Term
| What are the functions of the digestive system? |
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Definition
- Movement - control of rate
- Secretion - of saliva, enzymes, and hormones
- Digestion - breakdown of material
- Absorption - In the small and large intestine
- Circulation - nutrient transport to other areas
- Nervous and hormonal control - ENS, SNS, PSNS. Endocrine and paracrine hormones. |
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Term
| What are the components of the GI tract? |
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Definition
| Tongue --> oral cavity --> esophagus/pharynx --> stomach --> small intestine --> Large intestine |
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Term
| What are the areas of the small intestine? |
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Definition
| Duodenum --> Jejunum --> Ileum |
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Term
| What are the components of the large intestine? |
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Definition
| Cecum (appendix offshoot) --> Ascending colon --> transverse colon --> descending colon --> sigmoid colon --> Rectum --> anal canal |
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Term
| What are the 3 major motor functions of the GI tract? |
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Definition
- Segmentation - churning and mixing, no propulsion. Only uses circular muscles.
- Peristalsis - movement along the GI tract via contraction behind and relaxation ahead of content
- Sphincters - control allows for movement control, organs can act as reservoirs. |
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Term
| What is initially broken down in the mouth? |
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Definition
| Only carbohydrates, by amylase |
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Term
| What is the function of the pharynx? |
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Definition
| Propulsion of material into the esophagus |
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Term
| What kinds of membranes are in the abdominopelvic cavity? |
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Definition
- Serous membranes - all are serous, produce a lot of lubrication
- Mesentary - serous membranes form 2 layers to keep organs suspended
- Retroperitoneal - mesentary is resorbed and lost. Pancreas, duodenum, rectum |
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Term
| What are the different mesentary membranes? |
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Definition
- Lesser omentum - keeps stomach attached to cavity
- Falciform ligament - keeps liver in place
- Greater omentum - can get fat and grow, falls over entire abdomen. Attaches to stomach in the front.
- Mesentary proper - keeps small intestine suspended
- transverse mesocolon - keeps large intestine suspended
- Sigmoid mesocolon - keeps sigmoid colon suspended.
**Remember - double layer, may have lymphatics |
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Term
| What happens in the appendix? |
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Definition
| If material is trapped there, can cause appendicitis. May be important in immunity or have no function |
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Term
| What is splanchnic circulation? |
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Definition
Abdominal circulation - 25%. The amount of blood depends on if you're eating.
**Portal vein - nutrients absorbed go here --> to the liver through the sinusoids |
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Term
| What arteries supply blood to the digestive organs? |
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Definition
The celiac trunk comes down off the aorta, and the common hepatic artery branched to the liver, the splenic artery to the spleen. Lower, the left and right gastric artery branch to the stomach.
To the intestines, the celiac trunk branches to the superior and inferior mesenteric artery
**Celiac trunk brings blood back towards the heart - a vein. |
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Term
| How is bloodflow in a microvilli arranged? |
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Definition
In a countercurrent mechanism - arteries next to veins.
**fats bypass the liver by drainage into lymphatic system via thoracic duct. |
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Term
| What are the 4 layers of tissue of the alimentary canal? |
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Definition
- Mucosa - Epithelium (high rate of division), lamina propria (GALT and MALT - lymphoid tissues), muscularis. Presence of goblet cells. Inner layer, has muscle for peristalsis
- Submucosa - nervous tissue, elastic fibers
- Muscularis externa - mixes and churns
- Serosa - Epithelial, attaches to wall of abdomen. Outer later. Visceral peritoneum containing squamous cells |
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Term
| What are the 2 different nerve plexuses in the gut? |
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Definition
- myenteric nerve plexus/Aurbach's - between circular and longitudinal muscles, regulates their activity
- Submucosal plexus/Meissner's nerve plexus - regulates glands and smooth muscle --> glands and secretion in the intestines
**The enteric nervous system - local control for mixing, propulsion, and absorption. The 'mini-brain'.
**Both are involved in peristalsis |
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Term
| What enhances GI motility? |
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Definition
Distention
Ach
PSNS outflow
**always an action potential present = always movement |
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Term
| What is the Law of the Gut? |
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Definition
The peristaltic reflex moves material toward the anus
1) Contraction of circular muscles BEHIND the food
2) Shortening of longitudinal muscles
3) Contraction of circular muscle layers forces food forward |
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Term
| How is digestive function neurally controlled? |
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Definition
- Long reflexes: PSNS maintains normal GI function - outflow from 5th and sacral nerves, SNS can decrease GI activity and control sphincters by synapsing post-ganglionicly via alpha2- outflow from thoracolumbar region
- Short reflexes: The ENS bypasses the brain and spinal cord |
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Term
| How does the PSNS work on the GI tract? |
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Definition
Preganglionic fibers extend from the brain or sacrum to act on a post-ganglionic fiber or inter-neuron. Post-ganglionic fibers located in the ENS
** Ach mediated |
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Term
| How does serotonin affect peristalsis? |
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Definition
| Sensory nerve endings are not directly stimulated by food. Serotonin is released from enterochromaffin cells by distention signal --> IPAN signalling leads contraction of circular muscles behind the bolus, relaxation in front if the bolus --> shortening of the longitudinal muscles --> bolus moves forward. |
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Term
| How does serotonin get into platelets and blood? |
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Definition
It is ALL made in the GI tract, 90% by EC cells, 10% by the myenteric plexus. A very small amount could be produced by the brain.
EC cells also store serotonin, and are mostly found in the duodenum and rectum. |
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Term
| Do antidepressants affect serotonin in the GI tract? |
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Definition
No, because other cells in the GI tracts lack SERT.
**TPH is the rate limiting step in serotonin synthesis. |
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Term
| What stimulates serotonin release? |
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Definition
- Stretch/distention from food ingestion
- Vagal/PSNS stimulation
- Osmolarity
- End products of digestion - taste receptors
- Infection - leads to N/V - large amounts
- Cytotoxic anticancer agents - large amounts |
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Term
| What are the functions of serotonin? |
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Definition
- Signaling molecule - depolarizes sensory nerves to initiate peristalsis, or in large amounts extrinsic sensory neurons --> N/V
- Motor function - Direct action on smooth muscle (MMC) not involved in peristalsis. Material that's not supposed to be there
- Neurotransmitter - released to act on other neurons --> release excitatory or inhibitory mediators (Ach, NO, VIP)
**A concentrated 5-HT release through 1P and 3 receptors leads to N/V |
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Term
| What serotonin receptors are involved in the GI tracts? |
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Definition
- 5-HT 1A - relaxation of peristalsis via VIP/NO
- 5-HT 1P - key receptor in the initiation of peristalsis - IPAN signalling leads to Ach release
- 5-HT 3 - located on extrinsic afferents. On ascending nerves, Ach is release. On descending nerves, NO is released. Involvement in the MMC.
- 5-HT 4 - located pre-synaptically, allows Ach release. Reglan works here. |
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Term
| What are the steps to the peristaltic reflex? |
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Definition
Serotonin is released from the enterochromaffin cells --> bind to 5-HT 1P in an IPAN reflex --> Ach release to act on Nn --> contraction of circular muscle behind bolus by M3. Serotonin also binds to 5-HT 4 --> release of more Ach.
**If no Ach is present, stimulation of 5-HT 4 does NOTHING.
**5-HT 3 on extrinsic reflexes --> contraction, N/V, or in at IPAN |
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Term
| What will be the effect of blocking the 5-HT3 receptor? |
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Definition
- Blockage does NOT affect peristalsis or secretions, since response also mediated by 1P and 4 receptors
- Excess stimulation results in N/V, cramping, and bloating |
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Term
| What reflexes exist in the GI tract? |
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Definition
- Gastroenteric tract - signals intestine that food is coming
- Gastroileal reflex - Processing food prepares last part of intestine for food
- Gastrocolic/Duodenocolic reflex - food in stomach or beginning or small intestine, prepares large intestine for food.
- Enterogastric reflex - INHIBITORY, a nerve reflex. Delays gastric emptying through contraction of the pyloric valve in the presence of too much food or acid.
- Intrinsic defecation reflex - Short reflex in the ENS, signals to
- PSNS defecation reflex - sacral outflow results in evacuation
- Orthocolic - defecation on arisin |
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Term
| What are the types of hormones that control GI tract activity? |
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Definition
1) Endocrine
2) Paracrine
3) Neurocrine |
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Term
| What are the major endocrine hormones? |
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Definition
- Gastrin - stimulates gastric release, histamine, mucus growth, and may inhibit gastric emptying
- Secretin - from the pancreas, stimulates secretion of bicarbonate/water into duodenum. Slows gastric emptying in response to acid.
- CKK - releases bile from gall bladder, and pancreatic enzymes from acinar cells
- GIP - slows motility and secretion
- Motilin - promotes motility during interdigestive period
** Secretin, CCK, and DIP are enterogastrones. |
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Term
| What cells are responsible for the release of GI hormones? |
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Definition
- Gastrin - released from G cells - Stimulus is protein. Too much acid provides a negative feedback to inhibit gastrin.
- Secretin - released from S cells
- CCK - released from I cells
- GIP - released from K cells - stimulus is fat and carbs
- Motilin - released from M cells |
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Term
| What paracrine and neurocrine hormones affect the GI tract? |
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Definition
PARACRINE:
- Somatostatin from D cells - INHIBITORY. Inhibits secretion of endocrine hormones. Relaxes vascular smooth muscle in the liver.
- Histamine - from EC-like cells --> release of gastric acid
- Serotonin - from EC cells due to distention and noxious stimuli
NEUROCRINE:
- Ach and NE
- Sub P - on sensory afferents, GI smooth muscle, and the CTZ. N/V and contraction
- VIP - in enteric nerves, distention releases. relaxes sphincters. Effects similar to secretin and GIP. |
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