NPB101: Digestive Tract

Hamada lectures 42-45 SP2025

Where does the digestive tract begin and end

From the mouth to anus

Carbohydrate Digestion Order

Starts in the mouth, pauses in the stomach, and continues and finishes in the small intestine. Large intestine does not significantly digest carbs

What are the 4 major tissue layers of the digestive tract? (inner to outer)

1. Mucosa

2. Submucosa

3. Muscularis Externa (smooth muscle)

4. Serosa

Also includes lumen and enteric nervous system (not tissues)

Mucosa

innermost layer of the GI tract; specialized for secretion and absorption and contains epithelial cells

Muscularis externa

Responsible for motility (peristalsis and segmentation)

Serosa

Outermost layer; primilary provides structural support and protection

What is significant about the digestive lumen’s relationship to the external environment?

• It is continuous with the external environment.

• It is exposed to:

  • pH changes

  • Harsh enzymes

  • Microorganisms

What are the 4 basic digestive processes?

1. Motility

2. Secretion

3. Digestion

4. Absorption

Motility

1. Propulsive:

   • Move contents forward through the digestive tract

2. Mixing movements:

   • aid digestion by mixing food with digestive juices

   • facilitate absorption by exposing food to absorbing surfaces

Secretion

Digestive juices into the lumen by exocrine glands

Digestion

biochemical breakdown of food

Absorption

Primarily in the small intestine; transfer of small absorbable units along water, vitamins, electrolytes from the digestive lumen into the blood or lymph

What anatomical structures are involved in the mouth and salivary gland function?

• Lip & Tongue: contain food in mouth; guide food during chewing and swallowing

• Teeth: begin mechanical breakdown by chewing of food

• Palate: roof of oral cavity that separates oral cavity from nasal passage, allows chewing and breathing to occur simultaneously

• Uvula: soft tissue that hands from the rear of the mouth and seals off nasal passage during swallowing

Cephalic Phase Response (CPR)

• Important to prepare food digestion and nutrient absorption efficiently

• Crucial for energy homeostasis and metabolism

What are the effects of no CPR?

• Nutrient absorption delay

• Slow-digesting speed

• longer digestion period

Secrete saliva in response to ____ stimulation

autonomic

What does saliva contain?

1. mucus moisten food and lubricate

   • beings digestion of carbohydrate

2. lysozymes lyse bacteria (antibacterial effect/oral hygiene)

3. Bicarbonate buffer neutralize acids

4. Amylase (begins chemical digestion of carbohydrates)

   Helps facilitate speech

What are the types of salivary secretion reflexes?

1. Simple reflex: Pressure receptors and chemoreceptors

2. Conditional reflex (cephalic phase response): Comes from cerebral reflex when thinking about food/eating food

What is the function of the pharynx in digestion?

• Located at the rear of the mouth

• A common passageway for both digestive and respiratory systems

What is the esophagus and what does it connect?

A muscular tube connecting the pharynx and stomach

What are the two sphincters of the esophagus and their functions?

• Upper sphincter (pharyngoesophageal): normally closed to prevent air entering the esophagus

• Lower sphincter (gastroesophageal): normally closed to prevent acid reflux from the stomach

What is swallowing and how is it controlled?

• An all-or-none reflex

• Triggered by pressure of bolus in pharynx → activates pressure receptors → sends signal to swallowing center in the medulla (brain stem)

What are the stages of swallowing?

• Oropharyngeal stage

  • Moving bolus from mouth through pharynx into esophagus

• Esophageal stage

  • involves propulsive movements

  • peristaltic wave moves bolus through esophagus and into stomach

What are the main regions of the stomach and their features?

• Fundus: above gastroesophageal sphincter

• Body: middle portion

• Antrum: bottom portion; thick smooth muscle; connects to small intestine via pyloric sphincter

What are the primary functions of the stomach?

• Store ingested food and deliver it to small intestine at proper rate

• Secrete HCl and enzymes to start protein digestion

• Produce chyme: a thick liquid of food + gastric secretions

What region of the stomach contains oxyntic mucosa, and what structures does it include?

• Lines the body and fundus

• Located in gastric pits lining the mucosal wall

What are the functions of mucus in the stomach?

• protect the stomach lining from acid and enzymes

• lubricates food and protects mucosal surfaces

What are the 4 types of cells associated with gastric pits?

1. Mucous neck cells (secrete thin, watery mucus)

2. Chief cells (secrete pepsinogen - precursor of pepsin)

3. Parietal (oxyntic) cells (secrete HCl, intrinsic factor)

4. Surface epithelial cells (secrete thick, alkaline mucus)

What do chief cells secrete, and what is the role of it?

• Secrete pepsinogen (inactive)

  • inactivated form prevents damage to stomach lining

• Pepsinogen is cleaved by HCl to form pepsin

• Pepsin:

  • Activates more pepsinogen (autocatalytic)

  • Begins protein digestion by splitting amino acid linkages

  • Works best in acidic environment

What do parietal (oxyntic) cells of the gastric mucosa secrete?

• H+ and Cl- (intrinsic factors) are actively secreted by separate pumps HCl-:

  • Activates pepsinogen

  • Aids breakdown of connective tissue

  • Kills ingested microorganisms

What are parietal (oxyntic) cells of the gastric mucosa intrinsic factor?

• Needed for vitamin B12 absorption

• B12 is essential for normal RBC function

What do mucous neck cells and surface epithelial cells secrete?

• Mucous neck cells: secrete thin, watery mucus

• Surface epithelial cells: secrete thick, alkaline mucus

What do mucous neck cells and surface epithelial cells do?

• Protects against mechanical injury

• Prevents self-digestion

• Shields against acid injury

What secretions are produced in the pyloric gland area (antrum)?

• Mucus

• Pepsinogen

• Gastrin (hormone)

Describe how gastric mixing and emptying works

• Strong antral peristaltic contractions

• mix food with gastric secretions to produce chyme

• propel chyme towards pyloric sphincter where small amount is pushed into the duodenum

• upon reaching the pyloric sphincter, cause the sphincter to close and the remaining chyme is tumbled back into the antrum

• strength can vary depending on a number of factors

What hormone stimulates gastric mixing and secretions?

Gastrin → stimulates parietal and chief cells to produce HCl and pepsinogen

What factors control gastric mixing and emptying? Stomach

1. Volume of chyme

2. Fluidity of chyme

What factors control gastric mixing and emptying? Duodenum

The presence of any of these would inhibit It until the duodenum has coped with factors already present. Chyme inhibits. Gastrin promotes

1. Fat

2. Acid

3. Hypertonicity

4. Distention

What is the primary site of digestion and absorption?

• Small intestine

• No digestion occurs after the small intestine

What is segmentation in the small intestine?

• Type of oscillating, ringlike contraction (not peristaltic wave)

• Mixes and propels chyme

• absent btwn meals but vigorous during and after a meal

• Chyme moves forward because contraction frequency gradient gradually decreases along the intestine

What enzymes aid digestion in the small intestine?

• lumen is accomplished by pancreatic enzymes

• Bile secretions enhance fat digestion

Pancreas and Liver Secretions

Provide secretions that mix with the contents of the duodenal lumen

What does the exocrine pancreas secrete and which cells are involved? Duodenum

• Acinar cells: release 3 types of enzymes

  1. Proteolytic enzymes (trypsinogen)

     • Activated by enterokinase in the duodenum → becomes trypsin (active form)

     • Trypsin autocatalyzes more trypsinogen

     • Trypsin inhibitor prevents premature activation

  2. Pancreatic amylase (carbohydrate digestion)

  3. Pancreatic lipase (fats) → only fat-digesting enzyme in GI tract

• Duct cells: release sodium bicarbonate (NaHCO₃) to create alkaline environment

Chyme in the duodenum triggers the release of what 2 major enterogastrones?

• Secretin: released in response to acidic chyme

• Cholecystokinin (CCK): triggered by fat and protein in chyme

• All 3 enzymes (proteolytic, amylase, lipase) are packaged into common vesicles → fat/protein triggers release of all 3

• Carbohydrates do not trigger enzyme release

What are the major components and functions associated with bile?

• Bile salts: aid digestion and absorption of fats in the duodenum

• Gallbladder: stores bile

• Sphincter of Oddi: controls bile entry into duodenum

• Bile salts are recycled

What anatomical regions are bypassed in Roux-en-Y gastric bypass surgery?

• Antrum

• Duodenum

  • Reduces absorption of nutrients and caloric intake that is mostly done within small stomach (esp within duodenem)

What does bile act on?

Fat in the duodenum

What is the brush border and what enzymes does it contain?

• microvilli on intestinal epithelial cells

• Contains 3 enzymes:

  1. Enterokinase

  2. Disaccharidases

  3. Aminopeptidases

Brush Border: Enterokinase

activates pancreatic enzyme trypsinogen

Brush Border: Disaccharides

• maltase, sucrase, lactase

• complete digestion of carbohydrates

Brush Border: Aminopeptidases

Complete digestion of proteins by hydrolyzing most of the small peptide fragments into their amino acid components

Where is digestion of different macromolecules completed?

• Fat digestion: completed extracellularly

• Protein and carbohydrate digestion: completed intracellularly

How are carbohydrates digested and absorbed?

• In lumen:

  • Polysaccharides → disaccharides (via salivary and pancreatic amylase)

• At brush border:

  • Disaccharides → monosaccharides

• Absorption: Monosaccharides → capillaries (blood)

Function of Bile Salts

1. Aid fat digestion by acting as a detergent (emulsifying fat)

2. Facilitate fat absorption by forming fat micelles

How do bile salts aid fat digestion by acting as a detergent (emulsifying fat)?

• Convert large fat globules → lipid emulsion of small droplets

• Increases surface area for pancreatic lipase

How do bile salts help form micelles for absorption?

• Micelles = tiny structures (smaller than fat droplet)

• Composed of: bile salts, cholesterol (hydrophobic), and lecithin

• Micelles have hydrophobic core and hydrophilic shell

How is fat digested and absorbed?

1. Fat is emulsified by the detergent action of bile salts

2. Lipases hydrolyze triglycerides into monoglycerides and free fatty acids

3. water insoluble products move in the interior of micelles to cell surface

4. Monoglycerides and free fatty acids diffuse into cell

5. Monoglycerides and free fatty acids resynthesizes into triglycerides

6. Triglycerides Coates with lipoprotein and form chylomicrons that are exocytosed from cell

7. Chylomicrons enter lymphatic vessels

How does content flow from the small to large intestine?

• One-way flow from ileum → cecum

• Prevents colonic bacteria from entering small intestine

What structures regulate this one-way flow?

• Ileocecal valve

• Ileocecal sphincter

What effects does gastrin have on intestinal flow?

• Enhances gastric motility

• Stimulates ileal motility

• Relaxes ileocecal sphincter

What are the two types of movements in the large intestine?

• Haustral contractions: slow shuffling contents of large intestine to aid absorption (mainly water and salts)

  • colon’s main motility

• Mass movements: large, strong contractions in ascending and transverse colon

  • Occur 3–4 times/day

  • Triggered by gastrin (after meals) and extrinsic autonomic nerves

What initiates the defecation reflex?

• Mass movement pushes feces into rectum

• Rectal wall distension activates stretch receptors → initiate reflex

• Reflex:

  • Internal anal sphincter (smooth muscle) → relaxes

  • Rectum and sigmoid colon → contract

  • External anal sphincter (skeletal muscle) must also relax for defecation under voluntary control

What is constipation (large intestine) and what causes it?

• Occurs when defecation is delayed

• Too much water is reabsorbed → feces become dry and hard

What is appendicitis (large intestine) and how can it occur?

• Caused by hardened feces lodged in appendix

• Blocks normal circulation and mucus secretion

What are the 4 main mechanisms that regulate digestive function?

• Autonomous smooth muscle (e.g., Interstitial Cells of Cajal)

• Intrinsic nerve plexuses (Enteric Nervous System)

• Extrinsic nerves (Autonomic Nervous System: Sympathetic & Parasympathetic)

• Gastrointestinal hormones (e.g., Sympathetic Nervous System → Gastrin, Secretin, CCK)

What are Interstitial Cells of Cajal and what do they do?

• Non-contractile autonomous smooth muscle cells

• Act as pacesetters with slow-wave potentials that spread through gap junctions

• Self-induced electrical activity referred to as:

  • slow wave potential

  • basic electrical rhythm (BER)

  • pacesetter potential

• Regulate:

  • Rate of contraction

  • Intensity of contraction

Intrinsic Nerve Plexuses

• functions to coordinate local activity within the digestive tract

• contains both sensory neurons and neurons that innervate smooth muscle, exocrine, and endocrine cells

• output cells can be excitatory or inhibitory

Extrinsic Nerves

• Originate outside the digestive tract

• Come from both sympathetic and parasympathetic nervous system

• influence motility and secretion by

  • modifying ongoing activity in the internal nerve plexuses,

  • altering the level of gastrointestinal hormone secretion,

  • acting directly on smooth muscle and glands

Sympathetic Nervous system in digestive function Extrinsic Nerves

• dominates in fight or flight situations

• inhibits motility and secretion

Parasympathetic Nervous System in Digestive Function Extrinsic Nerves

• dominates during rest

• stimulates motility and secretion

• acts primarily via vagus nerve

Gastrointestinal Hormones - Endocrine gland cells

• located within the mucosa of certain regions of digestive tract

• release gastrointestinal hormones into the blood

Gastrointestinal Hormones

• released primarily in response to specific changes in the luminal contents (fats, proteins, acid)

• carried by the blood to other areas of the digestive tract where they can exert either excitatory or inhibitory influences

  • Ex: Gastrin, Secretin, CCK

Which section of the stomach has the thickest musculature?

antrum

Large intestine is primarily a drying and storage organ that consists of:

• Cecum (blind-ended pouch below illeocecal valve)

• Appendix (finger-like projection of lymphoid tissue)

• Colon (ascending, transverse, descending, and sigmoid)

• Rectum (“straight”, connected to anal canal)

Carbohydrate Digestion and Absorption

1. Dietary polysaccharides starch and glycogen converted to disaccharide maltose

2. maltose, lactose, and sucrose converted to their respective monosaccharides (glucose, lactose, and fructose) by lactase, maltase, and sucrase. Molecules enter cell by Na+ and energy dependent secondary active transport

3. Glucose and galactose enter the blood down their concentration gradients by passive carriers

4. Fructose enters blood by facilitated diffusion

Protein Digestion and Absorption

1. Proteins hydrolyzed into peptide fragments and individual amino acids by pepsin and pancreatic proteolytic enzymes

2. Amino acids absorbed into cell via Na+ and energy-dependent active transport

3. Small peptides are broken down into amino acids by intracellular peptidases and aminopeptidases.

4. Amino acids then enter blood down their concentration gradients