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What are the functions of the GIT
Motility, secretion, digestion, absorption, communication, and immune system
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Components of the GI tract
- mouth
- pharynx
- esophagus
- stomach (4 compartments)
- small intestine
- large intestine
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Components of the accessory organs?
- teeth
- tongue
- salivary glands
- liver
- gall bladder
- pancreas
- appendix
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What are/describe four layers of the GI tract
- Mucosa - absorptive/secretory layer
- epithelium, lamina propria,
- lining epithelial cells, muscle portion, and also goblet cells
- made up of vili and microvilli
- Submucosa - it is thick, highly vascular, supports the mucosa, takes up nutrients
- glands and nerve plexuses
- Muscularis - segmented contraction and peristalsis
- Serosa - membrane that wraps, protects and lubricates
- -has a secretory epithelial layer and underlying connective tissue layer
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Key points of the control of the GI function
- GI TRACT HAS an independent nervous system within the gut wall
- -Enteric nervous system - has receptors sensory and motor neurons
- - receives extrinsic innervation from autonomic nervous system that has afferent neurons to signal to the CNS
- GI tract has an intrinsic endocrine system
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Explain the nerves involved with the enteric nervous system?
- Myenteric plexus - located between circular and longitudinal smooth muscle
- Submucosal plexus - located in the submucosa; interneurons minor
- Neurons - Sensory and motor
- input from the mechanoreceptors (wall) and chemoreceptors (lumen)
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Autonomic effector tissues?
Heart, blood vessels, respiratory airways, GI tract.
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What are the sympathetic effects?
- fight or flight responses
- postganglionic neurons release norepinephrine
- decrease GI tract motility
- decrease exocrine secretion
- decrease endocrine secretion
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Parasympathetic effects?
- postganglionic neurons release acetylcholine
- increase GI tract motility
- exocrine secretion
- endocrine secretion
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How is the neural control of the GI tract controlled?
- CNS (long reflex)
- ENS (short reflex; Myenteric plexus and submucosal plexus)
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How is the gut hormones of the GI orchestrated?
- endocrine cells - produce true hormones
- paracrine hormones - local factors
- columnar cells - broad base narrow tip at lumen
- collectively known as "Neurohumoral regulatory molecules"
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What are the neurohumoral regulatory molecules (peptides and nonpeptides)
- peptides
- cholecystokinin
- enteroglucagon
- gastrin releasing peptide
- gastrin
- motilin
- somatostatin
- VIP
- non-peptides
- ACH (acetetylcholine)
- NO (nitric oxide)
- ATP (adenine triphosphate)
- serotonin (5-hydroxytryptamine)
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What does gastrin stimulate?
- gastric secretion
- mucosal growth
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what does CCK stimulate?
- pancreatic enzyme secretion
- gallbladder contraction
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what does secretin stimulate?
pancreatic HCO3 secretion
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what does GIP inhibit?
gastric secretion
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What are the 3 steps to the endocrine system
- 1. Signaling cell - physiological stimulus, hormone synthesis, storage, and secretion
- 2. transport by the circulatory system
- 3. target cells - receptor, second messenger, and biological effects
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Hydrophilic hormones descriptions
- hormones stored in intracellular vesicles
- hormones released by exocytosis
- attach to transmembrane cells
- amplify signals via second messengers (cAMP, Ca, and IP3
- rapid biological effects
- act quickly and go away quickly
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description of hydrophobic hormones?
- hormones synthesized on demand
- hormones released by diffusion
- diffuse and attach to intracellular receptors
- hormone receptor complex binds to target gene
- mRNA transcription is altered
- slow but more sustained biological effects
- lasts a longer time
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description of endocrine?
- released into general circulation
- specificity is a property of the target tissues
- specific receptors are present
- Ex. gut hormones are gastrin, secretin, CCK, and motilin
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description of paracrine?
- released from endocrine cells and diffuse thru extracellular space to the target tissue
- somatostatin (D cells) inhibits gastrin release (G cells)
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description of autocrine?
a released substance that regulates the cells own function
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description of neurocrine?
sensory cells secrete neurotransmitter
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What are the 3 sphincters that divide the GI tract?
- esophageal sphincter - prevents heartburn
- pylorus sphincter - stomach to SI, when closed allows the stomach to have a high acid content
- ileocecal sphincter - bewteen SI and LI, you dont want to lose partially digested food to the LI this prevents that from happening.
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What is segmentation reflex and why does it happen?
- mixing of digesta
- contraction of circular muscle
- not propagated in one direction
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what is peristalitic reflex and why does it happen?
- programmed by the ENS and moves food down the GI tract
- it is accomplished by the contraction of longtitudinal muscle relaxation of circular muscle and then the opposite.
- moves in one direction
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What is the mechanism for contraction throughout the GI tract
- Basic Electrical Rhythm - smooth muscle has an intrinsic rhythm of depolarization and repolarization. Frequency is stomach ever 3 min, duodenum every 12 min, ileum every 9 min
- Contraction occurs when depolarization is high enough to cause an action potential
- Regulation - acetylcholine increases the resting basal membrane potential and stimulates action potentials and contraction, Norepinephrine decreases the resting basal membrane potential and prevents generation of actions potentials and contraction
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What is smooth and striated muscle controlled by?
- smooth - myenteric nerve plexus
- striated - somatic nerve
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What is the function of the fundus, corpus, and Antrum of the stomach?
- Fundus - store contents by adapting volume
- Corpus - mixing vat for saliva, food and gastric secretions
- Antrum - propulsion of food thru the pyloric sphincter to the duodenum
- controlled by distention, and parasympathetic
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Why is there lots of folding on the stomach?
allows the stomach to change size and stretch
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Factors that increase gastric emptying?
- neural control - distention of gastric wall, increased parasympathetic
- endocrine control (gastrin)
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factors decreasing gastric emptying?
- neural control - chemoreceptor, osmoreceptor mechanoreceptor at the duodenum (hypertonicity, H plus)
- increased sympathetic
- endocrine control (CCK, gastric inhibitory peptide, secretin)
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What is the segmentation reflex of the SI?
- major type of motility
- mix luminal content
- regulated by ENS
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What is the migrating myoelectric complex?
- peristalic activity
- interrupted during feeding in carnivores but not in herbivores
- has 3 phases - 1. quiescent period, 2. irregular contraction, 3. regular contraction
- motilin stimulate stomach and duodenum
- intrinsic nerves stimulates distal duodenum and jejunum
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Describe LI motility (functions)?
- functions - microbial digestion, reabsorption of water and electrolytes
- stationary haustral contractions
- peristaltic contractions
- antiperistaltic movement (fill cecum)
- aboral mass movement (evacuate entire length of colon
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Why does acid and pepsin not digest the stomach?
- epithelial cells are protected from acid by mucous layer
- mucous secretion increased by: cholinergic stimulation and mechanical stimulation
- bicarbonate secreted by epithelial cells is trapped in the mucous gel
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