Gastrointestinal Physiology Flashcards
what are the 5 main functions of the GI system?
propulsion secretion digestion absorption barrier function
what are 3 things involved with propulsion?
deglutition (swallowing)
peristalsis
mass movements (elimination)
what is transcytosis? what is it involved in?
transporting macromolecules across the interior of a cell
involved in absorption
what are components of the alimentary canal?
mouth esophagus stomach small intestine (duodenum, jejunum, ileum) caecum (appendix) large intestive rectum anus
what is notable about the structure of the esophagus?
starts as skeletal muscle then transitions to smooth muscle
swallowing starts as a voluntary action and becomes involuntary
what is the function of the appendix?
contributes to immunity
what is the major function of the large intestine?
formation of feces not water absorption
what are three accessory organs to the alimentary canal?
liver
gall bladder
pancreas
what are the 7 sphincters of the GI tract? which is voluntary?
upper esophageal lower esophageal pyloric ileorectal colorectal internal anal external anal - voluntary
what are the 4 major tissue layers?
mucosa (innermost)
submucosa
muscularis
serosa (outermost)
what are three components of the mucosa layer?
epithelium lamina propria (connective tissue) muscularis mucosae (produces the mucus)
what kind of epithelial cell is found in the mucosa layer? what structure is it different in and what is the specific epithelial cell?
simple columnar epithelium
esophagus - stratified squamous epithelium
what is the submocosal plexus?
a neuron network
what are 3 components of the muscularis?
circular smooth muscle
myenteric plexus
longitudinal smooth muscle
what is the function of the circular smooth muscle?
constrict and dilate lumen
what is the function of the longitudinal smooth muscle?
antagonistic muscle
run perpendicular to the gut and shorten the gut upon contraction and elongate it upon relaxation
what is the serosa?
visceral peritoneum covering the gut
What is a characteristic of the small intestine ultrastructure?
massive surface area
what allows the small intestine to have such a large surface area?
villi increases SA 10x
microvilli on villi increase SA 50x
what two proteins are responsible for smooth muscle contraction?
actin and myosin
what is different about smooth muscle compared to skeletal muscle?
no sarcomeres
there is less myosin and it is organized in different ways
dense bodies: intermediate filaments and actin
What are the 9 steps for smooth muscle contraction and relaxation?
1) high calcium comes in from extracellular fluid (SR)
2) calmodulin binds to Ca
3) MLCK (myosin light chain kinase) activated by calmodulin bound to Ca
4) MLCK adds phosphate group to myosin (myosin-P)
5) cross bridging occurs
6) Low calcium in the system now
7) Calmodulin unbinds from Calcium (inactivates MLCK)
8) Phosphate removed from myosin, shutting it off (removed by myosin phosphatase)
9) cross bridging no longer occurs (myosin can’t bind to actin)
What are 3 things that control GI movements?
segmentation
peristaltic waves
migrating motor complexes
what is segmentation?
squeeze one area to force contents away from that area
what are peristaltic waves?
waves of peristalsis that are weak and repetitive and are efficient in moving contents around
what is the function of lamina propria?
a thin layer of connective tissue on which the epithelium rests; houses GALT
what are two kinds of gastric movements?
mixing/churning waves
peristaltic waves
what is mixing or churning waves?
the first wave
production of acidic liquid chyme occurs
what are the peristaltic waves associated with gastric movements?
second stronger wave that occur when there is enough liquid chyme in the stomach
opens the pyloric sphincter
what is the function of the pyloric sphincter?
controls how much stomach contents get passed onto the duodenum
since chyme is acidic, it does not allow much through at one time
what is the myenteric plexus?
collection of neurons with local control of the enteric nervous system (sensory, motor, and interneurons)
what 4 excitatory functions does the myenteric plexus have?
Increases.. muscle tone contraction intensity contraction rate peristaltic velocity
what 2 inhibitory functions does the myenteric plexus have?
sphincter tone (when material needs to go from one place to another) back flow (prevent contraction from moving contents backwards)
How does intrinsic control of GI function work?
1) slow waves at rest: undulating changes
2) depolarization to threshold due to stimulation (~-40mV)
3) peaks of waves reach threshold causing spiked potentials (Ca influx)
4) The greater the stimulation, the greater the depolarization and higher the frequency, the stronger the contraction will be
5) hyperpolarization occurs after spikes to prevent excitability
what 3 things stimulate depolarization to cause spikes?
Stretch of gut
Ach
Parasym
what 2 things stimulate hyperpolarization?
NE
sym
is extrinsic control an independent or dependent function? what does extrinsic control coordinate with?
independent
coordinates with myenteric plexus
what are 3 components of extrinsic control of GI function?
ANS (Parasym and Ach; Sym and NE)
Hormones
Cognitive/emotional control
how can anger or stress modulate gastric function?
can increase stomach acid production, motility, and blood flow to counteract increased acid production
what regulates gastrointestinal reflexes?
intragut communication and various stimuli sending signals (distension, chemicals, and irritants)
what is an example of proximal to distal reflexes?
stomach to large intestine or small intestine
how does gastrointestinal reflexes work when going from proximal to distal reflexes?
deals with new contents
when stomach is stretched, excitatory effects on distal sections increase motility in large intestine to make room for more material coming
how does gastrointestinal reflexes work when going from distal to proximal reflexes?
deals with old contents
large intestine communicates with stomach to say there is enough material in the intestine and to slow motility (inhibitory)
are both reflexes working at once?
it is common that we have food in our stomach and contents in our intestine, so yes
which reflex is predominant? why?
distal to proximal will override proximal to distal because it’s more efficient in getting all nutrients out of the food and not cause distension
what is disturbed motility?
when motility is increased or decreased
what does increased motility lead to? why?
diarrhea
everything is being hurried along and can’t get all the water and nutrients out of the material
what is the protective benefit to increased motility?
bacteria that may cause bacterial infections are moved out as quickly as possible
what does decreased motility lead to? why?
constipation
things are moving slower so the body has more time to get nutrients/water out, leading to a solid feces that has trouble moving
what could constipation lead to?
distention of large intestine and could possibly tear the tissue
may also lead to perforation or death
what is chagas disease?
destruction of the myenteric plexus
what does codeine metabolism have to do with constipation?
binds to opioid receptors, alleviating pain and incr. sym NS leading to NE release
in myenteric plexus, it binds to u-opioid receptors, signalling 5-HT And NE release
NE: leads to inhibitory functions in gut and slow motility, contributing to constipation
what are 2 characteristics of NE that contribute to constipation?
1) antisecretagogue - blocks water secretion
2) decreases the release of Ach which decreases motility
how do prokinetics help treat constipation?
increase motility by targeting upstream ENS neurons (via Ach) and agonize 5-HT (turn off inhibory signals, turn on excitatory signals) and antagonizes DA-R (turn off inhibitory signals)
what is retropulsion?
vomiting (emesis)
why and how does retropulsion occur?
its a protective reflex
emetic centre in medulla that controls vomiting and there are many things going on (sensory, memory, bumpy car rides) that could regulate the change in direction of peristalsis
what needs to be done in order for vomiting to occur?
close epiglottis
control abdominal movement to get forceful contraction to force material up quickly
all needs to be coordinated in the right order and right time
why is excess vomiting significant to someone’s health?
losing stomach acid (low pH) causes to body tissue pH to go up and get alkalosis. This can lead to death
what are some treatment options for vomiting?
antiemetics: block pathway to emetic centre Receptor antagonists: serotonin-R histamine-R muscarinic-R dopamine-R
how much is secreted by the GI tract daily?
8.5L
what are 3 various functions of fluid secretion?
liquefaction - converting semisolid food into liquid chyme
lubrication - water and mucus secreted to keep things moving smoothly and w/o damage to epithelium
digestions - salivary glands and pancreatic secretion contain digestive enzymes
what locally controls secretion?
submucosal plexus and response to luminal contents
how much saliva is secreted a day?
1.5L
what are 4 functions of saliva?
1) moistening
2) lubrication to make trip down esophagus smooth (mucin found in mucous)
3) defence - contains lysozyme (ABA) and IgA
4) Digestion of carbohydrates
what controls salivation secretion?
1) local/central control
2) tactile stimulation (food in mouth)
3) parasym stimulation
4) cognitive stimulation (thinking about food)
5) autonomic NS (sym inhibits salivation)
what is a gastric pit?
depression in the gastric wall containing goblet (mucus cells)
and chief cells (secrete gastrin)
what is the gastric gland?
further depression after gastric pit that houses chief cells and pariteal cells (produce acid)
what 3 endocrine cells associated with gastric secretion?
ECL cells
G cells
D cells
how much gastric juice is secreted each day and what structure on the parietal cell is responsible for HCl production
~2500ml/day
H+/K+ pump
what are 3 functions of gastric acid?
1) bacteriocidal role - low pH makes it difficult for bacteria to survive
2) digestion of protein - denatures protein
3) digestion of bone
what 2 conditions could result from overproduction of gastric acid?
peptic ulcer disease
GERD
how is gastric acid produced?
K is pumped out, Cl- is pumped in
Cl- gets pumped out at another pump and HCO3 is pumped out as well
H+ is pumped out and joins Cl to form HCl
what are 3 secretagogues that increase secretion?
Ach
Gastrin
Histamine
what are 2 antisecretagogues that inhibit secretion?
PGE2 (prostaglandin)
NE
what are 5 things that are also considered gastric secretions?
pepsinogen - by chief cells
intrinsic factors - by parietal cells
alkaline mucus - by surface goblet cells
thin mucus - by mucus neck (goblet cells)
minor digestive enzymes (gastric lipase, gastric amylase, gelatinase)
what is a proenzyme? give an example
a non-functional enzyme that needs to be activated to become an enzyme
pepsinogen
what is the function of intrinsic factor?
vitamin B12 uptake
escorts vit B12 through the intestine to the site where it is going to be absorbed (ileum)
what is the function of alkaline mucus?
protection against acid and abrasion
what is the function of thin mucus?
lubrication
what is the alkaline tide?
a huge concentration/pH gradient, causing H+ to want to move to higher pH side
what side do H+ and HCO3 get pumped out into?
H - apical side
HCO3 - basolateral side
