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 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 ...

(ICCs) in the

smooth muscle Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...

of the stomach, small intestine, and large intestine. This electrical rhythm is spread through

gap junction Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regula ...

s in the smooth muscle of the

GI 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 an ...

. 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.Widmaier, Eric P., Hershel Raff, and Kevin T. Strang. ''Vander's Human Physiology: The Mechanisms of Body Function''. New York, NY: McGraw-Hill Education, 2016. The frequency of contraction differs at each location in the GI tract beginning with 3 per minute in the

stomach 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 i ...

, then 12 per minute 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 m ...

, 9 per minute in the

ileum The ileum () is the final 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 posterior intestine or distal intestine m ...

, and a normally low one contraction per 30 minutes in the

large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces before bein ...

s that increases 3 to 4 times a day due to a phenomenon called mass movement. The basal electrical rhythm controls the frequency of contraction but additional neuronal and hormonal controls regulate the strength of each contraction.

Physiology

Smooth muscle Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...

within the GI tract causes the involuntary

peristaltic 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, whic ...

motion that moves consumed food down the

esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...

and towards the

rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the l ...

. The smooth muscle throughout most of the GI tract is divided into two layers: an outer longitudinal layer and an inner circular layer. Both layers of muscle are located within the

muscularis externa The muscular layer (muscular coat, muscular fibers, muscularis propria, muscularis externa) is a region of muscle in many organs in the vertebrate body, adjacent to the submucosa. It is responsible for gut movement such as peristalsis. The Latin, ...

. The stomach has a third layer: an innermost oblique layer. The physical contractions of the smooth muscle cells can be caused by

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...

s in efferent

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirect ...

s of the

enteric nervous system 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 ...

, or by receptor mediated calcium influx. These efferent motor neurons of the enteric nervous system are cholinergic and adrenergic neurons. 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 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 ...

, 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) 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

are specialized pacemaker cells located in the wall of the stomach, small intestine, and large intestine. These cells are connected to the smooth muscle via

s and the

myenteric plexus The myenteric plexus (or Auerbach's plexus) provides motor innervation to both layers of the muscular layer of the gut, having both parasympathetic and sympathetic input (although present ganglion cell bodies belong to parasympathetic innervati ...

. The cell membranes of the pacemaker cells undergo a rhythmic

depolarization 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 ess ...

and repolarization from -65 mV to -45mV. 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 A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...

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.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.

Calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

release from IP3 sensitive calcium stores activates calcium dependent

chloride channel Chloride channels are a superfamily of poorly understood ion channels specific for chloride. These channels may conduct many different ions, but are named for chloride because its concentration ''in vivo'' is much higher than other anions. Several ...

s. These chloride channels trigger spontaneous transient inward current which couples the calcium oscillations to electrical activity.

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.

Factors that impact gastric motility

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. Gast ...

- Stimulates increased contraction force and motility in the stomach, small intestine, and large intestine. * CCK - Suppresses motility in the stomach and duodenumWu T, Rayner C, Young R, Horowitz M. Gut motility and enteroendocrine secretion. ''Current Opinion In Pharmacology'' erial online December 1, 2013;13(Gastrointestinal * Endocrine and metabolic diseases):928-934. Available from: ScienceDirect, Ipswich, MA. Additionally, CCK stimulates secretion of PPY and inhibits the secretion of ghrelin. * PPY - inhibition of upper GI tract motility *

GLP-1 Glucagon-like peptide-1 (GLP-1) is a 30- or 31-amino-acid-long peptide hormone deriving from the tissue-specific posttranslational processing of the proglucagon peptide. It is produced and secreted by intestinal enteroendocrine L-cells and certa ...

- Functions as an "Ileal Brake" to inhibit upper GI tract motility when the distal gut is exposed to unabsorbed nutrients. Slows gastric emptying to promote nutrient absorption. * Distension of the stomach increases

motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

of the stomach. * Distension of the duodenum inhibits stomach motility in order to prevent the over filling of the duodenum. * Presence of fat, low pH, and

hypertonic In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane- ...

solutions cause a decrease in motility of the stomach. *

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

innervation inhibits gastric motility. *

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 o ...

innervation stimulates gastric motility. * Rate and motility are also dependent upon the meal composition. Meals that are solid and contain a greater

macronutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excr ...

composition require slower and more forceful contraction in order to extract the maximum amount of nutrients throughout the GI tract. 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. The frequency of the BER, and thus the contractions, changes throughout the GI tract. The

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

in the stomach is 3 per minute, while the

is 11 to 12 per minute and the

is 9 per minute. 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.

References

{{Gastrointestinal physiology Digestive system Electrophysiology

Physiology

Frequency

Factors that impact gastric motility

See also

References