Gastric motor activity plays a role in filling, churning, and emptying
Gastric motor activity has three functions: (1) The receipt of ingested material represents the reservoir function of the stomach and occurs as smooth muscle relaxes. This response occurs primarily in the proximal portion of the stomach. (2) Ingested material is churned and is thereby altered to a form that rapidly empties from the stomach through the pylorus and facilitates normal jejunal digestion and absorption. Thus, in conjunction with gastric acid and enzymes, the motor function of the stomach helps to initiate digestion. (3) The pyloric antrum, pylorus, and proximal part of the duodenum function as a single unit for emptying into the duodenum the modified gastric contents (chyme), consisting of both partially digested food material and gastric secretions. Gastric filling and emptying are accomplished by the coordinated activity of smooth muscle in the esophagus, lower esophageal sphincter, and proximal and distal portions of the stomach, as well as the pylorus and duodenum (Box 42-4).
Vomiting, a frequent sign and symptom in clinical medicine, represents a complex series of multiple afferent stimuli coordinated by one or more brain centers, leading to a coordinated neuromuscular response. Nausea is the sensation that vomiting may occur. The act of emesis involves several preprogrammed coordinated smooth- and striated-muscle responses. The initial event is the abolition of intestinal slow-wave activity that is linked to propulsive peristaltic contractions. As the normal peristaltic contractions of the stomach and small intestine wane, they are replaced by retrograde contractions, beginning in the ileum and progressing to the stomach. These retrograde contractions are accompanied by contraction of abdominal and inspiratory muscles (external intercostal muscles and diaphragm; see pp. 606–607) against a closed glottis, which results in an increase in intra-abdominal pressure. Relaxation of the diaphragmatic crural muscle and lower esophageal sphincter permits transmission of this increase in intra-abdominal pressure into the thorax, with expulsion of the gastric contents into the esophagus. Movement of the larynx upward and forward and relaxation of the upper esophageal sphincter are required for oral propulsion, whereas closure of the glottis prevents aspiration.
Three major categories of stimuli can potentially induce the foregoing series of events that leads to vomiting. First, gastric irritants and peritonitis, for example, probably act by vagal afferent pathways, presumably to rid the body of the irritant. Second, inner ear dysfunction or motion sickness acts through the vestibular nerve and vestibular nuclei. Third, drugs such as digitalis and certain cancer chemotherapeutic agents activate the area postrema in the brain (see p. 284). Pregnancy can also cause nausea and vomiting, by an unknown mechanism. Although several central loci receive these emetic stimuli, the primary locus is the area postrema, also called the chemoreceptor trigger zone. Although no single brainstem site coordinates vomiting, the nucleus tractus solitarii (NTS; see p. 348) plays an important role in the initiation of emesis. Neurotransmitter receptors that are important in various causes of vomiting include the NK1 receptor (which binds substance P) in the NTS, 5-hydroxytryptamine type 3 (5-HT3) receptors in vagal afferents, and dopamine D2 receptors in the vestibular nucleus.
The pattern of gastric smooth-muscle activity is distinct during fasting and after eating. The pattern during fasting is referred to as the migrating myoelectric (or motor) complex (MMC). This pattern is terminated by eating, at which point it is replaced by the so-called fed pattern. Just as the proximal and distal regions of the stomach differ in secretory function, they also differ in the motor function responsible for storing, processing, and emptying liquids and solids. The proximal part of the stomach is the primary location for storage of both liquids and solids. The distal portion of the stomach is primarily responsible for churning the solids and generating smaller liquid-like material, which then exits the stomach in a manner similar to that of ingested liquids. Thus, the gastric emptying of liquids and of solids is closely integrated.
Filling of the stomach is facilitated by both receptive relaxation and gastric accommodation
Even a dry swallow relaxes both the lower esophageal sphincter and the proximal part of the stomach. Of course, the same happens when we swallow food. These relaxations facilitate the entry of food into the stomach. Relaxation in the fundus is primarily regulated by a vagovagal reflex and has been called receptive relaxation. In a vagovagal reflex, afferent fibers running with the vagus nerve carry information to the central nervous system (CNS), and efferent vagal fibers carry the signal from the CNS to the stomach and cause relaxation by a mechanism that is neither cholinergic nor adrenergic. The result is that intragastric volume increases without an increase in intragastric pressure. If vagal innervation to the stomach is interrupted, gastric pressure rises much more rapidly.
Quite apart from the receptive relaxation of the stomach that anticipates the arrival of food after swallowing and esophageal distention, the stomach can also relax in response to gastric filling per se. Thus, increasing intragastric volume, as a result of either entry of food into the stomach or gastric secretion, does not produce a proportionate increase in intragastric pressure. Instead, small increases in volume do not cause increases in intragastric pressure until a threshold is reached, after which intragastric pressure rises steeply (Fig. 42-14A). This phenomenon is the result of active dilation of the fundus, called gastric accommodation. Vagotomy abolishes a major portion of gastric accommodation, so increases in intragastric volume produce greater increases in intragastric pressure. However, the role of the vagus nerve in gastric accommodation is one of modulation. It is generally believed that the ENS (see pp. 855–856) is the primary regulator permitting the storage of substantial amounts of solids and liquids in the proximal part of the stomach without major increases in intragastric pressure.
FIGURE 42-14 Gastric filling and emptying. (B, Data from Dooley CP, Reznick JB, Valenzuela JE: Variations in gastric and duodenal motility during gastric emptying of liquid meals in humans. Gastroenterology 87:1114–1119, 1984.)
The stomach churns its contents until the particles are small enough to be gradually emptied into the duodenum
The substance most rapidly emptied by the stomach is isotonic saline or water. Emptying of these liquids occurs without delay and is faster the greater the volume of fluid. Acidic and caloric fluids leave the stomach more slowly, whereas fatty materials exit even more slowly (see Fig. 42-14B). Solids do not leave the stomach as such, but must first be reduced in size (i.e., trituration). Particles >2 mm do not leave the stomach during the immediate postprandial digestive period. The delay in gastric emptying of solids occurs because solids must be reduced in size to <2 mm; at that point, they are emptied by mechanisms similar to those for liquids.
Movement of solid particles toward the antrum is accomplished by the interaction of propulsive gastric contractions and occlusion of the pylorus, a process termed propulsion (Fig. 42-15A). Gastric contractions are initiated by the gastric pacemaker, which is located on the greater curvature, approximately at the junction of the proximal and middle portions of the stomach. These contractions propel the luminal contents toward the pylorus, which is partially closed by contraction of the pyloric musculature before delivery of the bolus. This increase in pyloric resistance represents the coordinated response of antral, pyloric, and duodenal motor activity. Once a bolus of material is trapped near the antrum, it is churned to help reduce the size of the particles, a process termed grinding (see Fig. 42-15B). Only a small portion of gastric material—that containing particles <2 mm—is propelled through the pylorus to the duodenum. Thus, most gastric contents are returned to the body of the stomach for pulverization and shearing of solid particles, a process known as retropulsion (see Fig. 42-15C). These processes of propulsion, grinding, and retropulsion repeat multiple times until the gastric contents are emptied. Particles >2 mm are initially retained in the stomach but are eventually emptied into the duodenum by MMCs during the interdigestive period that begins ~2 hours or more after eating.
FIGURE 42-15 Mechanical actions of the stomach on its contents.
Modification of gastric contents is associated with the activation of multiple feedback mechanisms. This feedback usually arises from the duodenum (and beyond) and almost always results in a delay in gastric emptying. Thus, as small squirts of gastric fluid leave the stomach, chemoreceptors and mechanoreceptors—primarily in the proximal but also in the distal portion of the small intestine—sense low pH, high caloric content, lipid, certain amino acids (i.e., tryptophan), or changes in osmolarity. These signals all decrease the rate of gastric emptying by a combination of neural and hormonal signals, including the vagus nerve, secretin, CCK, and GIP released from duodenal mucosa. Delayed gastric emptying involves (1) the coordinated function of fundic relaxation, (2) inhibition of antral motor activity, (3) stimulation of isolated, phasic contractions of the pyloric sphincter, (4) and altered intestinal motor activity.