The

nervous system In Biology, biology, the nervous system is the Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals to and from different parts of its ...

, and

endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...

collaborate in the digestive system to control gastric secretions, and motility associated with the movement of food throughout the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans a ...

, including

peristalsis Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which ...

, and

segmentation contractions Segmentation contractions (or movements) are a type of intestinal motility. Unlike peristalsis, which predominates in the esophagus, segmentation contractions occur in the large intestine and small intestine, while predominating in the latter. Whil ...

. Gastric activity involved in

digestion Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...

is divided into three phases of digestion known as the

cephalic phase 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 contract ...

, the gastric phase, and the intestinal phase. These phases overlap and all three can occur simultaneously.Saladin Kenneth S.(2007) Anatomy & physiology the unity of form and function. Dubuque, IA: McGraw-Hill 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.

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

for food processing, which relays signals to the

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...

. 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). 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 The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and rig ...

in the medulla (activating the

parasympathetic nervous system The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part ...

). 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 Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

(ACh) which stimulates

parietal cell 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 ...

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 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 ant ...

to produce and release

gastrin 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. Gastrin ...

. Gastrin stimulates

gastric acid 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 ...

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.Boron, W.F., E.L. Boulpaep. 2003. Medical Physiology. 1st ed. Elsevier Science, USA.

Activation of gastric chief cells

Gastric chief cells A gastric chief cell (or peptic cell, or gastric zymogenic cell) is a type of gastric gland cell that releases pepsinogen and gastric lipase. It is the cell responsible for secretion of chymosin in ruminant animals. The cell stains basophilic upo ...

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 The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine ...

can stimulate

S cell S cells are cells which release secretin, found in the jejunum and duodenum. They are stimulated by a drop in pH to 4 or below in the small intestine's lumen. The released secretin will increase the secretion of bicarbonate (HCO3−) into the lu ...

s to secrete

secretin Secretin is a hormone that regulates water homeostasis throughout the body and influences the environment of the duodenum by regulating secretions in the stomach, pancreas, and liver. It is a peptide hormone produced in the S cells of the duoden ...

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. Stretch activates two reflexes: a short reflex mediated through the myenteric nerve plexus, and a long reflex mediated through the vagus nerves and brainstem.

Distention path

1.)

Vagovagal reflex 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 ...

: 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.

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). Small peptides also buffer stomach acid so the pH does not fall excessively low. Gastric secretion is stimulated chiefly by three chemicals:

(ACh),

histamine Histamine is an organic nitrogenous compound involved in local immune responses, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Since histamine was discovered in ...

, and

. ACh is secreted by

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part o ...

nerve fibers of both the short and long reflex ,.ml; pathways.

Histamine Histamine is an organic nitrogenous compound involved in local immune responses, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Since histamine was discovered in ...

is a paracrine secretion from the enteroendocrine cells in the gastric glands.

Gastrin 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. Gastrin ...

is a hormone produced by enteroendocrine G cells in the pyloric glands. All three of these stimulate parietal cells to secrete hydrochloric acid and intrinsic factor. The chief cells secrete

pepsinogen Pepsin is an endopeptidase that breaks down proteins into smaller peptides. It is produced in the gastric chief cells of the stomach lining and is one of the main digestive enzymes in the digestive systems of humans and many other animals, w ...

in response to gastrin and especially Ach, and ACh also stimulates mucus secretion.

Inhibitory pathway

Low intragastric pH stimulates antral D cells to release

somatostatin Somatostatin, also known as growth hormone-inhibiting hormone (GHIH) or by several other names, is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G protein-co ...

. Somatostatin inhibits gastrin release from G cells. Reduced gastrin secretion reduces acid secretion.

Intestinal phase

5-10% of gastric secretion occurs during this phase. The intestinal phase is a stage in which the

responds to arriving

chyme Chyme or chymus (; from Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenum duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine ...

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 cell 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 a ...

s 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

trigger the enterogastric reflex – the

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. 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.

Basal state

There is small continuous basal secretion of gastric acid between meals of usually less than 10 mEq/hour.Page 192 in:

Basal electrical rhythm

The

basal electrical rhythm The basal or basic electrical rhythm (BER) or electrical control activity (ECA) is the spontaneous depolarization and repolarization of pacemaker cells known as interstitial cells of Cajal (ICCs) in the smooth muscle of the stomach, small intestine ...

controls the smooth muscle of the stomach and intestines, and controls the actions of

, and

References

{{Reflist, 2

External links

* Regulation of gastric function by gastrin releasing peptid

*Vagal Regulation of Gastric Function Involves Thyrotropin Releasing Hormone in the Medullary Raphe Nuclei and Dorsal Vagal Comple

Digestive system