L19: GI tract motility Flashcards
normal motility helps facilitate what 2 functions of the GI tract
- digestion/absorption
- maintain aboral propagation
what are the 3 stages in swallowing
- voluntary initial stage: movement of bolus to back of oral cavity
- involuntary pharyngeal phase: bolus passes from oral cavity to esophagus
- involuntary esophageal stage: food passes into stomach
what generates peristalsis
the straited muscle portion of the esophagus is innervated by brainstem LMN from the nucleus ambiguus in which a central pattern generator facilitates coordinate contration
what are the steps in the reflex arc
- stretch receptors in the esophageal wall are stimulated by distension
- sensory afferents transmit impulses to dorsal motor nucleus (DMN)
- DMN activates somatic and vagal efferent neurons
- sphincters prevent food and air from entering esophagus
what sphincter in the esophagus acts as a barrier to prevent regurgitaton of stomach content
lower esophageal sphincter
what are the 3 mechanisms involved in the regulation of basal LES tone
- excitatory cholinergic nerves (ACh) favor contraction
- inhibitory nitrergic (NO) pathway favors inhibition
- tonic myogenic property favors contraction
the balance b/w inhibition (NO) and excitation (ACh) is required for normal LES activity
in the excitatory pathway of LES control, neurons from the DMN stimulate _______ neurons which release _______ and _______
cholinergic postganglionic ENS
ACh and substance P
in the inhibitory pathway of LES control, neurons from the DMN stimulate _______ neurons which release _______ and _______
cholinergic postganglionic neurons in the enteric ganglia
release NO, VIP, ATP
what occurs in dysmotility associated w/ or w/out megaesophagus in Mysthenia gravis
antibodies block or destroy Ach receptors at the NMJ, inhibiting contraction of the esophagus; food is unable to be propelled forward
neurodegenerative disease in which degeneration of inhibitory neurons of the myeneteric plexus occurs
Achalasia
explain Achalasia
LES smooth muscle excitation is unopposed (LES remains contracted) due to the loss of inhibitory neurons in the myenteric plexus
ingesta can’t be transported across the LES
what can cause esophageal diverticula
- granulomatous inflammation of mediastinal LNs - inflammed nodes attach to esophagus and pull on it, creating an outpouching
- can also develop as a consequence of a motility abnormality in the esophagus that causes pressure stress on the wall which eventually causes a herniation of esophageal mucosa
what generates baselines motility in the stomach, SI and LI
ICC - the pacemaker cells of the GI tract - cause spontaneously undulating electrical waves of partial depolarization = slow waves
what things can affect the rate of slow waves
- stretch
- pH
- nutrients
- parasympathetic stimulation
- GI hormones
what are factors that can hyperpolarize ICC cells, dimishing or abolishing muscle activity
- inhibitory hormones / neuroeffectors substances
- sympathetic stimulation
- VIP and NO
why does new food trigger increased motility
- food enters GI tract, activating intrinsic endocrine and neural mechanisms that stretch GI tract wall
- ICC activity exceeds threshold potential when cations enter into cell
- Ca+ is released from SR, generating excitation of smooth muscles
what allows for the spread of motility along the GI tract
gap junctions
exist between ICC and longitudinal and circular smooth muscle cells and between smooth muscle cells
they facilitate transmission of electrical activity between smooth muscles and between ICC and smooth muscle cells
the frequency of slow waves varies by GI segment; where do they occur most and least often
3-5 / min in stomach
12-20/min in small intestine
6-8/min in colon
the ______ region of the stomach serves a storage function while the ______ regions grinds food into smaller pieces
proximal
distal (antrum)
what are the 3 components of stomach motility
- adaptive relaxation of the proximal stomach to accept food from esophagus
- mixing to reduce bolus size
- propulsion of chyme past the pyloric valve into the duodenum
what are the 3 basic motility patterns in the stomach
- adaptive relaxation
- intensive slow waves
- interdigestive motility complex
explain the adaptive relaxation reflex
- mechanosensitive n. endings release ACh, activate interneurons of the ENS and afferents of the DMN
- ENS neurons release NO –> relaxes fundus
- feedback comes from SI via the ENS, CCK and Secretin
what suppresses contraction in the proximal region of the stomach
vagus stimulation, causes intense slow wave activity in the distal region of the stomach
strong waves of peristalsis from body of stomach to the antrum that cause mixing
inense slow waves
during intense slow waves of the stomach, the _____ sphincter closes and only the smallest particles can squeeze through
pylorus
why does the emptying of the stomach need to be precisely controlled
- digestion/absorption in the small intestine takes time
- the rate for food digestion/abs in the SI is a limiting factor of stomach emptying
what things inhibit stomach motility / slow the emptying of stomach
distension, low pH and high fat in duodenum
how does the endorcrine system help regulate food transport to the duodenum
- CCKsecreted in response to fat in duodenum - it inhibits antrum motility
- Secretin secreted in response to low pH in duodenum, also inhibits antrum motility
the rate of gastric empying matches the small intestion rate of digestion and absorption; this is called the _____ reflex
enterogastric
what is the interdigestive motility complex
- the relaxation of the pylorus as strong peristalsis waves move over the antrum
- stimulated by motilin
- forces remaining items into duodenum
what are the 3 main motility patterns of the small intestine
- segmentation (non-propulsive mixing)
- peristalsis (propulsive)
- migrating motility complex (empties contents of SI into LI)
what sphincter of the LI prevents retrograde movement into the ileum
ileo-cecal sphincter
main functions of the LI
- nutrient and H20 abs
- fermentation
- storage
- release of fecal matter
what are the 3 main motility patterns of the LI
- segmentation (mixing, abs of water and electrolytes)
- retropulsion/propulation (slow waves)
- mass movements (segments contract in sequence to drive feces distally)
what are the 2 sphincters of the anal opening
- internal sphincter of smooth muscle
- external sphincter of striated muscle
sympathetic (constriction) and parasympathetic (relax) control of the inner anal sphincter comes from …?
sacral spinal cord
what is the rectospincteric reflex
when feces enters the rectum, the internal sphincter relaxes and the rectal wall contracts peristaltically –> defecation
congenital condition that results from the absence of inhibitory neurons in the submucosal and myenteric plexus of the colon
Congenital Megacolon
the aganglionic intestinal segment is spastically contracted and the proximal segment becomes dilated and hypertrophied
where is the vomiting center
reticular formation of the medulla
where does the vomiting center recieve information from
- higher center
- vestibular apparatus
- vagal afferent fibers from inside and outside of GI tract
- chemoreceptors trigger zone (responds to drugs and toxins in blood)
what is the chemoreceptor trigger zone (CRTZ)
area in the reticular formation of the medulla under the 4th ventricle where the BBB is very weak/absent
emetric drugs are able to stimulate the CRTZ
what is the sequence of events that occurs after the vomiting center is stimulated
- diaphragm moves into an inspiratory position above the stomach; glottis closes
- abdominal wall muscles contract, increasing pressure
- stomach, esophagus and LES relax; stomach gets squeezed b/w diagphragm and viscera
- UES relaxes, vomit projects into mouth
- closed glottis prevents aspiration
LES relaxation is mediated by ?
VIP
