Gastric and Intestinal Motility Flashcards
What are the gastric motor functions of the stomach?
- reservoir of food ingested at a single meal - stored in fundus + body of stomach 2. Break food into smaller particles and mix it with gastric secretions, until chyme is formed
- Protects the body by destroying many bacteria and pathogens
- Slow emptying of chyme from stomach into duodenum at a controlled rate
What is receptive relaxation?
first pattern of stomach motility
What mediates the receptive relaxation?
- vagal nerve 2. triggered by movements of the esophagus - distension of oesophagus during swallowing evokes relaxation
Describe the mechanism caused by the distension of the esophagus?
- Oesophageal peristalsis triggers reflex that causes the lower oesophageal sphincter (LES) to relax 2. Relaxation of LES is followed by receptive relaxation of the fundus and the body of stomach
Describe the size of the fundus?
fundus and body can accommodate volume increases as much as 1.5 liters without a great increase in intragastric pressure
Which nerves are major efferent pathways for reflex relaxation of stomach?
vagal nerves
Which neurotransmitters are released in RR?
- vasoactive intestinal peptide 2. nitric oxide
When does mixing begin?
Gastric contractions begin when food enters the stomach
Where do gastric contractions begin?
in the middle of body of stomach and move towards the pylorus - contractions increase in force and velocity as they reach gastroduodenal junction
What is the purpose of the mixing process?
contractions mix ingested material with gastric juice, facilitating 1. digestion 2. solubilising some constituents 3. reducing particle size
Where does the major mixing activity occur?
antrum
Describe mixing in the fundus and the body?
- contractions are weak 2. Fundus contents settle in layers according to density, with fats on top 3. gastric contents may remain unmixed for 1 hour
Antrum contraction is followed by?
the contraction of the pyloric region and the duodenum
Stomach emptying is promoted by?
the intense peristaltic contractions of the stomach antrum
What is the function of the pylorus?
- to limit the size of particles emptied into the duodenum after a meal - Food particles larger than 2 mm do not pass through the narrow pyloric opening 2. acts to prevent reflux of the duodenal contents into the stomach
Gastric emptying is regulated by?
neural and hormonal mechanisms
Gatsric emptying is slowed by?
- hypertonic solutions in the duodenum 2. duodenal Ph less than 3.5 3. presence of amino acids and peptides 4. presence of fatty acids
How does the GIT sense these changes?
duodenal and jejunal mucosa contain receptors that sense acidity, osmotic pressure, products of fat digestion, products of protein digestion
How does acidity of chyme delay gastric emptying?
when pH falls below 3.5-4 acid in the duodenum
- stimulates secretin release which reduces antral contractions and increases contraction of pyloric sphincter
- presence of acid in the duodenum evoke neural reflexes that increases contraction of pyloric sphincter
How do fat digestion products in the duodenum and jejunum decrease gastric emptying?
- stimulates cholecystokinin (CCK) which decreases emptying
- stimulates release of gastric Inhibitory Polypeptide (GIP), that also decreases emptying
Describe how osmotic pressure of duodenal contents decrease gastric emptying?
Osmotic pressure of duodenal contents slow down gastric emptying by release of unknown hormone
Describe how peptides and amino acids in the duodenum decrease gastric emptying?
release gastrin from G cells in the stomach antrum and duodenum -
- gastrin increases strength of antral contractions
- gastrin relaxes pyloric sphincter
- increases rate of gastric emptying
What is vomitting?
the forceful expulsion of gastric and intestinal contents through the mouth
Reflex behaviour of vomiting is controlled by?
medulla oblangata
How does it start?
salivation - sensation of nausea
What is vomitting preceded by?
retching -
What is the primary function of the small intestine?
digestion and absorption of nutrients, hence motility patterns are organised to optimise these functions
Intestinal motility serves the folloring functions?
- mixing of contents with digestive enzymes and other secretions 2. further reduction of particle size and solubilisation 3. net propulsion of contents through the small intestines into large bowel
What causes contractions of intestinal smooth muscles?
pacemaker activity in the interstitial cells of Cajal
Describe hoe interstitial cells of Cajal work?
- produce slow waves which trigger action potential when above threshold 2. this triggers action potential by opening of voltage gated Ca2+ channels
What is the law of the intestine?
When a bolus is placed in the small intestine, the intestine typically contracts behind the bolus and relaxes ahead of it
What is the intestinointestinal reflex?
Overdistension of one segment of the intestine relaxes the smooth muscle in the rest of the intestine
What is the gastroileal reflex?
Elevated stomach secretory and motor functions of stomach accelerate movement of material through the ilocecal sphincter
Describe intestinal reflexes?
- After chyme has arrived in the duodenum, weak peristaltic movements move it slowly towards the jejunum 2. These contractions are myenteric reflexes not under CNS control 3. Their effects are limited to within a few centimetres of the site of original stimulus 4. Periodically even without stimulation
What are the 2 types of small intestinal movements?
- segmentation movements 2. migrating motility movements
What are segmentation movements?
localised, mixing type of contraction
Where do these segmentation movements occur?
in portions of the intestine distended by large volumes of chyme
Function of segmentation movements?
§Mix chyme with digestive juices and brings particles of food in contact with the mucosa for absorption
Describe segmentation movements?
- Starts with the contraction of the circular muscle fibres in a portion of the small intestine, an action that constricts the intestine into segments 2. Muscle fibres that encircle the middle of each segment also contract dividing each segment again 3. As this sequence of events repeated, the chyme sloshes back and forth 4. Segmentation occurs most rapidly in the duodenum, about 12 times per minute and progressively slowly, about 8 times per minute in the ileum
When does segmentation end?
After most of the meal has been absorbed, which lessens distension of small intestinal wall, segmentation stops and peristalsis begins
What is migrating motility complex?
a type of peristalsis in the small intestine
Function of migrating motility complex?
MMC moves any undigested material still in the small intestine into large intestine and prevents bacterial growth in the small intestine - moves chyme anal-ward at a velocity of 0.5 to 2 cm/sec
When does this peristalsis movement occur?
after most of the meal has been absorbed
Describe the MMC?
- begins in the lower portions of the stomach and pushes chyme forward along a short stretch of small intestine before dying out 2. The MMC migrates down the small intestine reaching the end of the ileum in 90 - 120 minutes Then another MMC begins in the stomach 3. All together chyme remains in small intestine for 3-5 hrs
Describe the neural regulation of intestinal motility?
- Excitability and frequency of contractions are enhanced by parasympathetic and decreased by sympathetic stimulation (extrinsic control) 2. Enteric nervous system
Describe the hotmonal regulation of intestinal motility?
- Gastrin, CCK (cholecystokinin), insulin and serotonin enhance motility 2. Secretin, glucagon inhibit intestinal motility
What is the function of the ileocecal sphincter?
regulates passage of chyme into cecum from ileum
Function of the gastroileal reflex after a meal?
intensifies peristalsis in the ileum forcing chyme into cecum
What triggers the closure of the iliocecal sphincter?
distension of cecum
When does the motility of the colon start?
when food passes through the ilocecal sphincter
Name the 2 types of movements in the large intestine?
- haustrations 2. mass movements
What are haustrations?
mixing movements
Describe haustrations?
- Simultaneous constrictions of circular and longitudinal muscles at the same points create haustrations 2. After another few minutes, new haustral contractions occur in the nearby area This allows faecal material to be slowly dug into and rolled over and exposed to surface of the large intestine 3. Fluid and dissolved substances are progressively absorbed until only 80 to 200 ml of faeces are expelled each day
What are mass movements?
Movement from transverse colon through the rest of the large intestine results from powerful peristaltic contractions
Function of mass movements?
propel material towards the anal opening
What is the gastrocolic reflex?
The stimulus is the distension of the stomach and the commands are relayed over the intestinal nerve plexuses
Describe mass movements?
- Mass movements occur 3 or 4 times a day 2. The contractions force faecal materials into the rectum and produce the conscious urge to defecate
What is the defecation reflex?
Distension of rectum with faeces initiates reflex actions to defecate
Describe the normal state of the rectum?
normally empty/ almost empty
Defecation is what kind of reflex?
sacral reflex
Describe the defecation reflex?
- In response to distension of the rectal wall, the receptors send sensory nerve impulses to the sacral spinal cord
- Motor impulses from the spinal cord travel along the parasympathetic nerves back to descending colon, sigmoid colon rectum and anus
- The resulting contraction of longitudinal rectal muscles shortens the rectum, thereby increasing pressure in it 4. The pressure + voluntary contractions of the diaphragm, abdominal muscles open internal sphincter muscles
What is voluntary defecation?
The actual release of faeces requires the conscious effort to open the external sphincter
What is the valsalva maneuver?
tensing the abdominal muscles or making expiratory movements while closing the glottis elevate intra-abdominal pressures and help force faecal materials out of rectum
What action inhibits defecation?
Normally ≈ 90⁰ angle between rectum and anus & contraction of puborectalis muscle inhibit defecation
What action causes defecation to occur?
Straining causes abdominal muscles to contract, puborectalis muscles to relax - Pelvic floor lowered 1-3 cm
Describe the nervous regulation of defecation?
- Sympathetic nerve supply to the internal anal sphincter is excitatory 2. Parasympathetic supply is inhibitory 3. External anal sphincter muscle is skeletal and is innervated by the pudendal nerve 4. Sphincter maintained in tonic contraction
What is diarrhea?
when an individual has frequent watery bowel movements
What is diarrhea a casue of?
results when the colonic mucosa becomes unable to maintain normal levels of absorption or the rate of fluid entry into the colon exceeds its maximum reabsorptive capacity
Consequence of diarrhea?
Can be life threatening due to cumulative loss of fluids and ions
What can cause darrhea?
In cholera, bacteria bound to intestinal lining release toxins that stimulate a massive fluid secretion across the intestinal epithelium
What is constipation?
Infrequent defecation, generally dry hard faeces
When does constipation occur?
when faecal material move through the colon so slowly that excessive water reabsorption occurs Faeces then become extremely compact, difficult to move, and highly abrasive
What causes constipation?
consumption of food material with indigestible fibre adds bulk to the faeces, retaining moisture and stimulating stretch receptors that promote peristalsis
What assists in the moveemnt of fecal material in the colon?
Active movements during exercise
