By Rene Fester Kratz. When a neuron is inactive, just waiting for a nerve impulse to come along, the neuron is polarized — that is, the cytoplasm inside the cell. Voltage-gated potassium channels are either open or closed. There are three main events that take place during an action potential: A triggering event occurs that depolarizes the cell body. This signal comes from other cells connecting to the neuron, and it causes positively charged ions to flow into the cell body. Thus, the cell fires, producing an action potential. The principal ions involved in an action potential are sodium and potassium cations; sodium ions enter the cell, and potassium ions leave, restoring equilibrium. Relatively few ions need to cross the membrane for the membrane voltage to change drastically.Cardiac action potential · Membrane potential · Saltatory conduction.
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This sets up the possibility for positive feedbackwhich is a key part of the rising phase of the action potential. The voltages and currents of the action potential in all of its phases were modeled accurately by Alan Lloyd Hodgkin and Andrew Huxley in[i] cell action potential which they were awarded cell action potential Nobel Prize in Physiology or Medicine in In reality, there are many types of ion channels,  and they do not always open and close independently.
This depolarization is often caused by the injection of extra sodium cations into the cell; these cations can come from a wide variety of sources, such as chemical synapsessensory neurons or pacemaker potentials.
For a neuron at rest, there is cell action potential high concentration of sodium and chloride ions in the extracellular fluid compared to the intracellular fluid while there is a high concentration of potassium ions in the intracellular fluid compared to the extracellular fluid.
The increasing voltage in turn causes even more sodium channels to open, which pushes Vm still cell action potential towards ENa. This positive feedback continues until the sodium channels are fully open and Vm is close to ENa.
A membrane that has just fired an action potential cannot fire another one immediately, since the ion channels have not cell action potential to the deactivated state.
The period during which no new action potential cell action potential be fired is called the absolute refractory period.
The period during which action potentials are unusually difficult to evoke is called the relative refractory period.
At the peak of the action potential, the sodium permeability is maximized and the membrane voltage Vm is nearly equal to the sodium equilibrium voltage ENa. However, the same raised voltage that opened the sodium channels initially also slowly shuts them off, by closing their pores; the sodium channels become inactivated.
At the same time, the raised voltage opens voltage-sensitive potassium channels; the increase cell action potential the membrane's potassium permeability drives Vm towards EK. In addition, further potassium channels open in response to the influx of calcium cell action potential during the action potential.
The potassium permeability of the membrane is transiently unusually high, driving the membrane voltage Cell action potential even closer to the potassium equilibrium voltage EK.
Hence, there is an undershoot or hyperpolarizationtermed an afterhyperpolarization in technical language, that persists until the membrane potassium permeability returns to its usual value.
Neuroscience For Kids - action potential
When closing after an action potential, sodium channels enter an "inactivated" statein which they cannot be made to open regardless of the membrane potential—this gives rise to the absolute refractory period. Even after a sufficient number of sodium channels have transitioned back to their resting state, it frequently happens that a fraction of potassium channels remains open, making it difficult for the membrane potential to depolarize, and thereby giving rise to the relative refractory period.
Because the density and subtypes of potassium channels may differ greatly between different types of neurons, the duration of the relative refractory period is highly variable. The absolute refractory period is largely responsible for the unidirectional propagation of action potentials along axons.
Nerve conduction velocity The action potential generated at the axon hillock propagates as a wave along the axon. If sufficiently strong, this depolarization provokes a similar action potential at the neighboring membrane patches.
This basic cell action potential was demonstrated by Alan Lloyd Hodgkin in After crushing or cooling nerve segments and thus blocking the action potentials, he showed that an action potential cell action potential on one side of the block could provoke another action potential on the other, provided that the blocked segment was sufficiently short.
At the molecular level, this absolute refractory period corresponds to the time required for the voltage-activated sodium channels to recover from inactivation, i. Some of them inactivate fast A-type currents and some of them inactivate slowly or not inactivate at all; this variability guarantees that there will be always an cell action potential source of current for repolarization, even if some of the potassium channels are inactivated because of preceding depolarization.
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On the other hand, all neuronal voltage-activated sodium channels inactivate within several milliseconds during strong depolarization, thus making following depolarization impossible until a substantial fraction of sodium channels have returned to their closed state.
Although it limits the frequency of firing,  the absolute refractory period ensures that the action potential moves in only one direction along cell action potential axon.
In cell action potential usual orthodromic conductionthe action potential propagates from the axon hillock towards the synaptic knobs the axonal termini ; propagation in the opposite direction—known as antidromic conduction —is very rare.