User:Diberri/Action potential lead

Action potentials are waves of electrical activity that travel along cell membranes. The best understood examples are found in the nervous system, where they are fundamental for communication between neurons. Action potentials are nearly ubiquitous in nature, however. They are found in nearly all cell types, from muscle and nerve to plant cell.

Each action potential is generated by complex machinery made of proteins located in the membrane of these cells. These proteins are organized into channels that allow charged particles called ions to move across the membrane. Ions such as sodium and potassium move across these channels and change the electrical properties of the membrane, specifically its voltage. Changes in voltage trigger the ion channels to open or close, and therefore change how sodium and potassium move across the membrane. These changes can be observed by measuring the voltage across the cell membrane.

When voltage is plotted against time, the resulting graph demonstrates the rising phase, peak, falling phase, and undershoot characteristic of the action potential. The rapid opening of sodium channels gives rise to the rising phase of the action potential. Its falling phase is produced by the opening of channels that allow potassium to leave the cell. The peak voltage achieved is determined by the properties of sodium and potassium channels. The undershoot is produced by potassium channels that are slow to close.

For an action potential to begin, it must be initiated. Initiation comes in the form of a positively charged stimulus. This stimulus increases the membrane voltage and opens both sodium and potassium channels. Roughly speaking, if more sodium channels open than potassium channels, then the feed-forward cycle of the action potential will begin. This requirement for open sodium channels to outnumber open potassium channels effectively produces a threshold voltage that must be met to trigger action potential initiation.

Once an action potential is initiated, the peak voltage it achieves is fixed. This peak voltage is determined by

The action potential is an all-or-none phenomenon.

A typical action potential is initiated at the axon hillock when the membrane potential is depolarized sufficiently (i.e. when its voltage is increased sufficiently). As the membrane potential is increased, both the sodium and potassium ion channels begin to open up. This increases both the inward sodium current and the balancing outward potassium current. For small voltage increases, the potassium current triumphs over the sodium current and the voltage returns to its normal resting value, typically −70 mV. However, if the voltage increases past a critical threshold, typically 15 mV higher than the resting value, the sodium current dominates. This results in a runaway condition whereby the positive feedback from the sodium current activates even more sodium channels. Thus, the cell "fires", producing an action potential. Once initiated, the action potential travels through the axon. Since the axon is insulated, the action potential can travel through it without significant signal decay. Nevertheless, to ensure the signal does not fail, regularly spaced patches, called the nodes of Ranvier, help to boost the signal. The process here is much the same as that at the axon hillock. The action potential depolarises the membrane patch at the node of ranvier, sparking another action potential there. In effect, the action potential is created afresh at each node of ranvier. The axon then branches along its length, and the action potentials travel down each branch. At this point, the axon sheds its insulation, and instead, the action potential is propagated by the voltage activated sodium channels. Here, the inward current may not quite suffice to trigger a new action potential in some of these branches. The action potential may thus fail. Action potentials that do reach the ends of the axon generally cause the release of a neurotransmitter into the synaptic cleft. This may combine with other inputs to provoke a new action potential in the post-synaptic neuron or muscle cell.

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. The ions exchanged during an action potential, therefore, make a negligible change in the interior and exterior ionic concentrations. The few ions that do cross are pumped out again by the continual action of the sodium–potassium pump, which, with other ion transporters, maintains the normal ratio of ion concentrations across the membrane. Calcium cations and chloride anions are involved in a few types of action potentials, such as the cardiac action potential and the action potential in the single-celled alga Acetabularia, respectively.