An action potential is a short event within the axon of a neuron in which the voltage potential across the plasma membrane rapidly changes to send a signal, or nerve impulse. The action potential is possible because of the resting electrical potential that exists across the membrane. lons, or charged atoms, on either side of the membrane create an electrical difference, called a potential. The resting membrane potential for a neuron is -70mV in relation to the inside of the axon.
The voltage potential is created by many ions, most importantly sodium (Na+) and potassium (K+). An excess of Na+ is present on the outside of the membrane while an excess of K+ exists on the inside. K+ plays a very important role in creating the resting potential as it passes through the membrane through K+ leak channels. Na+ leak channels exist as well but are fewer in number.
The action potential starts in response to a local potential. Local potentials happen in the dendrites of a neuron and sets off the action potential if it reaches the axon hillock. When the dendrites receive a signal, a ligand gated channel opens and Na+ rush into the cell, causing local depolarization. If the depolarization reaches the hillock and is strong enough, it will set off the action potential. The "threshold" is the necessary potential change required to start an action potential. At this point, the voltage-gated Na+ channels open and Na+ from outside the cell ruches into the cell.
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This causes a rapid depolarization in the cell. K+ voltage-gated channels begin to open at this point. As K+ voltage-gated channels open fully, Na+ channels close. A large amount of K+ rushes out of the cell, causing repolarization of the membrane. These K+ channels stay open longer than Na+ channels, causing a hyperpolarization, a slightly more polarized environment than the resting membrane potential. This process continues down the length of the axon to send the signal. After the action potential passes through a section of the axon, there is a period in which it is impossible to stimulate that area or the neuron, called the refractory period.
The absolute refractory period begins when the action potential starts and lasts until the membrane returns to the resting potential, the period in which Na+ channels are open. The relative refractory period lasts until hyperpolarization ends, the period where K+ channels are open. During an action potential, Na+ rushes into the cell causing depolarization. K+ then rushes out of the cell, causing repolarization. The resting membrane potential is returned to by a few mechanisms. Firstly, the amount of Na+ entering the cell and K+ leaving the cell is very, very little compared to the total concentrations of those ions. Secondly, astrocytes help remove extracellular K+. The Na+/K+ pumps help maintain a constant electrical potential.
Due to concentration gradients, Na+ and K+ move across the cell membrane. The Na+/K+ pumps pump out 3 Na+ from the inside of the membrane and pump in 2 K+ from the outside. This process requires ATP as an energy source for driving this movement up the gradient. Local potentials occur at the dendrites while the action potential occurs in the axon, starting at the axon hillock.
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