Introduction to GI System (I & II)

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Dr. Cukierman March 16, 2011

What two systems regulate secretion and motility in the GI system?

CNS (central nervous system)
ENS (enteric nervous system)

What percentage of Cardiac Output goes to the GI system?

25%

List in order the layers of the GI tract.

Epithelium, lamina propria, muscularis mucosa, submucosa, circular muscle, longitudinal muscle, and serosa.

Where is Meissner's (submuscosal) plexus? Aurbach's (myenteric)?

Meissner's:
-Between the submucosa and circular muscle layers.
Aurbach's:
-Between the longitudinal and circular muscle layers.

What is the primary functions of the two plexi?

Meissner's (submucosal):
-Secretion and blood flow
Aurbach's (myenteric):
-Motility

What three interconnected mechanisms control the GI system?

Endocrine, paracrine, and neural inputs.

What specialized GI cell type starts the process of hormone release in the GI system? How can they be stimulated?

EEC (enteroendocrine cells)
Neural inputs, hormones, and substances in the GI lumen.

How do EEC signals reach their targets?

Through paracrine/autocrine stimulation. NOT through the circulation.

What are the two most powerful paracrine regulators of the GI system? What are their functions?

Histamine
-powerful stimulus for acid secretion
Serotonin:
-modulates contraction of smooth muscle

Describe the secretion site, stimuli, and action of Gastrin.

1. G cells of stomach.
2. small peptides/amino acids/distention/vagal stimulation
3. increases gastric H+ secretion and mucosal growth

Describe the secretion site, stimuli, and action of CCK.

1. I cells of the duodenum/jejunum
2. small peptides/amino acids/fatty acids
3. contraction of gall bladder

Describe the secretion site, stimuli, and action of Secretin.

1. S cells of the duodenum
2. H+/Fatty acids in the duodenum
3. Increased pancreatic secretion/decreased gastric H+...

Describe the secretion site, stimuli, and action of GIP.

1. K cells of the doudenum/jejunum
2. Fatty acids and glucose (oral)
3. Increased insulin secretion/decreased gastric H+ secretion

What is the normal function of sympathetics on the GI system? What is the exception?

Inhibitory.
Sphincter muscles.

What is the normal function of Parasympathetics on the Gi system?

Enhances GI function.

Is the CNS necessary for GI function?

No, the GI system can have full functionality without CNS input.

Do parasympathetic signals travel directly to the myenteric or mucosal cells? What about sympathetics?

Parasympathetics: No
Sympathetics: Yes (mostly to mediate blood flow)

Law of Intestine

Stimulation at one point in the intestines causes contraction behind the point and relaxation in front of it.

What are the actions of Acetylcholine?

1. Contraction of smooth muscle
2. INCREASED SALIVA, H+, and pancreatic secretions
3. Relaxes sphincters

What are the actions of NE?

1. Relaxation of smooth muscle
2. INCREASED SALIVA
3. Contraction of sphincters

What are the actions of VIP?

1. Relaxation of smooth muscle
2. Increased intestinal, pancreatic, and gastric secretions.

What are the actions of NO?

1. Stimulates VIP release

What are the actions of GRP?

1. Increased H+ secretion
2. Increased Pancreatic secretion

Does atropene block vagal stimulation of parietal cells?

No, the stimulation to increase H+ is mediated by GRP, not Ach.

What are the actions of Enkenphalins (opiates)

1. Contraction of smooth muscle
2. Decreased intestinal secretion

What are the actions of Neuropeptide Y?

1. Relaxation of smooth muscle.
2. Decreased intestinal secretions

What are the actions of Substance P?

1. Contraction of smooth muscle
2. Increased salivary secretion

What is commonly secreted with Ach?

Substance P

What generates slow wave depolarization in the GI system? Repolarization?

Depolarization: Na+/Ca2+ channels
Repolarization: K+ channels

Do neural inputs cause slow wave depolarization in the Gi system?

No, they are generated by "pace-maker" activity and depolarization/repolarize on their own.

How are neural inputs related to slow waves?

They augment or increase the amplitude of slow waves.

How does sympathetic activity influence slow waves? Parasympathetics?

Sympathetics:
-Decreases slow wave amplitude
Parasympathetics:
-Increases slow wave amplitude

A. Single twitch summation of mechanical responses
B. Rythmic activity produced by slow waves
C. Tonic contractile activity
D. Pharmacomechanical coupling

Identify what type of inputs are causing the graph.

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