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ENTERIC NERVOUS SYSTEM
SMOOTH MUSCLE: All smooth muscle is innervated by the autonomic nervous system.
- General Properties:
- Caveolae: Micro-pits allow for increased surface area on smooth muscle.
- No Striations: Thin and thick filaments run through in a random order. Smooth
muscle has relatively more thin filaments than thick.
- Plasticity: Smooth muscle is able to stretch to a greater length and compress
to a shorter length than skeletal.
- Calcium supply comes more from outside the cell rather than inside (in the SR),
as compared to skeletal.
- Slow, Sustained contraction as compared to skeletal muscle.
- MULTI-UNIT SMOOTH MUSCLE: Has high innervation density. This is the type of
smooth muscle found in Ciliary Muscle and Ductus Deferens.
- UNITARY SMOOTH MUSCLE: The type of smooth muscle found in gut.
- Sparse innervation compared to multi-unit muscle
- Functional Syncytium: Gap junctions allow intercellular communication.
- Shows spontaneous (basal) electrical activity even in the absence of innervation.
- High basal resting potential (-57 mV -vs- -80 mV) as compared to skeletal
muscle. Smooth muscle is more permeable to Na+ which accounts for
spontaneous electrical activity.
- SMOOTH MUSCLE CHANNELS:
- Electromechanical Channels: Channels that transduce electrical activity, in one
form or another, to mechanical activity of actin and myosin.
- Slow-Leaking Ca+2-Channels
- Ligand-Gated Channels
- Voltage-Gated Na+-Channels
- Pharmaco-mechanical Channels: Channels that employ a second messenger,
causing contractility without a change in the cell's electrical potential.
- SMOOTH MUSCLE CONTRACTION:
- Ca+2 enters cell ------> Calmodulin then activates Myosin Light-Chain Kinase
(MLCK) ------> MLCK then phosphorylates myosin, turning it on and enabling
it to interact with actin ------> contraction occurs.
- Regulatory step is binding of Ca+2 with Calmodulin.
SLOW-WAVES: The basal electrical tone of smooth muscle. No contraction occurs with
- Also called the Basal Electrical Rhythm (BER)
- Magnitude of change is 5 - 15 mV, caused by entrance of Na+ into cell. No Ca+2 is
associated with these waves so no contraction occurs with them.
- Basal Rhythm in Different Regions: Remember these waves are only electrical -- not
- STOMACH: 3 waves per minute
- DUODENUM: 12 waves per minute. In the duodenum, 30-40% of slow-waves
are associated with Ca+2 as Ca+2 is added to the cells.
ENTERIC NERVOUS SYSTEM: The GI nervous system is independent of the CNS. Activity
can go on without any CNS input.
- GI Plexes:
- MYENTERIC PLEXUS: Outermost plexus located between the two layers of
musculature -- between the muscularis circularis and muscularis longitudinalis.
- SUBMUCOSAL PLEXUS: Located in the submucosa, just outside the Muscularis
- EXTRINSIC REGULATORY INPUT:
- Chemoreceptors and mechanoreceptors from the GI-Lumen are an important
source of input. They are the origin of short reflexes (not involving the CNS)
that go through the two GI plexes in the enteric NS.
- VAGO-VAGAL (long) REFLEX: Generally stimulatory (increase motility,
- The Vagus carries both afferents (70%!) and efferents. Luminal receptors
send afferent signal back to the CNS via the Vagus.
- INTESTINO-INTESTINAL (short) REFLEX: Generally inhibitory, involving only
the Enteric NS, and completely independent of the Autonomic NS.
- SYMPATHETICS are inhibitory to the GI-Tract. They work primarily by presynaptic inhibition, thus inhibiting release of ACh. In this way we get smooth
- Norepinephrine binds to alpha1-Adrenoreceptors on parasympathetic nerve
terminals and thereby inhibit the release of ACh.
- Acetylcholine increases GI-Motility when it acts on smooth muscle.
- Norepinephrine decreases GI-Motility when it acts on smooth muscle.
- Enkephalin (Opioid) decreases GI-motility by inhibiting the release of ACh.
- VASOACTIVE INTESTINAL PEPTIDE (VIP): Acts directly on smooth muscle to
cause smooth muscle relaxation.
- It is localized with ACh in the Vagus Nerve.
- VIP is in local neurons, and is released when Vagal Fibers excite these
inhibitory neurons to cause relaxation: Vagus (Excitatory synapse) ------>
Turn on VIP neurons (Inhibitory synapse) ------> Relaxation.
- COLOCALIZATION: Enkephalins, VIP, NO, Serotonin, and a whole bunch of other
transmitters are localized along with ACh and NorE in the autonomic nervous
system. Depending on the nerve, whenever the ACh and NorE are released, so
will the other substances be released.
MYOGENIC CONTRACTILITY: The gut has some contractility without any nervous input
- Luminal contents will cause basal contractility without any nervous influence at all.
- Thus there is a constant inhibitory tone of VIP and NO on the gut, to prevent / slow
down this contractility.
PARALYTIC ILEUS: Loss of GI contractility.
- It can occur chronically from overproduction of Sympathetics.
- Post-Operative (Physiologic) Ileus is a very common occurrence with abdominal
TYPES OF MOTILITY:
- PERISTALSIS: Propulsion of material in the aboral (away from mouth) direction.
- Rate of peristalsis varies in region, but peristaltic generally gets slower as we
move down the tract.
- Peristalsis occurs by segmental hyperpolarization followed by depolarization of
- Mechanism: Bolus of food in a particular location stimulates mechanoreceptors
and chemoreceptors in the GI lumen, ultimately resulting in peristalsis:
- Relaxation of the muscle occurs distal to the bolus, so that the food can go
forward. This is mediated by VIP / NO.
- Contraction of Longitudinal Muscle layer also occurs distal to bolus,
because longitudinal contraction causes widening of the GI lumen.
- Contraction of the muscle occurs proximal to the bolus, in order to propel the
- There is a basal level of VIP inhibition in the muscle, and a bolus of food
turns off this inhibition: distension of lumen by a bolus will cause inhibition
of release of VIP / NO ------> contraction of proximal region.
- RHYTHMIC SEGMENTATION: Mixing and churning of materials without propelling
them forward in the tract.
- Only involved the circular muscle -- not longitudinal
- Common in small and large intestine
- TONIC CONTRACTION: Blocking of the passage of material, as in sphincters.
- Tonic Contraction is myogenic -- it doesn't depend on innervation.
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HORMONES, ENZYMES, REGULATORY SUBSTANCES AND STUFF
NEUROENDOCRINE HORMONES: All of below are either exclusively endocrine (glandular
secretions into bloodstream), exclusively neural (neurotransmitter) or both. All of below
serve regulatory (as opposed to digestive) functions.
- GASTRIN: Endocrine.
- STRUCTURE: Active part of peptide is on carboxy-end. It shares the last four
residues in common with CCK (Trp-Met-Asp-Phe), and it has a protective NH2
on the carboy end to help prevent degradation.
- PENTAGASTRIN Drug that mimics Gastrin, containing the last four residues
in gastrin, and therefore containing similar biological activity.
- Distribution: Gastrin is made by G-CELLS in the ANTRUM of the Stomach.
- It stimulates release of HCl in Parietal Cells.
- Also stimulates growth of gastric mucosa and proliferation of intestinal
- Intestinal Resection: If you cut out part of the intestine, higher levels
of Gastrin will result.
- Gastrin release is inhibited by acid in the stomach. Primary negative
- Gastrin release is stimulated by digested proteins and by Acetylcholine.
- CHOLECYSTOKININ (CCK): Endocrine and neural
- STRUCTURE: Biological activity is contained in last seven residues on carboxy-end, with last four residues in common with Gastrin, and with a protective NH2
on the carboxy terminus.
- Activity on PARIETAL CELLS: CCK in the stomach can bind to Gastrin
receptors to BLOCK the effects of Gastrin.
- Distribution: CCK is made from I-CELLS
- Stimulates contraction of the gall bladder
- Stimulates secretion of pancreatic enzymes.
- Inhibits gastric emptying as part of the Entero-Gastric Reflex. The presence
of CCK indicates that the duodenum is currently full and gastric emptying
should be slowed.
- CCK-release is stimulated by the presence of peptides in the duodenum.
- SECRETIN: Endocrine and neural
- Distribution: Secretin comes from S-CELLS in the duodenum.
- It inhibits stomach motility when released in Duodenum bia the Entero-Gastric
- Secretin-release is stimulated by acid in the Duodenum.
- SOMATOSTATIN: The universal inhibitory substance. It acts in endocrine, neural,
and paracrine fashion.
- Distribution: Somatostatin is all over the place.
- GASTRIC INHIBITORY PEPTIDE (GIP): Endocrine.
- Inhibits the release of Gastrin by a pharmacological mechanism. Thus the
effect is dose-dependent, and a large (non-physiological) dose is required
to elicit a response.
- Dr. Greenwald thinks this effect is secondary importance because it is
- Major fnxn = GIP stimulates release of Insulin from Pancreas
- DISTRIBUTION: Antrum of stomach + duodenum.
- VASOACTIVE INTESTINAL PEPTIDE (VIP): Primarily neural
- MOTILIN: Endocrine.
- FNXN: It elicits the Migrating Motor Complex in the small intestine, to propel
- GASTRIC RELEASING PEPTIDE (GRP) (Bombesin): Neural. Involved in the release
of Gastrin. Its release is Non-Adrenergic Non-Cholinergic.
- REGULATION: Its release stimulated during the Cephalic Phase of gastric
- ENKEPHALIN (an Opioid):
- FNXN: Decreases GI-motility by inhibiting the release of ACh.
- Pregnant women tend to gain weight because they have increased levels of CCK
(higher fat and protein absorption) and lower levels of Somatostatin.
- Higher CCK is especially marked during first trimester.
- INFANTS have very high levels of Gastrin to accompany their very high calorie-per-body-weight intake. Gastrin interacts with hypothalamus to somehow promote anabolic
growth in infants.
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- Anatomy and Pressures:
- Upper Esophageal Sphincter (UES): Skeletal muscle, essentially comprising
the cricopharyngeus muscle.
- Resting pressure = 50-60 mm Hg to prevent swallowing of air.
- Muscle tone is neurogenic and depends on CNS neural input from swallowing center to remain active.
- Body: Combination of skeletal and smooth muscle.
- Resting pressure = -5 mm Hg
- Lower Esophageal Sphincter: Smooth muscle, normally closed in order to
prevent gastric reflux.
- Resting pressure = 30 mm Hg
- LES contractility is myogenic. The way we relax the LES is by putting tonal
amounts of VIP / NO on the sphincter.
- VIP inhibition of LES is Non-Adrenergic, Non-Cholinergic (NANC). We
know this because Atropine does not prevent the inhibition:
- Give atropine, and the LES will still relax because VIP is not stopped.
- Give a VIP-Antibody and the LES will no longer relax because inhibition
has been removed.
- SWALLOWING REFLEX: Can be studied with Manometry (esophageal pressure)
- Oral Phase: 1 second, voluntary.
- Pharyngeal Phase: 1 second, involuntary. It is stimulated by the presence of
the slightest food or liquid (saliva) in the back of the throat.
- You cannot swallow if your mouth is absolutely dry.
- Aspiration of food is prevented:
- Respiration is inhibited from this point forward.
- Epiglottis is NOT important in preventing aspiration. Rather it is
adduction vocal cords that prevents food getting into trachea..
- Esophageal Phase: 8-10 second, involuntary Esophageal Peristalsis
- Esophageal Peristalsis is a Vago-Vagal (CNS mediated) Reflex.
- RELAXATION of Lower Esophageal Sphincter occurs early in the swallowing
reflex -- before the end of peristalsis of the esophagus.
- At the end of swallowing the LES should tighten up again to prevent reflux
of gastric contents.
- Types of peristalsis:
- PRIMARY PERISTALSIS: The initial peristalsis, initiated by the swallowing reflex.
- SECONDARY PERISTALSIS: Any subsequent peristalsis, to get any remaining
food out of the esophagus. It is initiated by distension of esophagus and
mechanoreceptors on smooth muscle.
- The UES does NOT open with secondary peristalsis. It doesn't need to open.
- ACHALASIA: Tonic high pressure at the LES, making it difficult to swallow. Failure
of LES to relax due to lack of VIP or because enteric system has been knocked out.
- ETIOLOGY: Could be caused by sympathetic over expression (Sympathetics will
cause relaxation via stimulation of VIP neurons) or by VIP under expression.
- Distended esophagus because food can't easily get to stomach.
- Lacking or uncoordinated peristalsis; or no peristalsis at all.
- Spastic uncoordinated contractions following meal.
- GASTRO-ESOPHAGEAL REFLUX DISEASE (GERD): Having an incompetent or
over-relaxed LES. Heartburn.
- Newborn babies don't have a competent LES, hence they burp up food a lot.
- Secondary peristalsis can help alleviate the symptoms by pushing unwanted
chyme back into the stomach.
- Esophagitis and Esophageal Cancer can result from chronic cases.
- Lying down after a meal (i.e. lack of gravity) worsens the reflux.
- PROPULSID = drug that causes contractions of the LES, hence a treatment for
GERD. It acts on ACh receptors to amplify the effect of ACh.
- RECEPTIVE RELAXATION: The stomach is a reservoir of food and can
accommodate large changes in volume. Pressure increase with more food is
- Mechanism: Vago-Vagal. More food ------> distend stomach wall and
activate mechanoreceptors ------> more VIP on stomach wall ------>
- It converts food to chyme.
- It controls the rate of Gastric Emptying, so duodenum doesn't get overloaded with
- BASAL ELECTRIC RATE: Stomach BER is about 3 events per minute.
- This number represents the maximum number of contractions that can occur per
- GASTRIC EMPTYING: The rate of movement of food from the antrum of the stomach,
through the Pyloric Sphincter (a true sphincter), and into the duodenum.
- General Properties:
- Retropulsion: Stomach contractions originating at antrum and going
backward, to prevent too rapid of gastric emptying.
- Liquids empty before solids.
- Fats are slowest emptying of all substances. CHO's and proteins empty
- Isotonic contents empty before hypotonic contents.
- Stomach acid impedes the rate of gastric emptying.
- ENTERO-GASTRIC REFLEX: Negative feedback from duodenum will slow down
the rate of gastric emptying, by multiple mechanisms. Basically, whenever there
is food in the duodenum, gastric emptying will be down-regulated.
- Acid in duodenum ------> stimulate Secretin release ------> inhibit
stomach motility via Gastrin inhibition
- Fats in duodenum ------> stimulate CCK and GIP ------> inhibit stomach
- Hypertonicity in duodenum ------> (unknown hormone) ------> inhibit
- ABNORMAL EMPTYING: The major role of the duodenum is to restore isotonicity.
- Dumping (Gastric Emptying) Syndrome = TOO RAPID emptying, which
can result from resection of part of the stomach
- SYMPTOM: Too rapid emptying ------> hypertonic bolus in duodenum
------> pull fluid in from circulation ------> Severe cardiac problems
- DELAYED EMPTYING can occur from diabetic neuropathy. It can cause
nausea, heartburn, and reflux.
- Basal Electrical Rate:
- Fastest is in duodenum (12 cycles / min). It gets increasingly slower as you move
- LAW OF THE INTESTINE: The decreasing electrical rate as you move through
tract is ultimately responsible for the movement of food in an aboral (i.e. forward)
direction. The general movement of food aborally is a result of the basal electrical
- MYOGENIC: Small intestinal motility is myogenic. If you give tetrodotoxin to kill
all the nerves, you still get motility.
- ILEOCECAL SPHINCTER: Smooth muscle sphincter which acts by short (intestino-intestinal) reflexes. Atropine has no effect on it.
- Distension on ileal side of sphincter ------> sphincteral relaxation ------> bolus
can pass through.
- Distension on colonic side of sphincter ------> sphincteral contraction ------>
bolus is prevented from moving backward.
- Basal Electrical Rate: The colon has the slowest of all BER's.
- HAUSTRATIONS: Slow segmental movements that move food very slowly through
colon. This movement is going on continually.
- Mass Movements result from GASTRO-COLIC REFLEX: Food entering into stomach
can cause much more rapid and forceful peristalsis in colon, ultimately resulting in
- This phenomenon will esp. happen in the morning.
- ANAL SPHINCTER: Internal Anal Sphincter is smooth and external anal sphincter
- As you increase pressure in rectum (distend it), two things happen:
- The Internal Anal Sphincter relaxes to accommodate the fecal matter.
- The External Anal Sphincter contracts to prevent defecation.
- Pooping is voluntary (usually).
- FIBER is food that is not digested nor digestible. A large constituent of fiber is
cellulose which human can't digest.
- Fiber lowers bowel transit time, especially through the colon.
- MIGRATING MOTOR COMPLEX (MMC): Housekeeping function throughout the small
intestine, to sweep bacteria aborally.
- The MMC occurs post-prandially, after a meal.
- MMC is caused by Motilin and Acetylcholine -- it is blocked by atropine.
- VOMITING (EMETIC) CENTER: In the medulla. The following receptors feed into
the vomiting center.
- Chemical Trigger Zone: Floor of the fourth ventricle, controlled by higher
- Apomorphine is a drug that stimulates the chemical trigger zone.
- Labyrinthine Receptors in the inner ear, effected by balance.
- Touch Receptors in throat (as in gagging reflex)
- Mechanoreceptors and Chemoreceptors in stomach and duodenum.
- RETCHING AREA: That region of the brain responsible for the act of retching
- OUTPUT: The vomiting act. Four muscle groups are stimulated in synchronous order
to produce vomiting.
- Four groups of muscles are stimulated: this is the process of retching, which is
reverse peristalsis accompanied by relaxation of esophageal sphincters.
- Inspiratory Muscles: Deep inspiration ------> negative thoracic pressure
to facilitate upchucking.
- Abdominal Muscles ------> positive intraabdominal pressure to facilitate
- Esophageal, gastric, and duodenal muscles all undergo reverse peristalsis.
- Esophageal sphincter (LES and UES) must relax for vomiting to occur.
- Massive Autonomic Discharge occurs with vomiting. combined sympathetic /
parasympathetic on salivary glands causes hypersalivation
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SECRETIONS and ABSORPTION
SALIVARY SECRETIONS: Average about 1500 mL a day.
- Secreted Substances: Major function of saliva is protection and digestion.
- Salivary Amylase: Secreted primarily by Parotid gland.
- Amylase normally operates at pH 7-8 and is therefore inactivated once in
the stomach. However, if it is inside a bolus of food and protected on all
sides then it can still be active even in stomach.
- Mucus: Secreted by the other glands (Mandibular and sublingual).
- FNXN: Lubrication of food and it serves as a buffer.
- Lactoferrin: Binds Fe in mouth, preventing bacteria from getting it. It thereby
serves as an antibacterial role.
- Lingual Lipase: Released from tongue itself, allows easy movement of fats on
- It can serve a backup function in case pancreatic lipase is lacking.
- Secretory IgA: Antibacterial secretions.
- Lysozymes: Antibacterial secretions.
- XEROSTOMIA: Dry mouth. It can lead to caries (cavities) because the anti-bacterial
salivary secretions are lacking. It also leads to impaired speech because saliva is
required for speech.
- HYPOTONICITY: Salivary secretions are hypotonic and concentrated in HCO3- and
K+, due to exchangers in the salivary ducts.
- High in HCO3- and low in Cl-, giving basic pH, from a HCO3-/Cl- exchanger in
- Low in Na+ and higher in K+, as a result of a Na+/K+-ATPase exchanger in salivary
- The salivary ducts are impermeable to water, so they retain fluid which results
- Nervous Stimulation of Salivation: Both sympathetic and parasympathetic cause
salivation, but parasympathetic is the primary one. Increased salivation generally
results from vasodilation ------> increased blood flow to salivary glands.
- Parasympathetic: Two pathways
- Cholinergic Pathway causes vasodilation via two mediators: It causes
production of kallikrein (a vasodilator) and it causes conversion of Plasma
Kininogen ------> Bradykinin (another vasodilator).
- VIP Pathway causes vasodilation directly on the vascular bed.
- GASTRIC EPITHELIAL CELL TYPES:
- Parietal Cells: Produce HCl
- G-Cells: Produce Gastrin.
- Mucous Neck Cells (along the length of the villus): Produce soluble mucous
- Chief cells: Produce pepsinogen
- Surface Mucous Cells: Produces insoluble mucous, which secretes HCO3-
which serves as a pH-Buffer for the mucosa -- especially in stomach.
- Stem Cells in crypt
- PARIETAL (OXYNTIC) CELLS: Produce gastric acid. Stomach pH 1.3
- Carbonic Anhydrase: Parietal cell creates carbonic acid via this enzyme: CO2
+ H2O <====> H2CO3 <====> HCO3- + H+
- The H+ is then secreted into the lumen.
- ALKALINE TIDE: The HCO3- is transported into the portal circulation and
goes to the duodenum. Transport occurs by a HCO3-/Cl- antiport.
- The Cl- then comes in and goes to the lumen where it joins the H+ to form
- PARIETAL CELL STIMULATION: Three things stimulate parietal cells in synergy
-- the effects are additive, but the effect of all of them together is greater than the
sum of the individual effects.
- Histamine: H2-Receptors are coupled to a G-Protein and act via the beta-adrenergic (cAMP) pathway.
- H2-Blockers block the histamine receptor.
- There is an H1-Receptor for histamine in the lungs, which mediates lung
- Acetylcholine: Muscarinic receptor that acts by the alpha-adrenergic
- Atropine blocks the ACh receptor, duh??
- Gastrin: Gastrin also acts by the alpha-Adrenergic pathway (IP3).
- Proglutamide blocks the gastrin receptor.
- CCK will block the gastrin receptor.
- H+/K+-ATPase symport brings the H+ into the cell. K+ gradient is maintained by
the traditional Na+/K+-ATPase.
- OMEPRAZOLE blocks the H+/K+-ATPase. Good drug for antacid.
- PHASES OF GASTRIC SECRETION:
- BASAL PHASE: 15% of secretion.
- CEPHALIC PHASE: 30% of secretion, occurs when food is seen, smelled, or
tasted. Stimulus originates from higher centers (hence "cephalic").
- Vagus Nerves releases Acetylcholine during this phase. ACh has two
- Stimulates release of Gastrin from G-Cells
- Inhibits Somatostatin release from enteroendocrine.
- Gastric Releasing Peptide is also released in stomach. This release is
Non-Adrenergic Non-Cholinergic. It also stimulates release of Gastrin.
- GASTRIC PHASE: When food enters stomach, about 50% of secretion.
- INTESTINAL PHASE: Post-gastric-emptying.
- NEGATIVE FEEDBACK: ACID is the primary inhibitor of Gastric secretions.
- Acid stimulates the release of Somatostatin, which turns off Parietal Cells and
- GASTRIC MUCOSAL ISCHEMIA: Ischemia of mucosa causes increased permeability
------> Gastric Ulcers
- Etiology: Lots of things; shock, burns, sepsis, trauma.
- Treatment: Use acid-reducers like H2-Blockers
- VICIOUS CYCLE: The excess acid can cause conversion of pepsinogen to pepsin
which will stimulate further acid release. That normally only occurs in lumen but
with a lesion it can occur in mucosa, and that is not good.
- HELICOBACTER PYLORI: Those little critters in the stomach that have been recently
proven to cause ulcers.
- Urease: These bacteria can survive in acid because they have high urease which
can take urea and create HCO3- and NH3 out of it, forming a good acid-buffer.
- ULCER treatment should include antibiotics to fight these bacteria, but H.Pylori
is not always found in ulcer patients! Criteria for determine presence of H-Pylori:
- Do a biopsy and identify histologically
- Grow cells in culture
- Measure the amount of the enzyme urease.
- INTRINSIC FACTOR (IF): Produces by parietal cells in stomach, it is necessary for
- Saliva: Vit-B12 combines with R-Protein.
- Stomach: Secretes intrinsic factor into bolus.
- Intestine: Vit-B12 lets go of R-Protein and binds to Intrinsic Factor
- Ileum: The Vit-B12/IF Complex is absorbed through special transporters. Without
the IF, only 20% of B12 is absorbed.
- PERNICIOUS ANEMIA: Autoimmune disease destroys parietal cells, thereby
destroying intrinsic factor source and resulting in B12-deficiency.
- ACHLORHYDIA is an overgrowth of bacteria in stomach resulting in low HCl secretion
which will cause high Gastrin levels.
- PEPSIN: Released as pepsinogen in chief cells. Acid converts the proenzyme to
pepsin. Pepsin is an endopeptidase.
- Pepsin can continue to activate itself once active.
- REGULATION: Following factors stimulate pepsinogen secretion, from most to
- Pepsinogen I found in Chief Cells.
- Pepsinogen II found in duodenum and correlates with duodenal ulcers.
- ETIOLOGY: Pancreatic tumor ------> Under secretion of Pancreatic Enzymes
------> Over secretion of GASTRIN due to no CCK.
- Peptic Ulcer Disease
- Increased Gastric Emptying.
- Diarrhea from hypergastrinemia
- Steatorrhea (fat in stool):
- Denaturation of pancreatic lipase due to acidic environment in the duodenum.
- Reduced Intrinsic Factor activity.
- General Properties:
- They are basic (pH = 8.2). Higher HCO3- at increased flow levels until it plateaus.
- They are isotonic (unlike salivary hypotonic)
- REGULATION: Secretin and CCK both stimulate pancreatic secretions.
- H+ ------> stimulates S-Cells to secrete Secretin ------> stimulates pancreatic
- Fats in duodenum ------> stimulate I-Cells to secrete CCK ------> stimulates
- BOTH CCK and Secretin are required for maximal (or near maximal) pancreatic
- Phenylalanine stimulates the release of CCK, and it coupled with Secretin results
in maximal HCO3- secretion from pancreas.
- ACID TIDE: Pancreatic Ductal Cells counter the alkaline tide with an acid tide.
- Na+/H+ Antiport transports H+ into the blood (which counteracts the HCo3- from
- Carbonic Anhydrase can then make lots of HCO3-, which it secretes into the
GALL BLADDER / BILIARY SECRETIONS:
- Gall Bladder concentrates Bile from the liver.
- NaCl is pulled out of gall-bladder cells, and H2O follows, so that bile becomes
- CHOLAGOGUE: Any substance that causes contraction of the gall bladder, such
- CHOLERETIC: Any substance that increases the flow of bile down the bile duct,
but does not affect bile synthesis. Example = bile salts.
- HYDROCHOLERETIC: Any compound that promotes the secretion (synthesis)
of bile in the liver, such as Secretin.
- Sphincter of Oddi keeps the bile in the gall bladder. It is tonically contracted when
no food is in the duodenum.
- CCK causes contraction of the gall bladder and relaxation of the Sphincter of Oddi.
- MICELLES: Bile Salts + Cholesterol + Lecithin
- LECITHIN (PHOSPHATIDYLCHOLINE): MICELLES require lecithin to function
at maximum efficiency. It increases threefold the fat-emulsifying capacity of
SYNTHESIS / STRUCTURE OF BILE ACIDS: Bile acids are synthesized from cholesterol
in the liver, stored in the gall bladder, and secreted through the common bile duct.
- SYNTHESIS of BILE ACIDS: 7alpha-Hydroxylation of Cholesterol: The key, rate-limiting step in bile-acid synthesis. This hydroxylation destines the product to become
- CHOLATE: The major bile acid. It has three OH-groups in the rings, and a
carboxylic acid at the end of the side chain.
- It is more amphipathic than deoxycholate because it has one more hydroxy
- DEOXYCHOLATE: The minor bile acid. It is missing one of the OH-groups (at
the 12 carbon)
- REGULATION of BILE SYNTHESIS: Negative Feedback from the Bile Acids
themselves. They inhibit 7alpha-Hydroxylase.
- SYNTHESIS OF BILE SALTS: LIVER -- Bile Acids esterified to Glycine or Taurine.
Bile Salts are even more polar than their corresponding acids.
- Glycocholate: Cholate with glycine added as an amide function.
- Taurocholate: Cholate with Taurine added as an amide function. Taurinine is
a modified cysteine.
- Glycochenodeoxycholate and Taurochenodeoxycholate are the minor bile salts.
- SECONDARY BILE ACIDS AND SALTS: Bacteria in the INTESTINE modify bile salts
by removing the 7alpha-carbon, to the "secondary" bile salts.
- ENTEROHEPATIC CIRCULATION: The circulation of bile between the intestine and
- New Synthesis: A small of bile acids are newly synthesized every day. This is
mixed in with bile acids that are recirculated.
- Conjugation: Adding the glycine or taurine to the bile acid to form a bile-salt,
before secretion. This occurs in the liver.
- Secretion: We secrete daily about 15-30 grams of bile acids into the GI tract.
- Deconjugation: Deconjugation and reduction of bile salts often occurs in the
intestine, aided by intestinal bacteria.
- Reabsorption: 90% of the bile acids are reabsorbed in the intestinal tract -- in
the ileum, after most nutrients have already been absorbed.
- Reabsorption sends the acids through the portal circulation and ultimately
back to the liver.
- Excretion: Some bile acids are excreted in the feces on a daily basis, about the
same as the amount that is newly synthesized every day. This is one way to get
rid of cholesterol.
- GALLSTONES occur when bile is composed of more than 15% cholesterol. This
makes cholesterol precipitate out of the bile solution and form stones.
- WOMEN are at higher risk for gall stones because estrogen tends to yield higher
cholesterol levels. Women taking BIRTH CONTROL are at even higher risk,
relatively, for same reason.
BILIRUBIN METABOLISM AND EXCRETION:
- Bilirubin is the normal product of heme-breakdown. It is carried by Albumin in the
blood stream, where it goes to liver.
- LIVER: Bilirubin is conjugated to Bilirubin Glucuronide, making it water soluble and
- Bilirubin Glucuronide is then secreted in bile to intestine.
- INTESTINE: Bilirubin Glucuronide is converted to Urobilinogen by stepwise
reductions mediated by intestinal bacteria.
- UROBILINOGEN FATE:
- 80% of it is them excreted in feces.
- 20% is resorbed.
- Most of that goes back to liver and is re-secreted.
- Some of that goes to systemic circulation and is then excreted in urine.
JAUNDICE: Caused by bilirubin build up, indicating problems with the liver.
- HEMOLYTIC JAUNDICE: Greater amount of heme breakdown ------> increased
bilirubin in intestine ------> excessive urobilinogen in feces
- This can be caused by Pernicious Anemia.
- OBSTRUCTIVE JAUNDICE: Obstruction of bile-duct, preventing Bilirubin Glucuronide
from entering intestine; it is therefore diverted to kidneys ------> excessive bilirubin
in urine + big drop in fecal urobilinogen
INTESTINAL ABSORPTION: Absorption is ultimately dependent on the Na+/K+ Pump to
create the gradient.
- SODIUM: Na+ is transported into the enterocytes by three mechanisms:
- Electrochemical Channels: (40%) Na+ simply moving with its electrochemical
- Na+/Nutrient Cotransport: (30% )There are several Na+-Cotransporters, for
glucose and for individual amino acids.
- Na+/Cl- Cotransport: (30%) "Neutral" cotransport of Na+ and Cl-, bring water in
- PERICELLULAR: Recent evidence says that majority of fluid appears to be absorbed
between cells rather than through cells (transcellular), although both occur.
- CHLORIDE: Cl- is absorbed throughout the intestine.
- POTASSIUM: Passively absorbed in small intestine, and secreted in large intestine.
- Three secretomotor neurotransmitters for enterocytes:
- Substance P
- ACh, both preganglionic and postganglionic
- VIP, Postganglionic
- Inhibitory neurons act on the preganglionics to prevent secretomotor excitation:
- Opioids: Patients on opioids can become quite constipated and get a condition
called "Narcotic Bowel."
- VIPOMA: Cancer causing excess release of VIP ------> excess secretion ------>
- There are two types of Diarrhea: Secretory and Osmotic
- SECRETORY DIARRHEA: Diarrhea caused by hypersecretion, via too much
secretomotor stimulation or too little inhibition.
- No Solute Gap -- fecal analysis is osmotically normal or hypoosmotic.
- The Diarrhea does not go away after the meal is gone.
- CHOLERA TOXIN: It causes increased basal levels of cAMP in enterocytes.
cAMP is not normally active ever in enterocytes. The result is that Cl-
channels on luminal membrane are blocked open ------> perpetual
- Even worse, the toxin also blocks Na+-Cl- Cotransport, reducing
absorption of fluid.
- Gastric Tumor ------> Over secretion of Gastrin.
- Zollinger-Ellison Syndrome
- OSMOTIC DIARRHEA: Diarrhea caused by hyperosmotic bolus in intestine -- it is
- SOLUTE GAP: Fecal analysis shows a large "solute gap," i.e. hyperosmotic
- The diarrhea goes away when food is out of GI-Tract.
- LAXATIVES cause osmotic diarrhea because their contents (magnesium) are like
fiber in that they are not absorbed ------> higher bolus tonicity.
- Gluten Enteropathy: Intolerance for wheat (gluten), causing blunting of
enterocyte brush-border ------> decreased surface area for absorption ------>
- Celiac Sprue also causes osmotic diarrhea.
- PEPSIN: Secreted by stomach. Endopeptidase.
- Pepsinogen ------> Pepsin by the action of H+ in the stomach.
- Mainly splits bonds between Tyr and Phe.
- TRYPSIN: Endopeptidase
- It is a Serine-Protease, i.e. it uses Serine as its active site to cleave proteins.
- Trypsin is activated by ENTEROKINASE, which is secreted in the intestinal brush-border.
- It converts Trypsinogen ------> Trypsin
- CLEAVAGE-SPECIFICITY: Trypsin cuts amino acids that are adjacent to Lysine
- Auto-Catalytic: Activated trypsin acts on trypsinogen to make more of itself.
- Trypsin also acts on Chymotrypsinogen to make Chymotrypsin!
- CHYMOTRYPSIN: Endopeptidase
- Chymotrypsinogen is activated by Trypsin.
- CLEAVAGE-SPECIFICITY: It cleaves aromatic and non-polar side-chains. It is
not as specific in its cleavage site as Trypsin. It will cleave any of the following
residues: Trp, Phe, Tyr, Met, Leu
- ELASTASE: Endopeptidase
- Elastase is activated by Trypsin, too.
- It converts Proelastase ------> Elastase
- CLEAVAGE-SPECIFICITY: It cleaves residues adjacent to Alanine, Glycine, and
- CARBOXYPEPTIDASE: These are exopeptidases that cleave at the carboxy-end.
- They are both activated by Trypsin just like above: Procarboxypeptidase ------>
- They are both Metalloprotease which require Zinc for catalysis. This is a
different mechanism than the endopeptidases which are serine proteases.
- These guys are secreted in the pancreas.
- Carboxypeptidase-A: Cleaves neutral and acidic side-chains (on the carboxy
end), such as Alanine, Valine, Isoleucine, Leucine.
- Carboxypeptidase-B: Cleaves basic residues -- Lysine, Arginine
- Every time Trypsin cuts a protein, you are left with a protein piece that has either
Lys or Arg at the carboxy-end! Carboxypeptidase can then take over to remove
- This in effect gives us a free amino acid of Lysine or Arginine which can then be
- AMINOPEPTIDASE: Exopeptidases that cut at the amino end of a peptide.
- This is secreted by the intestinal mucosa further along the small intestine
- This is a metalloprotease that requires Zinc and Manganese for catalysis.
- This cuts, usually on smaller peptides, one acid at a time off the amino end.
- DIPEPTIDASE: An Aminopeptidase, similar to above, which cuts two acids at a time
from the amino end of a peptide.
- PROTEIN ABSORPTION:
- About 70% of proteins are taken in as dipeptides and tripeptides. They can then
be further degraded by intracellular peptidases.
- About 30% are taken in as free amino acids, via Na+-Cotransport. There are
multiple Na+ transporters for the different classes (neutral, basic, acidic) of amino
- SALIVARY AMYLASE: Begins breakdown of starch in mouth.
- PANCREATIC AMYLASE: Breaks down alpha-1,4 (starch) linkages into disaccharide
components. Secreted by pancreas into lumen of duodenum.
- Cellulose = beta-1,4 linkage. That is not digestible by humans and thus
- alpha-1,6 Limit Dextrins branches can also exist in starch. They are broken down
- SUCRASE: Breaks down Sucrose ------> Glucose + Fructose
- Located in the intestinal brush-border.
- MALTASE: Breaks down Maltose ------> Glucose + Glucose
- Located in the intestinal brush-border.
- LACTASE: Breaks down Lactose ------> Glucose + Galactose
- Located in the intestinal brush-border.
- LACTOSE DEFICIENCY is a very common problem. It results in osmotic
- ISOMALTASE: Breaks down alpha-1,6 LIMIT DEXTRANS Branches in starch.
- Located in the Intestinal Brush Border
- SUGAR ABSORPTION: Disaccharides goto the enterocytes where they are broken
down by disaccharidases (as above). The monosaccharides are then transported as
- Glucose: Enters enterocyte via Na+-Glucose Cotransport, and enters bloodstream via facilitated diffusion on other side.
- Galactose: Uses the same Na+-Glucose transporter as above, but it has a lower
affinity for the transporter than glucose. So, if both sugars are present then
glucose will preferentially bind.
- Fructose: Passes through membrane by simple diffusion.
- Glucose-Galactose Malabsorption syndrome: Congenital disorder; mutation
in Na+/Glucose Cotransporters. The child was successfully raised on a Fructose
- GENERAL PROCESS of Digestion and Absorption:
- EMULSIFICATION by micelles. Bile salts facilitate the attachment of Pancreatic
Lipase to the lipids.
- There is an unstirred water layer right at the villus border. Due to its
detergent properties, micelles can penetrate that border.
- DIGESTION: LIPASE then breaks down the triglyceride ------> monoglyceride
+ free fatty acids.
- Fatty Acids diffuse through enterocytes by simple diffusion.
- RE-ESTERIFICATION: Fatty acids are re-esterified to triglycerides inside the
- CHYLOMICRON FORMATION: Chylomicrons are formed as protein coats +
cholesterol is added to the triglyceride.
- LYMPH: Chylomicrons enter circulation through lacteals ------> lymphatic
- Short Chain fats go directly into the portal blood -- not into lymph.
- LINGUAL LIPASE: In the tongue.
- PANCREATIC LIPASE: The main triglyceride cutter. It attaches to micelles, with aid
of bile salts, to facilitate Triglyceride ------> 2-Monoglyceride + 2 Fatty Acids
- PHOSPHOLIPASE A2:
- COLIPASE: Forms a "wedge" in fat globules which facilitates attachment of lipase.
- OVERSUPPLY of Pancreatic Enzymes: 25% intact pancreas is sufficient for adequate
fat absorption. There is great redundancy in the supply of pancreatic exocrine
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Copyright 1999, Scott Goodman, all rights reserved