CASE 10 Flashcards
long reflex integrated in the CNS
- begins with stimulus transmitted along a sensory neuron to CNS
- digestive reflexes integrated in the CNS are called long reflexes
- long reflexes that originate completely outside the digestive system are called cephalic reflexes
short reflexes integrated in the ENS
- ENS can work in isolation, but it also coordinates function with ANS bringing signals from CNS
- processes controlled by ENS include motility, secretion, growth
reflexes involving GI peptides
- By GI cells released peptides may act as hormones or paracrine signals. GI hormones are secreted into the blood and act on GI tract and brain.
- paracrine molecules in ECF act locally, on cells close to where they were secreted
- in GI- peptides, excite or inhibit motility and secretion
The enteric nervous system
- one of the main division of the ANS
- functions autonomously, independent on efferent signals from CNS, shares many features with CNS:
1. intrinsic neurons
2. neurotransmittres and neuromodulators
3. glial support cells
4. diffusion barrier
5. integrating center
intrinsic neurons
- lie completely within the wall of the gut
1. sensory
2. inter
3. motor neurons - autonomic neurons that bring signals from the CNS to the digestive system are called extrinsic neurons
neurotransmitters and neuromodulators
- ENS neurons release neurotransmitters and neuromodulators
- sometimes called nonadrenergic, noncholinergic to distinguish them from traditional autonomic neurotransmitters norepinephrine and acetylcholine
Glial support cells
- similar to astroglia of the brain
diffusion barrier
- capillaries that surround ganglia in ENs are not very permeable and create a diffusion barrier that is similar to the blood brain barrier of cerebral blood vessels
integrating center
- reflexes that originate in GI can be integrated and acted on without neural signals leaving ENS.
Composed of two plexuses embedded in wall of GI
- submucosal nerve plexus: ascending and descending part connected to mucosa and circular muscle, senses and secretes
- myenteric nerve plexus: ascending and descending part connected to circular and longitudinal smooth muscle layers; controls motility of intestines.
submucosal plexus, Meissner
- regulates the configuration of luminal surface
- controls glandular secretions
- alters electrolyte and water transport
myenteric plexus, Auerbach
- regulate peristaltic waves
- control local muscular contractions that are responsible for stationary mixing and churning
SNS on digestive activity
- inhibits digestion activity
PNS on digestive activity
- stimulates digestive activity
Parasympathetic stimulation
- increases activity of ENS
- divided into:
1. cranial: in the vagus nerves. Provide extensive innervation to esophagus, stomach and pancreas
2. Sacral divisions: originate in 2nd 3rd 4th sacral segments of spine and pass through pelvic nerves to distal half of large intestine and all the way to the anus.
3. post-ganglionic neurons: located mainly in myenteric and submucosal plexuses. Stimulation of these nerves causes general increased activity of entire ENS
Sympathetic stimulation
- usually inhibits GI-activity
- fibers mainly secrete norepinephrine
- inhibiting causes many effects opposite to parasympathetic system:
1. to a slight extent by direct effect of secreted norepinephrine to inhibit intestinal tract smooth muscle
2. to a major extent by an inhibitory effect of norepinephrine on the neurons of the ENS - strong stimulation can inhibit motor movements, it can block movement of food through GI tract
GI hormones divided into three families
- the gastrin family
- the secretin family
- third family
The gastrin family
- both gastrin and CCK can bind to and activate the CCKB receptor found on parietal cells
a. gastrin: secreted in stomach
i. stimulus: peptides and amino acids ; neural reflexes
ii. target: ECL cells and parietal cells
iii. effect: stimulates gastric acid secretion and mucosal growth
iv. other info: somatostatin inhibits its release
b. Cholecystokinin (CCK); secreted in intestine
i. stimulus: fatty acids and some amino acids
ii. target: gallbladder, pancreas, stomach
iii. effects: stimulates gallbladder contraction and pancreatic enzyme secretion. Inhibits gastric empyting and acid secretion.
iv. other info: promotes satiety
the secretin family
a. secretin: released in small intestine
i. stimulus: acid in small intestine
ii. target: pancreas and stomach
iii. effect: stimulates HCO3- secretion and inhibits gastric emptying and acid secretion
b. vasoactive intestinal peptide (VIP): secreted in small intestine. It is a neurocrine molecule. It maintains general homeostasis
c. glucose-dependent insulinotropic peptide (GIP): secreted in small intestine
i. stimulus: glucose, fatty acids and amino acids in small intestine
ii. target: beta cells pancreas
iii. effect: stimulates insulin release and inhibits gastric emptying and acid secretion
d. glucagon-like-peptide-1 (GLp-1): secreted in small intestine
i. stimulus: mixed meal that includes carbohydrates or fats in lumen
ii. target: endocrine pancreas
iii. effect: stimulates insulin release and inhibits glucagon release and gastric function
third family
- contains peptides that don’t fit in the other two families
a. motilin: primary member of this family. Increases in motilin secretion are associated with the migrating motor complex. Secreted in small intestine
i. stimulus: fasting: periodic release every 1.5-2 hours
ii. target: gastric and intestinal smooth muscle
iii. effect: stimulates migrating motor complex, which are waves of electrical activity that sweep through intestines in a regular cycle during fasting:
iv. other info: inhibited by eating a meal
b. histamine: effect in the small intestine: histamine induces intestinal secretion by stimulating the production of prostaglandins, which then activate the secretory process
three phases
- cephalic phase
- gastric phase
- intestinal phase
cephalic phase
- CNS most prominent in this phase
- smelling or seeing food create a response
- stimulus of food in oral cavity activate neurons in medulla oblongata –> sends efferent signal through autonomic neurons to salivary glands through vagus nerve to the ENS –> stomach, intestine begin secretion and increase motility to get ready for the food to come
- salivary amylase contains lysozyme, disable bacteria and viruses
- chemical digestion begins in mouth
- mechanical digestion begins in oral cavity with mastication
- deglutition: stimulus is pressure created when the tongue pushes the bolus against the soft palate and the back of the mouth
gastric phase
- stomach has 3 general functions:
1. storage: stomach stores food and regulates its passage into the small intestine
2. digestion: stomach chemically and mechanically digests food into chyme
3. protection: stomach protects the body by destroying many of the bacteria and pathogens swallowed with food or trapped in airway mucus. - receptive relaxation: when food arrives in stomach, it relaxes and expands
- acid secretion
- enzyme secretion
- paracrine secretion
- hormone secretion
acid secretion
- parietal cells secrete gastric acid –> activates pepsinogen and denatures proteins –> makes peptide bonds between amino acids accessible to pepsin
- gastric acid also helps killing bacteria and inactivates salivary amylase, which stops carbohydrate digestion
enzyme secretion
- chief cells secrete inactive enzyme pepsinogen, which is cleaved to active pepsin in the lumen of stomach by the action of H+.
- endopeptidase that carries out initial digestion of proteins, particularly active on collagen, meat digestion
- chief cells also secrete gastric lipase, although very little fat digestion happens in stomach
paracrine secretion
- enterochromaffin-like cells (ECL cells) secrete histamine, which is a paracrine signal that promotes acid secretion by parietal cells
- D cells secrete somatostatin (SS), shuts down acid secretion directly and indirectly and also inhibits pepsinogen secretion
hormone secretion
- G cells secrete the hormone gastrin into the blood
- gastrin release is stimulated by the presence of amino acids and peptides in the stomach
mucous cells
in the neck of gastric glands secrete both mucus and bicarbonate.
intestinal phase
- chyme in intestine activate the ENS which slows gastric motility and secretion –> three hormones reinforce motility feedback: secretin, CCK and GIP
- secretin inhibits acid production and gastric motility and stimulates production of pancreatic HCO3- to neutralize acidic chyme
- CCK is secreted in bloodstream if a meal contains fats.
- GIP and GLP1 are released if the meal contains carbohydrates –> promote insulin release by the endocrine pancreas
- the mixture of acid, enzymes and digested food in chyme usually forms a hyperosmotic solution. When osmoreceptors are stimulated by high osmolarity the receptors inhibit gastric emptying
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hepatic portal system
- most absorbed nutrients move into capillaries in villi and from there into hepatic portal system
- specialized region has two sids of capillary beds: one picks up absorbed nutrients at intestine and another delivers the nutrients directly to the liver
ileocecal valve
- large intestine concentrates water
- chyme enters large intestine through ileocecal valve –> contracted region of muscularis that narrows the opening between ileum and cecum (first part of large intestine)
tenia coli
- discontinuous longitudinal muscle layer concentrated into three bands
- contractions pull wall into bulging pockets called haustra
mass movement, colonic contraction
chyme in colon is moved primarily via mass movement.
- The defecation reflex removes undigested feces from the body
gastral ileal reflex
stomach relaxes when food leaves the stomach
little bit liver
gets blood by hepatic artery and portal hepatic vein, which supplies liver with nutrients and toxins.
- fats are first taken up into the lymphatic vessels, than enter the blood system –> go through heart –> go to liver –> blood leaves liver via hepatic vein
