What is the Action Potential?

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Courtesy : internal.psychology.illinois.edu
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Our central nervous systems is made up of the Brain, Spinal cord and neurons(a bunch of neurons together is called a nerve). When a message has to be sent from the brain to the specific site of action it is the neurons and their network of axons that transmit these messages from the brain. Axons conduct electrical messages by action potentials. These axons are insulated by Schwann cells which help in transmitting the action potential rapidly. When you move your hand to hold or drop, how does it happen?It is the travelling of an electrical message from the neuron through the axon to the next neuron that helps in transmitting the message.

 

What is Action Potential?

An action potential is an important process which is associated with the neurons and which occurs when the neuron fires. The process can be simply explained as the exchange of positively charged ions (potassium and sodium) from the inside of neural membrane with the external negatively charged (protein and chloride) ions. This state is known as the resting potential. Owing to the exchange the positive charge on the nerve fiber increases rapidly. Once the charge on nerve fiber reaches +40 mv, its electrical impulse propagates down the nerve fiber due to which different series of action potential takes place.

 

What are the different stages of action potential?

The process of action potential in neuron takes place in different stages, they are the, the resting potential, reaching the threshold level, depolarization of the membrane, propagation of action potential.

What is the The Resting Potential?

At this stage no impulsive propagation is going on and the neuron is resting. Excess of sodium ions (Na+) are present outside the cell and similarly excess of potassium ions (K+) are present inside the cell. Extracellular fluid is present outside the cell membrane and the voltage is maintained constantly at -70 mV inside the membrane. The closed channels of ions along with the sodium potassium pump maintain this difference.

What is the threshold level for action potential?

The sodium channels are made to open due to external stimulus which can be caused by the neurotransmitters in the nerve cells placed adjacently or even due to the sensory receptor cells. Now at this stage, sodium ions start moving freely into the cell due to which the polarity inside the cell membrane is decreased. But the action potential is not generated till the potential difference of ions does not reach the threshold level of -55 mV.

How is depolarization of the membrane ?

When the threshold value is reached by the membrane, action potential is created. During this stage the channels of sodium are fully opened and the ions of sodium surge into the neural axon thereby depolarizing the membrane locally. Due to this fast diffusion the membrane gets oppositely polarized and overshoots for gaining neutral polarity. During the neutral polarity stage its potential becomes +30 mV. Not surprisingly though the external ions of the cell are comparatively negative during this stage.

How is action potential propagated?

After reaching the reversal stage of polarity, the cell membrane of neuron automatically closes the sodium channel. But the change activates the other cell membrane of the adjacent location and the sodium channel there becomes open. In this way the process propagates a wave of opening and closing and is transmitted through the whole length of axon with a speed ranging between 1 to 100 m/s.

 

What is Repolarization ?

Repolarization is a process of decline in the phase of action potential. In this stage the channels of potassium open. Due to this the potassium ions mix with the extracellular fluid and re-establish the resting potential a little thereby resulting in the closing of potassium channels.

How is the original Ion concentration restored ?

At this stage, the polarity is restored at its original stage of the membrane and neurons are not in a stage to propagate impulse. Hence to resettle the earlier ion distribution and concentration the sodium-potassium pump makes use of ATP (adenosine triphosphate) as energy and the potassium ions are pumped inside the cell membrane while the sodium ions are pumped outside.

 

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