2 - Gastrointestinal 01 Flashcards
List the layers of the gastrointestinal wall, from the outside to the inside
serosa
longitudinal smooth mm
circular smooth mm
submucosa
mucosa
Gastrointestinal smooth mm fibers are connected by _____ junctions
gap
allows low-resistance movement of depolarization between cells
Electrical signals can travel faster along the muscle fiber _____ than _____
lengthwise
sideways
Each muscle layer in the GI tract functions as a _______
syncitium
when an action potential is elicited anywhere in the muscle mass, it travels in all directions at once
Muscle fibers in the GI smooth muscle have two basic types of electrical waves:
slow waves (cause undulating change in membrane potential)
spikes (true action potentials that cause muscle contractions)
What is the average resting and threshold potential in the gut?
Resting -50 to -60
Threshold -40
What are factors that depolarize gut membranes?
stretching
acetylcholine release from PS
gastrointestinal hormones
What are factors that hyperpolarize the gut membrane?
catecholamines
sympathetic nn stim
The GI tract has its own nervous system, called the:
enteric nervous system
What are the two plexi of the enteric nervous system, and where are they located?
Auerbach’s Plexus (between longitudinal and circular layers)
Meissner’s Plexus (in the submucosa)
The myenteric plexus controls ______,
and the submucosal plexus controls ________
GI movement
GI secretion and blood flow
When the myenteric plexus is stimulated, what are its principal effects? (4)
- increased tone
- increased intensity of contraction
- increased rate of contraction
- increased velocity of conduction within the gut wall
The myenteric plexus inhibits _______
sphincters that prevent movement of food
Acetylcholine _______ GI activity
Norepinephrine ______ GI activity
excites
inhibits
The two types of movements in the GI tract are:
- Propulsive movements
- Mixing movements
What is the usual stimulus for peristalsis?
Distention of the gut
What is the “law of the gut?”
Why is it universally true?
All peristaltic movement moves toward the anus with downstream relaxation
The myenteric plexus is polarized in the anal direction
Peristalsis does not occur where the myenteric plexus is not present. Ever.
Blood vessels of the gut are part of the ________ circulation
splanchnic
All the venous blood from the gut, pancreas, and spleen empties into the:
protal vein
What happens to all the fat absorbed from the intestines?
It isn’t put into the portal vein with everything else
It’s absorbed into the intestinal lymphatics and carried to the circulating blood through the thoracic duct
It bypasses the liver
Blood flow to the gut increases with gut activity.
How is this increase stimulated?
- Vasodilating peptide horomones released from the mucosa
- Kinins released from GI glands
- Decreased O2 concentration
- Increased adenosine
Why are the villi particularly sensitive to low blood flow states?
Counter-current flow
At baseline, about 80% of oxygen passes directly from arterioles into adjacent venules without ever going to the tips of the villi
This is adaptive for high absorption, but leaves little wiggle room for hypoxia
What is the difference between hunger and appetite?
Hunger is an intrinsic desire for food
appetite is the type of food a person preferentially seeks
Most of the muscles of chewing are innervated by what nerve?
motor branch of CN5 (Trigeminal)
Why is chewing so important for digestion?
Digestive enzymes only work when they directly contact the surfaces of food particles
the more varied the surfaces, the more digestion occurs
What are the three stages of swallowing?
- Voluntary
- Pharyngeal (involuntary)
- Esophageal (involuntary)
Essentially all sensory impulses from the mouth wind up in which area of the brain?
tractus solitarius in the medulla
The esophagus exhibits two kinds of peristalsis:
Primary (continuation of the wave that began in the pharynx)
Secondary (esophagus initiates additional waves if the primary one is insufficient to move food out)
What kind of muscle controls the pharyngeal wall and the upper 1/3 of the esophagus?
Striated!
controlled by skeletal nerves from the glossopharyngeal and vagus nerves
NOT SMOOTH MUSCLE
What happens to the stomach as a wave from the esophagus approaches?
It relaxes and is inhibited
Describe the gastroesophageal sphincter
Tonically contracted at baseline
receptively Relaxes in response to waves from the esophagus
What prevents stomach reflux?
- Gastroesophageal Sphincter
- The esophagus has a valve like closure at the end
What are the three functions of the stomach?
- Storage of food until space opens up in the intestines
- mixing of food with secretions until it forms chyme
- Slow emptying of chyme into SI at a rate the intestines can handle
What is the capacity of a fully relaxed stomach?
0.8-1.5 L
The distal opening of the stomach is called the ______
pylorus
When a large amount of fats enters the duodenum, how does it communicate with the stomach to inhibit gastric emptying?
Hormones!
The fats extract these hormones from the wall of the duodenum
the hormones are carried via the blood to the stomach, where they inhibit the pyloric pump and increase the tone of the pyloric sphincter
Why do fats in particular play a role in inhibit gastric empyting?
They take the longest time to digest
they are the rate limiting factor, so the amount of fat should determine the speed of digestion
The number one hormone that inhibits gastric emptying is:
Cholecystokinin
Inhibits the motility caused by gastrin
Gastric inhibitory peptide (GIP) is also released when fat enters the duodenum. What does it do?
inhibits gastric motility slightly
mainly stimulates secretion of insulin from the pancreas
What chyme characteristics prevent further emptying into the duodenum?
Hyper or hypotonic
Excessively acidic
Too much unprocessed protein or fat
Highly irritating (for whatever reason)
The frequency of segmentation contractions in the small intestines is determined by:
the frequency of slow waves
maximum frequency is about 12 per minutes (but this is extreme)
What drug would cause profound inhibition of SI peristalsis?
Anything that block acetylcholine
esp. Atropine
What is the rate of net movement along the small intestines?
1 cm/min
3-5 hours to get from the pylorus to the ileocecal valve
Which hormones enhance intestinal motility?
gastrin
CCK
insulin
motilin
serotonin
Which hormones inhibit intestinal peristalsis?
secretin
glucagon
What is the gastroileal reflex?
when someone eats a meal, it triggers release from the ileum into the cecum of the large intestines (making room)
What is peristaltic rush?
powerful and rapid peristalsis caused by irritation of the bowel
goal is to sweep the irritant out of the SI and into the colon
The villi in the mucosa have tiny muscles that contract. Why is this important?
milk the villi so that lymph flow freely into the lympatic system
Which valve prevents backflow from the colon to the SI?
ileocecal
How much chyme normally empties into the cecum each day?
1500-200ml
Someone with an inflamed appendix will have almost no passage of chyme from the ileum to the LI. Why?
Inflammation and irritation in the LI inhibit the ileocecal sphincter and ileal peristalsis
In the colon, mixing movements are known as _______
and propulsive movements are known as ________
haustrations
mass movements
How long does it usually take to move chyme from the ileocecal valve to the rectum?
8-15 hours
Series of mass movement usually persist for ______ minutes
10-30
What reflexes initiate mass movements in the colon?
Gastrocolic and duodenocolic
Also: massive irritation in the colon
Why do patients with ulcerative colitis have frequent bowel movements?
The irritation causes almost perpetual mass movements in the colon
The internal anal sphincter is composed of _______ muscle
the external anal sphincter is composed of ______ muscle
smooth
striated
The external anal sphincter is innervated by what nerves?
Pudendal
To effectively lead to defecation, two reflexes are generally necessary:
myenteric defecation reflex
parasympathetic defecation reflex
need innervation from both nervous systems
Why does putting off pooping lead to severe constipation?
It’s not very effective to poop without reflexes
If you wait to poop until the reflexes are gone, it’s really difficult
so the poop just sits there and gets harder
What is the peritoneointestinal reflex?
Renointestinal reflex?
Vesicointestinal reflex?
If the peritoneum is irritated, it inhibits enteric excitation
kidney irritation inhibits enteric excitation
bladder irritation inhibits enteric excitation
What are the four types of alimentary glands?
- Goblet cells (single celled mucous glands)
- Pits (invaginations that contain specialized secretory cells)
- Tubular glands (stomach and upper duodenum)
- Complex glands (salivary, pancreas)
What types of stimuli activate the enteric nervous system?
tactile stimulation
chemical irritation
distention of the gut wall
What is the effect of parasympathetic stimulation on alimentary tract glandular secretion rate?
This is primarily true in which portions of the gut?
Increases
upper portions of the SI and some in distal large intestines
Secretion in the lower SI and the first 2/3 of the LI occurs in response to:
local neural and hormonal stimuli
not a whole lot of PS stimulation
How does sympathetic stimulation effect glandular secretion rate?
If only sympathetic: increases secretion
If superimposed of PS stimulation: reduces secretions due to vasoconstriction of blood supply to the gland
How do hormones regulate the secretion rate of alimentary glands?
Released from the GI mucosa in response to presence of food
absorbed into the blood and carried to the glands, where they stimulate secretions
Chemically, all GI hormones are ________
peptide/polypeptide
(water soluble)
How do gland cells develop their secretory products?
Just like any other protein
Ribosomes in the ER -> Golgi -> Vesicle Storage ->
Calcium mediated exocytosis
What’s so special about mucus?
- adheres tightly to food
- has enough body to coat the wall and prevent contact of food with the mucosa
- very slippery, allowing particles to slide along the epithelium
- makes feces adhere to itself
- Resistant to digestion
- amphoteric: can buffer small amounts of acid, and contains enough bicarb to neurtalize acids
What are the principal glands of salivation?
Parotid
Submandibular
Sublingual
Daily secretion of saliva is between ______ and _____ ml
800
1500
Saliva contains two major types of protein secretion:
- serous secretion (digests starches)
- mucus secretion (lubricates and protects)
Which salivary glands secrete serous substance?
Which secrete serous and and mucus substance?
Parotid
Submandibular and sublingual
Saliva contains large quantities of _____
and relatively low quantities of ______
K and HCO3
Na and Cl
The salivary glands are primarily controlled by _______ nervous signals from _______
parasympathetic
salivatory nuclei in the brainstem
Do smooth or rough objects in the mouth cause more salivation?
Smooth
Why do you salivate so much when you’re feeling nauseated?
when irritating foods are swallowed or there’s something wrong in the gut, saliva is producted
when swallowed, it helps to distance the irritating factor from the intestinal wall and dilute it
Esophageal secretions are entirely ______
mucus
In the stomach, oxyntin tubular glands produce which substances?’
Which substances do the pyloric glands produce?
Hydrochloric acid
pepsinogen
intrinsic factor
mucus
mucus and gastrin
A stomach oxyntic gland contains which three types of cells?
- Mucus neck cells
- Peptic (or chief)
- Parietal (or oxyntic)
What prevents hydrochloric acid from backing up into the stomach wall?
alkaline mucus
tight junctions in epithelial cells
The basic factors that stimulate gastric secretion are:
- acetylcholine
- gastrin
- histamine
Acetylcholine excites secretion of _______
Gastrin and histamine excite secretion of _______
pepsinogen, acid, and mucus
acid only
When does inactivate pepsinogen become activated into pepsin?
When it contacts hydrochloric acid
What does pepsin do?
proteolytic
only works in an extremely acidotic environment (pH 1.8-3.5)
Intrinsic factor is essential for absorption of ______
Vitamin B12
Why does chronic gastritis lead to pernicious anemia?
destroys parietal cells, which secrete intrinsic factor
Vit B12 can’t be absorbed and bone marrow won’t be stimulated to form RBCs
________ are the only cells that secrete hydrochloric acid
parietal cells of the oxyntic glands
What factors stimulate gastric secretion?
Acetylcholine
gastrin
histamine
What are the primary roles of gastrin and histamine
Strongly stimulate acid secretion by parietal cells,
but have little effect on the other cells
Describe the cephalic phase of gastric secretion
occurs before food enters stomach
sparked by sight, smell, thought or taste of food
30% of the gastric secretion associated with eating a meal
Describe the gastric phase of secretion
Vasovagal reflexes, enteric reflexes, and gastrin all cause secretion of gastric juice while food is in the stomach
60% of gastric secretion
Describe the intestinal phase?
presence of food in the upper portion of the small intestines continues to cause stomach secretion
Accounts for 10% of secretion
The most abundant enzyme found in pancreatic secretions is:
Trypsin
What is the role of trypsin and chymotrypsin?
split whole and partially digested proteins into peptides
do not cause the release of individual amino acids
Trace the pathway for pancreatic secretions leaving the pancrease
Produced in the acini
long pancreatic duct
joins up with the hepatic duct immediately before emptying into the duodenum
through the papilla of Vater
Which is surrounded by the sphincter of oddi
What prevents the pancreas from autodigesting?
The same cells that secrete proteolytic enzymes also secrete trypsin inhibitor
prevents activation of trypsin inside the cells
Since it inhibits trypsin, and trypsin activates all of the other proteolytic enzymes, trypsin inhibitor effectively inhibits them all
