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22 Cards in this Set
- Front
- Back
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Primary functions of the GI tract
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Digestion:
Mechanical -Mouth (chewing) -Stomach -SI Enzymatic -GI tract (Mouth, stomach, SI) -Accessory glands (salivary, pancreas, liver) Absorption (mostly upper 2/3 of SI, some in colon): -Diffusion -Transporters or carriers Elimination of Waste -GI tract (colon and rectum) -Accessory glands (liver) |
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Active transport vs diffusion substances
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Active transport
-AA -Sugars -Vitamins Diffusion -Water -Minerals -Vitamins -Lipids |
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Pathologies associated with motility, secretion, active transport, diffusion
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Motility
- Swallowing, GERD, vomiting, gastroparesia, ileus, diarrhea, constipation Secretion - Ulcer disease, malabsorption, diarrhea, pernicious anemia Active transport - Malabsorption, diarrhea Diffusion - Malabsorption, diarrhea |
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Levels of control of GI innervation
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Effector level
-Muscle, mucosa, vasculature Control level 1 -Enteric nervous system (myenteric and submucosal plexus) Control level 2 -Autonomic nervous system (parasympathetic and sympathetic) Control level 3 -CNS |
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Autonomic innervation of GI tract
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Extrinsic
Parasympathetic -Long preganglionic and short postganglionic -Ganglion embedded in wall of GI tract -ACh as neurotransmitter -Vagus nerve (esophagus to transverse colon) -Sacral spinal cord (remainder of colon) -Innervates enteric nervous system -Sensory fibers responsible for visceral reflexes -Results in increased motility and secretion Sympathetic -From paravertebral ganglia (T1-L3) -May synapse again in prevertebral ganglia (celiac, superior and inferior mesenteric) -Ganglion outside of GI tract -E/NE -Innervates enteric nervous system -Innervates blood vessels directly -Inhibits intestinal motility and secretions. Increases tone of sphincters |
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Enteric innervation of GI tract
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Myenteric plexus (Auerbach's)
-Controls motility Submucosal plexus (Meissner's) -Controls exocrine and endocrine secretions and blood flow Deep muscular plexus |
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Neurotransmitters in enteric nervous system
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ACh
-Activating NE -Deactivating At effector level: VIP and NO -Regulate blood flow -Inhibit intestinal motlilty (smooth muscle) -VIP also is excitatory to epithelial and gland cells. Vasodilatory ACh and SP -Smooth muscle contraction Opioids (dinorphin, enkephalin) -Inhibit contraction ATP -Inhibits smooth muscle |
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Interstitial cells of Cajal
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Intermediate between smooth muscle and neuron
- express c-Kit In close association to nerves Are innervated Establish gap-junctions with smooth muscle cells Play an important role in GI motility At least five different types of ICC 1. Associated with the myenteric plexus the ICC-MY (Auerbach) -Pacemaker -Between circular and longitudinal muscular layers 2. Associated with the submucosal plexus in the colon ICC-SM -Neuromodulation 3. Associated with the deep muscular plexus in small intestine ICC-DMP -Neuromodulation 4. Found within longitudinal muscle layers ICC-IM -Neuromodulation 5. Found on surface of muscle bundles in antrum of large animals ICC-SEP -Conduction |
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How do interstitial cells of Cajal work
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Slow wave electrical activity (basal electrical rhythm)
- Requires c-Kit stimulation by steele factor -Transmitted to smooth muscle through gap junction Are themselves innervated by ACh, NA, NO |
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Cephalic phase of response to meal
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The sight, smell and taste of food as well as mechanical stimulation of the oral cavity participate.
Salivation is stimulated by cholinergic nerves. Vagal reflexes inhibit contractile activity in the proximal stomach [receptive relaxation], stimulate acid secretion by parietal cells in the stomach and secretion of histamine by enterochromaffin-like (ECL) cells. In addition, vagal reflexes to the antrum stimulate secretion of gastrointestinal hormones and also induce a relatively small stimulation of enzyme secretion by the exocrine pancreas. |
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Gastric phase of response to meal
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Distention of the stomach activates stretch receptors initiating a number of reflexes which alter gastric, intestinal, colonic and pancreatic activities.
Gastric acid secretion is stimulated by vago-vagal reflexes and reflexes mediated by the enteric nerve plexus. If the meal contains protein, protein digestion products produced will stimulate secretion of gastrin further enhancing acid secretion. In addition to stimulating gastric acid secretion, these reflexes stimulate gastrointestinal motility. (increase in the contractile activity of the ileum and relaxation of the ileocecal sphincter [gastroileal reflex] and mass movement in the distal colon [gastrocolic reflex]. Vagal reflexes also cause a relatively small stimulation of enzyme secretion by the exocrine pancreas. |
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Intestinal phase of response to meal
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Chyme entering the small intestine initiates neural and hormonal mechanisms which alter the activities of the small intestine, pancreas, gallbladder and stomach.
As chyme empties from the stomach into the small intestine mechanical (distention) and chemical signals (osmolarity, pH, breakdown products) stimulate contractile activity and secretion. |
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Interdigestive phase of response to meal
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The interdigestive phase is the period of fasting after most of the components of the previous meal have been digested and absorbed and the stomach and small intestine are relatively empty.
This phase is characterized by brief periods of intense peristaltic contractions appearing first in the stomach and then in the upper, middle and finally the distal small intestine. These periods of intense contractile activity last for only 5-10 min and are followed by longer relatively quiescent periods. This pattern repeats at intervals of approximately 90 min and has been called the migrating motor complex (MMC). |
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Characteristics of GI smooth muscle
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Don't have arrangement of thick and thin contractile filaments
Increased ratio of actin to myosin No sarcomeres Have dense bodies which serve as internal skeleton for contractile proteins Caveolae -Small invagination in membrane -Similar role as t-tubules Cells arranged in bundles and are mechanically and electrically coupled -Mechanical (collagen and intermediate junction) -Electrical (gap junctions) Contractile activity regulated by ICC |
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GI smooth muscle myosin phosphorylation
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Need increase in intercellular calcium
Calcium binds calmodulin Activates myosin light chain kinase Phosphorylates myosin Phosphorylated myosin can interact with actin to make a complex and start shortening Removal of phosphate group by phosphatases yields relaxation |
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GI smooth muscle calcium concentration
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Neurotransmitter receptor coupled with calcium channel
-ACh binds to receptor which activates channel and calcium enters Membrane potential dependent channels -Channels open on depolarization Calcium release from sarcoplasmic reticulum upon membrane receptor stimulation Calcium removed by ATPases outside cell or goes back into sarcoplasmic reticulum |
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GI smooth muscle phasic vs tonic contraction
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Phasic
-Electrical stimulation -Rapid increase in calcium in cytoplasim -Cross bridges between actin and myosin -When calcium decreases, bridges decrease and force is removed Tonic -Sphicters -UES, LES, pyloric -Stimulus is maintained -Calcium spike and cross bridging is not maintained but force is maintained |
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GI smooth muscle: Basal electrical rhythm
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Originated in ICC-MY
-Requires c-kit (a membrane receptor) which is stimulated by steele factor Smooth muscle contraction results from an action potential that occurs as a consequence of a slow wave -Frequency of contraction can't exceed frequency of basal electrical rhythm # of smooth muscle cells recruited depends on how much ICC was depolarized Elecrical to mechanical is not always 1:1 |
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Motility patterns for different GI sites
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Esophagus, stomach, small intestine
-Peristalsis -Propulsive, causes transport -Nonpropulsive, causes mixing Rhythmic segmentation -Small and large intestine -Mixing Tonic contraction -GI sphincters -Blocking passages, separation |
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ICCs found in esophagus, stomach, small intestine, colon
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Esophagus
-ICC-IM (only in smooth muscle) Stomach -ICC-MY and ICC-IM in antrum -ICC-IM in fundus -ICC-IM in antrum Small intestine -ICC-MY, ICC-IM, and ICC-DMP Colon -ICC-MY, ICC-IM, and ICC-SMP |
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Mechanical activity of smooth muscle cells
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Altered by autonomic or enteric nerves
Nerve terminals preferentially form synapses with ICC which relay information to smooth muscle cell -ICC are targets of enteric excitatory and inhibitory nerve terminals |
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Integration of innervation (reflexes)
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Afferent fibers from chemoreceptors and mechanoreceptors in the intestinal wall travel to the myenteric and submucosal plexuses and to the CNS.
Reflex arch is completed either locally by motor and sensory interneurons (short reflex) or through central efferent fibers carried by the parasympathetic and sympathetic systems (long reflex). Wall distension, deformation of mucosa, and chemical stumulation are stimuli for intestinal reflexes. Activation of a sensory neuron initiates a motor response oral to the stimulus and an inhibitory response anal to the stimulus -Modulated by parasympathetic, sympathetic, and ICC |
