What are the tissue layers of the generic GI tract?
Mucosa-Composed of epithelium, lamina propria, muscularis mucosae
Submucosa- Submucosal Plexus (plexus of Meissner), also contains loose connective tissue that has blood vessels, glands, lymph tissue.
Muscularis externa- composed of longitudinal and circular muscle. The Auerbach plexus is located between the longitudinal and circular muscles. This is the plexus responsible for peristalsis.
Serosa- Composed of connective tissue and epithelium.
Describe the mucosal layer as you proceed through the GI tract.
Epithelium- In the esophagus and anus, the epithelium is stratified squamous, this provides protection to these to structures. In the esophagus the stratified squamous is nonkeratinized, in the anus it is keratinized. In the rest of the gut, the epithelium is simple columnar. The simple columnar cells absorb nutrients, secrete enzymes and contain Goblet cells (secretes mucous) and enteroendocrine cells (secrete hormones).
Lamina Propria- Connective tissue layer
Muscularis mucosa- Mostly multiple layers, except for the esophagus where it’s a single layer (longitudinal).
What are the types of muscle found in the muscularis externa?
There are both skeletal and smooth muscle found in the muscularis externa. The skeletal muscle is only found in the mouth, pharynx, larynx and anus. Smooth muscle is found everywhere else, there are 2 layers, longitudinal and circular layers (responsible for peristalsis). The Auerbach (Myenteric)plexus is located between these 2 layers it initiates peristalsis, when activated.
What makes up the Enteric Nervous system?
Both the Myenteric plexus and submucosal plexus are part of the enteric nervous system.
How are the neurons that form the enteric nervous system derived?
Neural crest cells migrate down to the wall of the GI and form the enteric nervous system.
Describe how a bolus of food can initiate peristalsis.
Bolus of food lead to GI stretching which activates enteroendocrine cells on the epithelium of the gut
Activated EC cells release serotonin that act on the submucosal plexus. The submucosal plexus has secretory-motor neurons, it releases hormones and peptides.
Activated submucosal plexus will innervate the mucosa and pass info to the myenteric plexus
Activated myenteric plexus leads to peristalsis. Myenteric neurons are motor neurons.
What is the relationship between the parasympathetic and sympathetic nervous system and the gut?
The parasympathetic (through Acetylcholine) and sympathetic nervous system (through norepinephrine) act upon the enteric nervous system, which then acts on the gut. The effects of the parasympathetic system are to contract the gut, while the sympathetic system relaxes the gut. However, those 2 systems do not directly act on the gut.
What are ganglions, plexi?
A ganglion consists of a group of neurons in a general area, a plexus represents those neurons working together.
Describe the tissue layers in the esophagus.
Mucosa- epithelium: stratified squamous unkeratinized cells; muscularis mucosa- single layer of longitudinal smooth muscle cells; lamina propria- Has dense irregular CT, diffuse lymphoid tissue, arterioles, venules contain esophageal cardiac glands that are located in the pharynx and gastric regions.
Submucosa- Dense fibroelastic CT, contains esophageal glands that are seromucous, release pepsinogen and lysozyme
Muscularis externa- Main division between different parts of esophagus. The top 1/3 is all skeletal muscle, the middle 1/3 is a mix between skeletal and smooth muscle and the bottom 1/3 is all smooth muscle.
Final layer is called adventitia in the diaphragm, once the esophagus crosses into the abdomen the outer layer is called serosa.
What are the chemicals released by the stomach to promote chemical digestion?
Hydrochloric acid, renin, pepsin, gastric lipase.
What are the layers of muscle found in the stomach?
There are 3 layers of muscle found in the stomach: outer longitudinal, middle circular and inner oblique. The middle circular is most evident in the pylorus, to form pyloric sphincter
What makes up the ruggae? Gastric pits? Gastric glands?
The ruggae are made from folds of mucosa and submucosa. Gastric pits are small invaginations of the epithelium, gastric glands are parts of the lamina propria
What is the basic cell types located in the epithelia of gastric gland of the stomach?
Surface lining cells- these cells release lysozyme
Regenerative cells- located towards the top of the pit, regenerate new cells.
Mucous neck cells- release a thick mucous
Parietal cells- Release HCL and Intrinsic Factor (factor required to absorb B12)
Chief Cells- Release pepsinogen, gastric lipase and renin
Enteroendocrine cells-secrete all sorts of stuff including gastrin, glucagon and gastric inhibitory peptide
How do parietal cells release HCl?
Parietal cells are activated by gastrin (stretch EC cells), histamine (stretch EC cells)and acetylcholine (vagus nerve)
In jejunum- villi are narrow, shorter, less dense, there are many goblet cells.
In ileum- villi are shortest, narrowest and smallest in number. Peyer’s Patches are located here.
What is the difference between stem cell renewal in the stomach versus the small intestine?
In the stomach cells are renewed in 2 different directions. The surface cells only take 4-7 days to renew while the glandular cells take longer.
In small intestine, cells renew in 1 direction and take 3-6 days
What are Brunner’s Glands? Where are they located?
Brunner’s glands are glands located in the submucosa of the duodenum that release an alkaline fluid to neutralize chyme.
What is the name of the specific lymphoid tissue in the small intestine?
These are Peyer’s Patches and they are ONLY located in the ileum. They are located in the side opposite to the mesentery. There is lymphoid tissue similar to this in the appendix, but in the appendix the lymphoid tissue goes all the way around the lumen.
What are the types of cells in the large intestine? What are the surface features?
Absorptive cell, goblet cell, regenerative cell, enteroendocrine cells. There are still crypts of Leiberkuhn. NO PANETH CELLS HERE. No villi either
What are teniae coli?
Teniae is the longitudinal layer of muscle in the colon. Here it is not a sheet but instead a condensed sheet of smooth muscle. There are 3 strings that make up the teniae coli. The teniae coli are constantly contracted, this contraction then forms the haustra and semilunar folds. If the teniae coli were relaxed, the haustra would go away. If the teniae coli were released, the muscle length and the gut length is the same. (review notes if this sucks)
What is the difference in epithelium between the rectum and anal canal?
The epithelium changes to keratinized stratified squamous, this occurs at the pectinate line.
What causes the hemorrhoids?
If the internal hemorrhoidal plexus gets distended.
What happens to the 2 muscle layers as you enter the anus?
The circular muscle becomes the internal anal sphincter, the longitudinal muscle disappears.