Lecture 1 & 2: (44/45) GI Regulation & Anatomy Flashcards
What are the overall functions of the GI tract?
- Absorb nutrients/water into circulation
- Participate in excretion of waste substances
Motility
Secretion
Digestion
Absorption
+ excretion
What are the different regulatory mechanisms in the GI tract?
- Endocrine
- Paracrine
- Neural
What is the only voluntary aspect of eating?
Swallowing
- the rest of MOTILITY is based on contractions of smooth muscle to reduce size of food and mix food with enzymes before it is propelled from the mouth to the rectum
What is secretion?
is the release of enzymes, biological detergents, mucus, ions and water in the GI lumen by the:
GI epithelia and
associated glands
(salivary, pancreas, liver, gall bladder)
What is the stomach’s defense mechanism to protect it from the acidic environment?
- Mucosal Lining (mucus protects)
2. High pH (bicarbonate secreted to neutralize the HCl)
What is the function of the liver?
Does NOT provide enzymes, but adds BILE which is important for digestion of lipids
What is digestion?
Macromolecules are converted to smaller absorbable molecules (glucose, maltose, etc)
- physical & CHEMICAL modification into carbs, amino acids, and lipids
What is absorption?
Process by which nutrients, electrolytes and water are absorbed from the GI lumen into the bloodstream.
- source of energy
- Components of biological structures
- Essential metabolic pathways
- Restore physiological activities
What is excretion? What gets excreted from the liver?
Excretion:
The GI tract :
stores and excretes waste substances from ingested food materials
Liver :
Cholesterol
Steroids
Drug metabolites
Colon STORES the fecal material and prevents it from backing up into the GI system since it is mixed with bacteria
What is the largest immune organ of the body?
GI tract
- open to external environment and vulnerable to infectious microorganisms
- many complex immune cells
Where are most enzymes found?
Duodenum
What are the associated GLANDULAR organs? Which of these is the only one NOT innervated by Vagas ?
- Salivary
- Pancreas
- Liver
- Gall Bladder
- Endocrine Glands or Cells
Salivary glands –> innervated by 9 and 10
What are the 6 sphincters of the GI system?
Which prevents the food from entering the airway? Which regulates the gastric reflux when abdominal pressure increases (ex: pregnancy)
- Upper Esophageal - (between pharynx and esophagus)
- Lower Esophageal - (between esophagus and stomach)
- Pyloric
- Sphincter of Oddi
- Ileocecal
- Internal and
- external anal
Upper Eso. = protects airway
Lower Eso. = regulates gastric reflux
What are the 3 major arteries supplying the abdominal organs and which organs specifically?
Splanchnic circulation = 25% of C.O.
- Celiac artery: supplies the liver, spleen and stomach.
- Superior mesenteric artery:
supplies the pancreas, small intestine, and proximal colon.
- Inferior mesenteric artery: supplies the distal colon.
Where does the venous drainage from the GI tract return to?
PORTAL VEIN!
- portal circulation to liver first, then hepatic veins into the Inferior Vena Cava
What is the purpose of Lymphatic Drainage in the gut?
This is important for the transport of lipids and lipid-soluble molecules (including some vitamins and drugs)
- too large for capillaries
- drained via thoracic duct to systemic circulation
What are the 4 layers of the gut wall?
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
What is the innermost layer of the lumen of the GI tract called?
MUCOSA
made of
- Epithelium (tight junctions)
- Lamina Propria
- Muscularis Mucosae
What are the 4 specialized cells within the epithelial layer of the MUCOSA?
- Absorptive Enterocytes
- majority
- digestion/absorption - Enteroendorcrine cells
- regulatory peptides released to regulate GI fx. - Gastric Mucosal Cells –> produce protons (parietal cells produce acid)
- Mucin - Producing Cells –> glycoprotein production
Which epithelium has no absorptive function:
- Esophogeal
- Intestinal
What type of cells make up both?
no absorptive = ESOPHOGEAL
- transports swallowed food
- SQUAMOS
Intestinal
- columnar type
- absorption/uptake
What is the surface area of small intestinal epithelium?
What is unique about these?
Villi and Crypts
- continuously renewed via programmed cell death (3-5 days)
- proliferative cells in the zone of intestinal stem cells
What is the function of villi? (named for this)
What extensions do they have?
What is a clinical correlate associated with Reduced Surface Area of the Villi?
- Unit of REABSORPTION
- Microvilli or BRUSH BORDER
- Celiac Disease:
- malabsorption
- flattened = less surface area and malnutrition due to malabsorption
What layer of the Mucosa is find immediately below the epithelium?
What is it composed of?
What is the next layer?
- Lamina Propria
- connective tissue, collagen, elastin
- glands, nodes, capillaries, nerve fibers - Muscularis Mucosae
- thin layer of SMOOTH muscle
What is the next layer after Muscular Mucosa?
What is the name for the dense network of nerve cells here? often referred to as the Little Brain in the Gut! (2 names)
TEST!!!
- Submucosa
- connective, collagen, elastin
- glands
- nerve trunks, blood vessels, lymph - SUBMUCOSAL PLEXUS OR
MEISNER’s Plexus**
of the enteric nervous system
- integrates motor & secretory activities
What is the function of the muscular externa/propria?
What 2 layers does it consist of (which is first off of the luminal side)
What lies between these 2 layers?
- Contractions help mixing and propelling contents of the GI tract
- Inner Circular Muscle layer
- Outer Longitudinal Layer
MYENTERIC PLEXUS in between
(Auerbach’s plexus) –> the other part of little brain in the gut
What is the outermost layer of the GI tract?
What cells does it consist of?
What is its function?
- Serosa
- Squamous Epithelial
- Keeps organs suspended in cavity
- Viscous secretions to lubricate the abdominal organs and REDUCE FRICTION
Which is GI most active?
intermeal period or the post-prandial period
- POST - PRANDIAL
- Intense activity
Relative Quiscence = intermeal
What secretions are completely neural and do not require endocrine or paracrine mechanisms?
SALIVARY secretions
CN 9 and 7
What are the sensor cells of the GI system?
What happens when their receptors are activated?
Enteroendocrine cells
(EEC’s)
- respond to stimulus by secreting regulatory peptide or hormone
- signal transduction cascade occurs!
(can be stimulated w/ a meal & WITHOUT neural input
Endocrine regulation requires that what occurs? (2)
- EEC hormones bind to membrane receptor in target cell to trigger a SIGNAL TRANSDUCTION cascade
- Enters the circulation!
ex: Gastrin (hormone) is released by G cells in the distal part of stomach – stimulates acid secretion by the ECL and parietal cells (gastric corpus or body).
What is paracrine regulation?
How is it different from endocrine regulation?
- Where a chemical messenger or regulatory peptide released from a sensing cell (often EECs) - acts on a nearby target cell by DIFFUSION through the interstitial space
- not transported via circulation!!
The following are regulated via paracrine or endocrine mechanisms?
- Histamine
- Seratonin
- Cholecystokinin
- Histamine - PARACRINE
- Seratonin - PARACRINE
- Cholecystokinin
- BOTH
it is released from the duodenum in response to dietary protein, lipid- acts locally on nerve terminals (paracrine effect) and on pancreas (endocrine effect).
Besides protecting the cells in the GI system from bacteria, what else do the immune cells release?
PARACRINE FACTORS
Is Celiac’s Disease a dysfunction in paracrine or endocrine regulation?
What is the main pathology of Celiac Disease?
- PARACRINE
- Allergic response to GLIADIN in gluten
- causes inflammation in small intestine and reduces density & length of microvilli
= causes malabsorption of nutrients
Is CCK paracrine or endocrine regulated?
What does it do to the following:
- Gastric Emptying
- H+ secretion
- Food intake
- Pancreatic Enzyme secretion
- Contraction of gallbladder
BOTH!
DECREASES:
Gastric emptying and
H+ secretion;
- Food intake;
INCREASE:
Pancreatic enzyme
secretion;
Contraction of gallbladder;
What is the action of Peptide YY (PYY)? Is this paracrine or endocrine?
BOTH!
Decreases:
1. Gastric Emptying
& H+ secretion
- Pancreatic Enzyme secretion
- Intestinal Motility
- Food intake
What are the two sets of nerves innervating the gut?
How do each relate to the cell body of the gut wall?
Which is autonomic?
Intrinsic & Extrinsic
- Intrinsic = cell body is INSIDE gut wall
- submucosal plexus (little brain of gut)
- myenteric plexus - Extrinsic = cell body is OUTSIDE gut wall
- AUTONOMIC
What are the 2 parasympathetic subdivisions of the Extrinsic Neural System?
- Vagal
- esophogus, stomach, gallbladder, pancreas, 1st part of intestine, colon (proximal) - Pelvic
- distal part of colon & anorectal
Parasympathetic post-ganglionic cells secrete what? What type of neurons are these?
- Cholinergic & Peptidergic
- release acetylcholine or peptides (substance P, vasoactive intestinal peptide VIP)
long fingers, short arms
- long preganglionic fibers synapse in ganglia in the walls of the organ
Vagovagal reflexes are only efferent.
True or False
False
- mixed afferent (75%) and efferent (25%)
What brings information to the afferent fibers before they synapse with interneurons and stimulate the Efferent fibers?
SENSORY information from periphery
- Mechanoreceptors
- Chemoreceptors
What is the main function of the sympathetic nervous system in relation to the GI tract?
What are the exceptions?
INHIBITS GI tract
- exception:
GI sphincter muscles are activated by sympathetic innervation to constrict the muscles
= TONIC STIMULATION
post-ganglionic fibers are adrenergic and release epinephrine
What is the percentage of afferent to efferent in parasympathetic and sympathetic innervation?
parasympathetic
75 - afferent
25 - efferent
Sympatheitc
50 afferent
50 efferent
Which system
communicates with the following: (synapses in these areas)
- Longitudinal Muscle
- Circular Muscle
- Mucosa
- Myenteric Plexus
- Submucosal plexus (Meissner’s plexus - little brain)
Parasympathetic = muscle
Sympathetic:
myenterix plexus
& submucosal plexus
The majority of the gut activity is regulated by what neural system?
ENTERIC Nervous System (ENS –> Intrinsic)
- Myenteric Plexus
- Submucosal plexus
The myenteric plexus - between the longitudinal and circular muscle layers.
The submucosal plexus - in the submucosa.
Neurons in the two plexuses are linked by interganglionic strands.
What is released by the neurons of ENS?
Can the ENS act autonomously?
Neurotransmitters & neuromodulators
YES! Autonomic activity also called “little brain in the gut”
- receive input from extrinsic (parasympathetic & sympathetic) neurons which modulate their activity
Sympathetic & Parasympathetic
Synapses of the ENS (Intrinsic) are mostly where?
Submucosal & Myenteric Plexus
- they communicate with secretory cells, endocrine cells, and mechanoreceptors and chemoreceptors
as well as circular muscle, longitudinal muscle, and mucosa
What are the effectors of the Enteric Nervous system?
Motility
Secretion
Blood Flow
What is the function of the following:
- Acetylcholine
- NE
- VIP
In terms of :
- Smooth muscle (relaxed or contracted)
- Salivary and gastric/pancreatic secretions
- Acetylcholine
- contraction of smooth muscles in wall
- increase salivary, gastric, and pancreatic secretions - NE
- relax smooth muscles in wall (opposite)
- contract sphincters
- increase salivary secretion - VIP
- relax smooth muscle
- increase intestinal & pancreatic secretion
What is the function of the following:
- Neuropeptide Y
- Substance P
- Neuropeptide Y
- relax smooth muscle
- decrease intestinal secretion - Substance P
- contract smooth muscle
- increase salivary secretion
What permits the rapid cell-to cell spread of action potentials in the GAP JUNCTIONS of smooth muscles of the GI tract?
Interstitial Cells of Cajal!
low resistance gap junctions!
(test)
What is phasic contraction of the GI tract?
periodic contractions followed by relaxation – found in esophagus, gastric antrum, small intestine – involved in mixing and propulsion
What is tonic contraction of the GI tract?
a constant level of contraction or tone WITHOUT REGULAR PERIODS OF RELAXATION
– found in orad (upper) region of stomach and lower esophagheal, ileocecal and internal anal sphincters.
( no state of relaxation to prevent chyme from moving backwards)
When does contraction occur during an action potential?
At the end once the actin potentials have summated
What are unique features of GI smooth muscle electrical activity?
Do these elicit action potentials?
- SLOW WAVES
- oscillating depolarization and depolarization of the membrane potential
- NOT large enough to elicit an Action Potential
- if at the peak of the slow wave the membrane potential is depolarized up to threshold, the AP occurs on “top of” the slow wave
Where do slow waves originate?
Often called the “pacemaker cells for GI smooth muscle”
(Test)
Interstitial Cells of Cajal
- between longitudinal and circular layer of muscular external
** PACEMAKER CELLS for GI smooth muscle**
What do sub-threshold slow waves produce?
What about when AP’s occur on top of the slow waves?
- TONIC contractions (w/o action potentials)
- Phasic contraction
- when AP occurs on top of the slow waves
How are skeletal muscles different from smooth muscle?
Individual action potentials in the smooth muscle are NOT followed by separate twitches
- summate into one LONG CONTRACTION
The frequency and amplitude of the slow wave are controlled by what 3 influences?
- Neural
- Hormonal
- Paracrine
What are the 2 patterns of GI motility?
- Segmentation
2. Peristaltic
Where do Segmentation contractions occur primarily?
What is their main function?
Is there forward movement?
- Small & Large intestine
- MIXING of luminal contents w/ GI tract secretions & increase exposure to mucosal surfaces for absorption
- NO FORWARD movement
- allows chyme to merge & mix
Where do Peristaltic contractions occur primarily?
What is their main function?
Is there forward movement?
- Pharynx, esophogus, gastric antrum, small & large intestine also*
- PROPEL the chyme forward along the GI tract
- Contraction at the ORAD (behind) to the bolus, and relaxation in the CAUDAD (forward)
- chyme propelled toward CAUDAD directon
What neurotransmitters are involved in ORAD contractions? (behind)
In CAUDAD? (front)
2 for each
- ORAD
- ACh & Substance P - CAUDAD:
- VIP and Nitric Oxide
What is the function of the following:
- GRP (gastrin Releasing Peptide) /Bombesin
- Enkephalins (Opiates)
In terms of:
Gastric Secretion & Smooth Muscle contraction/relaxation
- GRP (gastrin Releasing Peptide) /Bombesin
* INCREASE gastric secretion - Enkephalins (Opiates)
- CONTRACT smooth muscles
- decrease intestinal secretion