Slow-wave Potential
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A slow-wave potential is a rhythmic electrophysiological event in 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 organ (biology), organs of the digestive syste ...
. 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 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 ...
, which also act as intermediates between
nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
s and
smooth muscle cells 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 mus ...
. Slow waves generated in interstitial cells of Cajal spread to the surrounding smooth muscle cells and control motility.


Description

In the human
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 ...
, the slow-wave threshold is the slow-wave potential which must be reached before a slow wave can be propagated in gut wall
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 mus ...
. Slow waves themselves seldom cause any smooth muscle contraction (Except for, probably in the stomach). When the amplitude of slow waves in smooth muscle cells reaches the slow-wave threshold — the L-type Ca2+ channels are activated, resulting in calcium influx and initiation of motility. Slow waves are generated at unique intrinsic frequencies by the interstitial cells of Cajal, even within the same organ.
Entrainment Entrainment may refer to: * Air entrainment, the intentional creation of tiny air bubbles in concrete * Brainwave entrainment, the practice of entraining one's brainwaves to a desired frequency * Entrainment (biomusicology), the synchronization o ...
of these different intrinsic frequencies through electrical coupling allows these unique intrinsic frequencies to occur at a single frequency within the stomach and segments of the small intestine. Electron microscopic and dye coupling studies to date have confirmed gap junctions as the major coupling mechanisms between interstitial cells of Cajal. Coupling between ICC and smooth muscle cells is uncertain. Gap junctions have been demonstrated in rare circumstances as one coupling mechanism between ICC and smooth muscle cells. Another potential coupling mechanism is the "Peg and Socket" theory which demonstrates that the membranes of smooth muscle cells have the ability either form physical narrow "sockets" or "pegs" to lock onto other smooth muscle cells and/or interstitial cells of Cajal.


Types

Gastric slow waves occur at around 3 cycles-per-minute in humans and exhibit significance variances in both amplitudes and propagation velocities in the stomach due to the existence of a gradient of resting membrane potential gradient, interstitial cells of Cajal distributions, and gastric wall thickness. Gastric slow waves frequency, propagation velocity, and amplitude demonstrate significant inter-species differences. Extracellular bioelectrical recording studies have demonstrated that gastric slow waves originate from a pacemaker region located on the greater curvature of the stomach. Human gastric slow waves propagate slower in the corpus than in the pacemaker region and antrum of the stomach. Up to four simultaneous slow wave wavefronts can occur in the human stomach. Intestinal slow waves occur at around 12 cycles-per-minute in the duodenum, and decreases in frequency towards the colon. Entrainment of intestinal slow waves forms "frequency plateaus" in a piece-wise manner along the intestine. Similar to the stomach, intestinal slow waves frequency, propagation velocity, and amplitude also demonstrate significant inter-species differences. In
uterine smooth muscle The myometrium is the middle layer of the uterine wall, consisting mainly of uterine smooth muscle cells (also called uterine myocytes) but also of supporting stromal and vascular tissue. Its main function is to induce uterine contractions. Struc ...
, slow waves have not been consistently observed. Uterine muscle seems to generate action potentials spontaneously. In gastrointestinal smooth muscle, the slow-wave threshold can be altered by input from endogenous and exogenous innervation, as well as excitatory (
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. Part ...
and
Substance P Substance P (SP) is an undecapeptide (a peptide composed of a chain of 11 amino acid residues) and a member of the tachykinin neuropeptide family. It is a neuropeptide, acting as a neurotransmitter and as a neuromodulator. Substance P and its clos ...
) and inhibitory (
vasoactive intestinal peptide Vasoactive intestinal peptide, also known as vasoactive intestinal polypeptide or VIP, is a peptide hormone that is vasoactive in the intestine. VIP is a peptide of 28 amino acid residues that belongs to a glucagon/secretin superfamily, the ligan ...
and
nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
) compounds.Pathophysiology. Porth. 7th Ed. pg.875–878


References

Textbook of Medical Physiology - Gyton and Hall (12th edition){{page needed, date=January 2014 Muscular system Graded potentials