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 on Action Potential
1. What exactly is an action potential in simple terms?
An action potential is a very fast, temporary change in the electrical charge across a neuron's membrane. You can think of it as a brief electrical signal or a "spike" that travels along a nerve fibre. This signal is the fundamental way that nerve cells communicate with each other and trigger responses in muscles and glands.
2. What are the main phases or steps of an action potential?
An action potential unfolds in a sequence of four main phases:
- Resting Potential: The neuron is at rest, with a stable, negative electrical charge inside compared to the outside.
- Depolarization: When a stimulus hits, channels for sodium ions open. Positively charged sodium ions rush into the cell, causing the inside to become positively charged.
- Repolarization: The sodium channels close, and potassium channels open. This allows positive potassium ions to exit the cell, which helps restore the negative charge inside.
- Hyperpolarization: The membrane briefly becomes even more negative than its resting state before the sodium-potassium pump fully restores the original balance.
3. Why do nerve impulses travel so much faster in myelinated neurons?
Myelinated neurons are wrapped in a fatty, insulating layer called the myelin sheath. This sheath isn't continuous; it has small gaps called Nodes of Ranvier. Instead of flowing along the entire length of the cell, the action potential "jumps" from one node to the next. This jumping process, known as saltatory conduction, is significantly faster and more energy-efficient than the continuous signal flow in unmyelinated neurons.
4. What does it mean when an action potential is described as an 'all-or-none' event?
The 'all-or-none' principle states that a stimulus must be strong enough to reach a certain level, called the threshold potential, to trigger an action potential. If the stimulus is too weak, nothing happens. If it reaches the threshold, a full and complete action potential is generated. There are no 'half' or 'weak' action potentials; they either occur at their full strength or not at all. This ensures that the signal is transmitted without losing its intensity over distance.
5. How is an action potential in a heart cell different from one in a nerve cell?
Yes, they are quite different. A cardiac action potential lasts much longer than a neuronal one. The main reason is a unique plateau phase in heart cells, where calcium ions enter the cell. This prolonged phase is crucial as it prevents the heart muscle from contracting again too soon, ensuring the heart chambers have enough time to fill with blood between beats.
6. What is the key difference between an action potential and a graded potential?
The main difference is in their strength and how far they travel. A graded potential is a localized change in membrane potential that varies in size depending on the stimulus strength, and it fades away quickly. In contrast, an action potential is an all-or-none signal that can travel long distances along an axon without losing any strength, making it ideal for long-range communication.
7. Why can't an action potential travel backwards up the axon?
An action potential cannot move backwards because of the refractory period. Right after a section of the axon depolarizes, its sodium channels become temporarily inactivated and cannot open again for a brief moment. This ensures that the wave of depolarization can only move forward, away from its starting point, which prevents the signal from becoming chaotic and ensures one-way information flow.
8. What would be the consequence if a neuron's sodium-potassium pump stopped working?
The sodium-potassium pump is vital for establishing and maintaining the resting potential of a neuron. If it stopped working, the neuron couldn't pump sodium out and potassium in after an action potential. The ion concentration gradients across the membrane would gradually disappear, and the neuron would lose its ability to generate any future action potentials, effectively shutting down its signalling function.
