Gastric Motility And Emptying

Question 1

Stomach layers

Answer 1

In contrast to the rest
of the intestine the stomach is composed of four
muscle layers; an outer longitudinal layer, a
circularly-oriented layer, an inner oblique layer and a
muscularis mucosae.

Question 2

Stomach fxs.

Answer 2

1. Storage……..to act as a reservoir
2. Mixing……..preparation of ingested materials and formation of chyme
3. Emptying….delivery of nutrients to the small intestine

Question 3

Upper 1/3 (orad) vs. lower 2/3 (caudad) stomach regions

Answer 3

The caudad region has a much thicker muscular wall and produces much stronger contractions than the orad region.

The resting membrane potential of the smooth muscle cells is less negative (about -48mV) in the upper portion and gradually becomes more negative moving towards the pylorus (-75mV).

The upper one third is electrically quiescent and has no basal electrical rhythm (BER) but it has active tone at rest. The lower two thirds exhibit a BER; this may reach threshold and initiate contractions. The BER originates in pacemaker cells located in the mid stomach along the greater curvature.

Question 4

Sympathetic innervation of stomach

Answer 4

stomach receives sympathetic innervation from the stellate, celiac, and superior mesenteric ganglia.

Question 5

Receptive relaxation

Answer 5

the stomach must fill without a significant increase in intraluminal pressure. To achieve this, the stomach undergoes a vagally mediated process known as receptive relaxation (adaptive relaxation) which allows for an increase in gastric volume with little rise in intragastric pressure.

Receptive relaxation is a vagovagal reflex (both the afferent and
efferent limbs of the reflex are carried in the vagus nerve).
Mechanoreceptors detect distention of the stomach and relay the
information to the CNS via sensory neurons. The CNS then sends
efferent information to the smooth muscle wall of the orad stomach
and relaxation occurs. The neurotransmitters responsible appear to
be Nitric Oxide (NO) and Vasoactive Intestinal Polypeptide (VIP).

Question 6

Vagotomy’s effect on receptive relaxation

Answer 6

Vagotomy (to decrease acid production) will eliminate
receptive relaxation. In these patients food will cause
an increase in tone and the stomach will empty at an
accelerated rate → “dumping syndrome”.

Question 7

Retropulsion

Answer 7

The opening of the
pylorus is so small that only a few milliliters of antral contents are
actually expelled into the duodenum with each peristaltic wave.
Thus, in association with pyloric contraction the bulk of the antral
contents are forced backwards towards the body of the stomach.

This retropulsion is extremely important in mixing the stomach
contents. When completely emptied the stomach then becomes quiescent.

Question 8

migrating motor (myoelectric) complexes (MMC)

Answer 8

During fasting periodic gastric contractions occur (approximately every 90 minutes) to clear the
stomach of any remaining food from the previous meal.

Question 9

Factors that regulate gastric emptying

Answer 9

1. Particle size: liquids empty faster than solids, which empty faster than indigestible solids. Retropulsion continues until solid food particles are reduced to the required size.
2. Volume: the stomach empties in proportion to the volume of the gastric contents (amount of chyme).
3. Acid: the presence of acid in the duodenum releases secretin which decreases gastric emptying by inhibiting antral contractions and by stimulating contraction of the pyloric sphincter.
4. Osmolality: hypertonic or hypotonic solutions empty slower than isotonic solutions. This is because activation of osmoreceptors in the duodenum results in inhibition of gastric motility.
5. Hormones: There are a variety of hormones released in response to the entry of food into the stomach (e.g. gastrin) and into the duodenum (e.g. CCK).
6. Neural mechanisms: Chemical or mechanical (distension) stimulation of the duodenal mucosa inhibits gastric peristalsis and slows gastric emptying. This is called the enterogastric reflex.
7. Meal composition: Glucose empties faster than protein, which empties faster than fat. Again, this relates to inhibition of gastric motility in response to the duodenal contents.
8. Vomiting: (emesis) is a forced expulsion of stomach and intestinal contents. It is usually preceded by nausea, salivation and retching. During retching increased intra-abdominal pressure compresses the
   duodenum and antrum forcing their contents into the relaxed body of the stomach. The large pressure gradient between the thorax (low) and the abdomen (high) then opens the LES and forces gastric contents into the esophagus and out of the mouth.

Question 10

GASTROPARESIS

Answer 10

delayed gastric emptying. A common problem often due to mechanical obstruction, complicated diabetes or surgical procedures involving partial gastrectomy.

Question 11

DUMPING SYNDROME

Answer 11

when there is a sudden delivery of hypertonic fluid to the duodenum. Occurs when the rate of liquid emptying from the stomach is accelerated or if the controls from the duodenum aren’t fast enough or sufficient to slow down emptying. Osmotic equilibrium across the intestinal mucosa may then induce hypovolemia, which could lead to nausea and dizziness. Can also be a side effect of gastric bypass surgery. If too large meals are eaten a rise in insulin secretion occurs due to rapid absorption of glucose from the intestine.