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