Week 1 Flashcards
aa’s & monosaccharides are absorbed in
duodenum & jejunum thru secondary active transport
Na/K ATPases keep Na levels within enterocytes low; coupled transport w/Na often drives uptake of nutrients
Cobalamin (B12) & bile salts/acids are absorbed in
the ileum
All blood leaving the small intestine is directed thru the
portal vein to the liver
Metabolic vasodilators
CO2, H+, K+, adenosine
Chylomicrons
are too large to pass through capillary cells so lipids are absorbed through lacteals (lymphatics)
which empty into the bloodstream via the thoracic duct
Mesenteric Ischemia
Causes:
Occlusive mechanisms including thrombi (mesenteric infarction)
Non‐occlusive mechanisms: prolonged reflex vasoconstriction (due to hypovolemia, heart
failure) or abnormal levels of circulating vasoconstrictors (e.g, epinephrine, angiotensin II)
o Effects:
Postprandial Pain, Sitophobia
Necrosis of the tips of the villi
Loss of barrier function of the wall of the gutuptake of vasodilator toxins (endotoxin) from
the gutresults in Septic Shock
Secretory diarrhea
Infection leads to excess secretion of chloride (occurring in the crypts) drawing water into lumen
GI Smooth Muscle – Contractile Characteristics
o Rhythmic “phasic” (seconds) contractions and long “tonic” contractions (minutes to hours).
o Basal resting tension or “tone” is maintained without elevation in intracellular Ca++ and without
energy expenditure. (Ex: sphincters are tonically contracted; don’t use energy, just general tone) o GI smooth muscle has a remarkable ability to shorten (e.g., to 50% of resting length!!!)
o Can initiate depolarization in response to stretch leading to contraction.
GI Smooth Muscle – Excitation‐Contraction Coupling
o Slow wave electrical activity (3‐12/min, 5‐15 mV) initiated by interstitial cells of Cajal are phasic and propagated over a few centimeters to neighboring cells
o Slow waves generated by increase in Ca followed by repol. by K+ channels o Amplitude, but not frequency, of slow waves can be altered by signals
releasing calcium from internal stores or opening Ca++ channels on plasma
membrane
o Muscle contraction accompanies action potential.

o Excitation‐contraction coupling initiated by increases in intracellular calcium ion concentration
Binding of ACh to muscarinic Rincreased influx of Ca into cellactivation of calmodulin‐
dependent myosin light chain kinasephosphorylation of myosinincreased myosin ATPase activity & binding of myosin to actincontractiondephosphorylation of myosin by myosin light chain phosphataserelaxation or sustained contraction due to the latch bridge & other mechanisms
Migrating Motor Complex
relaxation of sphincters (fully opens all sphincters) and contractions in stomach
and small intestine occurring during fasting (interdigestive) controlled by hormone motilinrepeated giant
waves that sweep from stomach to large intestine to empty everythingwhy gum doesn’t stay for 7 years
Submucosal nerve plexus
within small and large intestine, sensory and blood flow; Meissner’s
Myenteric nerve plexus
bw circular & longitudinal muscle layer from esophagus to internal anus; Auerbach’s
Afferent Sensory Neurons
o Excited by fast distension of the gut wall or chemical signals from the lumen of the gut transmitted to sensory neurons.
o Many of the sensory neurons are stimulated by serotonin (5‐HT) released from mucosal enterochromaffin cells (ECL)
Sensory neurons respond w/ a few AP’s followed by hyperpolarization (adaptation)
o Transfer information about gut environment to interneurons in myenteric plexus which relay signals
up and down the gut.
Efferent Motorneurons
o found primarily in the myenteric plexus
o usually unipolar in structure
o excited by fast EPSPsrespond with sustained trains of action potentials
o carry “efferent” information to GI smooth muscle, vascular smooth muscle, GI exocrine secretory
cells and GI endocrine secretory cells
o excitatory fibers release acetylcholine, neurokinin A and substance P
o inhibitory fibers release vasoactive intestinal peptide (VIP) and nitric oxide (NO) on smooth muscle
cellsrelaxation
Neural Reflexes influencing GI Function
o “Short” = involve only nerves of the enteric plexes; occur right in your GI wall
o “Long” = both CNS and ANS are involved
Extrinsic Autonomic Nervous System
o Afferent information leaving GI tract is carried by autonomic nerves
o Parasympathetic: mostly cholinergic of vagus nerve; stimulates activity of the enteric plexuses,
increases GI motility and secretory activity.
o Sympathetic: mostly adrenergic, generally inhibits activity of the enteric plexuses, decreases GI
motility, contracts GI sphincters, constricts GI microvasculature
Parasympathetic …
Sympathetic…
Parasympathetic increases activity: cholinergic, vagus“rest and digest”
Sympathetic stimulation decreases activity: noradrenaline, dopamine, neuropeptide Y
In general:
Parasympathetic …
Sympathetic…
Parasympathetic increases activity: cholinergic, vagus“rest and digest”
Sympathetic stimulation decreases activity: noradrenaline, dopamine, neuropeptide Y
Vago‐vagal Reflex
o Vagal afferent info is transmitted to autonomic centers in the medulla
o Vagal efferents coordinate excitatory (ACh) & inhibitory (NO) activity within the Enteric Nervous System to mediate peristalsis:
Acetylcholine
primary excitatory transmitter from sensory cells
& from motoneurons to muscle, epithelium, secretory cells and at
interneuronal junctions increase intracellular Ca++
Gastrin releasing peptide
released from vagal nerve endings to stimulate G cell secretion of gastrin.
Substance P
(tachykinin) ‐ an excitatory transmitter generally co‐released with acetylcholine.
Vasoactive Intestinal Peptide
(VIP) – Promotes motility
Relaxes smooth muscle in esophagus and stomach (“inhibitory”)
Stimulates fluid secretion and promotes dilation of the GI vasculature increase cAMP
Nitric Oxide
(NO) ‐ an inhibitory transmitter co‐released with VIP from inhibitory motoneurons,
hydrophobic‐ intracellular targets.
Hormone:Cholecystokinin CCK
Cell:
Cell detects:
Hormone release stimulates secretion of:
Hormone:
Cholecystokinin CCK
Cell: I cells (D/J)
Cell detects:
Fat
Amino acids
Hormone release stimulates secretion of:
Pancreatic enzymes Bile salts for fat uptake
Hormone: Gastrin
Cell:
Cell detects:
Hormone release stimulates secretion of:
Hormone:
Gastrin
Cell: G cell (stomach antrum)
Cell detects:
Amino acids
Pepsinogen
Hormone release stimulates secretion of:
H+ (from parietal cells)
Hormone:
Secretin
Cell:
Cell detects:
Hormone release stimulates secretion of:
Hormone:
Secretin
Cell: S cells (D/J)
Cell detects:
Acid
Hormone release stimulates secretion of: Pancreatic juice (bicarbonate)
Hormone:
Gastric Inhibitory Peptide or Glucose‐Dependent Insulinotropic Peptide (GIP)
Cell:
Cell detects:
Hormone release stimulates secretion of:
Hormone:
Gastric Inhibitory Peptide or Glucose‐Dependent Insulinotropic Peptide (GIP)
Cell: K cells (D/J)
Cell detects:
Carbohydrates Fat
Hormone release stimulates secretion of:
Inhibits gastric acid secretion
Stimulates insulin release from pancreas
Hormone:
Motilin
Cell:
Cell detects:
Hormone release stimulates secretion of:
Hormone:
Motilin
Cell:
Endocrine cells
Cell detects: Fasting state (released cyclically thoughout)
Hormone release stimulates secretion of:
Initiates Migrating Motor Complex
Paracrine Regulators
further modulate the response
Histamine (ECL cells) and somatostatin (D cells) = paracrine regulators
Borborygmi
rumbling noise created by movement of gas in bowels
Sympathetic
associated with T1‐L2 spinal cord segments = Fight or Flight
Function: inhibit peristalsis and vasoconstrict
Preganglionic: cell bodies lie in thoracolumbar lateral horn; relatively short
Single sympathetic preganglionic axon has many collateral branchesmay synapse with ≥ 20 postganglionic neuronssynapse with several visceral effectors
Terminate in paravertebral or prevertebral ganglia; ACh is neurotransmitter
Postganglionic: highly branched; long; release NE as neurotransmitter (except for sweat glands)
Parasympathetic
associated with brain stem (cranial nerves) and spinal cord segments S2‐S4 = Rest, Read, and Digest Function: stimulate peristalsis, relax sphincters, stimulate glandular secretion
Preganglionic: fibers originate in cranial and sacral regions; relatively long; use ACh
Postganglionic: short; release ACh
Esophagus innervation
fibromuscular tube extending from cricoid cartilage to stomach (11th thoracic vertebra) o Upper 1/3: parasympathetics from recurrent laryngeal nerve
o Esophageal plexus: below bifurcation of trachea
Parasympathetics: from vagus (CN X) – supply smooth muscle and glands
Post‐ganglionic sympathetics: from ~T1‐4 – directly or via cardiac or pulmonary plexuses innervate vascular
smooth muscle
Vagus provides VA from esophagus; thoracic splanchnic branches carry visceral pain from esophagus
