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Question 1

describe the anatomical structure of the GI tract

Answer 1

• tubular structure comprise of several layers
• layers include:
  Submucosa
  Submucosal plexus
  Circular muscle
  Myenteric plexus
  Longitudinal muscle

Question 2

what are the 2 layers of neurons of the ENS in walls of the GI tract

Answer 2

• myenteric plexus
  control motility
• submucosal plexus
  controls secretion, water, and ion movements across the intestinal epithelium and blood flow

Question 3

what digestive organs does the neurons of the ENS lie within?

Answer 3

• esophagus
• stomach
• small and large intestine

Question 4

what types of neurons does the ENS contains

Answer 4

• sensory neurons
• interneurons
• motor neurons

Question 5

describe the sensory neurons of the ENS (what where projections how)

Answer 5

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

Question 6

describe sensory neurons of the ENS at the R level

Answer 6

• 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)

Question 7

what happens when IPANs is activated

Answer 7

• activation of IPANs initiates motor and secretomotor reflexes
• IPANs release acetylecholine that binds to nicotinic R on interneurons and motor neurons

Question 8

describe interneurons function

Answer 8

• 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

Question 9

describe functions of interneurons and where they project

Answer 9

• 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

Question 10

describe orally/aborally projecting interneurons

Answer 10

• 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

Question 11

describe the motor neurons of ENS

Answer 11

• 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

Question 12

describe the layout of motor neurons of the ENS in myenteric/submucosal accordingly

Answer 12

• 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

Question 13

what are the various functions related to digestion thats carried out by cooperation of ENS neurons

Answer 13

• 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

Question 14

describe peristalsis

Answer 14

• 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

Question 15

describe peristalsis reflex at cell level

Answer 15

• 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

Question 16

how is coordination of peristalsis possible

Answer 16

• 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

Question 17

describe mixing segmentation

Answer 17

• 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

Question 18

describe the different segments during mixing segmentation

Answer 18

• 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

Question 19

how is secretion reflex activated

Answer 19

• mechanical or chemical stimulation of mucosa > activation of IPANs > IPANs direct/indirect activate secretomotor neurons
• effector cells are enterocytes and exocrine glands

Question 20

describe secretomotor neurons in the small intestine + mechanism

Answer 20

• 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

Question 21

describe hirschsprung disease + treatment

Answer 21

• 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

Question 22

how does ANS affect ENS; exceptions?

Answer 22

• 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

Question 23

how does PNS provide input to ENS

Answer 23

• through vagus nerve + pelvic nerves

Question 24

what are the PNS inputs to ENS

Answer 24

• 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

Question 25

how does SNS input to ENS + func

Answer 25

- 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)

Question 26

describe SNS inputs to the various ENS areas

Answer 26

- SNS postgang fibers > - myenteric plexus = inhibits motility - submucosal plex = inhibit secretion - sphincter muscles = direct innervation > constriction - vasculature = vasocon