Digestive system

Enteric motor system

• “Second brain”

• Governs GI tract from esophagus to anus

• Sensory and motor neurons within submucosal and myenteric plexuses

  

Short reflex

• Local reflex

• Only involves ENS

• Coordinates small segments of the GI tract

Long Reflex

• Involves sensory input to CNS and autonomic output

• Coordinates GI tract motility

• Secretes

• And controls accessory digestive organs

Phases of swallowing

1. Voluntary phase - Tongue pushing food/bolus toward pharynx

2. Pharyngeal phase - Uvula closes nasopharynx; epiglottis closes larynx

3. Esophageal phase - Bolus moves through esophagus

Gastric pit and glands

Parietal cells - HCl and intrinsic factor

Chief cells - Pepsinogen to break down protein

G-Cell - Gastrin

Surface mucous cell - Alkaline mucin

Mucous neck cell - Less alkaline mucin

Parietal cell creation diagram

Quick notes: HCO₃- goes out of cell and Cl- gets pulled into the cell

Gastrin

• Enteroendocrine cells

• Secreted by the G-cells

• Stimulates secretions and motility (more contraction)

Regulation of digestive processes in the stomach

Regulation is stimulated by Reflexes

• Cephalic

• Gastric

• Intestinal

Cephalic reflex

• Cerebral cortex sends sensory signals to medulla oblongata

• Activation: Sensory receptors

• Motor output: Vagus nerve to stomach

• Effect: Stimulated to increase force of contraction and release of secretions

Gastric Reflex

• Initiated by presence of food

• Activation: Stretch receptors in stomach; Chemoreceptors detecting protein or high pH in stomach contents

• Motor output: Increases motor output of vagus nerve

• Effect: Increases force of contraction and release of secretions

Intestinal reflex

• Initiated by acidic chyme in duodenum

• Activation: Chemoreceptors detecting low pH

• Motor Output: Is decreased

• Effect: Decreases force of contraction and release of secretions

Bile

• Secreted by liver; contains no digestive enzymes

• Water, bicarbonate ions, bile salts, pigments, cholesterol, lecithin, mucin

• Neutralizes acidic chyme and eliminates bilirubin

Common bile duct

• Merge between cystic duct of gallbladder and common hepatic duct

• Enters duodenum through major duodenal papilla

Glucagon

• Released by alpha cells of pancreas

• Effect: Increase glycogenolysis, and gluconeogenesis; decrease glycogenesis

Defecation

1. Rectum contents stimulate stretch receptors in rectal wall

2. Sensory input to spinal cord

3. Motor output along parasympathetic neurons

4. Smooth muscle of sigmoid colon contract; internal anal sphincter relaxes

5. Conscious decision to defecate is controlled by cerebral cortex; external anal sphincter relaxes

Fecal transplant

• Low cost and low risk

• Fecal matter is placed in colon of patient via enema or colonoscopy

• Treats: Crohn disease, ulcerative colitis, irritable bowel syndrome

Carbohydrate digestion

• Initiated first by salivary amylase via salivary glands

• Does not occur in stomach

• Resumes in small intestine

Salivary amylase in stomach

• Inactivated in stomach due to low pH

Protein digestion in small intestine

1. Enzyme release: Trypsinogen, chymotrysinogen, and procarboxypeptidase via pancreas

2. Enteropeptidase activates enzymes

3. Enzymes breakdown partial proteins into amino acids

4. Brush border enzymes dipeptidase and aminopeptidase completely breakdown the proteins into amino acids