Digestive System Overview

Question 1

What is the enteric nervous system and how is it organized?

Answer 1

The enteric nervous system is a subdivision of the autonomic nervous system that consists of two interconnected plexuses: the Meissner’s Plexus of the submucosa and the Auerbach Plexus between the two layers of the muscularis externa. These plexi are made up of sensory neurons, motor neurons and interneurons, post-ganglionic sympathetic neurons, and pre-ganglionic parasympathetic neurons.

Question 2

What is the role of the enteric nervous system in gut motility?

Answer 2

The neurons of the enteric nervous system are able to give rise to local peristaltic waves without CNS input. This occurs only with Meissner’s plexus because these plexus have reflex action (sensory neurons detect food and react with contraction or secretion locally. CNS input is required for coordinated peristalsis throughout the GI tract as well as modulation of motility.

Question 3

How do smooth muscle cells contract?

Answer 3

Smooth muscle cells use actin and myosin fibers to contract (like skeletal musc.), Ca2+ can enter the cell via VSCC or sarcoplasmic reticulum, and binds to calmodulin. Ca2+ & calmodulin -> activate myosin light chain kinase (MLCK) -> phosphorylation of myosin -> cross bridge cycling -> contraction. Unlike skeletal muscle, smooth muscle cells can maintain long contractions without ATP, called “latch state”, via high intracellular [Ca2+].

Question 4

How do smooth muscle cells connect and coordinate contractions?

Answer 4

Mechanical connections and gap junctions
Cell-to-cell communication of electrical, ion, small mol signals
Autonomic nerve input coordinates many cells
Pacemaker cells of the gut depolarize spontaneously, firing APs that cause contraction of many SM cells = tone
Tone is modified by PNS and SNS input

Question 5

What are the four main layers of the GI tract wall?

Answer 5

Mucosa
Submucosa
Muscularis externa (aka Muscularis própria)
Serosa or Adventitia

Question 6

What is the structure of the Mucosa throughout the GI tract?

Answer 6

Esophagus and anorectal canal: Stratified squamous epithelium for protection.
Stomach through large intestine: Simple columnar epithelium for secretion, digestion, absorption

Question 7

What are the three layers of the mucosa?

Answer 7

Epithelium
Lamina propria
Muscularis mucosa

Question 8

What does the lamina propria contain?

Answer 8

The lamina propria is immediately deep to the mucosal epithelium. it contains:
Arterioles, venules, capillaries, and lymphatics
Fibroblasts
Immune cells

Question 9

What does the submucosa contain?

Answer 9

Loose, irregular tissue containing larger vessels and nerves
Contains Meissner (Submucosal) Plexus: Post-ganglionic Sympathetic, Pre-ganglionic Parasympathetic, plus some cell bodies of Enteric nerves

Question 10

What does the muscularis externa contain?

Answer 10

Two layers of smooth muscle: Circular inner layer, longitudinal outer layer
Auerbach (myenteric) Plexus: Pre-ganglionic Parasympathetic, Post-ganglionic Sympathetic, cell bodies of Enteric nerves

Question 11

What does the serosa or adventitia contain?

Answer 11

If there is mesothelium present (i.e. Peritoneum) = serosa
No mesothelium (esophagus) = adventitia

Question 12

What glands are present in the submucosa of the esophagus, what muscle type is present in the externa, and does it have serosa or adventitia?

Answer 12

Mucous secreting glands
Contains skeletal muscle in upper 1/3, transitioning to entirely smooth muscle in lower 1/3
Lacks mesothelium in upper region, thus has adventitia in all but the lower 2-3 inches after it enters the abdominal cavity.

Question 13

What cells are present in the mucosa of the stomach?

Answer 13

Cells are simple columnar with gastric pits
Surface mucous cells (neutral mucous), mucous neck cells (acidic mucous)
Parietal cells: Secrete HCl and intrinsic factor
Chief cells: Secrete pepsinogen and lipase
Stem cells
Neuroendocrine Cells: Gastrin from G cells, Histamine from ECL cells

Question 14

What is present in the submucosa of the stomach?

Answer 14

Reissner’s Plexuses

Submucosa also contributes to the rugal folds of the stomach

Question 15

What is present in the muscularis externa of the stomach?

Answer 15

THREE layers of muscle: circular, longitudinal, and oblique

Auerbach Plexuses

Question 16

Does the stomach have serosa or adventitia?

Answer 16

The stomach has serosa that is continuous with the rest of the peritoneum.

Question 17

What cells are present in the mucosa of the small intestine?

Answer 17

Cells are simple columnar with crypts of lieberkuhn, plicae circularis, and villi.
Enterocytes: located on villus, have microvillae with glycocalyx extending up (digestion and absorption)
Gobet cells: on villus
Paneth cells: Antibacterial cells at base of crypts
Neuroendocrine cells: in crypts, secrete secretin to stimulate HCO3 secretion, CCK to stimulate pancreatic enzyme and bile release
Stem cells: in middle of crypts

Question 18

What does the lamina propria do in the small intestine?

Answer 18

Lamina propria forms villi and houses ECL cells (secrete Histamine). It is highly vascular and also contains lacteals.

Question 19

What is present in the submucosa of the small intestine?

Answer 19

Brunner’s glands: in duodenum, secrete neutral to alkaline mucous
Peyer’s Patches: lymphoid tissue, more prominent in ileum.
Meissner’s Plexuses

Question 20

Does the small intestine have serosa or adventitia?

Answer 20

The small intestine has serosa that is continuous with the peritoneum except for the the portion of the duodenum which is retroperitoneal and has adventitia.

Question 21

What cells are present in the large intestine?

Answer 21

Cells are simple columnar with crypts of Lieberkuhn, but lacks villi and plicae circulares
Absorptive cells: NOT enterocytes as they have no digestive enzymes
Stem cells in crypts
Goblet cells abundant
Endocrine cells

Question 22

What is contained in the muscularis externa in the large intestine?

Answer 22

The longitudinal layer is thinner than the circular layer except in the three longitudinal bands known as taenia coli.

Question 23

Does the large intestine have serosa or adventitia?

Answer 23

The ascending and descending colon are retroperitoneal and have adventitia.
The transverse and sigmoid colon are peritoneal and have serosa.

Question 24

What cells are present in the anorectal canal?

Answer 24

Stratified squamous epithelium for protection. Transition from columnar at pectinate line.

Question 25

What cells are present in the muscularis externa of the anorectal canal?

Answer 25

Internal sphincter: circular smooth muscle thickening

External Sphincter: Striated muscle of the pelvic floor

Question 26

Does the anorectal canal have serosa or adventitia?

Answer 26

the anorectal canal has adventitia

Question 27

What is the histology of the stomach mucosa?

Answer 27

The mucosa is simple columnar epithelium with pits that extend into the gastric glands which then extend down to the muscularis mucosa. There are five different cell types that make up the mucosa and glands, and three regions of the stomach that possess different glands.

Question 28

What is the histology of the three regions of the stomach?

Answer 28

Cardia: contains short, dense, branched glands that are involved in mucus production.
Fundus: Long, straight, glands involved in HCl, intrinsic factor, and Pepsinogen production.
Pylorus: Short, coiled glands that produce mucus to neutralize chyme and gastrin to stimulate HCl release.

Question 29

What are the five cell types found in the stomach mucosa?

Answer 29

Mucous cells
Parietal cells
Chief cells
Stem cells
Neuroendocrine cells