Phases of digestion

Last updated March 28, 2024

The nervous system, and endocrine system collaborate in the digestive system to control gastric secretions, and motility associated with the movement of food throughout the gastrointestinal tract, including peristalsis, and segmentation contractions.

A fourth phase of acid secretion is known as the basal state which occurs in the times between meals (interdigestive phase). The level of acid secretion during these times is regulated by body weight, individual, number of parietal cells, and time of day. Acid secretion is lowest in the morning before awakening and highest at night.^[2]

Phases

Cephalic phase

The cephalic phase of digestion is the stage in which the stomach responds to the mere sight, smell, taste, or thought of food. About 20% of total acid secretion occurs before food enters the stomach. These sensory and mental inputs converge on the hypothalamus to induce the responses needed for preparing the gastrointestinal tract for food processing, which relays signals to the medulla oblongata.^[3] Vagus nerve fibers from the medulla stimulate the parasympathetic nervous system of the stomach which, in turn, stimulates gastric secretion (via parietal and G cells).^[1] This enhanced secretory activity brought on by the thought or sight of food is a conditioned reflex. It only occurs when food is desired. When appetite is depressed this part of the cephalic reflex is inhibited.

Chain of events

Sensory stimuli from food activate dorsal motor nucleus of vagus nerve in the medulla (activating the parasympathetic nervous system). Insulin induced hypoglycemia also stimulates the vagus nerve. This results in four distinct physiological events.

1) In the body of the stomach, the vagal postganglionic muscarinic nerves release acetylcholine(ACh) which stimulates parietal cell H+ secretion.

2) In the lamina propria of the body of the stomach the ACh released from the vagal endings triggers histamine secretion from ECL cells. Histamine also stimulates H+ secretion from parietal cells.

3) In the antrum, peptidergic postganglionic parasympathetic vagal neurons and other enteric nervous system neurons release GRP which stimulates antral G cells to produce and release gastrin. Gastrin stimulates gastric acid secretion by directly stimulating parietal cells as well as by promoting histamine secretion by ECL cells.

4) In both the antrum and corpus, the vagus nerve inhibits D cells, thus reducing their release of somatostatin and reducing background inhibition of gastrin release.^[2]

Activation of gastric chief cells

Gastric chief cells are primarily activated by ACh. However the decrease in pH caused by activation of parietal cells further activates gastric chief cells. Alternatively, acid in the duodenum can stimulate S cells to secrete secretin which acts on an endocrine path to deactivate gastric chief cells.

Gastric phase

50-60% of total gastric acid secretion occurs during this phase. The gastric phase is a period in which swallowed food and semidigested protein (peptides and amino acids) activate gastric activity. Ingested food stimulates gastric activity in two ways: by stretching the stomach and by gastric contents stimulating receptors in the stomach.^[2] Stretch activates two reflexes: a short reflex mediated through the myenteric nerve plexus, and a long reflex mediated through the vagus nerves and brainstem.^[1]

Distention path

1.) Vagovagal reflex: Distention, or stretching, activates an afferent pathway which in turn stimulates efferent response from the dorsal nucleus of the vagus nerve. Stimulation of acid secretion occurs as it does in the cephalic phase.

2.) Local ENS Pathway: Activated ENS releases ACh stimulating parietal cells to secrete acid.^[2]

Chemical activation

As dietary protein is digested, it breaks down into smaller peptides and amino acids, which directly stimulate the G cells to secrete even more gastrin – a positive feedback loop that accelerates protein digestion. As discussed earlier gastrin stimulates by activating parietal cells and stimulating ECL to produce histamine (histamine stimulates parietal cells to produce acid).^[2] Small peptides also buffer stomach acid so the pH does not fall excessively low.

Gastric secretion is stimulated chiefly by three chemicals: acetylcholine (ACh), histamine, and gastrin. ACh is secreted by parasympathetic nerve fibers of both the short and long reflex ,.ml; pathways. Histamine is a paracrine secretion from the enteroendocrine cells in the gastric glands. Gastrin is a hormone produced by enteroendocrine G cells in the pyloric glands.^[1]

All three of these stimulate parietal cells to secrete hydrochloric acid and intrinsic factor. The chief cells secrete pepsinogen in response to gastrin and especially Ach, and ACh also stimulates mucus secretion.^[1]

Inhibitory pathway

Low intragastric pH stimulates antral D cells to release somatostatin. Somatostatin inhibits gastrin release from G cells. Reduced gastrin secretion reduces acid secretion.^[2]

Intestinal phase

5-10% of gastric secretion occurs during this phase.^[2]

The intestinal phase is a stage in which the duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes. The duodenum initially enhances gastric secretion, but soon inhibits it.

Duodenal stimulation

The presence of partially digested proteins and amino acids in the duodenum stimulates acid secretion in the stomach by four methods:

1.) Peptones stimulate duodenal G cells to secrete gastrin.

2.) Peptones stimulate an unknown endocrine cell to release an additional humoral signal, "enterooxytonin".

3.) Amino acids absorbed by the duodenum stimulate acid secretion by unknown mechanisms.

4.) Osmolarity due to products of digestion stimulate acid secretion

Duodenal inhibition

The acid and semi-digested fats in the duodenum trigger the enterogastric reflex – the duodenum sends inhibitory signals to the stomach by way of the enteric nervous system, and sends signals to the medulla that (1) inhibit the vagal nuclei, thus reducing vagal stimulation of the stomach, and (2) stimulate sympathetic neurons, which send inhibitory signals to the stomach. Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin. They primarily stimulate the pancreas and gall bladder, but also suppress gastric secretion and motility. The effect of this is that gastrin secretion declines and the pyloric sphincter contracts tightly to limit the admission of more chyme into the duodenum. This gives the duodenum time to work on the chyme it has already received before being loaded with more.^[1] The enteroendocrine cells also secrete glucose dependent insulinotropic peptide. Originally called gastric-inhibitory peptide, it is no longer thought to have a significant effect on the stomach, but to be more concerned with stimulating insulin secretion in preparation for processing the nutrients about to be absorbed by the small intestine.^[1]

Basal state

There is small continuous basal secretion of gastric acid between meals of usually less than 10 mEq/hour.^[4]

Basal electrical rhythm

The basal electrical rhythm controls the smooth muscle of the stomach and intestines, and controls the actions of peristalsis, and segmentation contractions.

Related Research Articles

<span class="mw-page-title-main">Vagus nerve</span> Main nerve of the parasympathetic nervous system

The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that carries sensory fibers that create a pathway that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right vagus nerves—but they are typically referred to collectively as a single subsystem.

The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach is involved in the gastric phase of digestion, following chewing. It performs a chemical breakdown by means of enzymes and hydrochloric acid.

Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. Mechanical digestion takes place in the mouth through mastication and in the small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small compounds that the body can use.

<span class="mw-page-title-main">Delta cell</span>

Delta cells are somatostatin-producing cells. They can be found in the stomach, intestine and the pancreatic islets. Delta cells comprise ca 5% of the cells in the islets but may interact with many more islet cells than suggested by their low numbers. In rodents, delta-cells are located in the periphery of the islets; in humans the islet architecture is generally less organized and delta-cells are frequently observed inside the islets as well. In both species, the peptide hormone Urocortin III (Ucn3) is a major local signal that is released from beta cells to induce the local secretion of somatostatin. It has also been suggested that somatostatin may be implicated in insulin-induced hypoglycaemia through a mechanism involving SGLT-2 receptors. Ghrelin can also strongly stimulate somatostatin secretion, thus indirectly inhibiting insulin release. Viewed under an electron microscope, delta-cells can be identified as cells with smaller and slightly more compact granules than beta cells.

Cholecystokinin is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. Cholecystokinin, formerly called pancreozymin, is synthesized and secreted by enteroendocrine cells in the duodenum, the first segment of the small intestine. Its presence causes the release of digestive enzymes and bile from the pancreas and gallbladder, respectively, and also acts as a hunger suppressant.

Gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the pyloric antrum of the stomach, duodenum, and the pancreas.

Gastric acid, gastric juice, or stomach acid is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid plays a key role in digestion of proteins by activating digestive enzymes, which together break down the long chains of amino acids of proteins. Gastric acid is regulated in feedback systems to increase production when needed, such as after a meal. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring a regulated pH. These cells also produce mucus – a viscous barrier to prevent gastric acid from damaging the stomach. The pancreas further produces large amounts of bicarbonate and secretes bicarbonate through the pancreatic duct to the duodenum to neutralize gastric acid passing into the digestive tract.

<span class="mw-page-title-main">Parietal cell</span> Epithelial cell in the stomach

Parietal cells (also known as oxyntic cells) are epithelial cells in the stomach that secrete hydrochloric acid (HCl) and intrinsic factor. These cells are located in the gastric glands found in the lining of the fundus and body regions of the stomach. They contain an extensive secretory network of canaliculi from which the HCl is secreted by active transport into the stomach. The enzyme hydrogen potassium ATPase (H⁺/K⁺ ATPase) is unique to the parietal cells and transports the H⁺ against a concentration gradient of about 3 million to 1, which is the steepest ion gradient formed in the human body. Parietal cells are primarily regulated via histamine, acetylcholine and gastrin signalling from both central and local modulators.

Digestive enzymes are a group of enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption into the cells of the body. Digestive enzymes are found in the digestive tracts of animals and in the tracts of carnivorous plants, where they aid in the digestion of food, as well as inside cells, especially in their lysosomes, where they function to maintain cellular survival. Digestive enzymes of diverse specificities are found in the saliva secreted by the salivary glands, in the secretions of cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the secretions of cells lining the small and large intestines.

<span class="mw-page-title-main">Enterochromaffin-like cell</span>

Enterochromaffin-like cells or ECL cells are a type of neuroendocrine cell found in the gastric glands of the gastric mucosa beneath the epithelium, in particular in the vicinity of parietal cells, that aid in the production of gastric acid via the release of histamine. They are also considered a type of enteroendocrine cell.

<span class="mw-page-title-main">Enterochromaffin cell</span> Cell type

Enterochromaffin (EC) cells are a type of enteroendocrine cell, and neuroendocrine cell. They reside alongside the epithelium lining the lumen of the digestive tract and play a crucial role in gastrointestinal regulation, particularly intestinal motility and secretion. They were discovered by Nikolai Kulchitsky.

<span class="mw-page-title-main">G cell</span> Type of cell in the stomach and duodenum that secretes gastrin

In anatomy, the G cell or gastrin cell is a type of cell in the stomach and duodenum that secretes gastrin. It works in conjunction with gastric chief cells and parietal cells. G cells are found deep within the pyloric glands of the stomach antrum, and occasionally in the pancreas and duodenum. The vagus nerve innervates the G cells. Gastrin-releasing peptide is released by the post-ganglionic fibers of the vagus nerve onto G cells during parasympathetic stimulation. The peptide hormone bombesin also stimulates gastrin from G cells. Gastrin-releasing peptide, as well as the presence of amino acids in the stomach, stimulates the release of gastrin from the G cells. Gastrin stimulates enterochromaffin-like cells to secrete histamine. Gastrin also targets parietal cells by increasing the amount of histamine and the direct stimulation by gastrin, causing the parietal cells to increase HCl secretion in the stomach. G-cells frequently express PD-L1 during homeostasis which protects them from Helicobacter pylori-induced immune destruction

Gastrinomas are neuroendocrine tumors (NETs), usually located in the duodenum or pancreas, that secrete gastrin and cause a clinical syndrome known as Zollinger–Ellison syndrome (ZES). A large number of gastrinomas develop in the pancreas or duodenum, with near-equal frequency, and approximately 10% arise as primary neoplasms in lymph nodes of the pancreaticoduodenal region.

Pancreatic juice is a liquid secreted by the pancreas, which contains a number of digestive enzymes, including trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, pancreatic lipase, nucleases and amylase. The pancreas is located in the visceral region, and is a major part of the digestive system required for proper digestion and subsequent assimilation of macronutrient substances required for living.

<span class="mw-page-title-main">Gastric glands</span> Glands in lining of the human stomach

The gastric glands are glands in the lining of the stomach that play an essential role in the process of digestion. All of the glands have mucus-secreting foveolar cells. Mucus lines the entire stomach, and protects the stomach lining from the effects of hydrochloric acid released from other cells in the glands.

<span class="mw-page-title-main">Vagovagal reflex</span> Reflex circuits in the gastrointestinal tract

Vagovagal reflex refers to gastrointestinal tract reflex circuits where afferent and efferent fibers of the vagus nerve coordinate responses to gut stimuli via the dorsal vagal complex in the brain. The vagovagal reflex controls contraction of the gastrointestinal muscle layers in response to distension of the tract by food. This reflex also allows for the accommodation of large amounts of food in the gastrointestinal tracts.

The enterogastric reflex is one of the three extrinsic reflexes of the gastrointestinal tract, the other two being the gastroileal reflex and the gastrocolic reflex. The enterogastric reflex is stimulated by duodenal distension. It can also be stimulated by a pH of 3-4 in the duodenum and by a pH of 1.5 in the stomach. Upon initiation of the reflex, the release of gastrin by G-cells in the antrum of the stomach is shut off. This in turn inhibits gastric motility and the secretion of gastric acid (HCl).

Enteroendocrine cells are specialized cells of the gastrointestinal tract and pancreas with endocrine function. They produce gastrointestinal hormones or peptides in response to various stimuli and release them into the bloodstream for systemic effect, diffuse them as local messengers, or transmit them to the enteric nervous system to activate nervous responses. Enteroendocrine cells of the intestine are the most numerous endocrine cells of the body. They constitute an enteric endocrine system as a subset of the endocrine system just as the enteric nervous system is a subset of the nervous system. In a sense they are known to act as chemoreceptors, initiating digestive actions and detecting harmful substances and initiating protective responses. Enteroendocrine cells are located in the stomach, in the intestine and in the pancreas. Microbiota play key roles in the intestinal immune and metabolic responses in these enteroendocrine cells via their fermentation product, acetate.

Gastrointestinal physiology is the branch of human physiology that addresses the physical function of the gastrointestinal (GI) tract. The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and excrete waste products. The GI tract is composed of the alimentary canal, that runs from the mouth to the anus, as well as the associated glands, chemicals, hormones, and enzymes that assist in digestion. The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation. The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.

The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion. Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.

References

1 2 3 4 5 6 7 Saladin Kenneth S.(2007) Anatomy & physiology the unity of form and function. Dubuque, IA: McGraw-Hill
1 2 3 4 5 6 7 Boron, W.F., E.L. Boulpaep. 2003. Medical Physiology. 1st ed. Elsevier Science, USA.
↑ Smeets, PA; Erkner, A; de Graaf, C (November 2010). "Cephalic phase responses and appetite". Nutrition Reviews. 68 (11): 643–55. doi: 10.1111/j.1753-4887.2010.00334.x . PMID 20961295.
↑ Page 192 in: Elizabeth D Agabegi; Agabegi, Steven S. (2008). Step-Up to Medicine (Step-Up Series) . Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 978-0-7817-7153-5.

External links

Regulation of gastric function by gastrin releasing peptide
Vagal Regulation of Gastric Function Involves Thyrotropin Releasing Hormone in the Medullary Raphe Nuclei and Dorsal Vagal Complex

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[Saladin-1] 1 2 3 4 5 6 7 Saladin Kenneth S.(2007) Anatomy & physiology the unity of form and function. Dubuque, IA: McGraw-Hill

[Boron-2] 1 2 3 4 5 6 7 Boron, W.F., E.L. Boulpaep. 2003. Medical Physiology. 1st ed. Elsevier Science, USA.

[Smeets-3] Smeets, PA; Erkner, A; de Graaf, C (November 2010). "Cephalic phase responses and appetite". Nutrition Reviews. 68 (11): 643–55. doi: 10.1111/j.1753-4887.2010.00334.x . PMID 20961295.

[agabegi2nd192-4] Page 192 in: Elizabeth D Agabegi; Agabegi, Steven S. (2008). Step-Up to Medicine (Step-Up Series) . Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 978-0-7817-7153-5.

[1]

[2]

[3]

[4]