Normal Structure and Function of the Oral Cavity & Upper GI Tract

Question 1

What is the general process by which the gastrointestinal system digests food and absorb nutrients?

Answer 1

1. A bolus of food generated by the oral cavity enters the esophagus by swallowing
2. In the stomach,
   
   • food is churned in the presence of digestive juices
   • generates a semi-fluid mass of partially digested food called chyme
3. Chyme leaves the stomach via the pylorus and enters the duodenum where it encounters pancreatic digestive enzymes and bile from the liver
4. Digestion is completed in the small intestine
   
   • digestive products are absorbed
5. Liquid residue enters the large intestine where water is absorbed
   
   • As water is absorbed, the residue becomes solid generating fecal material that is collected in the rectum

Question 2

How is food is moved down the GI tract by muscular action?

Answer 2

• Initially voluntary muscle action in the oral cavity and upper 1/3^rd of the esophagus moves food into the alimentary canal
• Involuntary muscle action (peristalsis) then takes over to propel the food and products of digestion through the remainder of the GI tract

Question 3

How is the structure of the GI tract generally conserved?

Answer 3

Four tunics define separate functional layers:

1. Mucosa
2. Submucosa
3. Muscularis externa/propia
4. Adventitia / Serosa

Question 4

Describe the layers of the mucosa:

Answer 4

1. Epithelium
   
   • major site of mucosal function, varies in type depending upon function
2. Lamina propria
   
   • loose connective tissue that supports the epithelium
   • rich in lymphatic and blood capillaries
   • contains lymphocytes and plasma cells
3. Muscularis mucosae
   
   • thin layer of smooth muscle
   • required for local movements of the mucosa and folding of the mucosa

Question 5

How does the mucosal epithelium change based on the organ?

Answer 5

• Stratified squamous non-keratinizing epithelium lines the esophagus and anal canal
  
  • serves to protect against abrasion
• Simple columnar epithelium lines the stomach, small intestine, and large intestine
  
  • functions to secrete, to absorb, and to absorb/protect depending upon the organ

Question 6

• What is the functional role of submucosa?
• What is the role of nervous tissue found here?

Answer 6

• Layer of loose collagenous and adipose tissue that supports the mucosa
  
  • Large blood vessels and lymphatic vessels
  • Mucous-secreting glands present in SM of esophagus and duodenum
• Nervous tissue submucosal (Meissner’s) plexus (PG, parasympathetic ganglia)
  
  • Control muscularis mucosae movement, blood flow, and gland secretion

Question 7

Muscularis Externae:

• Relationship to the submucosae:
• Arrangement of muscle fibers:
• Role of nervous tissue:

Answer 7

• Surrounds the submucosa with a thick layer of smooth muscle
  
  • except in the esophagus and anal sphincter
    
    • also contain skeletal muscle
• Muscle layers are usually arranged as an inner circular layer an outer longitudinal layer
  
  • responsible for peristalsis
• Nervous tissue:
  
  • Myenteric (Auerbach’s) plexus resides between the muscle layers
    
    • ​controls peristalsis

Question 8

What protects the GI tract?

Answer 8

Serosa/Adventitia

• Outermost covering of the GI tract

Question 9

Serosa/Adventitia:

• Adventitia
• Serosa
  
  • function of the epithelial component?

Answer 9

• Adventitia
  
  • loose connective tissue layer surrounding the GI tract located outside of the peritoneal cavity
• Serosa
  
  • simple squamous epithelium (mesothelium) along with connective tissue covers the GI tract within the peritoneal cavity
  • epithelial component secretes serous fluid
    
    • lubricates the interface between the visceral and parietal peritoneum
    • allows movement of the GI tract organs

Question 10

Role of Submucosal (Meissner’s) plexus vs. Myenteric (Auerbach’s) plexus

Answer 10

• Submucosal (Meissner’s) plexus
  
  • Controls mucosal movement, secretory activity and blood flow
• Myenteric (Auerbach’s) plexus
  
  • Controls peristaltic gut movements

Question 11

1. How many neurons are found in the enteric nervous system?
2. Why is it called the “second brain”?
3. What mediates the action of enteric neurons?

Answer 11

1. ~ 100 x 10⁶ neurons
2. division of the autonomic nervous system but can operate independent of the brain and spinal cord
   
   • hormones secreted by enteroendocrine cells also regulate the ENS
3. Enteric neurons mediate their action through the standard repertoire of neurotransmitters and neuromodulators found in the brain including acetylcholine, norepinephrine, and serotonin

Question 12

• Parasympathetic system stimulates _______ and ________
• Sympathetic system represses _______ and activates activity of ________

Answer 12

• Parasympathetic system stimulates secretion and peristalsis
• Sympathetic system represses peristalsis and activates activity of sphincters

Question 13

Functions of GI mucosa based on location

• Protective:
• Secretory:
• Absorptive:
• Absorptive/protective:

Answer 13

1. Protective:
   
   • stratified squamous epithelium mainly found in the upper G.I. and anus
2. Secretory:
   
   • found in the stomach
   • contains an abundance of tubular glands
3. Absorptive:
   
   • mucosa is folded into villi to maximize surface area
   • found throughout the small intestine
4. Absorptive/protective:
   
   • lines the large intestine
   • produces mucin to help fecal movement and absorbs water

Question 14

What is the function of the esophagus?

Answer 14

Muscular tube that propels a bolus of food to the stomach

Question 15

What can be found in the esophageal mucosa?

Answer 15

1. Epithelium (E):
   
   • stratified nonkeratinized squamous epithelium with Langerhans cells (APCs)
2. Lamina propria:
   
   • narrow with lymphoid aggregates (Ly)
   • 2 clusters of mucosal glands that secrete mucous to help propel food = esophageal cardiac glands
3. Muscularis mucosae (MM)
   
   • single layer of smooth muscle cells
   • more substantial near stomach

Question 16

Submucosa of the esophagus:

• What is abundant?
• What are the two cell types?
• What are the major secretory products?

Answer 16

• Esophageal glands (G) are abundant in the submucosa
  
  • arranged as acini that drain into a central duct similar to salivary glands
• Two cell types
  
  • mucous cells ⇒ basally located nucleus
  • serous cells ⇒ centrally located nucleus
    
    • less abundant
• Major secretory products of serous cells are pepsinogen and lysozyme that are deposited via ducts into the lumen

Question 17

What are the only two areas of the GI tract that have submucosal glands?

Answer 17

only the esophagus and duodenum contain glands in their submucosal layers

Question 18

Muscularis Externae of the esophagus

1. Arrangement of muscle layers:
2. Location of skeletal muscle:

Answer 18

1. inner layer of circular smooth muscle (CM) and an outer layer of longitudinal smooth muscle (LM)
2. upper the 1/3rd of the esophagus has skeletal muscle (Sk) interspersed with the smooth muscle
   
   • initial swallowing is under voluntary control

Question 19

Describe the Adventitia/Serosa of the esophagus:

Answer 19

• Prior to reaching the diaphragm, the esophagus is encased with an adventitia
• Once it traverses the diaphragm, it is covered with a serosa

Question 20

• What happens at the gastroesophageal junction?
• What are the 2 physiological sphincters of the esophagus?

Answer 20

• Sharp change from a stratified squamous epithelium to a simple columnar epithelium dedicated to secretion at the junction of the esophagus and stomach
• 2 physiological sphincters:
  
  • pharyngoesophageal sphincter prevents reflux from esophagus into the pharynx
  • gastroesophageal sphincter prevents reflux from the stomach into the esophagus

Question 21

1. What is the location and function of the stomach?
2. How are the mucosa and submucosa arranged in the empty stomach?
   
   • How much volume can the stomach hold?
3. What is in the gastric juice?

Answer 21

1. The stomach is a dilated sac between the intestine and esophagus where food undergoes mechanical and chemical digestion to form chyme
2. Mucosa and submucosa are arranged as prominent folds called rugae in the empty stomach; holds ~ 1.5 liters by distending
3. Churning food in gastric juice:
   
   • hydrochloric acid and** digestive enzymes**
     
     • pepsin, rennin, and gastric lipase

Question 22

What are the anatomical regions of the stomach?

Answer 22

1. cardia - small area at gastroesophageal junction
2. fundus - upper left to cardia
   
   • secrete acid and enzymes
3. body (corpus) - central area
   
   • secrete acid and enzymes
4. pyloris - constricted region at base
   
   • regulates release of chyme into duodenum (pyloric sphincter)
   • secretes gastrin

Question 23

What type of epithelium is present in the stomach?

Answer 23

simple columnar

Question 24

Mucosa of the stomach

• Epithelium
• **Lamina propria **
• Muscularis mucosae

Answer 24

1. **Epithelium: **​​epithelial cells lining the lumen generate a thick mucous
   
   • traps bicarbonate ions (alkaline)
   • protects the mucosa from the low pH in the stomach
   • soluble mucous acts as a lubricant
   • Invaginates into the lamina propria forming gastric pits (GP)
2. Lamina propria: loose connective tissue layer densely packed with gastric glands
   
   • Enterochromaffin-type cells secrete histamine in response to the vagus nerve to increase acid secretion
3. Muscularis mucosae (MM): smooth muscle
   
   • gastric glands extend to MM influences gastric gland outward flow

Question 25

**Submucosa** of the stomach: * What **nervous tissue** is found here?

Answer 25

* Thick layer of dense connective tissue * Numerous wandering cells including: * lymphocytes, eosinophils, mast cells, and plasma cells * May also have adipose cells * Vascular and lymphatic structures * **Submucosal (Meissner’s) plexus**

Question 26

**Muscularis Externa** of the stomach: * What are the **3 muscle layers**? * What **nervous tissue** is found here?

Answer 26

* **3 layers of smooth muscle: ** * circular and longitudinal always present * oblique fibers may also be present * **Myenteric (Auerbach’s) plexus** located between layers * **controls contraction**

Question 27

1. **What coats the surface of stomach mucosa?** 2. How are gastric glands **arranged**? 3. What are the **regions** of gastric glands?

Answer 27

1. Surface of the mucosa is pitted with indentations called **gastric pits** 2. **Gastric glands are tubular columns of epithelial cells** * One to seven gastric glands open into each gastric pit. 3. Gastric glands are **divided into 3 regions:** * isthmus, neck, base * different epithelial cell types in each region

Question 28

What are the **6 cell types** found in gastric glands? * Where are the cells located?

Answer 28

1. **Surface mucous/lining cells** * gastric pit 2. **Parietal (oxyntic) cells** * isthmus, neck, and base 3. **Neck mucous cells** * neck and base 4. **Chief (zymogenic, peptic) cells** * base 5. **Enteroendocrine cells** * base 6. **Stem (regenerative) cells** * neck

Question 29

Mucous differs based on type of gland cell: * **surface lining cells** * **mucous neck cells**

Answer 29

* **surface lining cells** mucous: * thick and dense * adheres to the mucosa * protects against acid digestion * **mucous neck cells** mucous: * soluble * lubricates the stomach contents to aid digestion

Question 30

What is the bioactive protein produced? 1. **G cells - ** 2. **EC cells - ** 3. **D cells - ** 4. **A cells - ** 5. **ECL cells - **

Answer 30

1. **G cells** - gastrin 2. **EC cells** - serotonin 3. **D cells** - somatostatin 4. **A cells** - enteroglucagon 5. **ECL cells** - histamine * **Note:** Substances act either on surrounding cells in * *paracrine** fashion or are transported through the bloodstream to act as **endocrine **molecules

Question 31

What is the function of **parietal cells**?

Answer 31

generate **HCl** and gastric **intrinsic factor**

Question 32

**Parietal cells:** 1. Cytoplasm: 2. Distinctive feature of apical surface: 3. What greatly increases the secretory surface area? 4. Tubulovesicular network vs. HCl secretion (i.e. relationship) 5. What is extremely abundant?

Answer 32

1. Cytoplasm stains heavily with eosin 2. Pyramidal cells distinctive: **secretory canaliculus** * invagination of the apical surface * can almost encircle the nucleus 3. Dense network of **microvilli** extend into the canaliculus * hugely increases the secretory surface area 4. **Tubulovesicular network** * inversely correlates with HCl secretion 5. **Extremely rich in mitochondria** * **​​**provide energy for HCl secretion

Question 33

What is the function of **chief cells**?

Answer 33

To **secrete digestive enzymes** into the stomach lumen

Question 34

**Chief cells** * **Location:** * **Secretory granules:** * **​**Proenzymes: * Packed with .... to accomplish ....

Answer 34

* Principal cell type at the **base of the gastric glands** * Full of apically located **zymogen secretory granules** (ZG) containing proenzymes including: * **pepsinogen, rennin, and gastric lipase** * basal nucleus * Packed with **rough endoplasmic reticulum** (rER) and **golgi** to accomplish **high volume protein production**

Question 35

How is **granule release controlled** in chief cells?

Answer 35

By both **hormonal** and **neural** action: * **hormonal:** * **secretin** binds to chief cell receptors ⇒ rapid exocytosis of secretory granules * **neural: ** * **​**stimulation of** vagus nerve** ⇒ enzyme release

Question 36

* Where are regenerative (stem) cells found? * How are they identified? * Describe what happens as stem cells divide:

Answer 36

* Stem cells are few in number but most commonly found in the **neck** of the gastric gland * Stem cells are most easily identified as containing **mitotic figures** in stained sections * As the stem cells divide, the daughter cells **differentiate** and **migrate** upward or downward to **replace** all types of epithelial cells

Question 37

1. How often is the gastric mucosa renewed? What is this dependent upon? 2. What is **mucosal restitution?**

Answer 37

1. Cells of the **gastric mucosa are completely renewed** **every three days** and is **dependent upon** the **proliferating stem cell compartment** 2. **Surface epithelium** is also **rapidly repaired** after injury (e.g., by aspirin) by the **rapid migration of epithelial cells** lying deep in the protected environment of the pits - **mucosal restitution**

Question 38

What are the **differences in mucosa** in **different areas** of the stomach?

Answer 38

* **Fundus and body** are histologically indistinguishable with **straight tubular glands** * **Pylorus** contains the same cell types as other regions, however, the **glands are branched and mucous neck cells predominate** * **Cardiac** region has **shallower gastric pits,** and the **glands are highly coiled** * abundance of enteroendocrine cells

Question 39

List the **3 phases** of the stomach response to food intake:

Answer 39

1. Cephalic phase 2. Gastric phase 3. Intestinal phase

Question 40

What happens in the **cephalic phase**?

Answer 40

1. **Brain anticipates meal** -- smells, visual cues 2. Parasympathetic signals via vagus nerve ⇒ Acetylcholine (ACh) release ⇒ acts on G cells ⇒ gastrin secretion 3. Gastrin, ACh, and histamine ⇒ act on parietal cells to stimulate limited HCl release **(\*30% total gastric acid)**

Question 41

What happens in the **gastric phase**?

Answer 41

1. **Food enters stomach** ⇒ distention ⇒ **increases G cell activity** 2. Increased gastric juice release **(\*60% total gastric acid)** 3. Onset of powerful smooth muscle contraction --\> _mechanical digestion_

Question 42

What happens in the **intestinal phase**?

Answer 42

1. **Chyme** released **into duodenum** 2. Gastric emptying slows 3. Distention of intestine ⇒ gastric-inhibitory impulses by ENS = **enterogastric reflex** 4. Cholecystokinin and secretin suppress gastric activity **(\*10% gastric acid)**