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 creation work, how does our body do it?

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

The Resting Membrane Potential

In the aqueous environment of our body, there are ions (charged atoms). These can spontaneously form when salts (e.g., table salt: sodium chloride NaCl) dissolve in polar solvents (like water H2O). Positive anions (in the case of table salt NaCl, sodium ions Na+) and negative cations (chloride ions Cl-) are formed. Proteins also become negatively charged cations in an aqueous environment.

So we have positively and negatively charged particles within us.

To shorten and simplify, here are some important ions and facts about their distribution in the human body:

• Negative protein ions are found mainly INSIDE the cell

• Positive potassium ions Ka+ are also primarily found INSIDE the cell

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

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

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

This resting potential varies in size in different cells. In nerve and muscle cells, it is about -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

Due to an external stimulus (neurotransmitter or electric signal to the dendrites), the permeability (known as permeability) of the cell membrane of a neuron is first altered for sodium and shortly thereafter for potassium ions; channels open and diffusion processes come into play:

Since there are almost no sodium ions in the cell, these positive Na+ ions (first) rush into the ICR, changing the polarity intracellularly. This leads to depolarization of the cell. If the stimulus is large enough and exceeds the threshold voltage (stimulus threshold) of around -55mV, complete depolarization of the nerve cell occurs: An action potential occurs (with an overshoot) and an all-or-nothing response of the nerve cell, temporarily increasing the voltage between ICR and ECR to around +20mV!

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

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

NB: This action potential, a signal triggering of a nerve cell, is only triggered if the stimulus threshold (approx. -55 mV) of a neuron is exceeded. If the threshold is not reached, there will still be a slight depolarization, but not according to the all-or-nothing principle and thus without an action potential!! Therefore, no signal is triggered or transmitted.

The Repolarization with Hyperpolarization

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

Then, through active processes using the Na-Ka pumps, the original distributions are restored (i.e., Na+ back into the ECR and Ka+ into the ICR), quickly achieving the original resting voltage of -60 to -90 mV again.

This hyperpolarization makes direct stimulus with the initiation of an action potential more difficult (NB: the stimulus threshold must be reached to trigger an action potential); any incoming stimulus would have to be stronger than a normally sufficient one and is hardly possible in a normal anatomical way! There are no endogenous stimuli that could excite such a cell in hyperpolarization.

This ensures that a membrane is not immediately excitable again and allows the electric signal to be transmitted in only one direction (unidirectionally) along the axon!! This also acts as feedback protection. The action potential must and will be driven in one direction.

All of this occurs incredibly quickly, in about 1-2 mS. A nerve needs to be quickly excitable again to transmit further signals. This can only happen if the action potential and hyperpolarization occur swiftly!

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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.