In physiology, an action potential occurs when the membrane potential of a specific cell location rises and falls: this depolarization causes adjacent locations to depolarize. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, endocrine cells, glomus cells, in some plant cells. In neurons, action potentials play a central role in cell-to-cell communication by providing for—or with regard to saltatory conduction, assisting—the propagation of signals along the neuron's axon toward synaptic boutons situated at the ends of an axon. In other types of cells, their main function is to activate intracellular processes. In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction. In beta cells of the pancreas, they provoke release of insulin. Action potentials in neurons are known as "nerve impulses" or "spikes", the temporal sequence of action potentials generated by a neuron is called its "spike train".
A neuron that emits an action potential, or nerve impulse, is said to "fire". Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane; these channels are shut when the membrane potential is near the resting potential of the cell, but they begin to open if the membrane potential increases to a defined threshold voltage, depolarising the transmembrane potential. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential; this causes more channels to open, producing a greater electric current across the cell membrane and so on. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential; the rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, the ion channels rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, they are actively transported back out of the plasma membrane.
Potassium channels are activated, there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization. In animal cells, there are two primary types of action potentials. One type is generated by the other by voltage-gated calcium channels. Sodium-based action potentials last for under one millisecond, but calcium-based action potentials may last for 100 milliseconds or longer. In some types of neurons, slow calcium spikes provide the driving force for a long burst of emitted sodium spikes. In cardiac muscle cells, on the other hand, an initial fast sodium spike provides a "primer" to provoke the rapid onset of a calcium spike, which produces muscle contraction. Nearly all cell membranes in animals and fungi maintain a voltage difference between the exterior and interior of the cell, called the membrane potential. A typical voltage across an animal cell membrane is −70 mV.
This means. In most types of cells, the membrane potential stays constant; some types of cells, are electrically active in the sense that their voltages fluctuate over time. In some types of electrically active cells, including neurons and muscle cells, the voltage fluctuations take the form of a rapid upward spike followed by a rapid fall; these up-and-down cycles are known as action potentials. In some types of neurons, the entire up-and-down cycle takes place in a few thousandths of a second. In muscle cells, a typical action potential lasts about a fifth of a second. In some other types of cells, in plants, an action potential may last three seconds or more; the electrical properties of a cell are determined by the structure of the membrane that surrounds it. A cell membrane consists of a lipid bilayer of molecules in which larger protein molecules are embedded; the lipid bilayer is resistant to movement of electrically charged ions, so it functions as an insulator. The large membrane-embedded proteins, in contrast, provide channels through which ions can pass across the membrane.
Action potentials are driven by channel proteins whose configuration switches between closed and open states as a function of the voltage difference between the interior and exterior of the cell. These voltage-sensitive proteins are known as voltage-gated ion channels. All cells in animal body tissues are electrically polarized – in other words, they maintain a voltage difference across the cell's plasma membrane, known as the membrane potential; this electrical polarization results from a complex interplay between protein structures embedded in the membrane called ion pumps and ion channels. In neurons, the types of ion channels in the membrane vary across different parts of the cell, giving the dendrites and cell body different electrical properties; as a result, some parts of the membrane of a neuron may be excitable. Recent studies have shown that the most excitable part of a neuron is the part after the axon hillock, called the initial segment, but the axon and cell body are excitable in most cases.
Each excitable patch of membrane has two important levels of membrane potential: the resting potential, the value the membrane potential maintains as long as nothing pe
The 2011 Championship Cup, was the 10th season of the rugby league football competition for clubs in Great Britain's Co-operative Championship and Championship One. Leigh Centurions won the final against Halifax by the score of 20-16; the match was played at Bloomfield Road in Blackpool. The twenty teams are split up into two pools. Pool A: Featherstone Rovers, London Skolars, Widnes Vikings, Toulouse Olympique, York City Knights, Keighley Cougars, Barrow Raiders, Rochdale Hornets, Dewsbury Rams. Pool B: Swinton Lions, Halifax RLFC, Batley Bulldogs, Leigh Centurions, Gateshead Thunder, Sheffield Eagles, Hunslet Hawks, Workington Town, Whitehaven RLFC; each team played two home games and two away games against teams in their pool. The top four teams in each pool following the conclusion of the group stage fixtures progressed into an open draw for the knock-out quarter-final stage; the competition started on 5 February. Toulouse Olympique competed in the competition for the first time after the withdrawal of Blackpool Panthers who have entered administration.
The South Wales Scorpions did not compete. Source: Northern Rail Cup Table – The RFL Classification: 1st on competition points. Competition Points: For win = 3. Competition Points: For win = 3; the Northern Rail Cup Final took place at Bloomfield Road, Blackpool on Sunday 17 July at 17:00 GMT live on Sky Sports 3. The Match was contested by Leigh Centurions; this was Halifax's first appearance in the National League Cup final and was he fourth time Leigh have appeared in the final after making the finals in 2003 and winning the final in 2004 & 2006. The game was won by Leigh 20 points to 16 with Leigh scoring in the final minute thanks to Tom Armstrong; the victory fulfils the on-field criteria that clubs must meet to be allowed to apply for a Super League licence for the 2015–17 period. Leigh Centurions have become the first club to have won the competition on 3 occasions. Https://web.archive.org/web/20080831022046/http://www.therfl.co.uk/index.php http://www.northernrail.org/northernrailcup
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