Download Nervous and hormonal control of GIT Learning Objectives

Nervous and hormonal control of GIT
Learning Objectives
By the end of the lecture, the student should be able to :
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Describe the importance of GIT.
Mention the gastrointestinal peptides.
Describe the control of digestive functions
by nervous system.
What is mean by the term enteric nervous system.
Give us the role played by the gastrointestinal hormones in gastrointestinal motility.
6 ESSENTIAL ACTIVITIES OF DIGESTIVE SYSTEM
1. Ingestion - taking food into GI tract
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usually by mouth
2. Mechanical digestion - physically prepares food for chemical digestion by enzymes
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chewing, mixing w/saliva by tongue, churning of stomach, segmentation of intestines (food mixed over different parts of intestinal walls)
3. Chemical digestion - catabolism of foodstuffs into building blocks
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starts in mouth-->ends mostly in small intestine
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mostly in small intestine
4. Secretion – water, acids, enzymes, buffers, and salts
5. Absorption - passage of digested end products (+ vitamins, minerals, & H2O) from lumen of GI tract into blood or
lymph
6. Excretion (defecation) - elimination of indigestible substances from the body via the anus in the form of feces
Control pathways
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Both hormonal and neural
Short pathways: involves automatic regulation within the enteric system itself
Long pathways: involves the CNS (somatic and autonomic)
Three phases: cephalic, gastric and intestinal phases
Cephalic phase: salivary and gastric secretions
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Salivary secretion stimulated by parasympathetic NS by odors, sight, taste  saliva fluid and
rich in enzymes
Stimulated by sympathetic NS thick secretion, rich in proteins
Gastric secretion: increase acid and enzymes secretion in response to sight, smell and taste
of food
Gastric phase
• Stimuli: presence of food in the stomach (both distention and nutrients)
• Stimulation of the parasympathetic NS and secretion of gastrin (hormone)
• Response: increased motility and juice secretion
Intestinal phase:
• Arrival of nutrients in duodenum  decreased gastric secretion and motility
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Promotes secretion of cholecystokinin (CCK) and secretin
- CCK promotes:
- increased pancreatic enzyme secretion
- gallbladder contraction and sphincter of Oddi relaxation
- secretin promotes:
- bicarbonate ion secretion (pancreas)
- bile secretion
Enterogastric Reflex
CONTROL OF DIGESTIVE FUNCTIONS
BY NERVOUS SYSTEM
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Autonomic nervous system (ANS) is divided into
- ENS
Parasympathetic
Sympathetic
CONTROL OF DIGESTIVE FUNCTIONS
BY NERVOUS SYSTEM
Parasympathetic Nerves:
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Located in brain stem & sacral region
Projection to the G.I. are preganglionic efferents
Vagus & pelvic nerves
Vagus nerves synapse with neurons of ENS in esophagus, stomach, small intestine, colon,
gall bladder & pancreas
Pelvic nerves synapse with ENS in large intestine
Neurotransmitter is Ach
CONTROL OF DIGESTIVE FUNCTIONS
BY NERVOUS SYSTEM
Sympathetic nerves:
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Located in thoracic & lumbar regions
Neurotransmitter is NE
NE increases sphincter tension
Inactivate the motility
Enteric nervous system
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Entero = gut
Linked to CNS by PsNS (+) & SNS (-)
Contains 100 million neurons (as many as the entire spinal cord!)
Submucosal nerve plexus
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Regulates gland activity & smooth mm activity in the mucosa tunic
Myenteric nerve plexus
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B/t circular & longitudinal smooth mm layers in muscularis externa
Control GI tract mobility
CONTROL OF DIGESTIVE FUNCTIONS
BY NERVOUS SYSTEM
Enteric Nervous System (minibrain)
 Has as many neurons as spinal cord
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Located close to the effector systems such as:
- Musculature
- Glands
- Blood vessels (from esophagus to the anus)
Consists of ganglia & fibers projecting to the effector systems
CONTROL OF DIGESTIVE FUNCTIONS
BY NERVOUS SYSTEM
Enteric Nervous System (minibrain)
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Composes of two plexuses:
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1- myenteric plexus: excitatory or inhibitory (outer plexus)
increases intensity of rhythm of contraction
increases tone
increases rhythm rate
increases velocity of conduction of excitatory waves
2- Submucous plexus (inner plexus)
Regulation of Gastric Secretions
Digestive Hormones
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At least 18 hormones that affect:
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Are peptides
Are produced
cells in
Reach target
distribution in
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– most aspects of digestive function
– activities of other systems
by enteroendocrine
digestive tract
organs after
bloodstream
Local Mechanisms
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Coordinating response to changing conditions:
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Affect only a portion of tract
– e.g., variations in local pH, chemical, or physical stimuli
Constriction of pyloric sphincter
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Hormones promote constriction
1. CCK
2. Secretin
3. Gastrin
4. GIP
Sympathetic innervation
Excitatory Motor Neurons Evoke Muscle Contraction & Intestinal
Secretion
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Neurotransmitters of motor neurons:
Substance P
Ach
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Neurotransmitters of secretomotor neurons (releasing of water, electrolytes and mucus from crypts of Lieberkuhn):
Ach
VIP
Histamine (neurogenic secretory diarrhea)
Inhibitory Motor Neurons Suppress Muscle Contraction
Neurotransmitters:
 ATP
 NO
 VIP
N.B. Longitudinal muscles do not have inhibitory motor innervation
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Hormonal Control of
Gastrointestinal Motility
Gastrin:
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secreted by “G” cells of antrum of
Stomach.
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Secreted in response to stimuli associated with
ingestion of a meal, such as:
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distention of the stomach,
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products of proteins, and
gastrin releasing peptide.
The primary actions of gastrin are:
(1) stimulation of gastric acid secretion and
(2) stimulation of growth of the gastric mucosa.
Cholecystokinin
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secreted by “I” cells in duodenal mucosa and jejunum.
in response to digestive products of fat, fatty acids, and monoglycerides in intestinal contents.
strongly contracts gallbladder, expelling bile into small intestine.
bile is important for emulsifying fatty substances, allowing them to be digested and absorbed.
Cholecystokinin also inhibits stomach contraction moderately. Therefore, at the same time that this hormone causes
emptying of the gallbladder.
slows emptying of food from stomach to give adequate time for digestion of fats in upper intestinal tract.
Secretin
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first gastrointestinal hormone discovered
secreted by the “S” cells in duodenal mucosa in response to acidic gastric juice emptying into
the duodenum from the pylorus of stomach.
mild effect on motility of gastrointestinal tract.
promote pancreatic secretion of bicarbonate which in turn helps to neutralize acid in the small
intestine.
Gastric inhibitory peptide
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secreted by mucosa of upper small intestine, mainly in response to fatty acids
and amino acids but to a lesser extent in response to carbohydrate.
mild effect in decreasing motor activity of stomach.
Slows emptying of gastric contents into duodenum when upper small intestine
is already overloaded with food products.
Motilin
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secreted by the upper duodenum during fasting.
increase gastrointestinal motility.
Motilin is released cyclically and stimulates waves of gastrointestinal motility called
interdigestive myoelectric complexes that move through the stomach and small intestine every
90 minutes in a fasted person.
Motilin secretion is inhibited after ingestion.