Action Potential

The action potential is a scientific description that explains a neural phenomenon. The impetus to describe this phenomenon can be traced to several origins in thought. The Greek physician, Galen, provided one of the first dominating theories of the nervous system, describing the motion of fluids secreted from brain tissue and conveyed by nerves.

It is important to note that most descriptions of the action potential are meant to describe general motifs. The most basic definition of an action potential describes the properties which produce the active propagation of a signal through a cell: a brief 1-10 ms large amplitude depolarization which is propagated in an all-or-none fashion. An action potential (AP) is "all-or-none" as compared to passive, graded signaling; an AP is like a flipping a light switch on and off, while graded signaling (like neurotransmitter release) are like adjusting a light dimmer.

Like a switch that goes on and off, an AP represents a fundamental unit of information in the nervous system. However, the light-switch analogy only demonstrates the idea that an AP contains a discrete unit of information. In reality, the AP is a complex, though ordered, biochemical and electrophysiological process. First, let's discuss what is taught in introductory courses to establish our framework.

A neuron spikes when the membrane potential is depolarized past a threshold where a cascade of voltage-gated ion channels open leading to a rapid change in membrane potential that propogates down the length of an axon. The phospholipid bilayer maintains a certain difference in electrical potential on either side; while resting, this potential is typically modeled to be -65 mVA but this can range from -40 to -80 mV where muscle cell is may be around -90 mV. Another important feature of the membrane is its selective permeability to certain molecular species, particularly Na+ and K+ ions. Sodium-potassium pumps maintain this distribution by actively pumping sodium ions out of the cell and pumping potassium ions into the cell. While at rest, potassium ions leak out of the cell through ion channels, proteins mebedded in the cell membrane that form pores, as K+ follows the concentration gradient. This state of affairs leaves the inside of the cell more negatively charged. It is the inward current of positively charged ions (Na+ and Ca2+) that perturb the resting state. The excitability of the neuron can be understood by the ability of the membrane potential to be quickly and significantly altered. As positive charge rushes in, the negative charge inside the cell is neutralized, further increasing the influx of sodium ions. This is the basis of the action potential, which is actively propagated down an axon, maintaining its amplitude until it reaches the postsynapse where it may be transduced into a chemical signal of neurotransmission.
Alls signaling in the nervous system involves input, integration, conduction, and output. The input may be mechanical, chemical, or electrical. These produce graded local signals that may begin at a receptor region like a postsynapse. The information is translated into a sum of shift in membrane potential that is integrated in a region called the Axon Hillock where a high density of Na+ channels can be triggered to amplify the inputs into action potential. Information can be encoded in the frequency and and duration of spiking which can determine the amount of neurotransmitter is released
The result is an explosive shift in the membrane potential of ~+40 mV. reduced model of the action potential may describe the fast persistent Na+ current as an amplifying current while the