Gastric motility

Definitions & Key takeaways

Gastric motility is the coordinated movement of the muscles and secretions of the stomach. This coordinated activity propels food and drink through the stomach and comprises three main phases. First, there is receptive relaxation characterized by the relaxation of lower esophageal sphincter, to let food pass through into the stomach. Next, there is secretion in which the gastric juice is released in the stomach, and peristalsis helps to mix contents and break apart food particles. Finally, there is gastric emptying which occurs when food leaves the stomach and goes into the duodenum.

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Introduction0:00–1:11

When we eat food, it’s first chewed up in the mouth and mixes with saliva to form a mushy mass called a bolus, and then it travels down a muscular tube called the esophagus.
Next, it enters the stomach through the lower esophageal sphincter, which is a ring of muscle surrounding an opening. The stomach has three main parts: the fundus, the body and the antrum.
The body is further divided into a proximal portion and a distal portion. The fundus, and proximal body make up the orad region, or top half of the stomach.
The distal body and the antrum make up the caudad region or bottom half of the stomach. Once the food bolus is partially digested in the stomach, it’s called chyme and it passes through the pyloric sphincter and into the duodenum, which is the first part of the small intestine.
The pancreas is connected to the duodenum through the pancreatic duct, and it secretes many digestive enzymes to further break down food.
All these part play a role in gastric motility which is the contraction of the stomach to break up food and move it into the intestine.

Anatomy1:11–4:47

Like other parts of the gastrointestinal tract, the stomach has 4 layers. Starting from the innermost layer to the outermost layer, these layers are the mucosa, submucosa, muscularis externa, and serosa.
The mucosa and muscularis externa layers are modified in the stomach. The mucosa contains various glands filled with different cells that secrete the components of gastric juice.
In the body of the stomach, there are parietal cells that secrete hydrochloric acid or HCl, a strong acid that helps to break down protein, and chief cells that secrete pepsinogen, an inactive enzyme.
When pepsinogen is exposed to HCl it activates to becomes pepsin - an enzyme that helps break down proteins. In the antrum, there are mucous cells which secrete mucus which protects the stomach lining from the acidic environment.
There are also G cells which secrete a hormone called gastrin. Unlike the others secretions, gastrin is not secreted into the stomach lumen but instead, it’s released into the bloodstream where it can reach the basolateral surface of other stomach cells - which is the cell surface that faces the blood.
Gastrin increases the HCl secretion of the parietal cells, it increases pepsinogen secretion by the chief cells, and it also increases the contraction of the stomach muscles.
The next layer is the submucosa is mainly made up of connective tissue, blood vessels, and nerves - which supply the mucosa.
The nerves in the submucosa branch out and connect with one another forming the submucosal plexus of nerves which actually stretch throughout the gastrointestinal tract.
In the stomach, the submucosal plexus regulates gastric secretions of various glands and controls blood flow to the stomach.
Next, there’s the muscularis externa layer of the stomach which contains three layers of smooth muscle that involuntarily contract in waves called peristalsis.
These waves of peristalsis help the stomach mix, digest and empty the food that it has received. The three muscular layers include an outer longitudinal layer, a middle circular layer, and inner oblique layer which is unique to the stomach.
This extra layer of inner oblique muscles provide more forceful contraction in the stomach. In addition, the thickness of the muscular layers increases as you go from the orad region to the caudad region and that helps push food towards and past the pyloric sphincter.
Innervation of these muscles is from the myenteric plexus which lies between the circular and longitudinal muscle layers.
Both the myenteric and submucosal plexuses makes up the enteric nervous system which can function autonomically without input from the central nervous system and is often called the second brain because of how many interconnected nerves are there.
In fact, the myenteric and submucosal plexuses contain just as many neurons as the spinal cord! In addition, they are connected to the central nervous system.
They receive parasympathetic input through the vagus nerve, which increases gastric secretion and motility, and sympathetic input through nerve fibers from the celiac ganglion which inhibits those functions.
Finally the outermost layer is the serosa which is a thin, slippery membrane that surrounds the entire gastrointestinal tract, kind of like the casing around a sausage.

Receptive relaxation4:47–5:46

When food travels down the esophagus towards the stomach it’s propelled downwards by peristalsis, gravity, and the lower pressure in the abdominal cavity.
When food reaches the distal part of the esophagus, it triggers a process called receptive relaxation. That’s where food distends the esophagus and that’s detected by mechanoreceptors in the muscularis externa layer of the distal esophagus.
Next the mechanoreceptors relay that information up to the brain via the vagus nerve. Once the brain processes that information, a motor signal is sent back down the vagus nerve to the myenteric plexus.
The nerves in the myenteric plexus secrete a neurotransmitter called vasoactive intestinal peptide or VIP which cause the smooth muscles in the esophagus and the orad region of the stomach to relax.
This “relaxation” lets the food through and it’s “received” by the stomach - hence the term receptive relaxation! Once food has entered the stomach, it distends the stomach, and this is sensed by receptors in the submucosa called mechanoreceptors which triggers the release of gastrin from the G cells in the stomach.

Secretion and Peristalsis5:46–6:55

Gastrin promotes the secretion of HCl from parietal cells and pepsinogen from chief cells as well as increase contraction of the stomach muscles.
Similar to the esophagus, the stomach has waves of peristalsis that begin near the lower esophageal sphincter and moves toward the pyloric sphincter.
This motion is initially just a gentle ripple, but it becomes more powerful due to the thick muscular walls of the caudad region of the stomach.
The enteric nervous system regulates the rate that the stomach contracts and it’s typically about 4 contractions per minute.
When food reaches the stomach, the rate remains the same but the force of the contraction increases. These contractions break apart food into particles that are about 1 millimeters in size so they can pass through the pyloric sphincter.
Any large particles pushed back into the body of the stomach - a process called repulsion - so that they can be further broken down.

Gastric Emptying6:55–8:48

Once the food has been broken down, it moves into the duodenum. Gastric emptying usually happens in a few hours, and it varies a bit depending on the stomach contents.
Liquids empty faster than solids, and liquids that contains lots of calories like juices empty slower than liquids like water.
Another factor is the amount of food eaten and the volume of content in the stomach. Eating a single chip with salsa won’t distend the stomach very much, so the stimulus for gastric motility is relatively small, meaning that the chip might hang out in the stomach for a while.
On the flip side, eating three burritos will distend the stomach a lot, and that will also slow down gastric emptying since there’s a lot to break down.
Like most things the most efficient gastric emptying takes place somewhere in between. Foods high in fat also slow down gastric emptying - and that’s because fat stimulates release of the hormone cholecystokinin or CCK from I-cells of the mucosa of the duodenum.
CCK binds to its receptors on the stomach to slow down the rate of gastric emptying by decreasing stomach contractions so that there’s enough time for fat to be digested and absorbed in the small intestine.
Acidic substances containing lots of protons also slow down gastric emptying - and that’s because H+ receptors in the duodenal mucosa detect the high concentration of protons in the chyme and send that information to gastric smooth muscle cells through interneurons in the myenteric plexus.
This slows down the delivery of gastric contents to the duodenum and ensures enough time for neutralization of the protons by pancreatic HCO3−.
This neutralization of duodenal contents is necessary for pancreatic enzymes to work in the duodenum. Alright, as a quick recap, gastric motility is made up of three main events: receptive relaxation where the orad and esophageal sphincter relaxes to let food into the stomach, mixing and digestion, where food is broken down by gastric acid, enzymes, and the contraction of the stomach, and gastric emptying, where food leaves the stomach and goes into the duodenum.

Review8:48–9:11