Action Potential

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Meaning of Action Potential

One of the fascinating physiological phenomena is action potential. It is the reversal of the electric polarization of the regions on the nerve cell or neuron and muscle cell membrane that causes a potential difference. It happens to produce nerve impulse and contraction of muscle cells and takes only 1/1000th of a second to complete. In this article, we will discuss its properties, steps, and functions elaborately.

What is an Action Potential?

The wave of propagation or excitation of the cell membrane of neurons and muscles due to a sudden change in their electric polarization is called the action potential. It is also called propagated potential as the excitation wave transmits from one region to the next in neurons and muscle fibres.

The speed of propagation can range from 3 to 300 feet per second. It entirely depends on the physiological properties and environment of the nerves and muscles fibres. The carrying of the nerve impulse from one neuron to the other requires a potential difference. It is created by the reversal of polarization throughout a neuron cell. At the end of a neuron, this action potential is then transmitted to the next neuron ending for propagation.

Phases of Action Potential

An action potential is generated and passed from one location to the other in the following steps.

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1. The Resting Potential

It is the potential when a neuron is at rest. During this time, a small set of K+ ion pumps open to exchange these ions to maintain the electrochemical forces. A balance is maintained by exchanging this ion across the membrane during resting potential.

2. Threshold phase

During this phase, a depolarization stimulus is sent to the neuron. It is then Na+ channels open letting these ions enter the neuron. There is a quick increase in the number of positive ions in that part of the neuron resulting in depolarization (the internal area becomes less negative). The potential inside the neuron comes closer to the threshold generating an action potential.

3. The Rising Phase

When the depolarization effect reaches the maximum threshold potential, more Na+ channels open resulting in an inflow of more ions. There is a quick reversal of the voltage in the membrane and eventually, the action potential of neuron of that area reaches its highest positive value. This is also called the peak phase.

4. The Falling Phase

After the peak phase of the action potential is over, two simultaneous processes run in the excited neuron. Most of the voltage Na+ channels close resulting in the conservation of these ions. On the other hand, the K+ channels start to open resulting in the flow of these ions outside the cell. It means that this part of the neuron membrane starts losing positive charge. The spike in the positive charge starts to reside due to the loss of K+ ions and the potential starts to decrease considerably. Eventually, the neuron membrane achieves its resting potential.

5. The Recovery Phase

This phase is called the recovery phase as almost all the K+ channels are open. The outflow of the ions causes a significant drop in the potential of the membrane. It eventually recovers the previous stage of resting membrane potential. It is during this time the membrane starts re-polarizing and even reaches beyond the resting potential. This causes the escape of more K+ ions on the go.

The next step of this phase is when a few of the K+ channels close to stop the K+ drain and to achieve the normal resting potential again. These are the action potential steps that occur in the specific location of the cell membrane of the neuron. It is propagated from one location to the other and then transmitted to the next neuron.

Properties of Action Potential

As mentioned earlier, the action potential of neuron is very short spanned. It happens only for 1 or 2 milliseconds. It happens due to the sudden changes in the ion concentration of the internal part of the cell membrane and in the extracellular fluid.

The ions involved in this action potential mechanism are Na+, K+, and Cl- (partially). There are ion-specific pumps and channels that bring in or flow out ions according to this natural biological phenomenon.

You will be surprised to know that when the stimulus intensity is escalated, there is no escalation in the level of action potential rather the frequency of the action potential gets increased. It means the number of times the cell membrane reaching and dropping this potential will increase.

During the resting phase, the potential inside the cell membrane will also always be slightly lower than the outside or the extracellular fluid. During the development of the action potential, the charge increases considerably to reach a higher potential and then drops even below the resting phase.

The unmyelinated neurons continue this process in a conductive method where the axon will depolarize, maintaining a sequence. On the other hand, the myelinated ones show saltatory conduction. It means that the nodes of Ranvier show only depolarization. In the second case, depolarization and conduction occurs much faster than the first one.

FAQs (Frequently Asked Questions)

1. What is the action potential of heart?

Ans: The action potential of the cardiac cells of the sinu-auricular node (SA Node) is a brief change in the potential or voltage in order to convey a stimulus for beating the auricles and ventricles continuously. This action potential is produced nearly 60 to 100 times in the SA node within a minute resulting in the heartbeat rate of a person. It can vary depending on the situations an individual is facing.

2. Why do the myelinated neurons conduct better than the unmyelinated ones?

Ans: We know what is an action potential and how it transmits from one part of the cell membrane to the other of a neuron. From this description, we can conclude that the transmission of the stimulus occurs from one node of Ranvier to the other in myelinated neurons resulting in faster conduction and stimulus carriage.