Physiology Flashcards
What are the seven sphincters of the GI tract?
UES LES pyloric sphincter sphincter of Oddi ileocecal internal anal external anal
What area of the GIT is Meissner’s plexus best developed in? What does it control?
best developed in the SI and controls GI secretions
Where is Auerbach’s plexus found? What does it control?
it is between circular and longitudinal layers of muscle in muscularis externa and is well developed throughout the GIT; controls GI movements
What does the enteric nervous system control?
motility, exocrine and endocrine secretions, and microcirculation of the GIT
How is the enteric nervous system connected to the autonomic neural network in CNS?
by parasympathetic and sympathetic nerves
What are examples of intermediate cells which the ENS is able to indirectly influence effector systems?
endocrine, interstitial cells of Cajal, cells of immune system (mast cells)
Where are primary vagal afferent neuron cell bodies?
nodose ganglia
Where are primary sympathetic afferent neuron cell bodies?
dorsal root ganglia
The presence of extrinsic nerves from the autonomic nervous system is not necessary for may of the functional characteristics of GIT, but is needed for ____________
coordination of most GI functions
Afferent fibers come from what, within the gut wall and then run to the CNS, which then influence central and neural control
chemoreceptors and mechanoreceptors
What nerves contribute the parasympathetic innervation of the GIT and what area do they go to?
vagus nerves (above and first 2/3 of the transverse colon) pelvic (sacral) nerves (below and distal of the transverse colon)
Where do parasympathetic, pre-g fibers terminate? what do they share their post-g fibers with?
ganglia of the intramural plexuses and share their post-g with the ENS
When the vagus nerves are activated, what happens?
mostly excitatory - increasing motility or secretion
What two neurotransmitters are considered excitatory? What two neurotransmitters are considered inhibitors?
excitatory - ACh and SubP
inhibitory - VIP and NO
Where do sympathetic post-g adrenergic fibers arise from?
prevertebral ganglia
Most sympathetic fibers synapse where? This releases norepinepherine that does what?
intramural plexuses
NE inhibits parasympathetic ACh release by activating alpha2 presynaptic receptors
ultimately decreasing motility and secretion
Pathways which influence the muscularis externa (____________) and muscularis mucosae (__________) act through the intramural plexuses.
inhibitory
stimulating
Sympathetic fibers innervate blood vessels of the gut, directly causing what? through what mechanism?
causing vasoconstriction (alpha1 mediated) which reduces secretion
Circular and longitudinal smooth muscle is arranged in ________, connected together via _______________ allowing ion movement
bundles
low resistance gap junctions
What is a major determinant of how far the electrical signal is transmitted in smooth muscle down the GI tract?
excitability of cells in adjacent muscle bundles
What are slow waves?
oscillations in resting membrane potential (from ~-40 to -80 mV) that occur at a given rate
What is the basic electrical rhythm?
the rate at which slow waves occur
What are pacemaker cells called?
interstitial cells of Cajal
What is the BER in the stomach? in the duodenum? ileum?
3/min in stomach, 12/min in duodenum, slows on way to ileum
The BER of each segment is set by a group of faster cells called what?
pacemakers
What characteristic of slow waves can be modulated by intrinsic and extrinsic neural input? What increases or decreases this?
amplitude;
sympathetic input decreases amplitude,
parasympathetic increases amplitude
If _______________ exceeds threshold, action potentials may be triggered. The more APs that occur, the greater the contraction and force (tension) developed)
slow waves
What phase of AP is due primarily to Calcium entry into the cell through L-type Ca channels? Repolarization occurs coincident with increasing what?
initial rising phase
Gk
What is a major source of calcium for concentration?
extracellular calcium entering visceral smooth muscle cells during an AP
Initial AP occurs near what part of the slow wave?
peak on the rising phase
What contracts only in response to APs?
intestine
What can contract in the absence of APs?
stomach
Mixing movements of the GIT that are local constrictive contractions are called what?
segmentation
What is the stimulus for peristalsis?
distention
What is the peristaltic reflex?
The law of the gut - segment on the orad side of a distention contracts first and moves sequentially caudad. simultaneous reflex relaxation of the smooth muscle in front of the distention through ENS
What is the strong wave of contractile activity that spreads down through the GIT between meals called? When does this occur? What is its purpose?
Migrating Motor Complex (MMC); occurs once every two hours during the interdigestive (fasted) period; sweep clean the GI lumen between meals, moving undigested contents >2mm in diameter to move out of stomach and into SI
What are four things that control contractile activities of the GI system?
- CNS/ANS
- ENS
- electrical coupling between cells
- humoral factors
Neuromuscular contractions- the circular layer of smooth muscle makes the gut __________________. the longitudinal layrer of smooth muscle makes ___________________.
circular layer - longer and thinner
longitudinal layer - shorter and wider
What two transmitters excite smooth muscle contraction?
ACh and Sub P
What two transmitters relax GI smooth muscle?
NO and VIP (vasoactive intestinal polypeptide)
What transmitter is released by enteric interneurons?
serotonin (5-HT)
What hormone increases the force of stomach contractions?
gastrin
what hormone is responsible for MMC (migrating motor complex?
motilin
Where is CCK released and what does it cause?
secreted in the SI in response to acid chyme entering the duodenum;
decreases gastric motility
increases GB contractility
What does secretin do?
Decreases motility of most of GIT
What does Gastric Inhibitory Peptide (GIP) do?
mildly decreases gastric motility
What are the three phases of swallowing?
- oral or voluntary phase
food bolus to back and tactile receptors initiate swallowing reflex at pharyngeal level - pharyngeal phase
prevention of food going into nasopharynx and trachea; UES relaxes, superior constrictor initiates peristalsis (coordinated by swallowing center in reticular formation) - esophageal phase
UES constricts, primary peristalsis continues down and then secondary peristalsis is a second wave occurring d/t continued distention if the primary wave did not completely clear (regulated by swallowing center)
Describe the pressure at the UES at rest and when swallowing.
at rest - about 40 mmHg
when swallowing, pressure rapidly drops, bolus passes, pressure rapidly increases, prevents entry of air
Describe the LES pressure and tone at rest
resting pressure about 30 mmHg
sphincter is tonically contracted by extrinsic and intrinsic nerves, hormones, and neuromodulators
ENS alone can maintain constriction (extrinsic nerves can be cut and maintained tone)
What is LES relaxation mediated by?
a vagovagal reflex that causes relaxation of the circular muscle layer
What is achalasia? What is it associated with?
a condition where the LES does not fully relax during swallowing. associated with degeneration of nerve ganglion cells within the myenteric plexus (often a result of an autoimmune attack)
Describe the LES pressure in relation to gastric pressure
in normal person, the LES pressure is always higher than gastric pressure except during swallowing
What are three functions of the stomach and its associated motility?
- large reservoir
- fragment and mix food into chyme
- control rate of emptying from antrum to duodenum
What are folds of the mucosal lining of the stomach called?
rugae
swallowing causes receptive relaxation of the orad portion of the stomach; what is this mediated by?
vagal efferents: VIP and NO are the transmitters
Where are the interstitial cells of Cajal located? What do they initiate?
centrally in the body of the stomach
initiate the slow waves about 3/min
As pressure builds in the antrum, what will happen to chyme?
some is forced into the duodenum, most is forced back into the body of the stomach by retropulsion
The vagus does what two things in regard to emptying the stomach into the duodenum?
organizes both the peristaltic wave and influences force of the contractions
Describe the tone of the pyloric sphincter at rest.
tonically constricted, allows fluid and small particles to pass but prevents most of the chyme from entering the duodenum even when there are strong antral contractions
emptying of the stomach is regulated by bother neural and humoral mechanisms, with receptors in the SI that can sense and respond to what three things?
acidity, fat content, and osmotic pressure
What hormone is released in response to food in the stomach? What does this do?
gastrin - increases stomach motility and thereby promotes gastric emptying
What four factors tend to decrease gastric motility?
acids, fats, hypertonicity, and amino acids
What hormonally stimulates vomiting?
GI irritation causes paracrine release of serotonin 5-HT from enterochromaffin cells which stimulates vagal afferents to medullary vomiting center
What receptors mediate the “emetic trigger zone”? Where is this zone located?
dopamine, histamine, and serotonin receptors
located in the fourth ventricle
What is the first step of vomiting? Where does this begin?
reverse peristalsis, starting as low as the mid SI, copious salivation begins
The feeling of nausea is the indication that what two things have begun?
pyloric sphincter and stomach relaxation have begun
What is the physiology behind retching?
stomach contents are being pushed up in the esophagus and the UES remains closed
What is segmentation in the SI?
mixing contractions (chopping) and stretch (distention) by chyme initiating localized contraction of circular muscle layer at spaced intervals; muscle relaxes and it starts over at a location a segment away
What is the rate of segmentation determined by? What is the innervation of this?
that segment’s BER; duodenum 12/min ileum 8-9/min, frequency decreasing along length;
the ENS - myenteric plexus
What is intestinointestinal reflex?
overdistention of one segment of the intestine will cause reflex relaxation in the rest of the intestine through extrinsic nerves
What is gastroileal reflex?
stomach and ileum interact reflexly; increases activity in the stomach will increase motility of the ileum and accelerate the movement of chyme through the ileocecal sphincter
What is the gastrocolic reflex?
gastric distention will increase activity in the colon
What are the four phases of MMC?
- quiescence; slow waves, minimal mechanical activity
- irregular APs and contractions increase frequency and intensity
- intense electrical and mechanical activity lasting 3-6 minutes
- rapid decline in electrical and mechanical activity
Distention of the cecum causes the ileocecal valve to do what?
contraction
What three things relax the ileocecal valve?
- distention in the terminal ileum
- gastrin
- gastroileal reflex
Circular muscle in the colon contracts in intervals and causes what to occur? What is this similar to in the SI?
haustra formation similar to segmentation in the SI
Three strips of longitudinal muscle in the colon that can contract are called what?
taenia coli
What are propulsive peristaltic waves in the colon called? How often do they occur?
mass movements, occur 1 to 3 times a day
What two reflexes increase mass movement activity?
gastrocolic and duodenocolic reflexes
Describe the resting state of the rectum and anal sphincters,
rectum is usually empty, the internal anal sphincter is tonically constricted while the external anal sphincter is relaxed at rest
What is the rectosphincteric reflex?
a mass movement from the sigmoid colon fills the rectum, which then contracts, while reflexively relaxing the IAS and contracting the EAS
How is the EAS innervated and controlled?
striated muscle under voluntary control via the pudendal nerve
What physiological changes can cause diarrhea?
increased osmotic load, inflammatory exudate, increased secretion, and/or decreased absorption time
Irritation of the lower GIT can cause release of what hormone causing increasing water and motility?
serotonin
What four other reflexes of the intestine and its pair reduce gastric motility?
- peritoneointestinal reflex (peritoneum irritation)
- renointestinal reflex (kidney irritation)
- vesicointestinal reflex (bladder irritation)
- somatointestinal reflex (irritation of skin over stomach)
What are the three sources of gas in the GIT?
swallowed air, formed by bacteria, diffusion from blood
What are three roles of secretion?
facilitate digestion - enzyme secretion
provide protection - mucous secretion
provide lubrication - serous secretion
What are three types of mucous cells?
goblet cells, neck mucous cells, and surface mucous cells
What are the two principle locations of acinus glands and what kind of secretions do they have?
salivary glands and pancreas
produce watery secretions containing proteins such as digestive enzymes
What cells secrete a large volume of extracellular fluid into the GIT? Where are these located?
crypts of liberkuhn (water and electrolyte secretion)
in mucosa of SI
What four receptive type things will stimulate secretion?
tactile, chemical, distention, motility
What does sympathetic stimulation due to secretion
decreases secretion by inhibiting parasympathetic ACh release and decreasing blood flow (alpha1)
What are the glycoproteins that lubricate food and where are they made?
mucins made in the submandibular and sublingual glands
At what pH does the function of salivary amylase stop?
<4
What three things in saliva have antibacterial action?
lysozyme, lactoferrin and peroxisomes
Parasympathetic stimulation of salivary glands causes what to happen?
vasodilation and increased blood flow
ACh activates endothelial NO synthase, and NO causes relaxation of VSMC
Why is saliva hypotonic?
because sodium and chloride are readily reabsorbed in excretory ducts leaving water behind.
What are four characteristics unique to salivation?
- saliva always hypotonic
- almost totally under neural control
- parasympathetic stimulation vasodilates
- sympathetic stimulation causes secretion (initially)
Acinar cell secretion of amylase, fluid, and electrolytes is dependent on what?
second messengers including cAMP, IP3, and Ca
increased cAMP (from NE) means increased what? increased Ca2+ (from ACh, and SubP) means increased what?
amylase
volume
What are six things in gastric juice?
salts, water, HCl, pepsins, intrinsic factor, and mucous
What can cause hypokalemia in regards to gastric acid? Why?
vomiting, because gastric acid contains more potassium than plasma does
What is the major anion in gastric acid?
Cl-
What is the tonicity of gastric acid in relation to plasma?
ranges from isotonic (high rates of secretion) to hypotonic
What converts pepsinogen to pepsin?
HCl
What are the morphological changes of parietal cells during acid secretion?
At rest, internalized intracellular canaliculi with tubovesicles in cytoplasm. Active cells have tubulovesicular system fused with intracellular canaliculi to open the lumen of the gland, exposing HCl-pumps
What is the alkaline tide?
When bicarb ion leaves the cell and Cl enters the cell against gradient. bicarb into blood, raising blood pH
Where is pepsin stored and how is it released?
stored in zymogen granules, released by exocytosis stimulated by histamine, ACh, gastrin, secretin, and CCK
What secretes insoluble mucous? soluble mucous?
insoluble mucous by surface epithelial cells (surface mucous cells) and insoluble mucous by pyloric glands
When cell damage leads to ulceration, what causes vasodilation?
histamine release
What increases mucosal blood flow, bicarb and mucus secretions, while stimulating mucosal cell repair and renewal?
prostaglandins
How does ACh stimulate HCl secretion? What blocks this?
from vagus, stimulates muscarinic receptors, opens calcium channels
atropine blocks
How does histamine stimulate HCl secretion? What blocks this?
from enterochromaffin-like cells (ECL) diffuses to parietal cells and stimulates H2 receptor (then activates cAMP); blocked by H2 receptor antagonists like cimetidine
Where is gastrin from that stimulates HCl secretion?
from G cells in the stomach antrum and duodenu; opens calcium channels
What are three phases of gastric acid secretion?
- cephalic - vagal reflexes to parietal and G cells (ACh and gastrin released)
- gastric - distention causes local and vagovagal reflexes to parietal and G cells (ACh and gastrin released)
- intestinal - protein digestion products in duodenum lead to stimulation of intestinal G cells and release of gastrin
When is HCl secretion high? low?
high HCl secretion in the evening, low in the morning
Acid less than 3 pH in the antrum of the stomach stimulates what?
somatostatin release by D cells, which decrease gastrin release by G, INHIBITING gastric acid secretion
Acid in duodenum causes what?
enterogastric reflex which inhibits acid secretion
stimulates release of both secretin and bulbogastrone which inhibit gastrin release as well as directly inhibiting parietal cells
Hyperosmotic solutions, fatty acids, and monoglycerides in the duodenum cause what?
GIP and CCK release, which inhibit acid secretion
Despite secretin and CCK inhibiting acid secretion, they stimulate what?
pepsinogen secretion
Someone losing gastric juices (vomiting or aspirating) can’t get just NaCl and dextrose replacement because why?
it will result in hypokalemic metabolic alkalosis (fatal)
How are secretions of the pancreas different when stimulated by secretin and CCK?
secretin - aqueous pancreatic juice
CCK - rich in pancreatic enzymes
What is in aqueous pancreatic juice?
Na, K similar to plasma
HCO3 and Cl major anions
initially hypertonic because of HCO3 but equilibriates with water to make it isotonic
What four proteases are released in the pancreas in inactive form?
trypsin, chymotrypsin, carboxypeptidase, ribonuclease and deoxyribonuclease
What prevents activation of proteases while in the pancreatic duct?
trypsin inhibitor
What are three lipases found in pancreatic juice?
pancreatic lipase, cholesterol ester hydrolase, phospholipase A2
What is the pancreas condition between meals in regard to neural control?
sympathetic stimulation, reducing enzyme secretion, secondary to reduced blood flow
How is parasympathetic neural regulation acting on the pancreas?
during cephalic and gastric phases of stomach secretions, accumulate enzymes, low secretion level
What causes pancreatic juices to be released?
chyme into duodenum triggers secretin to release pancreatic juice. CCK regulates proportion of enzymes in juice to chyme consistency
ACh, CCK, gastrin, and Sub P in the pancreas work on what?
Ca channels
Secretin and VIP in the pancreas work on what?
cAMP
What are Brunner’s glands stimulated by?
tactile stimuli
How are crypts of Libekuhn stimulated?
mechanical stimuli
How is splanchnic circulation arranged?
both in parallel (intestines and pancreas) and series (hepatic artery and portal vein into liver)
Sympathetic stimulation causes what kind of control on circulation?
temporary expulsion of up to 1 L of blood into systemic circulation from capacitance veins - providing autotransfusion
Parasympathetic stimulation causes what kind of control on circulation?
(PRIMARY)increases both the secretory and contractile activities of the gut - most are vasoactive and cause dilation(SECONDARY) - increasing metabolic activity and increases the production of vasoactive metabolites - causing increased blood flow
What protects capillaries in the gut from high systemic pressures and excess fluid filtration?
myogenic vasoconstriction
Low perfusion pressures result in what?
decreased blood flow, reduced substrates delivery and the accumulation of metabolic factors that cause vasodilation and increased blood flow
Control of local blood flow is closely linked to what in the hepatosplanchnic ciruclation?
metabolic activity of the tissue (secretory or muscular)
What is the escape phenomenon?
despite continued stimulation the splanchnic arterioles but not veins, will spontaneously vasodilate within minutes - protective mech to prevent consequences of countercurrent exchange - hypoxia
What are three reasons for biles secretion?
- digest and absorb lipids
- eliminate excess cholesterol and bile pigments
- eliminate heavy metals or phenothiazines
How does bile remain isosmotic?
the large number of inorganic cations present in the GB are bound to bile acids
What causes contraction of the GB?
vagal stimulation and CCK cause contractions of the GB and relaxation of the sphincter of Oddi
What is the standing gradient osmotic mechanism?
Na/K pump moves Na out of cell Cl follows outside cell is hypertonic water goes out of cell hydrostatic pressure moves water and electrolytes into capillaries
What is absorbed in the ileum?
bile acids - in portal blood, bound to plasma proteins
