Body processes

Action Potential – Function, Task & Diseases

Action potential

An action potential is a short-term change in the membrane potential . Action potentials typically arise at the axon hillock of a nerve cell and are the prerequisite for the transmission of stimuli.

What is the action potential?

The action potential is a spontaneous charge reversal in nerve cells. Action potentials arise at the axon hillock. The axon hillock is the origin of the transmitting processes of a nerve cell. The action potential then travels down the axon, i.e. the nerve process.

A potential can last from a millisecond to a few minutes. Each action potential has the same intensity. Accordingly, there are neither weak nor strong action potentials. They are more of an all-or-nothing response, that is, either a stimulus is strong enough to fully trigger an action potential, or the action potential is not triggered at all. Each action potential has several phases.

Function & task

Before the action potential, the cell is in its resting state. Most of the sodium channels are closed and some of the potassium channels are open. Due to the movement of the potassium ions, the cell maintains the so-called resting membrane potential in this phase . This is around -70 mV. So if you were to measure the voltage inside the axon, you would get a negative potential of -70 mV. This can be attributed to a charge imbalance of the ions between the space outside the cell and the cell fluid.The receiving extensions of the nerve cells, the dendrites , absorb stimuli and transmit them via the cell body to the axon hillock. Each incoming stimulus changes the resting membrane potential. However, in order for an action potential to be triggered, a threshold value must be exceeded at the axon hillock. This threshold value is only reached when the membrane potential increases by 20 mV to -50 mV. For example, if the membrane potential only rises to -55 mV, then nothing happens due to the all-or-nothing reaction.

Once the threshold is exceeded, the cell’s sodium channels open. Positively charged sodium ions flow in, the resting potential continues to rise. The potassium channels close. The result is a polarization reversal. The space within the axon is now momentarily positively charged. This phase is also referred to as overshoot.

The sodium channels close again before the maximum membrane potential is reached. The potassium channels open and potassium ions flow out of the cell. Repolarization occurs , which means that the membrane potential approaches the resting potential again. A so-called hyperpolarization even occurs for a short time . The membrane potential drops below -70 mV. This period of approximately two milliseconds is also called the refractory period. During the refractory period, an action potential cannot be triggered. This is to prevent over-excitability of the cell.

After regulation by the sodium-potassium pump , the voltage is again at -70 mV and the axon can again be excited by a stimulus. The action potential is now transmitted from one section of the axon to the next. Because the previous section is still in the refractory period, the stimulus can only be transmitted in one direction.

However, this continuous stimulus transmission is rather slow. The saltatoric stimulus transmission is faster . The axons are surrounded by a so-called myelin sheath. This acts like a kind of isolation tape. In between, the myelin sheath is repeatedly interrupted. These interruptions are called nodes . The action potentials now jump from one node to the next during the saltatory stimulus transmission. This greatly increases the propagation speed.

The action potential is the basis for the transmission of stimulus information. All functions of the body are based on this transmission.

Diseases & Ailments

If the myelin sheaths of the nerve cells are attacked and destroyed, serious disturbances in the transmission of stimuli occur. Due to the loss of the myelin layer, charge is lost during transmission. This means that more charge is needed to excite the axon at the next break in the myelin sheath. If there is slight damage to the myelin sheath, the action potential is delayed. If there is severe damage, the conduction of excitation can be completely interrupted, since no action potential can be triggered.The myelin sheaths can be affected by genetic defects such as Krabbe’s disease or Charcot-Marie-Tooth disease. The best-known demyelinating disease is probably multiple sclerosis . Here the myelin sheaths are attacked and destroyed by the body’s own defense cells. Depending on which nerves are affected, visual disturbances , general weakness , spasticity , paralysis , sensitivity or speech disorders can occur.

A rather rare disease is Paramyotonia congenita . On average, only one person in 250,000 people is affected. The disease is a disruption of the sodium channel. As a result, the sodium ions can penetrate the cell even in phases in which the sodium channel should actually be closed and thus trigger an action potential, even if there is actually no stimulus at all. As a result, there can be constant tension in the nerves. This manifests itself as increased muscle tension (myotonia). After a voluntary movement, the musculature relaxes with a significant delay.

The opposite way is also conceivable with Paramyotonia congenita. It may be that the sodium channel does not allow sodium ions into the cell even when it is excited. An action potential can only be triggered with a delay or not at all despite an incoming stimulus. There is therefore no reaction to the stimulus. The consequences are sensory disturbances, muscle weakness or paralysis. The occurrence of the symptoms is favored above all by low temperatures, which is why those affected should avoid any cooling of the muscles.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.