digestive motility Flashcards
4 functions of GI movements
- Propel ingesta from one location to next
- Retain ingesta for digestion, absorption or storage
- Physically break up food and mix it with secretions
- Circulate ingesta so can be absorbed at luminal surface
slow waves are a unique feature of GIT ____ muscle
smooth
spreading slow waves cause ______ and _____ in primed smooth muscle cells
action potentials
contractions
GI smooth muscle functions as a ____ meaning its a continuous interlocking sheet of muscle
syncytium
smooth muscle cells in GI are connected by
gap junctions
GI smooth muscle functioning as a syncytium allows changes in ______ to spread
membrane potential
resting membrane potential of GI smooth muslce
fluctuates rhythmically by 20-30 mV ie it undergoes a partial depolarization
resting membrane potential of GI smooth muscle undergoes partial depolarization, this id due to changes in
intracellular levels of Ca2+
what are the rhythmic fluctuations in the resting membrane potential known as
the GITs basic electrical rhythm or slow waves
are membrane fluctuations associated w slow waves alone enough to cause muscle contraction
NO!
slow waves frequency depends on
region of GIT and species
canine SI has approx how many slow waves per min
canine stomach and LI has approx how many slow waves per min
20
5
are slow waves intrinsic or extrinsic property of GIT tract
instrinsic
slow waves initated and controlled by what specialized cells in GIT tract
interstitial cells of cajal (ICC)
interstitial cells of cajal act as
pacemakers of the gut
interstitial cells of cajal are the ____ level of control of GI motility
first
Interstitial cells of cajal are to the GI as _____ are to the heart
purkinje fibers
where are slow waves initiated and where do they move
initiated in proximal duodenum and move towards large intestine
how do interstitial cells of cajal generate electrical rhythmicity in GIT muscle
they undergo spontaneous changes in membrane polarity
amplitude and frequency of slow waves can be modulated by what systems
nervous and endocrine systems
do slow waves cause smooth muscle contraction
not directly!
However if the resting membrane potential reaches a threshold value at the crest of a slow wave (about -40 mV), this will trigger an action potential
→ Muscle contraction
when does muscle contraction occur during a slow wave
ONLY at the crest of a slow wave and ONLY if that slow wave reaches the threshold value
the possibility that a slow wave will reach the
threshold required to trigger an action potential depends on what 2 things
both of these things are controlled by what systems
the starting baseline
membrane potential and the amplitude of the slow waves themselves
controlled by the enteric nervous system, and the endocrine system.
Sympathetic stimulation causes hyperpolarisation of the cell membranes. This means that the baseline membrane potential becomes more ______, reducing the chance of the slow wave crest reaching the threshold. Hence the muscle becomes ____
excitable
negative
less
Parasympathetic stimulation causes depolarisation of the cell membranes.
This means that the baseline membrane potential becomes more _____, increasing
the chance of the slow wave crest reaching the threshold for an AP. Hence the muscle
becomes _____ excitable or “primed”
positive
more
parasympathetic stimulation causes depolarization or hyperpolarization
depolarization ie more pos, more exitable
sympathetic stimulation causes depolarization or hyperpolarization
hyperpolarization, ie more neg, less excitable
what are other factors that make GIT smooth muscle more excitable (other than parasympathetic stimulation)
stretch
GIT hormones
muscle contractions always linked to ____ of slow waves
crest
will all crests of slow waves be high enough to cause muscle contraction
no
Frequency and propagation of muscle contraction dependent on inherent properties
of slow waves in that
region of GIT
mastication function
- Break down food to suitable size
- Moisten and lubricate
mastication uses what parts of body
muscles of the lips, tongue, jaw and face, as well as the use of teeth
abnormalities in mastication may be due to
dysfunction of the facial (VII), glossopharyngeal (IX) or trigeminal (V) cranial nerves,
with the teeth, jaws or muscles themselves,
or within
the CNS
deglutition transfers food from ____ to ____
oral cavity to stomach
deglutition initially has ____ movements
voluntary: bolus forced into oropharynx by tongue
what are the involuntary movements of deglutition
- Swallowing reflex
- Respiratory tract closed off, the oesophagus is dilated, and the bolus is propelled into the oesophagus
what tongue muscles move bolus caudally
what else aids this action?
styloglossus & hyoglossus mm
filiform papillae and palatine ridges
what occurs to the pharyngeal sphincters, breather, and soft palate when bolus enters oropharynx
- pharyngeal sphincter (pharyngeal circular muscles) relaxes
- breathing stops
- soft palate is elevated to seal nasopharynx
The bolus gets trapped in the oropharynx by the root of the tongue, which is pulled caudally by the ______ muscle and pressed hard against the ______
styloglossus
hard palate
during deglutition the hyoid and larynx are moved rostrally by what muscle
geniohyoid muscle
during deglutition the hyoid and larynx are moved rostrally, what does this result in
it swings the esophageal opening down to recieve bolus and pulls glottis under the epiglottis blocking laryngeal opening
epiglottis blocks the _____ opening during deglutition
laryngeal
Closure of the glottis to prevent food or liquid
entering the respiratory system is doubly ensured by constriction of the _____, causing approximation of the vocal folds
arytenoid cartilages
during deglutition pharyngeal muscles undergo a contractile wave which pushes bolus where
towards esophagus
as bolus reaches esophagus what happens to upper esophageal sphincter
what does tongue do
relaxes (cricopharyngeal muscle)
tongue returns to resting position
describe what happens to hyoid, larynx, soft palate, glottis and breathing once food has passed into esophagus
- hyoid and larynx return to resting position
- soft palate drops down
- glottis opens
- breathing resumes
how does bolus move down esophagus
by a wave of constriction by momentum of bolus entering the esophagus and gravity
peristalsis occurs
control centers for deglutition located in
brainstem
efferent nerves involved with deglutition
- Facial nerve
- Vagus nerve
- Hypoglossal nerve
- Glossopharyngeal nerve
- Trigeminal nerve
problems w deglutition usually involve
lesions in brainstem or cranial nerves
esophageal muscle is mostly
striated
where is smooth muscle present in esophagus, which species
in distal portions in horse and cat
esophagus striated muscle is innervated by
somatic motor neurons
smooth muscle of esophagus is under direct control of ____ and indirect control of ____
- ENS (myenteric plexus)
- ANS
where is myenteric plexus present in esophagus
throughout
peristalsis of esophagus is done by constriction caused by
contractions of circular muscle
in esophagus, contraction of circular muscle causes constriction for peristalsis
what does contraction of longitudinal muscle do
shortens the esophagus, helping to concentrate past the bolus
in esophageal transport, do secondary waves of peristalsis occur
yes
if bolus jams in esophagus, what occurs
repeated waves may cause muscle spasm and constriction around bolus
as food reaches distal esophagus, what happens to lower esophageal sphincter
it relaxes
upper and lower esophageal sphincters are normally open or closed
closed
the esophagus enters the stomach obliquely, why might this anatomy be important
distension of the stomach: oblique anatomy blocks entry of food back to esophagus
in horses why is vomiting rare
the anatomy of esophagus entering stomach obliquely is well developed, so overextension of stomach causes rupture rather than reflux
simple stomach: storage occurs in proximal or distal region
proximal
simple stomach; mechanical and chemical digestion occurs in proximal or distal region
distal
muscular activity in proximal or distal stomach maintains tone in wall ?
proximal:
- Relaxes as food enters and is stored
- Increases tone slightly to move ingesta to distal stomach
Strong peristaltic contractions migrate with slow waves from middle of stomach towards
pylorus
pylorus ____ as waves approaches
constricts
pylorus constricting allows what to enter duodenum
only liquid and small particles (<2mm)
pylorus mechanical function
crushes and grinds food
control of stomach motility has parasympathetic control via
vagus nerve (CN X)
how does parasympathetic control function in stomach
Parasympathetic control occurs via the vagus nerve.
It relaxes the muscle in proximal stomach, via the inhibitory neurotransmitter vasoactive intestinal peptide.
It also increases the
peristaltic activity in the distal stomach, via the excitatory neurotransmitter acetylcholine.
how is vagus nerve stimulated
The vagus nerve is stimulated by the presence of food in the stomach, as part of a positive feedback loop. It’s also stimulated by the CNS. This is known as the cephalic phase of digestion.
what hormones stimulates stomach motility
gastrin
what hormone inhibits stomach motility
Cholescystokinin (CCK)
rate of gastric emptying must equal the rate of
digestion and absorption in SI
(enterogastric reflex)
enterogastric reflex; rate of gastric emptying equals rate of digestion and absorption in SI
what do the receptors in the duodenum detect
- Low pH
- High osmolality
- High fat
enterogastric reflex; rate of gastric emptying equals rate of digestion and absorption in SI
what are the outputs
how do the outputs respond
The response will
either be via endocrine, enteric nervous system or central nervous system output (via
the brainstem and vagus nerve
These respond to decrease motility, which decreases gastric emptying, to allow the duodenum time to process the contents.
In order to clear the stomach of indigestible material between meals, what occurs (carnivores and herbivores)
strong gastric
contractions occur about one hour after meals in carnivores and hourly, regardless of
feeding, in herbivores
interdigestive motility
Clears stomach of indigestible material between meals
when does interdigestive moitlity occur in carnivores
~ 1 hr intervals after meals (mostly digested)
when does interdigestive motiltiy occur in herbivores
Hourly regardless of feeding
how does interdigestive motility occur
Pylorus relaxes and strong peristaltic contractions occur, forcing less digestible material into duodenum
vomiting is a complex activity coordinated from
vomiting centre in brainstem
GIT stimuli that trigger vomiting include (3)
- Pharyngeal mechanoreceptors
- Tension receptors
- Chemoreceptors in the gastric and duodenal mucosa
stimuli OUTSIDE of GIT can also trigger vomit reflex
Chemoreceptor trigger zone
- Area of the brain in contact with the CSF
- Senses toxins, drugs and products of inflammation the blood
(ex of stimuli outside GIT that trigger vomit reflex)
Disturbances of semicircular canals of inner ear can
trigger vomit centre: Motion sickness
(ex of stimuli outside GIT that trigger vomit reflex)
what happens to gastric muscles & lower esophageal sphincter
pylorus
abdominal muscles
breathing
Gastric muscles and lower oesophageal sphincter relaxes,
pylorus closes
Abdominal muscles contract, increasing intra-abdominal pressure
Breath in against a closed glottis, expands chest cavity, decreasing intra-thoracic pressure
Ingesta enters oesophagus, upper oesophageal sphincter opens, wave of reverse peristalsis
Can vomit intestinal contents
small intestine motility; what are the 2 phases
- digestive phase
- interdigestive phase
digestive phase of small intestine motility has 2 components
propulsive: Peristaltic contractions in sync with slow waves, die out after short span
nonpropulsive: segmentation, 3 to 4cm segments of circular muscle contract, mix food, bring it into contact with mucosa
interdigestive phase of small intestine motility:
powerful peristaltic contractions pass over
large length of intestine
interdigestive phase of small intestine motility:
waves known as
migrating motility (or myoelectric) complex (MMC)
interdigestive phase of small intestine motility:
the waves move at the rate of
slow waves
interdigestive phase of small intestine motility:
how far do the waves travel
Some travel entire length of SI, some die out earlier
interdigestive phase of small intestine motility:
what is the function
Clears undigested material and keeps microbial population in check
what is the Ileocaecal/Ileocolic sphincter
Well-developed ring of circular muscle that relaxes during peristaltic activity in ileum to allow passage of digesta into colon
Increased colonic pressure causes
_____ constriction of Ileocaecal/Ileocolic sphincter
increased
Increased colonic pressure causes
increased constriction of Ileocaecal/Ileocolic sphincter
what does this prevent
Prevents retrograde movement from
colon to ileum
colon functions
- Absorbs H2O and electrolytes
- Stores faeces
- Fermentation and absorption of organic matter
motility patterns in both simple colons and fermentative colons
similar in both
Retropulsion occurs in colon due to
antiperistaltic contractions
where do slow waves spread in colon
Slow waves spread orally (retropulsion) and aborally (propulsion) from mobile pacemaker regions from the Junction of transverse and descending parts of colon
retropulsion of colon causes
intense mixing activity
* Absorbing most H2O
* Many of the electrolytes
If taenia and haustra present in colon, what also occurs in addition to the mass movements
segmentation contractions also occur
colon motility
Periods of intense propulsive movements (mass movements)
what does the horse have at the pelvic flexure (colon)
Horse has a pacemaker at the pelvic flexure, resulting in prolonged retention of material in the larger ventral colon
*Pig and ruminants may have pacemaker at the centre of the spiral colon
segmentation in colon likely results in what forming
faecal balls in sheep and horses
megacolon
Megacolon relatively common problem in cats, causing severe constipation as the
colon loses motility.
anal spincter
External sphincter
* Striated muscle
* Pudendal nerve
Internal sphincter
* Circular muscle layer
* Normally contracted – tonic tone necessary for continence
* Parasympathetic input from pelvic nerve → relaxation → defecation
* Sympathetic input from hypogastric nerve → constriction
Rectosphincteric reflex
Faeces into rectum:
* Reflex relaxation of internal anal sphincter
* Peristaltic contractions of rectum
what does Rectosphincteric reflex require
Requires voluntary relaxation of external anal sphincter
* Can prevent defaecation in trained animals
* Rectum relaxes to accommodate faeces and internal sphincter regains tone, until another bolus enters
Posture associated with defaecation is ______
voluntary
* Contraction of diaphragm and abdominal muscles
* Allows complete emptying
slow waves are a ____ feature of GIT smooth muscle
unique
Spreading slow waves cause ____ in primed smooth muscle cells
action potentials and contractions
____ stomach stores food, _____ stomach grinds filters food
Proximal
distal
true or false: Control of gastric motility differs in proximal and distal stomach
true
Rate of gastric emptying matches SI rate of
digestion and absorption
true or false Between meals stomach is cleared of indigestible material
true
Vomiting is complex reflex coordinated from
brainstem
SI motility has _____ and _____ phase
digestive and interdigestive
______ prevents movement of colon contents back into ileum
Ileocaecal sphincter
true or false Colon motility causes mixing, retropulsion, and propulsion of ingesta
true
true or false Anal sphincter has 2 layers with the same innervation
false: Anal sphincter has 2 layers with separate innervation
_____ reflex important in defaecation
Rectosphincteric
Which of the following statements regarding slow waves is INCORRECT?
A.
The frequency of the slow waves depends on the region of the GI tract, as well as the species
B.
Slow waves are initiated and controlled by specialised cells within the GIT known as interstitial cells of Cajal
C.
Slow waves are a partial depolarisation of the smooth muscle membrane, where the resting membrane potential of the GI smooth muscle fluctuates rhythmically by 20 to 30 mV
D.
Slow waves directly cause muscle contraction
D.
Slow waves directly cause muscle contraction
Parasympathetic input __________ the smooth muscle membrane, __________ the chance of a slow wave crest reaching the threshold for an action potential.
Depolarises; increasing
The oesophagus is mostly striated, skeletal muscle, although some species have smooth muscle present in the distal oesophagus, such as the horse and the cat. Smooth muscle is under direct control of __________ and striated muscle is innervated by __________.
The enteric nervous system (myenteric plexus); somatic motor neurons
Food is propelled down the oesophagus by peristaltic movements. The __________ muscle contracts behind the food bolus. The __________ muscle layers ahead of the bolus contract.
Circular; longitudinal
The term cephalic phase is used in reference to a number of activities occuring in the GI tract. In general, the term means:
Digestive events that occur before the ingestion of food, and in response to central nervous system stimulation that is brought on by the anticipation of eating
Conditions in the duodenum, such as low pH or high fat concentration, can inhibit gastric emptying. Which reflex arc is involved in this inhibition?
A.
Parasympathetic nervous system
B.
GI enteric nervous system
C.
GI endocrine system
D.
All of the above
All of the above
Which of the following best describes the motility of the proximal region of the monogastric stomach?
Adaptive relaxation
Which of the following is characteristic of the interdigestive phase of small intestinal motility?
Migrating motility complexes consisting of waves of peristaltic contractions that pass over the entire length of the small intestine
Which of the following aspects of the physiology of the colon is common to many species, regardless of species-specific differences in the anatomical structure of the colon?
Retropulsion, or antiperistalsis
Colonic “pacemakers”:
Are involved in segmentation, but not peristalsis
