The Action Potential - All or Nothing

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

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

The Resting Membrane Potential

In the watery environment of our body, there are ions (charged atoms). These can arise spontaneously when salts (e.g. common salt: sodium chloride NaCl) are dissolved 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 we have positive and negative electrically charged particles within us.

To keep it short and simple, here are a few key ions and facts about their distribution in the human body:

• Negative protein ions occur mainly INSIDE the cell

• Positive potassium ions K+ also occur mainly INSIDE the cell

• Positive sodium ions Na+ occur mainly OUTSIDE the cell

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

This is what we call the resting membrane potential (or equilibrium potential).

This resting potential varies in magnitude for different cells. In nerve and muscle cells, it is approximately -70 to -90 mV.

Author of 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

An external stimulus (neurotransmitter or electrical signal to the dendrites) changes the permeability of the cell membrane of a neuron first for sodium 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 (first) rush into the ICR, altering the intracellular polarity. This leads to a depolarization of the cell. If the stimulus is strong enough and exceeds the threshold voltage (stimulus threshold) of approximately -55mV, complete depolarization of the nerve cell occurs: an action potential is triggered (with an over-shoot) resulting in an all-or-nothing response of the nerve cell, which temporarily increases the voltage to about +20mV between the ICR and ECR!

The sodium channels for the Na+ influx are then closed again.

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

Note: This action potential, i.e., the triggering of a nerve cell signal, is only initiated when the threshold of a neuron (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

During the subsequent repolarization phase, the abundant potassium ions continue to flow out of the cell (where K+ is scarce) through the channels that remain open longer than the sodium channels, leading to hyperpolarization (to approx. -100mV) after an action potential.

Afterward, active processes driven by Na-K pumps restore the original distributions (i.e., Na+ into the ECR and K+ into the ICR), quickly returning the cell to the original resting potential of -60 to -90 mV.

This hyperpolarization makes immediate re-stimulation to trigger another action potential more difficult (Note: the threshold would have to be reached to trigger an action potential); any incoming stimulus would have to be stronger than a normally sufficient one, which is virtually impossible through normal anatomical means! There are no endogenous stimuli that can excite such a cell during hyperpolarization.

This ensures that a membrane cannot be immediately re-excited, forcing the electrical signal to travel in only one direction (unidirectionally) along the axon! This also acts as a feedback protection mechanism. The action potential must and will be driven in one direction.

All of this happens in an incredibly fast 1-2 ms. After all, a nerve must be ready for excitation again as quickly as possible to transmit further signals. And this is only possible if both the action potential and hyperpolarization occur rapidly!

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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.