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123 Cards in this Set

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Motility
The movements that mix and circulate GI contents and propel them along the length of the tract.
Secretion
Release of water and substances into GI tract from glands associated with it.
Digestion
breaking down food/large molecules to smaller absorbable molecules.
Absorption
Entry of nutrient molecules into the cells lining the GI tract; subsequently enter bloodstream.
5 layers of the GI wall (ext to lumenal)
1. Serosa
2. Longitudinal muscle
3. Circular muscle
4. Submucosa
5. Mucosa
What overlies the serosa?
The mesentary (dorsal) that keeps the GI tract attached to the body wall.
Function of longitudinal muscle:
Allows the GI tract to bulge and increase its volume
Function of circular muscle:
Contraction to narrow the lumen diameter
What structure lies between the longitudinal and circular muscle layers?
Myenteric nerve plexus!
What structure lies beneath the circular muscle layer? What is it important for?
Submucosal/Meissner's nerve plexus; senses the contents of the GI tract.
What is directly underneath Meissner's nerve plexus?
Submucosa
What is just under submucosa?
Mucosal muscle
What's under the mucosal muscle?
Mucosal glands and mucosal layer
What structures are found within the folds of the submucosa?
-Submucosal glands
-Mucosal glands
What is at the very innermost layer of the GI wall?
Epithelial cells
How does GI muscle function and what enables it?
As a syncytium - due to gap junctions between adjacent cells.
Function of gap junctions:
-Fast spread of ionic flow between adjacent muscle cells
-Moreso longitudinal than sideways
Normal resting membrane potential of gut smooth muscle:
-50 -> -60 mV
What is GI muscle contraction like?
Continual, slow, and Spontaneous - intrinsicly stimulated
What are the 2 types of electrical waves in GI sm muscle?
1. Slow oscillations
2. Spike potentials
Function of slow waves:
Controls the rythmic contractions of the gut
What are slow waves like?
(ie what do they not do, and what is their amplitude)
-NOT action potentials
-Range from 5-15 mV in intensity
What generates slow waves?
Interstital cells of Cajal - act as electrical pacemakers for smooth muscle cells.
Where are interstial cells located?
Between the circular and longitudinal muscle layers (Myenteric nerve plexus)
How are interstitial cells connected to the muscle cells?
By gap junctions
How fast are the oscillations of interstitial cell generated contractions in the:
-stomach
-duodenum
-ileum
Stomach = 3/min
Duodenum = 12/min
Ileum = 8/min
What causes the periodic changes in membrane potential of slow waves?
Periodic changes in Na/K ATPase activity - Na influx when membrane depolarizes changes intracellular ion concentration.
What is NOT caused by slow waves?
-NO calcium entry
-NO action potentials
-NO muscle contraction
What IS caused by slow waves?
Resting membrane slow depolarizations to generate spike potentials
At what potential are spike potentials generated?
-40 mV
How do spike potentials compare to skeletal muscle potentials?
Much slower (5x); take ~20mSec
What is the primary cause of Spike potentials?
Influx of calcium via Ca/Na channels
What is the nature of Ca/Na channels that cause spike pots?
-Slow to open
-Slow to close
-Primarily allow Calcium influx into muscle cells
What is the effect of the calcium influx during spike potentials?
Causes smooth muscle contraction!
What are 4 things that stimulate spike potentials / membrane depolarization?
1. Stretch
2. Acetylcholine
3. PNS
4. Specific hormones
What are 2 things that stimulate membrane hyperpolarization?
1. Norepi/Epi
2. SNS
What causes smooth muscle contraction?
Calcium - Ca/Calmodulin activates MLCK - activates myosin allowing for powerstroke.
What are the 2 layers that make up the Enteric Nervous System?
-Myenteric nerve plexus
-Submucosal/Meissner's plexus
Another name for Myenteric plexus:
Auerbach's
Function of the ENS:
Independent and autonomous Integration of motor and sensory activities of the GI system
What does the ENS also have input from?
PNS/SNS
How far does the ENS extend?
From esophagus to anus
Function of Myenteric plexus:
Control of GI movements
Function of Meissner's plexus:
Control of GI bloodflow and secretions
What type of autonomic nerves input into the ENS?
SNS - mainly postganglionics
PNS - mainly preganglionics
Where do visceral afferents from the GI tract extend?
-Prevertebral SNS ganglia
-Spinal cord
-Vagal nerves in the brainstem
What is the structure of the myenteric nerve plexus like?
-A linear chain of many interconnecting neurons
-Extends entire length of GI tract between musc layers
Myenteric plexus is important for controlling:
Motor activity in the gut
4 things increased by myenteric plexus stimulation:
1. Tonic contraction of gut
2. Intensity of rythmic contractions
3. Rate of Rythmic contraction
4. Velocity of excitatory wave conduction along the gut wall
General consequence of myenteric plexus stimulation:
More Rapid Peristalsis
Are all nerves in the myenteric plexus excitatory?
No; some are inhibitory
What NT do inhibitory neurons of the myenteric plexus release?
VIP (vasoactive intestinal polypeptide)
Function of VIP releasing neurons:
Inhibition of sphincter tone (pyloric and ileocecal valves)
General functions of the Sumucosal (meissner's) plexus:
-Control of inner intestinal wall functions
-Sensory input from lumen
What 3 things are controlled by the submucosal plexus?
1. Intestinal secretions
2. Absorption
3. Contraction of submucosal muscle for mucosal infolding
2 divisions of the PNS that give autonomic regulation to ENS:
-Cranial - mainly vagal
-Sacral - S2-s4
What portions of GI tract get
-vagal PNS input
-sacral PNS input
Vagal - foregut up to first 1/2 of transverse colon
Sacral - distal 1/2 of colon and beyond
What is sacral PNS input important for?
Defecation reflexes
Where do postganglionic PNS neurons reside?
In both ENS plexuses
What does PNS stimulation do in general to the GI tract?
Stimulates activity
Where do SNS preganglionic fibers for GI tract innervation originate? Where do they synapse?
-Originate at T5-L2 in spinal paravertebral chain ganglion
-Synapse at prevertebral ganglia
What are the prevertebral sympathetic ganglia?
-Celiac ganglion
-Mesenteric ganglion
What do postganglionic SNS fibers to the ENS release?
Norepi
How does NE release affect GI functioning?
-Directly inhibits intestinal tract sm muscle (minor effect) except for mucosal muscle
-Inhibits ENS NEURONS (major)
How does norepinephrine release affect Mucosal muscle?
It stimulates it! Hence the exception to NE's otherwise inhibitory effect on GI muscle.
2 Types of Afferent systems from the gut:
1. Entirely within the ENS
2. Exiting the ENS - cell bodies in ENS extend to prevertebral SNS ganglia and DRG to medulla; elicit Vagal reflex impulses.
3 things that stimulate visceral afferents from gut:
1. Mucosal irritation
2. Gut distention
3. Chemical presences
Where do visceral afferents from the gut go?
-Cell bodies in the ENS extend to prevertebral symp ganglia
-Cell bodies in DRG extend to spinal cord or medulla
What happens to the afferent impulses transmited by DRG to the spinal cord/medulla?
They get integrated and sent back via the Vagus to provide reflex control of GI functions.
3 types of GI reflexes:
1. Reflexes entirely w/in ENS
2. Reflexes from gut -> prevertebral SNS ganglia -> GUT
3. Reflexes from gut -> SC or brainstem -> Gut
4 Reflexes contained entirely within the gut:
1. Secretion
2. Peristalsis
3. Mixing contractions
4. Local inhibition
What reflexes use the gut -> prevertebral SNS ganglia -> gut reflex?
Long distance reflexes for coordinating different areas of the GI tract.
3 reflexes that use the prevertebral SNS ganglia:
What is the nature of each type?
1. Gastrocolic (stimulatory)
2. Enterogastric (inhibitory)
3. Colonileal (inhibitory)
Function of Gastrocolic reflex:
Evacuation of the colon when the stomach fills
Function of the Enterogastric reflex:
Inhibition of stomach motility and secretion when intestins fill.
Function of colonoileal reflex:
Inhibition of ileal motility when the colon fills
3 types of reflexes from the gut to spinal cord/brainstem and back to the GI tract:
1. from stomach to control gastric motility/secretions
2. Pain reflexes (inhibit entire GI tract)
3. Defecation reflex
What is hormonal most important for? What is it less important for?
Most important:Gastric secretion
Less important: GI motility
5 hormones that control GI motility (to some extent)
1. CCK
2. Secretin
3. GIP
4. Gastrin
5. Motilin
Where is CCK made?
I cells in duodenal and jejunal mucosa
Stimulus for CCK release
Fat presence in intestines
What does CCK stimulate?
-Gall bladder activity/motility to expel bile and emulsfy fats
What does CCK inhibit?
Stomach motility
Where is Secretin made?
S cells in duodenal mucosa
What stimulates Secretin release?
Acid in the duodenum
How does Secretin affect GI motility?
Mildly inhibits GI motility
Where is GIP made? Stimulus for release?
in the mucosa of the upper small intestine; stimuladed by FA, AA, and CHO
How does GIP affect GI motility?
-Decreases stomach motility
-Slows gastric emptying
Where is Gastrin made? Stimulus for release?
-Made in Gcells of stomach antrum
-stimulated by stomach distention
Effect of Gastrin on GI motility:
STIMULATES GI motility!!
What is special about Motilin?
It is the only gastric hormone to stimulate GI motility when the stomach is EMPTY - that is the stimulus for its release.
Where is Motilin made?
Duodenum.
Function of Motilin:
prevents stagnation of the intestines in the unfed person
2 types of GI motility:
-Propulsive/peristaltic
-Mixing
Function of propulsion:
Moves food along at the appropriate rate for digestion and absorption.
What is peristalsis?
The forward movement of a contractile ring around the gut wall
Stimulus for peristalsis:
Distention of the gut
What does distention do?
Causes the gut to contract 2-3 cm behind where the gut wall stretched.
What is MANDATORY for peristalsis?
An intact myenteric plexus
Why does peristalsis always move forward toward the anus?
Because of the law of the gut - retrograde peristalsis dies out.
What blocks peristalsis? (2 spcf things)
Absence of the myenteric plexus
-Atropine
-Congenital abnormality
What is the law of the gut?
-Peristalsis is always stimulated BEFORE the site intended for receipt
-Receptive relaxation always occurs AFTER site of constrictn
What is the nature of mixing movements within the gut?
Heterogenous
-Peristalsis against a closed sphincter causes churning
What is the special name for GI bloodflow?
Splanchnic circulation
Where does splanchnic circulation flow?
Through Gut
-Spleen
-Pancreas
-Liver
Where does all splanchnic bloodflow drain?
Into the portal vein to pass thru sinusoids and leave via the Vena cava.
What is the purpose of sending nutrient rich blood through the liver?
To allow RES cells to clean out bacteria and particulate matter.
Where do the non-fat, water-sol nutrients absorbed by the liver go?
Reticuloendothel and Hepatic cells absorb and store 75%
Where do fat-based nutrients go?
To intestinal lymphatics -> blood via the thoracic duct.
-Bypass the liver
What supplies blood to the stomach?
Celiac artery
What supplies blood to the intestinal wall?
Arcades branching from Superior and Inferior mesenteric aa.
What 3 structures do the mesenteric arteries serve?
1. Muscle bundles
2. Intestinal villi
3. Submucosal vessels
What are intestinal villi?
Intricate webs of capillaries for absorbing nutrients
What is unique about villi vasculature?
They have highl muscular walls for tight control of villus bloodflow.
What is countercurrent flow in villi refer to?
The diffusion of 80% of oxygen directly from artery to vein.
What are intestinal villi vulnerable to?
Ischemia
What is the predominant way to control GI bloodflow?
Via metabolic regulation
3 cases where GI bloodflow increases:
1. After a meal (high local metabolic activity)
2. During nutrient absorption
3. During motility
What happens to GI bloodflow between meals?
It returns to low resting levels
3 Mechanisms for Metabolic regulation of GI bloodflow:
1. Release of vasodilators during digestion
2. Secretion of KININS as GI glands secrete other things.
3. Decreased O2 / increased adenosine
How do PNS nerves regulate GI bloodflow?
Increasing PNS activity increases bloodflow to the GI tract
How does SNS stimulation alter GI bloodflow?
Decreases it by causing intense vasoconstriction.
Why does SNS stimulation vasoconstrict bloodsupply to the GI tract?
So that in situations where SNS outflow is high - heavy exercise, circulatory shock, etc, you want to shunt blood AWAY from splanchnics to skel muscle, brain, etc.