Motillity of the Gastrointestinal Tract Flashcards
What are the functional layers that make up the majority of the GI tract
Mucosal Layer
- muscularis mucosae, consists of SM, its contractions change the shape and surface area of the epithelium
- epithelium
- Lamina propria
Submucosa
Muscle Layers (muscularis propria)
- SM layers that provide motility to the GI tract
- circular muscle
- longitudinal muscle
- myenteric plexus
Serosa
-near the blood
What are the two plexus in the GI tract that have control over the functional layers
Submucosal plexus
Myenteric Plexus
what are the functions of the circular and longitudindal muscle in the GI tract
Circular muscle: decreases the diameter of the segment
Longitudinal muscle: decreases the length of the segment
what are the two types of contractions that are key for motility in the GI tract
Phasic contractions: periodic contractions followed by relaxation
- i.e. esophagus, stomach (antrum), SI and all tissues involved in mixing and propulsion
- wave of muscle tension
Tonic contractions: maintain a constant level of contraction without regular periods of relaxation
-i.e. stomach (orad), lower esophageal, iliocecal, internal anal sphincters
What is the relationship between the slow waves, APs, and contractions in the smooth muscle
as a slow wave increase there is some muscle tension
as a action potential goes, there is much more muscle contraction
the more number of APs on a slow wave increase the size of contraction
however if their is no AP but there are some slow wave still will have muscle tension
what increases the number of APs in the GI tract?
1) stretch
2) Acetylcholine
3) Parasympathetics
all lead to depolarization
what decreases the number of APs in the GI tract
1) Norepinephrine
2) Sympathetics
all lead to hyperpolarization
what are the two plexus that make up the Enteric Nervous system and what do they each control
Submucosal Plexus
- In the submucosa
- mainly control GI secretions and local blood flow
Myenteric Plexus (Auerbachs)
- between the circular and longitudinal SM layers
- mainly control GI movements
What creates the pacemaker like slow wave activity in the GI smooth muscle
Pacemaker regions in the myenteric plexus generate spontaneous slow wave activity
these cells are the Interstitial cells of Cajal (ICC)
- create the slow wave that spreads rapidly in the SM via the gap junctions
- drives the frequency of contraction
What is important about mastication and what are the nerves that control this?
Important for breaking down the food with teeth and saliva
muscles of mastication are innervated are innervated by the motor branch of the 5th cranial nerve
Both voluntary and involuntary
controlled by nuclei in the brainstem
Mastication is caused by a chewing reflex
what are the three phases of swallowing
Oral phase (voluntary -initiates swallowing process
Pharyngeal phase (involuntary)
- soft palate is pulled upward
- epiglottis moves to cover trachea
- UES relaxes
- peristaltic wave of contraction is initiated in the pharynx
- food is propelled through the UES
Esophageal phase (involuntary)
- Controls by the swallowing reflex and the ENS
- Primary peristalic wave
- Secondary peristalic wave
What is the Involuntary swallowing reflex controlled by?
medulla
Afferent: sensory input of vagus/glossopharyngeal
-goes to swallowing center in medulla
then goes to brainstem nuclei
Efferent is in put to the pharynx
WHat are the two types of Peristalic waves?
Primary Peristalic wave:
- Continuation of the pharyngeal peristalsis
- controlled by the medulla
- cannot occur after vagotomy
Secondary Peristalic wave:
- occurs if primary wave fails to empty the esophagus or if gastric contents reflux into esophagus
- Medulla and ENS are involved
- Can occur in the absence of the oral and pharyngeal phases
- Occurs even after vagotomy
What is significant about the pressure change of muscle as the bolus travels down the esophagus
the pressure will increase subsuquenty as a wave as the bolus moves down the esophagus until reach the LES and the fundus where the pressure will decrease since these need muscle relaxation to open the sphincter and expand the stomach to allow for food to enter
What are the two problems that the intrathoracic location of the esophagus runs into and how are they solved?
- Keeping the air out of the esophagus at the upper end
- keeping acidic gastric contents out of the lower end
these are solved by the UES and LES being closed unless a food bolus is passing
What is achalasia of the esophagus?
Impaired peristalis
- Incomplete LES relaxation during swallowing
- Elevation of LES resting pressure
this is due to decreased number of ganglion cells in the myenteric plexuses
-degeneration preferentially involves inhibitory neurons that produce NO/VIP
results in backflow of food int the throat (regurgitation)
also difficulty in swallowing liquids and solids (dysphagia)
How is the esophagus physiologically affected in Gastroesophageal reflux disease (GERD)
Changes in the barrier between the esophagus and stomach LES becomes weakens
- abnormally low pressures in the LES
- caused by pregnancy, largemeal, heavy lifting, due to iincrease intragastric pressures
Persistent reflux leads to inflammation and GERD
- hearburn, acid regurgitation
- gastrointestinal bleeding
- esophagitis
- scar tissue
- Barretts esophagus
What are the functional divisions of the stomach
and what are the extrinsic and intrinsic innervation?
Two regions: orad and caudad
Extrinsic innervations: Parasympathetic and sympathetic
Intrinsic innervations: Myenteric and submucosal plexuses (ENS)
What occurs in the Orad region of the stomach, and how does it happen?
Receptive relaxation:
- decrease pressure and increase volume of the orad region
- Vagovagal reflex
- CCK decreases the contractions and increases gastric distensibillity
Orad exhibits minimal contractile activity
What occurs in the Caudad region of the stomach?
Mix and Digestion
Primary contractile event is peristalitic contraction (mid stomach to pylorus)
- contraction increase in force and velocity as it approaches the pylorus (decrease lag time)
- max frequency is 3-5 min
If particles cant fit through the pyloric sphincter what happens
Retropulsion of the contents back to the mid stomach where the process repeats again
What increases gastric contractions?
Parasympathetic stimulation
Gastrin and Motlin
all increase AP and the foce of contraction
what slows down gastric contractions?
Sympathetic stimulation
Secretin and GIP
all decrease AP and force of contractions
How does gastric emptying process increase via contractile activity of the stomach?
- Decrease distensibillity of the orad stomach
- increase force of peristalitic contractions of the caudad stomach
- decrease tone of the pylurus
- increase diameter and inhibition of segmenting contractions of the proximal duodenum
How does gastric emptying process decrease via contractile activity of the stomach?
- relaxation of orad (increase in distensibility)
- decrease force of peristaltic contractions
- increase tone of pyloric sphincter
- Segmentation contractions in intestine
What is the Entero-Gastric reflex?
Negative feedback from duodenum will slow down rate of gastric emptying
- acid in duodenum stimulates secretin release to inhibit stomach motility via gastrin inhibition
- fats in duodenum stimulate CCK and GIP to inhibit stomach motility
- hypertonicity in duodenum will inhibit gastric emptying
What is the slow gastric emptying and some of the causes?
also known as Gastroparesis
-slow emptying of the stomach leading to fullness, loss of appetite, nausea, and sometimes vomiting
causes: gastric ulcer, physical obstruction, eating disorders, vagotomy
Diabetes Mellitus
what is the migrating Myoelectric complex/migrating mortor complex
- Periodic bursting peristaltic contractions
- Occur at 90 min intervals, during fasting
- Help empy large undigested residue left in the stomach
- inhibited during feeding
- Motilin plays a significant role in the complex
- Important at getting rid of small intestinal bacterial overgrowth (SIBO)
How is motility important in the small intestine for digestive and absorptive functions?
- Mix the chyme with digestive enzyme and pancreatic secretions
- expose nutrients to the intestinal mucosa for absorption
- Propel the unabsorbed chyme along the small intestine to the large intestine
How does segmentation contractions affect the motility
Generates back and forward movements
Produces no forward propulsive movements along the small intestine
how does peristaltic contractions affect the motility
Circular and longitudinal muscles works in opposition to complement each others actions
-are recipriocally innervated
help push things down the tract
how do the slow waves in the Small intestine SM differ from slow waves in the stomach SM
Small intestine the slow wave activity does not generate muscle tone
-needs an AP on the top of a slow wave to generate muscle tension
how does the slow wave frequency change throughout the small intestine?
overall decrease
Duodenum: 12 cycles a min
Jejunum: 10 cycles a min
ileum: 8 cycles a min
what is the Hormonal control of the activities of the smooth muscle cells?
Contraction:
- serotonin
- prostaglandins
- CCK
- Motlin
- Gastrin
- insulin
INhibit contraction:
- Epinephrine
- secretin
- glucagon
What is the vomiting reflex and the events
Coordinated by medulla, nerve impulses are transmitted by vgus and sympathetic afferents to multiple brain stem nuclei
Events:
- reverse peristalsis in small intestine
- Stomach and pylorus relaxation
- Forced inspiration to increase abdominal pressure
- movement of the larynx
- LES relaxation
- Glottis closes
- Forceful expulsion of gastric contents
how does the flow of contents from the small intestine to the large intestine regulated by the ileocecal junction?
distention of the ileum causes relaxation of the sphincter and allows flow of contents from the ileum to the colon
Distention of the colon causes contraction of the sphincter and precents passage of contents from the colon to the ileum
what innervates the internal anal sphincter?
what innervates the external anal sphincter?
internal = Pelvic splanchnic n (parasympathetic)
external = Pudenal N
-somatic motor fibers (voluntary)
what are the 4 types of innervation of the Large intestine
- ENS
- Parasympathetic Nervous system
- Sympathetic Nervous system
- Somatic pudendal nerves
what is the Parasympathetic innervation distribution on the large intestine?
Vagus nerve: Cecum, ascending and transverse colon
Pelvic Nerves (S2-S4): Descending and sigmoid colon, rectum
what are the sympathetic nervous system innervation of the large intestine
T10-L2
Superior mesenteric ganglion: proximal regions
inferior mesenteric ganglion: Distal regions
hypogastric plexus: Distal rectum and anal canal
what are the somatic pudendal nerves innervation distribution of the large intestine?
External Anal sphincter
Motility in the large intestine is done in what way? and how does it affect absorption in both poor and excess motility?
Mass movements
occur in the colon over larger distances
-1-3 times a day
stimulate defecation reflex
A final mass movement propels the fecal content into the rectum
motility is key for absorption of water and vitamins and the conversion of digested food into feces
- poor motillity causes greater absorption and lead to constipation
- excess motillity leads to less absorption and diarrhea
Motility of the rectum and the anal canal
Rectum fills intermittently
- mass movements
- segmentation contractions
as it fills with feces, SM wall of the rectum contracts and internal anal sphincter relaxes
-rectosphincteric reflex
the external anal sphincter is tonically closed (and is under voluntary control)
Rectosphincteric reflex and what nerves is it under?
Under neural control
- controlled by the ENS
- reflex is reinforced by activity of neurons within the spinal cord
Sensation of rectal distention and voluntary control of the external anal sphincter are mediated by pathways within the spinal cord to the cerebral cortex
-destruction of these pathways causes loss of voluntary control of defecation
what is Hirschsprung disease?
Cause: Ganglion cells absent from segment of the colon
result: VIP levels low and smooth muscle constriction and loss of coordinated movement therefore colon contents will accumulate
- colon equivalent to achalasia
present at birth and leads to congenital mega colon
- failure to pass meconium
- poor feeding, jaundice, vomitting, constipation
- swollen bell, and malnutrition
treatment: surgical resection of colon segment lacking the ganglia
Summary of reflexes: Vago-vagal reflex
long reflex, generally stimulatory that increases motility, secretomotor, vasodilatory activities
-vagus carries both afferents 75% and efferents 25%
Summary of reflexes: Intestino-intestinal reflex
depends on the extrinsic neural connections
- inhibitory if an area of the bowel is grossly distended
- contractile activity in the rest of the bowel is inhibited
Summary of reflexes: Enterogastric reflex
Negative feedback from duodenum will slow down the rate of gastric emptying
Summary of reflexes: gastroileal reflex
gastric distension relaxes the ileocecal sphincter
Summary of reflexes: gastro and duodeno-colic reflex
distention of stomach/duodenum initiates movements
-transmitted by way of the ANS
Summary of reflexes: Defecation reflex
rectosphinteric
rectal distention initiates defecation
when the rectum is distended by feces, the internal sphincter relaxes
