Gastric Motility

Introduction

Gastric motility coordinates storage, mixing, and controlled delivery of food. The proximal stomach accommodates intake, while the distal region grinds contents into chyme. Neural and mechanical factors regulate emptying, and vomiting involves coordinated central and peripheral mechanisms.

• The stomach is functionally divided into proximal and distal regions, each with distinct roles.
• The proximal stomach acts as a reservoir and accommodates food through receptive relaxation. It maintains gentle tonic contractions that gradually move contents toward the antrum.
• The distal stomach functions as a pump with strong phasic contractions. It mixes and grinds food into smaller particles and combines it with gastric secretions.
• This process converts food into chyme suitable for intestinal digestion.
• Gastric motility enables storage of a large meal without a significant rise in pressure. It ensures efficient mechanical digestion and uniform mixing of contents.
• Controlled emptying allows small quantities of chyme to enter the duodenum. This regulation prevents overload and supports optimal digestion and absorption in the small intestine.

Functional Anatomy

Parts of Stomach

• The stomach is functionally divided into proximal and distal regions for understanding gastric motility.

Proximal Stomach

• The proximal stomach includes the fundus and upper body.
• It accommodates large volumes, approximately 2 to 4 liters, with minimal rise in intragastric pressure. This is achieved through receptive relaxation, which allows expansion during food intake.
• The contractile activity in this region is relatively weak, so contents remain less mixed. Therefore, the proximal part primarily acts as a reservoir for ingested food.

Distal Stomach

• The distal stomach mainly consists of the antrum and pyloric region. It functions as a mechanical pump, generating strong contractions. These contractions grind food into smaller particles and mix it thoroughly with gastric secretions.
• The processed content is converted into chyme for further digestion.
• The pyloric sphincter regulates emptying by remaining partially closed.
• Only small amounts of chyme pass into the duodenum during each contraction. This ensures controlled gastric emptying and efficient downstream digestion.

Structure of Stomach Wall

• The stomach wall contains typical gastrointestinal layers, including mucosa, submucosa, and muscularis externa.
• The circular muscle layer is more prominent than the longitudinal layer. ·  Muscle thickness is relatively low in the fundus and body. It increases toward the antrum and pylorus, enabling strong contractions. This well-developed muscle coat allows the antrum to function as an effective mechanical pump.

Gastroduodenal Junction

• The gastroduodenal junction is the transition between the stomach and duodenum, regulated by the pyloric sphincter. This sphincter is formed by a thickened ring of circular muscle fibers.
• Its primary function is to ensure controlled emptying of gastric contents into the duodenum.
• It allows only small volumes of chyme to pass at a rate suitable for intestinal processing.
• It also prevents reflux of duodenal contents into the stomach.

Gastroduodenal Mucosa

• The duodenal mucosa is relatively resistant to bile salts but sensitive to gastric acid.
• In contrast, the gastric mucosa tolerates acid but is vulnerable to bile reflux.
• Incompetence of the pyloric sphincter may lead to gastric mucosal injury due to bile exposure.
• Rapid gastric emptying can expose the duodenum to excess acid, increasing risk of duodenal ulceration.
• Entry of acidic chyme into the duodenum stimulates release of secretin.
• Secretin promotes secretion of bicarbonate-rich pancreatic fluid, which neutralizes acidity. This mechanism protects the duodenal mucosa from acid damage.
• The pyloric region is under neural control by both sympathetic and parasympathetic fibers.

Neural Control of Pyloric Part

• The gastroduodenal junction is the transition between the stomach and duodenum, regulated by the pyloric sphincter. This sphincter is formed by a thickened ring of circular muscle fibers.
• Its primary function is to ensure controlled emptying of gastric contents into the duodenum.
• It allows only small volumes of chyme to pass at a rate suitable for intestinal processing.
• It also prevents reflux of duodenal contents into the stomach.

Innervation of Stomach

• The stomach receives extensive innervation from the autonomic nervous system and the enteric nervous system.
• Parasympathetic (vagal) fibers enhance gastric motility and secretion.
• Sympathetic fibers from the celiac plexus inhibit both motility and secretion.
• Neurons from intramural plexuses directly innervate smooth muscle cells and secretory glands.
• Sensory fibers transmit information to the central nervous system through both vagal and sympathetic pathways. These afferent signals convey data on intragastric pressure, distension, chemical composition, pH, and pain.
• Such inputs regulate gastric activity and coordinate digestive responses.

Electrophysiology of Gastric Motility

• Gastric motility is governed by rhythmic slow waves generated by a pacemaker region in the body of the stomach. These waves occur at a frequency of about three per minute and propagate toward the pylorus.
• Slow waves have multiple phases and resemble cardiac electrical activity, but they do not overshoot and have longer duration.
• Contraction occurs when depolarization exceeds the threshold level.
• The force of contraction depends on the magnitude and duration of depolarization.
• In the antrum, action potential spikes occur during the plateau phase, producing stronger contractions.
• Acetylcholine and gastrin enhance contractility by increasing slow wave amplitude.
• Norepinephrine reduces contractility by decreasing excitability.
• These coordinated neural and electrical mechanisms ensure effective mixing, grinding, and propulsion of gastric contents.

Types Of Gastric Motility

• Gastric motility includes coordinated movements such as hunger contractions, receptive relaxation, peristalsis, migrating motor complex, and reverse peristalsis. These movements ensure storage, mixing, and controlled gastric emptying.

Hunger Contractions

• Hunger contractions occur when the stomach is empty for prolonged periods.
• They are strong, rhythmic contractions, especially in the antrum. These contractions may produce hunger pain and are often associated with partial relaxation of the pyloric sphincter.

Gastric Relaxations

Receptive Relaxation

• Receptive relaxation is the relaxation of the fundus and body during swallowing.
• It allows accommodation of large meals without significant rise in intragastric pressure.
• This response is mediated by vagal pathways involving noncholinergic transmitters such as nitric oxide and vasoactive intestinal peptide.

Adaptive and Feedback Relaxations

• Adaptive relaxation occurs in response to gastric distension after food entry. It is mediated by reflex pathways and enables further accommodation of ingested food.
• Feedback relaxation is triggered by the presence of chyme in the small intestine.
• Factors such as acidic pH and fatty acids inhibit gastric motility through neural and hormonal mechanisms.
• This slows gastric emptying to match the digestive capacity of the intestine.
• Peristaltic movements mix and grind food into chyme and propel it toward the pylorus.
• Reverse peristalsis and the migrating motor complex contribute to regulation of gastric contents. ·  These integrated mechanisms maintain efficient digestion, storage, and coordinated emptying of gastric contents.

Peristalsis

• Gastric peristalsis begins about 30 minutes after food enters the stomach. It is initiated by rhythmic slow waves originating from a pacemaker region near the greater curvature.
• Contraction waves start in the body and move toward the antrum at a frequency of about three per minute. These waves mix food with gastric secretions and propel contents toward the distal stomach.
• The pyloric sphincter remains mostly closed, causing retropulsion and enhancing grinding of food.

Migrating Motor Complex

• The migrating motor complex occurs during the interdigestive period.
• The stomach remains relatively inactive for about 75 to 90 minutes, followed by a burst of strong contractions. These contractions, along with pyloric relaxation, clear residual contents into the duodenum.
• The wave progresses distally, helping to clean the gastrointestinal tract. This cycle repeats approximately every 90 minutes until food intake resumes.

Reverse Peristalsis

• Reverse peristalsis is an abnormal movement in which contractions travel in the opposite direction. It begins in the distal stomach and moves toward the esophagus.
• Relaxation of esophageal sphincters allows expulsion of gastric contents, resulting in vomiting.

Gastric Emptying

• Gastric emptying is the controlled transfer of stomach contents into the duodenum. It occurs gradually to allow proper digestion and absorption in the small intestine.
• After food intake, the stomach undergoes receptive and adaptive relaxation to accommodate contents.

Mechanism of Gastric Emptying

• Subsequent peristaltic and antral contractions propel chyme toward the pylorus.
• Retropulsion enhances mixing and particle size reduction. These coordinated mechanisms ensure efficient and regulated delivery of chyme.

Peristaltic Contractions

• Peristaltic contractions originate in the mid-stomach and move as a ring toward the pylorus. These waves propel contents from the body into the antrum.
• The velocity and strength of contractions increase near the pylorus.
• The fundus and body have weak contractions, so they mainly serve a reservoir function.
• Significant mixing does not occur in the proximal stomach.

Antral Contractions

• Antral contractions are strong and play a major role in mixing food with gastric secretions.
• Forceful contractions push contents toward the pylorus.
• As the pyloric sphincter remains mostly closed, contents are driven backward into the stomach.
• After repeated cycles, the pylorus opens slightly, allowing small amounts of chyme to pass into the duodenum.

Retropulsion

• Retropulsion occurs due to powerful contractions in the terminal antrum.
• Chyme is forced backward toward the proximal stomach.
• This process enhances grinding and homogenization of food particles. It ensures that only adequately processed material approaches the pylorus.
• The pyloric sphincter then allows gradual passage of chyme into the duodenum.
• This coordinated activity forms the gastric pump mechanism.

Physiological Significance

• Weak contractions in the proximal stomach allow layering of contents based on density.
• Fats form a superficial layer and are emptied more slowly.
• Liquids move rapidly toward the antrum and are emptied faster than solids.
• These mechanisms ensure effective mechanical digestion, mixing, and controlled gastric emptying, optimizing subsequent intestinal processing.

Regulation of Gastric Emptying

• Gastric emptying is regulated by coordinated neural and hormonal mechanisms to match intestinal capacity.
• The duodenum and jejunum contain receptors that sense pH, osmolality, and digestion products. These signals adjust the rate at which chyme leaves the stomach.
• Acidic chyme in the duodenum lowers pH and slows gastric emptying. This effect is mediated by neural reflexes and hormones such as secretin.
• Secretin reduces antral contractions and increases pyloric sphincter tone.
• Fat digestion products, including fatty acids, strongly inhibit gastric emptying.
• Hormones such as cholecystokinin and gastric inhibitory peptide are released in response. These hormones decrease gastric motility and delay emptying.
• Increased osmolality of duodenal contents activates osmoreceptors.
• Hypertonic chyme triggers hormonal responses that slow gastric emptying.
• Protein digestion products stimulate release of gastrin, cholecystokinin, and gastric inhibitory peptide. Although gastrin enhances antral contractions, it also increases pyloric resistance. The overall effect is a reduction in emptying rate.
• The volume and composition of the meal influence emptying.
• Large meals delay emptying, whereas liquids empty more rapidly than solids.
• Duodenal distension activates the enterogastric reflex, which inhibits gastric motility. This reflex also responds to acid and chemical stimuli in the intestine.
• Neural control plays an important role.
• Vagal stimulation promotes gastric emptying, while sympathetic activity inhibits it.
• Loss of vagal input can lead to delayed gastric emptying.

• Most intestinal hormones, including secretin, cholecystokinin, and gastric inhibitory peptide, act to slow gastric emptying. These integrated mechanisms ensure controlled delivery of chyme for optimal digestion and absorption.

Applied Physiology

Dysfunctions of Gastric Emptying

Delayed Gastric Emptying

• Delayed gastric emptying occurs in conditions such as diabetic autonomic neuropathy, leading to impaired motility.
• Loss of vagal function after surgery may cause gastroparesis, resulting in gastric stasis.
• Structural obstruction, such as hypertrophic pyloric stenosis, can further delay emptying.

Rapid Gastric Emptying

• Rapid gastric emptying occurs with increased vagal activity and reduced sympathetic influence.
• Sympathetic overactivity during stress suppresses appetite and slows motility.
• Thyroxine enhances gastrointestinal motility, causing increased appetite and frequent bowel movements.
• Liquid-rich meals empty faster than solid food.

Vomiting

Definition

• Vomiting is the forceful expulsion of gastric and duodenal contents through the mouth.

Associated Features

• It is usually preceded by nausea, salivation, pallor, sweating, and increased heart rate.
• Retching involves rhythmic contractions that move contents into the esophagus before expulsion.

Stimuli and Vomiting Centers

• Vomiting is a reflex action coordinated by centers in the medulla.
• Multiple afferent pathways transmit signals from the gastrointestinal tract, vestibular system, and higher brain centers.
• Vestibular inputs trigger vomiting in motion sickness.
• Pharyngeal stimulation activates reflex pathways during gagging.
• The area postrema detects circulating toxins, drugs, and hormones.
• Emotional stimuli from higher centers can also initiate vomiting.
• Common triggers include gastric distension, throat irritation, visceral pain, and chemical substances.
• The reflex involves coordinated contraction of abdominal muscles, relaxation of the lower esophageal sphincter, and opening of the upper esophageal sphincter. This sequence allows expulsion of contents while protecting the airway.

Emetics and Antiemetics

• Emetics are agents that induce vomiting by stimulating central or peripheral receptors.
• The chemoreceptor trigger zone contains receptors sensitive to circulating substances.
• Antiemetic drugs act by blocking neurotransmitter receptors.
• Serotonin receptor antagonists reduce chemotherapy-related vomiting.
• Dopamine receptor antagonists are also effective.
• Additional agents, including corticosteroids and sedatives, are used in resistant cases. ·  Understanding these mechanisms helps in managing nausea and vomiting in clinical practice.

Mechanism of Vomiting

• The vomiting reflex follows a coordinated sequence of motor events.
• Reverse peristalsis begins in the proximal small intestine, moving contents toward the stomach.
• The pyloric sphincter relaxes, allowing duodenal contents to enter the stomach.
• A deep inspiratory effort against a closed glottis lowers the diaphragm and increases intra-abdominal pressure.
• Strong contraction of abdominal muscles further elevates pressure within the abdomen. This pressure forces gastric contents into the esophagus.
• The lower esophageal sphincter relaxes, permitting upward movement of contents.
• With continued retching, the upper esophageal sphincter opens to allow expulsion through the mouth.
• Protective mechanisms prevent aspiration.
• The glottis closes, vocal cords adduct, and respiration is briefly inhibited.
• These coordinated actions ensure effective expulsion while safeguarding the respiratory tract.

Dumping Syndrome

• Dumping syndrome occurs after gastric surgery due to rapid entry of food into the intestine.
• Patients develop weakness, dizziness, and sweating within a few hours after meals.
• Rapid glucose absorption causes hyperglycemia, followed by excess insulin release and reactive hypoglycemia.
• Symptoms result from this sudden fall in blood glucose levels.
• Entry of hypertonic chyme draws fluid into the intestine, leading to hypovolemia and hypotension. These changes contribute to fatigue and lightheadedness.
• Dietary modification and controlled meal patterns help reduce symptoms.