The Action Potential - All or Nothing

As described in our earlier blogs (Nerves, Pain Transmission), signals such as pain are generated and transmitted by your nerve cells and interpreted in your brain. But how exactly does this signal generation work, how does your body do it?

This happens through electrical voltage differences and is an exciting and fascinating process:

The Resting Membrane Potential

In the watery environment of your body, there are ions (charged atoms). These can arise spontaneously when salts (e.g. table salt: sodium chloride NaCl) dissolve in polar solvents (such as water H2O). This creates positive anions (in the case of table salt NaCl, sodium ions Na+) and negative cations (chloride ions Cl-). Proteins also become negatively charged cations in a watery environment.

So you have positively and negatively charged particles inside you.

To keep things brief and simple, here are a few key ions and facts about their distribution in your body:

• Negative protein ions are found mainly INSIDE the cell

• Positive potassium ions K+ are also found mainly INSIDE the cell

• Positive sodium ions Na+ are found mainly OUTSIDE the cell

Among other things, this results in a voltage difference in all cells with a negative charge between the intracellular space (ICS, inside the cell) and the extracellular space (ECS, outside the cell).

This is the so-called resting membrane potential (or equilibrium potential).

This resting potential varies between different cell types. In nerve and muscle cells, it is around -70 to -90 mV.

Creator Action_potential.svg: Original by en:User:Chris 73, updated by en:User:Diberri, converted to SVG by tiZom, Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

The Action Potential

Through an external stimulus (a neurotransmitter or an electrical signal to the dendrites), the permeability of a neuron's cell membrane changes—first for sodium ions and shortly after for potassium ions; channels open and diffusion processes take place:

Since there are almost no sodium ions inside the cell, these positive Na+ ions rush (first) into the ICS, changing the intracellular polarity. This leads to a depolarization of the cell. If the stimulus is large enough and exceeds the threshold voltage (stimulus threshold) of about -55 mV, complete depolarization of the nerve cell occurs: an action potential develops (with an overshoot) following an all-or-nothing response of the nerve cell, causing the voltage between the ICS and ECS to rise briefly to about +20 mV!

The sodium channels for the Na+ influx then close again.

The electrical signal of the action potential travels along the axon of the nerve cell.

Note: This action potential—the firing of a signal by a nerve cell—is only triggered if the neuron's stimulus threshold (approx. -55 mV) is exceeded. If the threshold is not reached, a slight depolarization still occurs, but not according to the all-or-nothing principle and consequently without an action potential! As a result, no signal is triggered or transmitted. 

Repolarization with Hyperpolarization

In the subsequent repolarization phase, the abundant potassium ions continue to flow out of the cell (where there is little K+) through channels that remain open longer (than those for sodium), leading to hyperpolarization (to about -100 mV) following an action potential.

Afterward, active processes using Na-K pumps restore the original distribution (Na+ into the ECS and K+ into the ICS), relatively quickly returning the cell to the initial resting voltage of -60 to -90 mV.

This hyperpolarization makes immediate re-stimulation and triggering of another action potential much harder (Note: the stimulus threshold must be reached to fire an action potential); any incoming stimulus would have to be stronger than normally required, which is practically impossible in a normal anatomical state! There are no bodily stimuli that could excite such a cell while it is hyperpolarized.

This ensures that a membrane cannot be re-excited immediately and guarantees that the electrical signal along the membrane travels in only one direction (unidirectionally)—straight along the axon! This also serves as feedback protection. The action potential must and will move forward in one direction.

All of this happens in an incredibly fast 1–2 ms. After all, your nerve needs to become excitable again as quickly as possible to transmit further signals. And that is only possible if both the action potential and hyperpolarization take place at lightning speed!

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You can find more information about this on our chronic pain page.

Cover Image Credit

Action_potential.svg: Original by en:User:Chris 73, updated by en:User:Diberri, converted to SVG by tiZom, Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.