The repolarization or falling phase is caused by the slow closing of sodium channels and the opening of voltage-gated potassium channels. As a result, the membrane permeability to sodium declines to resting levels. As the sodium ion entry declines, the slow voltage-gated potassium channels open and potassium ions rush out of the cell. This expulsion acts to restore the localized negative membrane potential of the cell. Hyperpolarization is a phase where some potassium channels remain open and sodium channels reset.
A period of increased potassium permeability results in excessive potassium efflux before the potassium channels close. This page describes how neurons work. I hope this explanation does not get too complicated, but it is important to understand how neurons do what they do. There are many details, but go slow and look at the figures.
Much of what we know about how neurons work comes from experiments on the giant axon of the squid. This giant axon extends from the head to the tail of the squid and is used to move the squid's tail. How giant is this axon? It can be up to 1 mm in diameter - easy to see with the naked eye. Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are "electrically-charged" -- when they have an electrical charge, they are called ions.
There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. What happens across the membrane of an electrically active cell is a dynamic process that is hard to visualize with static images or through text descriptions.
View this animation to learn more about this process. And what is similar about the movement of these two ions? The membrane potential will stay at the resting voltage until something changes. To begin an action potential, the membrane potential must change from the resting potential of approximately mV to the threshold voltage of mV. Once the cell reaches threshold, voltage-gated sodium channels open and being the predictable membrane potential changes describe above as an action potential.
Any sub-threshold depolarization that does not change the membrane potential to mV or higher will not reach threshold and thus will not result in an action potential.
Also, any stimulus that depolarizes the membrane to mV or beyond will cause a large number of channels to open and an action potential will be initiated.
This means that either the action potential occurs and is repeated along the entire length of the neuron or no action potential occurs. Either the membrane reaches the threshold and everything occurs as described above, or the membrane does not reach the threshold and nothing else happens. Stronger stimuli will initiate multiple action potentials more quickly, but the individual signals are not bigger. One is the activation gate , which opens when the membrane potential crosses mV.
The other gate is the inactivation gate , which closes after a specific period of time—on the order of a fraction of a millisecond. When a cell is at rest, the activation gate is closed and the inactivation gate is open. Timed with the peak of depolarization, the inactivation gate closes. During repolarization, no more sodium can enter the cell.
When the membrane potential passes mV again, the activation gate closes. After that, the inactivation gate re-opens, making the channel ready to start the whole process over again. Potassium continues to leave the cell for a short while and the membrane potential becomes more negative, resulting in the hyperpolarization overshoot. All of this takes place within approximately 2 milliseconds Figure While an action potential is in progress, another one cannot be initiated.
That effect is referred to as the refractory period. There are two phases of the refractory period: the absolute refractory period and the relative refractory period. During the absolute refractory period, another action potential will not start. The action potential is initiated at the beginning of the axon, at what is called the initial segment trigger zone.
Because of this, positive ions spreading back toward previously opened channels has no effect. The action potential must propagate from the trigger zone toward the axon terminals. Propagation, as described above, applies to unmyelinated axons. When myelination is present, the action potential propagates differently, and is optimized for the speed of signal conduction.
Because there is not constant opening of these channels along the axon segment, the depolarization spreads at an optimal speed. The distance between nodes is the optimal distance to keep the membrane still depolarized above threshold at the next node. If the nodes were any closer together, the speed of propagation would be slower.
Propagation along an unmyelinated axon is referred to as continuous conduction ; along the length of a myelinated axon it is referred to as saltatory conduction. Examples of cells that signal via action potentials are neurons and muscle cells.
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