As food leaves the esophagus into the stomach, stomach initially stores the food, and then mixes the food with stomach acid, pepsin, mucus and other gastric secretions to form a semi-fluid substance called chyme, after which stomach empties the chyme into the small intestine (duodenum) at a controlled rate suitable for adequate digestion and absorption by the small intestine.
This lecture shall therefore cover the function of the stomach in terms of motility, together with the mechanisms that control the motor functions of the stomach.
Physiologic anatomy of the stomach
A review of the specific functional regions of the stomach will facilitate our understanding of the motor functions of the stomach.
Anatomically, the stomach has three parts:
- Body, and
Receptive relaxation of the stomach
Just before the point where the esophagus meets the stomach, circular muscles of the esophagus form a sphincter called lower esophageal sphincter or gastroesophageal sphincter. This sphincter regulates the passage of food from the esophagus into the stomach. This sphincter usually remains tonically closed whereas the mid-portion of the esophagus usually remains relaxed. The lower esophageal sphincter (LES) sphincter relaxes ahead of each peristaltic wave that gets to the stomach to allow for easy passage of swallowed food into the stomach.
One motor function of stomach is to store food temporarily during which some digestive processes occur on the food. An entirely empty stomach has a volume that is as little as 50 mL; however, the same stomach can hold up to 1500 mL of substances without significant increase in intraluminal pressure. Receptive relaxation, as a phenomenon, allows the stomach to hold large volumes of food without significant rise in pressure in the stomach.
As one swallows a meal, smooth muscles found in the wall of the stomach relax even before the arrival of the food in the stomach. This phenomenon called receptive relaxation allows the stomach to increase its volume with very little increase in pressure.
As a peristaltic swallowing wave approaches the stomach, myenteric inhibitory neurons transmit a wave of relaxation, which cause the entire stomach to relax. This initial relaxation serves to prepare stomach to receive food that shall arrive soon from the esophagus.
Parasympathetic fibers that release nitric oxide (NO) at their nerve endings; (NO act on enteric neurons of the stomach to mediate stomach wall relaxation); are responsible for mediating the receptive relaxation. As more food enter the stomach, a vagal-mediated reflex arising from the stomach to the brainstem (CNS) and then back to the stomach causes more and more relaxation of stomach musculature. Receptive relaxation therefore makes the stomach wall to budge outward ultimately enabling the stomach to store even more food. Sensory impulses arise from stretch receptors (mechanoreceptors) in the stomach wall and run in vagus nerve to the brainstem; the brainstem integrates the impulses before sending motor commands back to the stomach through vagus nerve.
The motor command causes the stomach muscles to relax in order to enable the stomach accommodate more food. An important point to note is that the rise in volume of stomach does not cause much rise in pressure in the stomach. However, there is a limit to what the stomach can accommodate; at volumes exceeding about 1.5L, the intraluminal pressure of stomach rises significantly. If the individual ingests more food, the pressure may rise so much so that it forces gastric content back into the esophagus (a common cause of gastroesophageal reflux and resultant sensation of heartburn).
Stomach contractions mix food with stomach secretions to form chyme
Smooth muscles of the stomach contract in response to the arrival of food in the stomach. These contractions are weak constrictor waves, which begin in the body of the stomach producing just a ripple of constrictor waves as it proceeds toward the antrum. Note these contractions are too weak and not strong enough to produce significant mixing of stomach content with stomach secretions. However, as the mixing wave approaches antrum where the surrounding muscle is thicker the force of contraction becomes significantly stronger. By the time a stomach mixing wave arrives the pyloric sphincter, it causes the pyloric muscles to contract and close the sphincter.
The constrictor rings contract and dig deep into the luminal contents of the stomach, thereby mixing the stomach content.
Moreover, the strong antral contractions close off the pyloric sphincter blocking emptying of stomach contents into the duodenum. With each mixing wave of contraction, only a small amount of antral contents squeeze through the closed pyloric sphincter, much of the antral content squeeze upstream backwards towards the body of the stomach. This further contributes to the mixing effect in antral portion of the stomach.
It takes competent LES to prevent the forced backward stomach content from entering the esophagus.
BER of stomach determines the rate of stomach contractions
Electrical slow waves generated by enteric pacemaker cells found in the stomach wall initiate stomach mixing contractions. These specialized cells with smooth muscle properties undergo spontaneous cycles of depolarization and repolarization. The electrical waveform of these depolarization-repolarization cycles called slow waves or basic electrical rhythm of the stomach determines the rate of peristaltic waves of the stomach. Pacemaker cells found in the stomach generate slow wave at an exceedingly slow rate of 3/min, hence the basic electrical rhythm of stomach is 3/min.
Gap junctions between the pacemaker cells (specialized smooth muscle cells) conduct the slow waves from fiber to fiber and induces similar slow wave in the entire gastric musculature (including both circular and longitudinal muscle layers).
Excitatory neurotransmitters and hormones; their role in stomach contractions
Slow waves on their own are not true action potentials and are often too weak to induce significant contractions. Excitatory neurotransmitters and hormones act on the membrane of the smooth muscle cells to cause depolarization current on their membrane, consequently bringing the slow wave closer to the threshold voltage. If the amplitude of the slow wave reaches threshold voltage, action potentials occur superimposed on the peak of the slow wave. Excitatory neurotransmitters and hormones increase the frequency of the action potentials consequently increasing the force of smooth muscle contraction.
This means that stomach basic electrical rhythm (BER) determines the frequency of stomach contractions and remains essentially constant, whereas the force of contraction and consequently the amount of chyme emptied for each contraction depends on the availability of excitatory neurotransmitters and hormones.
Therefore, the presence or absence of excitatory neurotransmitters and hormones has significant effects on the force of contraction of the smooth muscles of the stomach.