Gastric motor function is regulated by both mechanical and chemical stimuli, such as distention and the presence of protein digestion products. These stimuli activate neural pathways, mainly through vagovagal reflexes initiated by extrinsic vagal afferent fibers that terminate in the stomach's muscle and mucosa. Chemical stimuli are detected by mucosal afferents, while mechanosensitive afferents respond to distention and smooth muscle contraction. This afferent stimulation leads to the reflex activation of vagal efferent (parasympathetic) outflow and the activation of enteric neurons that innervate the smooth muscle. The activation of enteric neurons has inhibitory and excitatory effects on gastric smooth muscle, which vary depending on the stomach region. Distention of the gastric wall inhibits smooth muscle in the proximal part of the stomach, allowing entry and storage of the meal with minimal increase in intragastric pressure.

In contrast, during the gastric phase of the meal, the predominant motor pattern in the distal part of the stomach is the activation of smooth muscle to produce and strengthen antral contractions. The rate of antral contractions is controlled by the gastric pacemaker, while the strength of the contractions is regulated by the release of neurotransmitters, including substance P and acetylcholine, from enteric neurons. These neurotransmitters increase the level of depolarization of the smooth muscle, resulting in stronger contractions. In this phase, the pylorus is mostly closed, causing the antral contractions to move the contents towards the pylorus but then return them to the more proximal part of the stomach. This movement allows for mixing of the gastric contents. Additionally, antral contractions can occlude the lumen, dispersing larger particles, a process known as grinding.

Coordinated activity in the smooth muscle of the proximal and distal portions of the stomach, along with the pyloric sphincter, results in mixing and grinding in the gastric antrum. The peristaltic wave moves down the gastric body and antrum towards the pylorus. If the pylorus is closed, the contents of the gastric antrum are retropulsed back into the more proximal part of the stomach. This motility pattern leads to grinding and mixing of food with secretions from the gastric wall, eventually reducing particle size and allowing digestive products to empty into the duodenum.

The stomach has two main functions: storage and initiation of protein digestion. Its regulation involves extrinsic and intrinsic neural pathways, as well as humoral (gastrin) and paracrine (histamine) mediators. The main secretions from the stomach are acid and pepsinogen, which begin protein digestion. Acid secretion occurs through the H+,K+-ATPase on the apical plasma membrane of parietal cells. The stomach also secretes intrinsic factor, which plays a role in vitamin B12 absorption.
The stimuli regulating gastric motor function that result from the presence of the meal in the stomach are both mechanical and chemical and include distention and the presence of products of protein digestion (amino acids and small peptides). The pathways regulating these processes are predominantly neural and consist of vagovagal reflexes initiated by extrinsic vagal afferent fibers that terminate in the muscle and mucosa. Mucosal afferents respond to chemical stimuli, and mechanosensitive afferents respond to distention and contraction of smooth muscle. This afferent stimulation results in reflex activation of vagal efferent (parasympathetic) outflow and activation of enteric neurons that innervate the smooth muscle. Activation of enteric neurons produces both inhibitory and excitatory effects on gastric smooth muscle; these effects vary depending on the region of the stomach. Thus distention of the gastric wall results in inhibition of smooth muscle in the proximal portion of the stomach and subsequent reflex accommodation, which allows entry and storage of the meal to occur with minimal increase in intragastric pressure.
In contrast, the predominant motor pattern of the distal part of the stomach in the gastric phase of the meal is activation of smooth muscle to produce and strengthen the antral contractions. The rate of antral contractions is set by the gastric pacemaker; however, the magnitude of the contractions is regulated by release of neurotransmitters from enteric neurons, including substance P and acetylcholine, which increase the level of depolarization of the smooth muscle and therefore produce stronger contractions. In this phase of the meal the pylorus is mostly closed. Thus antral contractions will tend to move the contents toward the pylorus; however, because the pylorus is closed, the contents will be returned to the more proximal part of the stomach. In this way the gastric contents will be mixed. In addition, antral contractions can occlude the lumen, and thus larger particles will be dispersed, a process referred to as grinding (
Fig. 29.14
Onset of terminal Complete terminal antral contraction antral contraction • Force for retropulsion is increased pressure in terminal antrum as the antral contraction approaches the closed pylorus.
•Fig.29.14Coordinated activity in the smooth muscle of the proximal and distal portions of the stomach and the pyloric sphincter results in mixing and grinding in the gastric antrum. The peristaltic wave moves down the gastric body and antrum toward the pylorus. If the pylorus is closed, the contents of the gastric antrum are retropulsed back into the more proximal part of the stomach. This pattern of motility results in grinding and mixing of the food with secretions from the gastric wall and eventually leads to a reduction in particle size and the presence of digestive products that will empty into the duodenum.
1.
The main functions of the stomach are storage and initiation of protein digestion.
2.
Regulation of gastric function is driven by extrinsic and intrinsic neural pathways together with key humoral (gastrin) and paracrine (histamine) mediators.
3.
The key secretions from the stomach are acid and pepsinogen, which together begin protein digestion.
4.
H+ is secreted across the apical plasma membrane of parietal cells via H+,K+-ATPase.
5.
The stomach also secretes intrinsic factor, which is involved in absorption of vitamin B12.
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