23 Flashcards
describe the anatomical structure of the GI tract
- tubular structure comprise of several layers
- layers include:
Submucosa
Submucosal plexus
Circular muscle
Myenteric plexus
Longitudinal muscle
what are the 2 layers of neurons of the ENS in walls of the GI tract
- myenteric plexus
control motility - submucosal plexus
controls secretion, water, and ion movements across the intestinal epithelium and blood flow
what digestive organs does the neurons of the ENS lie within?
- esophagus
- stomach
- small and large intestine
what types of neurons does the ENS contains
- sensory neurons
- interneurons
- motor neurons
describe the sensory neurons of the ENS (what where projections how)
what: intrinsic primary afferent neurons (IPANs)
where: located in myenteric and submucosal plexuses
projections: to submucosal plexus, circular muscle, myenteric plexus, and longitudinal muscle
how: IPANs receive sensory info from projections in the mucosa and muscles
describe sensory neurons of the ENS at the R level
- IPANS activated by mechanical and chemical stim
- mechanoR (sensitive to stretch and tension)
- chemoR (sensitive to acid, glucose, and aa)
- chemosensitive cells expresses taste R similar to mouth (ie couples to GPCR > PLC > increase cal > exocytosis of ex. GLP1 so must be sensing glucose)
what happens when IPANs is activated
- activation of IPANs initiates motor and secretomotor reflexes
- IPANs release acetylecholine that binds to nicotinic R on interneurons and motor neurons
describe interneurons function
- act as relay neurons (from sensory to motor)
- located in myenteric plexus only - projects to the plexuses ie both submucosal and myenteric plex)
- integrate information from IPANs and provide output to motor neurons
describe functions of interneurons and where they project
- functionally similar type of interneurons synaptically connected and form networks
- orally projecting: activate contraction on oral side of bolus
- aborally projecting: activate relaxation and secretion on aboral side of bolus
describe orally/aborally projecting interneurons
- involved in excitatory motor reflexes
- release ACh that binds to nicotinic R on excitatory motor neurons
- aborally projecting neurons involved in secretory reflexes and inhibitory motor reflexes
- release ACh that binds to nicotinic R on inhibitory motor neurons and secretomotor neurons
describe the motor neurons of ENS
- mediate motility and secretion reflexes
- located in submucosal (secretory) and myenteric plex. (smooth muscle cells)
- projections: submucosal > mucosa (secretory)
myenteric > circular + longitudinal muscle - motor neurons NOT synaptically connected to one another
describe the layout of motor neurons of the ENS in myenteric/submucosal accordingly
- myenteric: ascending + descending
- ascending MN = excitatory = release ACh + cause contraction of smooth muscle
- descending MN = inhibi = release nitric oxide and VIP + causes relaxation of smooth musc
- submucosal: secretomotor + vasodil
- secretomotor neurons = cause glandular secretion
- vasodil MN = cause vasodil
what are the various functions related to digestion thats carried out by cooperation of ENS neurons
- peristalsis = aboral movement of contents of GI tract
- mixing segmentation = mix intestinal contents with digestive enzy + increase absorp of nutri
- secretion = increases water content of GI contents in small intest
describe peristalsis
- unidirectional propulsive motility
- propagating ring of muscle contraction at the oral side of bolus = orally/ascending excitatory pathways
- relaxation of muscles below bolus = aborally/descending inhib pathways
describe peristalsis reflex at cell level
- mechanical or chemical stimulation of mucosa initiates reflex (IPAN activations > networks of IPANs stimulate 2 networks of interneurons (orally/aborally projecting interneurons) >
orally proj int = synaptically connected to excitatory motor neurons = release ACh onto SM > contraction of smooth musc
aborally proj int = connected to inhibit MN = VIP/nitric oxide onto SM > relaxation of smooth muscles
how is coordination of peristalsis possible
- requires ICC (interstitial cells of Cajal)
- pacemakers in ENS and located between enteric neurons and smooth muscle cells
- exhibit slow-wave depolarization in 3 phases
- depolarization of ICC must link up with activation of SM in order to cause contraction and overcome threshold
AP must fire during the plateau phase of ICC
either by itself is not enough to cross threshold
describe mixing segmentation
- circular and longitudinal musc work together to mediate mixing segmentation
- segmentation contractions = bidirectional motility = no net movement
- alternating segmental rings of contraction mix contents of small intestine = mixes intestinal contents with digestive enzyme + bring mixture (different surfaces) into contact with mucosa for absorption
describe the different segments during mixing segmentation
- propulsive segment: the constricted segment pushing material into receiving segment = contraction of circular but inhibition of longitudinal
- receiving segment: inhibition of circular but contraction of longitudinal
how is secretion reflex activated
- mechanical or chemical stimulation of mucosa > activation of IPANs > IPANs direct/indirect activate secretomotor neurons
- effector cells are enterocytes and exocrine glands
describe secretomotor neurons in the small intestine + mechanism
- they’re involved in liquefaction of the content of the lumen
- stimulate enterocytes to secrete Cl- into the lumen
- Cl ch is expressed on lumenal side > cystic fibro transmem conduc regulator = cAMP-dependent Cl- Ch secretes Cl- into the lumen
- as result: Na flows Cl- and H2O moves into lumen via osmotic pressure
- secretomotor reflex paralleled by vasodil reflexes to enhance the absorption of nutrients
describe hirschsprung disease + treatment
- congenital aganglionic megacolon = enlargement of colon caused by bowel obstruction which results from aganglionic section of bowel
- treatments: surgical resection of agang. section of colon - laxatives - high fiber diet
how does ANS affect ENS; exceptions?
- ANS mainly modulate intrinsic ENS activity
- doesnt not directly innervate most GI tissues
Exceptions:
Direct CNS innervation of smooth muscles of sphincters
Direct CNS innervation of vasculature that supplies the GI
tract
Direct CNS innervation at ends of GI tract
Colon- defecation reflex
Mouth- salivary glands, swallowing
how does PNS provide input to ENS
- through vagus nerve + pelvic nerves
what are the PNS inputs to ENS
- PNS preganglionic neurons synapse on ganglion cells of ENS (efferent fibers from ENS = PNS post gang)
- increase peristalsis
- increase secretion of glands (gastric + pancreatic + small intest)
- relaxes sphincters
how does SNS input to ENS + func
- SNS postgang neurons with cell bodies in collateral gang synapse on ENS neurons
- inhibit peristalsis
- inhibit secretion of glands
- contracts sphincters (direct innervation of sphincters and vasculature)
describe SNS inputs to the various ENS areas
- SNS postgang fibers >
- myenteric plexus = inhibits motility
- submucosal plex = inhibit secretion
- sphincter muscles = direct innervation > constriction
- vasculature = vasocon
