Basal electrical rhythm

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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, and large intestine. This electrical rhythm is spread through gap junctions in the smooth muscle of the GI tract. [1] These pacemaker cells, also called the ICCs, control the frequency of contractions in the gastrointestinal tract. The cells can be located in either the circular or longitudinal layer of the smooth muscle in the GI tract; circular for the small and large intestine, longitudinal for the stomach. [2] The frequency of contraction differs at each location in the GI tract beginning with 3 per minute in the stomach, then 12 per minute in the duodenum, 9 per minute in the ileum, and a normally low one contraction per 30 minutes in the large intestines that increases 3 to 4 times a day due to a phenomenon called mass movement. [2] The basal electrical rhythm controls the frequency of contraction but additional neuronal and hormonal controls regulate the strength of each contraction.

Contents

Physiology

Smooth muscle within the GI tract causes the involuntary peristaltic motion that moves consumed food down the esophagus and towards the rectum. [1] The smooth muscle throughout most of the GI tract is divided into two layers: an outer longitudinal layer and an inner circular layer. [1] Both layers of muscle are located within the muscularis externa. The stomach has a third layer: an innermost oblique layer.

The physical contractions of the smooth muscle cells can be caused by action potentials in efferent motor neurons of the enteric nervous system, or by receptor mediated calcium influx. [1] These efferent motor neurons of the enteric nervous system are cholinergic and adrenergic neurons. [2] The inner circular layer is innervated by both excitatory and inhibitory motor neurons, while the outer longitudinal layer is innervated by mainly excitatory neurons. These action potentials cause the smooth muscle cells to contract or relax, depending on the particular stimulation the cells receive. Longitudinal muscle fibers depend on calcium influx into the cell for excitation-contraction coupling, while circular muscle fibers rely on intracellular calcium release. Contraction of the smooth muscle can occur when the BER reaches its plateau (an absolute value less than -45mV)[ citation needed ] while a simultaneous stimulatory action potential occurs. A contraction will not occur unless an action potential occurs. Generally, BER waves stimulate action potentials and action potentials stimulate contractions.

The interstitial cells of Cajal are specialized pacemaker cells [3] located in the wall of the stomach, small intestine, and large intestine. [1] These cells are connected to the smooth muscle via gap junctions and the myenteric plexus. The cell membranes of the pacemaker cells undergo a rhythmic depolarization and repolarization from -65mV to -45mV.[ citation needed ] This rhythm of depolarization-repolarization of the cell membrane creates a slow wave known as a BER, and it is transmitted to the smooth muscle cells. The frequency of these depolarizations in a region of the GI tract determines the possible frequency of contractions. In order for a contraction to occur, a hormone or neurocrine signal must induce the smooth muscle cell to have an action potential. The basal electrical rhythm allows the smooth muscle cell to depolarize and contract rhythmically when exposed to hormonal signals. This action potential is transmitted to other smooth muscle cells via gap junctions, creating a peristaltic wave.

The specific mechanism for the contraction of smooth muscle in the GI tract depends upon IP3R calcium release channels in the muscle. [4] Calcium release from IP3 sensitive calcium stores activates calcium dependent chloride channels. [4] These chloride channels trigger spontaneous transient inward current which couples the calcium oscillations to electrical activity. [4]

Frequency

The number of action potentials during the plateau of a particular BER slow wave can vary. This variation in action potential generation does not impact the frequency of waves through the GI tract, only the strength of those contractile waves. [2]

Factors that impact gastric motility

The cells that respond to and secrete these substances include I cells and K cells in the proximal small intestine, whose stimulation is dependent on nutrient exposure and entry into the duodenum, and L cells in the distal small intestine and colon which are stimulated by unabsorbed nutrients and gastric emptying. [5]

The frequency of the BER, and thus the contractions, changes throughout the GI tract. The frequency in the stomach is 3 per minute, while the duodenum is 11 to 12 per minute and the ileum is 9 per minute. [1] The colon can have a BER frequency between 2 and 13 per minute. The electrical activity is oscillatory, so that the BER has peaks and valleys when graphed over time.

See also

Related Research Articles

Stomach Digestive organ

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.

Gastrointestinal tract Organ system within humans and other animals

The gastrointestinal tract 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 and other animals, including the esophagus, stomach, and intestines. Food taken in through the mouth is digested to extract nutrients and absorb energy, and the waste expelled at the anus as feces. Gastrointestinal is an adjective meaning of or pertaining to the stomach and intestines.

Peristalsis

Peristalsis is a radially symmetrical contraction and relaxation of muscles that propagates in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which is preceded by a simultaneous contraction of the longitudinal muscle and relaxation of the circular muscle in the lining of the gut.

Digestion Biological process of breaking down food

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 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 mouth through mastication and in small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small molecules the body can use.

Enteric nervous system Vital system controlling the gastrointestinal tract

The enteric nervous system (ENS) or intrinsic nervous system is one of the main divisions of the autonomic nervous system (ANS) and consists of a mesh-like system of neurons that governs the function of the gastrointestinal tract. It is capable of acting independently of the sympathetic and parasympathetic nervous systems, although it may be influenced by them. The ENS is nicknamed the "second brain". It is derived from neural crest cells.

Cardiac pacemaker Network of cells that facilitate rhythmic heart contraction

The contraction of cardiac muscle in all animals is initiated by electrical impulses known as action potentials. The rate at which these impulses fire controls the rate of cardiac contraction, that is, the heart rate. The cells that create these rhythmic impulses, setting the pace for blood pumping, are called pacemaker cells, and they directly control the heart rate. They make up the cardiac pacemaker, that is, the natural pacemaker of the heart. In most humans, the concentration of pacemaker cells in the sinoatrial (SA) node is the natural pacemaker, and the resultant rhythm is a sinus rhythm.

Depolarization Change in a cells electric charge distribution

In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is essential to the function of many cells, communication between cells, and the overall physiology of an organism.

Electrical conduction system of the heart Aspect of heart function

The electrical conduction system of the heart transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle. The pacemaking signal generated in the sinoatrial node travels through the right atrium to the atrioventricular node, along the Bundle of His and through bundle branches to cause contraction of the heart muscle. This signal stimulates contraction first of the right and left atrium, and then the right and left ventricles. This process allows blood to be pumped throughout the body.

Muscle contraction Activation of tension-generating sites in muscle

Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.

Enterochromaffin cell

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.

An electrogastrogram (EGG) is a computer generated graphic produced by electrogastrography, which detects, analyzes and records the myoelectrical signal generated by the movement of the smooth muscle of the stomach, intestines and other smooth muscle containing organs. An electrogastroenterogram or electroviscerogram is a similar display of the recording of myoelectrical activity of gastrointestinal or other organs which are able to generate myoelectrical activity.

Interstitial cell of Cajal

Interstitial cells of Cajal (ICC) are interstitial cells found in the gastrointestinal tract. There are different types of ICC with different functions. ICC and another type of interstitial cell, known as platelet-derived growth factor receptor alpha (PDGFRα) cells, are electrically coupled to smooth muscle cells via gap junctions, that work together as an SIP functional syncytium. Myenteric interstitial cells of Cajal (ICC-MY) serve as pacemaker cells that generate the bioelectrical events known as slow waves. Slow waves conduct to smooth muscle cells and cause phasic contractions.

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 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.

Enteroendocrine cell

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 plays 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.

A slow-wave potential is a rhythmic electrophysiological event in the gastrointestinal tract. The normal conduction of slow waves is one of the key regulators of gastrointestinal motility. Slow waves are generated and propagated by a class of pacemaker cells called the interstitial cells of Cajal, which also act as intermediates between nerves and smooth muscle cells. Slow waves generated in interstitial cells of Cajal spread to the surrounding smooth muscle cells and control motility.

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.

Cardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; myocardium structure; the electrical conduction system of the heart; the cardiac cycle and cardiac output and how these interact and depend on one another.

Gastrointestinal wall

The gastrointestinal wall of the gastrointestinal tract is made up of four layers of specialised tissue. From the inner cavity of the gut outwards, these are:

Human digestive system Digestive system in humans

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

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  3. ↑ Thomsen, L.; Robinson, T. L.; Lee, J. C.; Farraway, L. A.; Hughes, M. J.; Andrews, D. W.; Huizinga, J. D. (1998-07-01). "Interstitial cells of Cajal generate a rhythmic pacemaker current". Nature Medicine. 4 (7): 848–851. doi:10.1038/nm0798-848. ISSN   1078-8956. PMID   9662380.
  4. 1 2 3 Ju, Yue-Kun, Elizabeth A. Woodcock, David G. Allen, and Mark B. Cannell. "Inositol 1,4,5-trisphosphate Receptors and Pacemaker Rhythms." Journal of Molecular and Cellular Cardiology 53.3 (2012): 375-81.
  5. 1 2 3 4 5 6 Wu T, Rayner C, Young R, Horowitz M. Gut motility and enteroendocrine secretion. Current Opinion In Pharmacology [serial online]. December 1, 2013;13(Gastrointestinal * Endocrine and metabolic diseases):928-934. Available from: ScienceDirect, Ipswich, MA.
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