

• initiation of swallowing

• initiation of defecation

What is the nervous system of the GI tract called?

the enteric nervous system

What does the enteric nervous system do?

Controls motility and secretory activities of the gut

What are the two distinct neuronal circuits of the enteric nervous system?

• Submucosal plexus (Meissner’s plexus)

• Myenteric plexus (Auerbach’s plexus)

What is the major action of gastrin?

• Causes gastric acid secretion

• Enhances proliferation of cells in stomach wall

What is the major action of cholecystokinin?

• Increases enzyme secretion from pancreas

• Stimulates gallbladder emptying

What is the major action of secretin?

• Causes bicarbonate and water secretion from pancreas and bile ducts

What are the main salivary glands?

• Parotid

• Mandibular

• Sublingual

What does parasympathetic stimulation do to salivation?

• increases salivation in anticipation of eating

• Inhibition of parasympathetic system causes dry mouth

What does sympathetic stimulation do to salivation?

• Blocks salivation

• Fear, nervousness or distress causes dry mouth

What does the enzyme Amylase do for digestion?

• Enzyme in saliva of omnivores

• Breaks down amylose (starch)

Salivary amylase is also called what?

ptyalin

What does the enzyme Lipase do for digestion?

• enzyme that digests lipids

• In saliva of some young animals on high-milk diet

What do buffers do in the oral cavity for digestion?

• Sodium bicarbonate and phosphate in saliva of ruminants neutralize acids in the rumen

What does the esophagus do?

Transports swallowed material from pharynx to stomach

Peristalsis

The pattern of movement in the digestive tract

Describe what peristalsis movement is like:

• Wavelike movement along the digestive tract

• Contraction of circular muscle fibers narrows the tract

• Contraction of longitudinal muscle shortens the tract

Segmental contractions

• “Mixing contractions”

• Are caused by periodic and alternate contractions of circular muscle at neighboring sites in gut

What is the purpose of segmental contractions?

• Mixes digestive tract contents

• Slows movement through tract

• Good for digestion and absorption

What are the 5 different regions of the monogastric stomach?

1. Cardia

2. Fundus

3. Body (Corpus)

4. Pyloric antrum

5. Pylorus

What is the function of the pylorus?

• regulates the movement of chyme from stomach into the duodenum

• Prevents backflow of duodenal contents into stomach

What are the components of gastric juice?

• Hydrochloric acid

• Pepsinogen

• Intrinsic factor

• Gastrin

• Mucus

• Histamine

Gastric secretion is stimulated by what?

• Acetylcholine from parasympathetic neurons

• Gastrin made by G cells

• Histamine secreted by ECL-cells

What are the three phases of gastric secretion?

• Cephalic phase

  • anticipation of eating a meal

• Gastric phase

  • begins when food enters stomach

• Intestinal phase

  • begins when food enters intestine

What is propulsion in the stomach?

Contractions in body of stomach move food to pylorus

What is retropulsion in the stomach?

Pyloric sphincter closes and causes food to move back into pyloric antrum

What two mechanisms allow for the grinding and mixing of food in the stomach?

Propulsion and retropulsion

What does parasympathetic stimulation do for gastric motility?

• Causes fundus to relax

• Causes pyloric antrum to contract

What is the enterogastric reflex?

• Distention of the small intestine or increased acidity in duodenum inhibits stomach contraction

• This delays stomach emptying

What is Gastrin and what does it do?

• Hormone produced by G cells in the stomach

  • Increases production of hydrochloric acid

  • Stimulates contraction of pyloric antrum and constricts the pyloric sphincter

  • inhibits smooth muscle contraction in fundus of stomach

What is Secretin and what does it do?

• a hormone released from the duodenum in response to excess stomach acid in small intestine

• Causes fundus to relax

• Inhibits peristalsis of the body and pyloric antrum of stomach to slow down gastric emptying

What is Cholecystokinin (CCK) and what does it do?

• a hormone released from duodenum in response to fats or proteins

• CCK decreases contraction of the pyloric antrum, body, and fundus of stomach

Stomach emptying is initiated by what?

peristaltic contractions of the stomach

When the stomach is emptying what is happening to the pyloric antrum and pyloric sphincter?

Pyloric antrum is contracted; Pyloric sphincter is relaxed

What hormones released by the small intestine delay gastric emptying?

• Secretin

• Cholecystokinin (CCK)

• gastric inhibitory peptide (GIP)

How many stomachs do ruminants have?

Three forestomachs and one true stomach

What are the names of the ruminants forestomachs?

• Reticulum

• Rumen

• Omasum

What is the name of the ruminants true stomach?

Abomasum

What is the reticulum?

• Smallest, most cranial forestomach compartment

• Separated by the rumen by the rumino-reticular fold

• Lining composed of honeycomb arragement of mucosa

What is the rumen?

• One of the forestomachs, consisting of a series of sacs

• Sacs are separated by muscular folds called pillars

• Pillars mix and stir contents

• Ruminal mucosa has papillae

What does fermentation in the rumen produce?

Volatile fatty acids (VFAs)

What are the volatile fatty acids (VFAs) absorbed by?

Papillae of mucosa in the rumen

How many rumen contractions does a ruminant usually get in a minute?

1-3

Rumen motility is controlled by ——— ——- and ——-

parasympathetic system (vagus nerve) and brainstem

What are the three types of movements of the rumen:

• Mixing contractions (primary contractions) - for moving moving contents back and forth in rumen and reticulum

• Rumination contractions

• Eructation contractions (secondary contractions)

What is the purpose of secondary rumen contractions?

to help remove gas from rumen (eructation or belching)

What is the term for accumulation of gas?

Bloat or acute ruminal tympany

How often is mucus secreted in the digestive tract?

Mucus is secreted continuously

Rumination involves what mechanisms?

regurgitation, re-insalivation, re-mastication (re-chewing), re-swallowing into reticulum

What is gas in the rumen produced by?

ruminal microbes