GI 01 & 02: GROSS ANATOMY AND BASIC PHYSIOLOGY Flashcards
What are the 4 main functions of the GI tract (and what is the fifth that textbooks often leave out)?
Motility, digestion, absorption, secretion, and excretion
What causes motility?
Contractions of smooth muscle in the gut lining
With regard to food particle size, what does motility contribute?
Helps reduce the size of the particles (helps in the breakdown process of food size). This allows for increased contact of area of food with chemicals and the mixing of those chemicals with food
With regard to motility, what do we think of as its main main function
Propels food in the caudad direction
Is the rate of mobility important?
Yes, it is going to happen at a rate that is best for full breakdown (digestion) and absorption of ingested food (ex. apples down to fructose)
What is secretion
The release of enzymes from cells in the GI lumen
What kinds of cells participate in secretion?
Epithelial cells lining the GI tract and associated glandular organs
What are the major associated glandular organs that participate in secretion
Gall bladder, pancreas, salivary glands, liver
What is the digestion process?
The process by which large macromolecules are broken down into smaller micromolecules
Why is digestion critical?
A whole apple isn’t useful to us but individual fructose is- therefore digestion is required to get things to absorbable levels of size
What are the two aspects of digestion?
Physical and chemical breakdown (mixing via strong muscles and enzymes)
What is the process of absorption?
Nutrients and water are moved from GI lumen into the blood
What are the reasons we need to absorb nutrients
Energy
Building blocks for other shit
parts of metabolic pathways
Regulation of all physiological activitie
What is the process of excretion
Waste –> bye bye
This includes storage and excretion of ingested food that goes through, but also products from the liver such as steroids, drug metabolites, and cholesterol
Why is it important that the GI tract has an immunocological function?
It’s exposed to the external environment - we can consider the GI tract the largest immune organ in the body
Connection between gut microbiome and immunity
They’re linked (as gut microbiome increases so does immunity and this tracks with age)
What are the major functional segments of the GI tract
- mouth and pharynx
- esophagus
- stomach
- small intestine (duodenum, jejunum, ileum)
- large intestine (colons)
- rectum
- anus
Associated glandular organs of Gi tract
- salivary glands
- pancreas
- liver
- gall bladder
- associated endocrine glands in gastric walls
What are sphincters
specialized circular smooth muscle
What are the sphincters of the GI tract
- Upper esophageal
- lower esophageal
- Pyloric
- sphincter of oddi (the one at the GB/pancreas/small intestine junction)
- ileocecal
- internal (involuntary) and external (voluntary) anal sphincter
Layers of gut wall
- mucosa
- submucosa
- muscularis externa
- serosa
What is the mucosa
Innermost layer of gut wall, made of epithelium, lamina propria, and muscularis mucosae
What is gut epithelium?
Single layer of continuous cells linked by tight junctions - can consist of many specialized groups
absorptive enterocytes
Most abundant epithelial cell in gut, play vital role in digestion and absorption
Enteroendocrine cells
Release regulatory peptides, amines - regulate GI function
Gastric mucosal cells
Produce protons (important for stomach acid)
Mucin producing cells
Produce a glycoprotein that protect the GI wall
Esophageal epithelium
Hellps in transportation of swallowed food (no absorption) -thus they are squamous and simple
Intestinal epithelium
Helps in absorption and selectiveuptake of ions, nutrients, and water - they have a big job and are therefore big themselves (columnar)
the surface area and small intestine epithelium consist of (large structure, and the places in between those structures) to increase surface area
Villi and crypts
Lifespan of Gi cells
3-5 days
Where are the proliferative cells in the Small intestine?
Crypts (these are called intestinal stem cells)
What exactly are villi and crypts
Fingerlike projections of the small intestine (they are units of absorption)- crypts are the invaginations/folds
What does every villus have?
The epithelial cell on the suraface has numerous cytoplasmic extensions that create a “brush border” / microvilli to increase the surface area even more
What can happen if you have destruction of villi and microvilli?
Malabsorption and malnutrition - happens in celiac disease
Lamina propria
Immediately below epithelium, consists of connective tissue (collagen, elastin). This is rich in glands, and contains lymph vessels and nodes, capillaries, and nerve fibers
Muscularis mucosae
Thin layer of smooth muscle cells in a folding configuration caused by contractions
Submucosa
Consists of connective tissues (collagen, elastin). Also has glands, nerves that can sense stretch, blood vessels, and has a dense nerve plexus called the “submucosal plexus” (pt 1 of gut brain). Overall this helps with the integration of motor and secretory activities
What is the meissner’s plexus
This is the submucosal plexus in the submucosal layer and is pt 1 of gut brain
Muscularis externa / muscularis propria
Contractions in this layer help in mixing and propelling GI tract contents; there are two layers of smooth muscle (inner circular, outer longitudinal). Between the two layers is the second part of the gut brain, the myenteric plexus
What is auerbach’s plexus
Myenteric plexus, pt 2 of gut brain (this is the plexus impt in coordinating peristalsis
Serosa
Outermost layer of Gi tract; consists of squamous mesothelial cells and lines surface of abdominal wall to keep organs suspened. Also, secretions are viscous and lubricates abdominal organs - this is important to reduce friction between ab organs during contractions and relaxation of GI tract
Portal circulation
Venous blood arising from abdominal organs flows to liver first via portal vein (portal circulation). Blood from liver is transported via hepatic veins to inferior vena cava, and then pumped by the heart which will supply the rest of the tissues.
Importance of portal vein going to liver
The liver is going to detoxify from any bad shit that might have gotten absorbed - it also will keep some nutrients for itself
Splanchnic Circulation
This is the circulation that actually supplies the GI tract - it has a HUGE blood flow (25% of CO), and a large reservoir function as well as a huge diversity of organs it perfuses.
3 major arteries of splanchnic circulation
Celiac, superior mesenteric artery, inferior mesenteric artery
What does the celiac artery supply
liver, spleen, stomach
what does the superior mesenteric artery supply
pancreas, SI, proximal colon
What does the inferior mesenteric artery supply
Distal colon
Lymphatic circulation purpose
Helps return fluids, FA molecules, white cells to the blood - this is crucial for the absorption of lipids and lipid soluble molecules , as well as some vitamins and drugs. It also plays a role in immune fxn and filtering (ex, bacteria enter capillaries and pass into lymph vessels, eventually getting to nodes where there are many immune cells). They also pick up interstitial fluid to carry them back to circulation
Is fatty acid absorbed directly into blood
no it must go through lymphatic system
GI activity overall
GI goes through relative quiescence and intense activity which can vary through food intake, as well as communication between segments
How do we regulate GI function
Endocrine, paracrine, and neural siganling
Sensor cells (for endocrine fxn in GI tract) aka enteroendocrine cells
respond to stimulus by secreting a regulatory peptide or homrone that will travel endocrine style to a target cell at a distant location
are enteroendocrine cells / sensor cells typically open or closed type and are there exceptions?
Open (apex of cell is in contact with GI lumen for sensing, and base releases hormone that diffuses into local capillaries)
But some are closed - these do not have contact with Gi luminal surface (ECL cells of gastric epithelium are like this)
Paracrine signaling
A chemical messenger or regulatory peptide released from a signaling cell diffuses to nearby cell - it is not transported in circulation (targets are generally smooth muscle, absorptive enterocytes, secretory cells in glands, and other EECs)
Histamine
Paracrine signaler released by enterohromaffin-like cells in stomach, diffuses through Interstitial space and binds to neighboring parietal cells that then secrete HCl
Cholecystekinin
Paracrine or endocrine signaler, it is released from duodenum in response to dietary protein, lipids - acts locally on nerve terminals, but also goes into bloodstream to affect pancreas (paracrine; endocrine respectively)
What is another major system in Gi tract that relies on paracrine signaling
Immune system
Celiac disease
Response to Giladin in wheats that leads to destruction of microvilli
The gut is innervated by 2 sets of nerves. What are they (basically)?
Extrinsic and intrinsic
extrinsic NS of the gut
Nerves that innervate gut with cell bodies located outside of gut ; these are part of the autonomic nervous system
Intrinsic (enteric) nervous system
Cell bodies are inside of the gut wall; made of myenteric and submucosal plexi
Can intrinsic and extrinsic nervous systems communicate
YES
Extrinsic innervation is primarily through what branch of ANS
Parasympathetic (although sympathetic does happen)
How does parasympathetic innervation occur (via what nerves)
vagal and pelvic , primarily vagal
Vagus nerve innervates:
Esophagus, stomach, gallbladder, pancreas, 1st part of intetine, cecum, proximal colon
pelvic nerve innervates:
distal part of the colon and anorectal region
What does PSNS innervation of the gut result in
OVerall, activates GI function
Where does PSNS innervation of gut originate
Brain stem (medulla) for vagus nerve, and sacral spinal cord for pelvic nerve
What are the neurotransmitters for PSNS innervation of gut
Ach, substance P, VIP
What kind of nerve is the vagus nerve and how does that play into how it functions in the GI tract
mixed - 75% afferent (to brain) and 25% efferent (to gut from brain)
What are the afferent fibers of the vagus nerve telling the brain (what receptor types)
sensory information - mechanoreceptors and chemoreceptors
what is a vagovagal reflex
A vagus response due to vagal input
Generally how does SNS activity impact gut and what is the exception
inhibition of GI smooth muscle - exception is GI sphincters, which are activated (closed)
What are the SNS ganglia that serve Gi tract
celiac, superior mesenteric, inferior mesenteric
What do the SNS fibers release
NE
with regard to SNS innervtion how much is afferent and how much is efferent
50/50
Main fxn of myenteric plexus
regulate mobility
main fxn of submucosal plexus
modulate overall activity of cells (esp endocrine cells)
Can enteric nervous system function without CNS
yes
Long neural reflex
Sensory afference to medulla (ex. vagal), then efference back to motor to GI
Short neural reflex
Loop that stays entirely in enteric nervous system
What kind of neural reflex is a vagovagal reflex
Long (to cns and back)
What muscles are voluntary for sphincters, what kind of muscle do they have
Pharynx, upper 1/3 of esohagus, external anal sphincter
Two kinds of contractions of smooth muscle in GI tract
Phasic, tonic
Phasic contractions
Periods of contraction followed by relaxation - found in esophagus, gastric antrum, small intestine (involved in mixing and propulsion)
Tonic contractions
Constant level of contraction / tone withtout periods of relaxatio regulalry - found in orad (upper) region of stomach and lower esophgeal region, ileocecal and internal rectal sphincters
What are slow waves?
Slow electrical waves of oscilating de and re polarization of membrane potential that are not large enough to elicit an action potential or contraction. If, at the plateau/peak of slow wave, membrane is further depolarized to threshold then you get a full AP (strong contraction
Is slow wave frequency constant along Gi tract, or does it vary?
Frequency of waves varies based on area, has to do with function
Where do slow waves originate from?
Interstitial cells of Cajal (ICCs) - these are “GI pacemaker cells”
Segmental contraction
Predminantly in Small and large intestine - this is when you contract in the middle of the food bolus / chyme to break it up and then the chyme comes back together again - important for mixing but produces no forward movement
Peristaltic contraction
Contraction in orad direction and relaxation in caudal direction to result in net forward propulsion of food
NT’s associated with contraction
Ach and Substance P
NT’s associated with relaxation
NO and VIP
Which set of neurons, ascending or descending, go in the orad direction?
Ascending (will release Ach and Substance P to contract circular smooth muscle and will release NO and VIP to relax longitudinal muscle)
Reverse is true for descending in caudal direction
