Depolarization (Definition + Process)

Depolarization is one of the stages that take place in nerve conduction and action potential. Some ions play a significant role in nerve conduction. Due to differences in electrolyte content, the plasma membrane of our cells typically has a charge differential between the interior and exterior. These distinctions are known as polarization.

The loss of polarization is known as depolarization and is frequently accompanied by a nerve impulse. Depolarization is a quick alteration within a cell during which the cell experiences a significant electrical change. Most cells typically have an internal environment that is negatively charged.

Depolarization is an important medical concept that takes place in cells. Here is all you need to know about depolarization.

All You Need To Know About Depolarization

Suppose there is one nerve, and it communicates with another never. How is information traveling from one nerve to another if that is the case? Magic? Not exactly. A certain amount of action potential needs to be reached for the communication between nerves to take place.

A semi-permeable membrane that covers cells divides the inside from the cell’s external environment, which would be plasma in most cases. The proportion of positive and negative ions in the cell will vary in relation to the outside.

An electrochemical gradient develops as a result. It can be measured by comparing the total charge on the inside of the cell to the outside. Membrane potential refers to this distinction. The cell may regulate the ions’ concentration using a protein-based membrane channel.

Specific ions can pass across the membrane while the channels are open. Unique pumps in some channels use energy to force ions through. Let’s imagine that during polarization, the membrane is negatively charged on the inside and positively charged on the outside. Charge changes both within and outside of the cell are caused by electrolyte migration across the plasma membrane.

By segregating ions across their plasma membrane, biological cells, particularly electrically excitable cells, like cardiac and neuron cells, maintain a membrane potential. To put it another way, the membrane is electrically polarized, often with a negative inside to a positive exterior. Depolarization is defined as a decrease in the polarity of the membrane potential.

Depolarization is a shift in a cell’s electric charge that makes the cell’s interior increasingly more positive than the exterior. Electrical stimuli, or excitatory stimuli that raise the cell’s voltage, cause the voltage-gated ion channels to open in the cell. Through these ion channels, positive sodium ions flood the cell, shifting its internal electric charge from negative to positive.

A resting electrostatic potential exists in the membrane of a resting nerve cell. Sodium ions and potassium ions are actively pumped against the diffusion gradient on opposing sides of the membrane by a sodium-potassium pump, which keeps this equilibrium. These two ions travel quickly when the membrane’s permeability changes, depolarizing the resting membrane potential in waves.

How Does Stimulation Lead To Depolarization?

A cell opens its membrane channels in response to a specific signal, allowing ions to pass across the membrane and balance the positive and negative charges on each side. This means that the cell loses its electoral poles due to this equalization.

Therefore some kind of stimulation prompts this to happen. For example, if a bug crawls on your skin, this would create a sense of touch. This creates an action potential in the nerves that are present deep-seated under the skin surface.

The sense of touch triggers the nerves and can generate an impulse in the nerves. This trigger leads to the nerves communicating the information that there is a bug crawling on your skin. For nerves to be triggered, the impact must be so solid that an action potential of 150 mV is reached.

When cells are at rest, the membrane charge is negative compared to the outside. Consider spinal neurons as an illustration. Those motoneuron membranes have a resting membrane potential of -70 mV, meaning that the interior is -70 mV lower than the outside.

The membrane is considered depolarized when this potential decreases or when the inside has become less negative relative to the outside, roughly – 50 mV. Living cells often have resting voltages between 70 and 90 millivolts, making them polarized like tiny biological batteries.

These resting voltages are stated in negative terms, such as -70 mV, because the inner part of the cell membrane is typically negative compared to the outer surfaces. Simply put, neurons can react to stimuli like touch, noise, light, and other types of stimuli. Neurons carry out impulses and communicate with different cell types, including muscle cells, and among themselves.

Action potentials help neurons relay information. The environment outside the cell contains the ions sodium and potassium. The charge is -70mV and is referred to as the resting membrane potential as no stimulation occurs, and all the gates are closed. The electric current causes a voltage-gated sodium channel to open when a neuron is activated.

Because of their positive charges, the sodium ions rush through the neuron’s membrane and begin raising the charge within the cell. Local depolarization, also known as local potential, refers to the fact that this occurs at a specific location on a cell. Although the event may spread across a limited area from the place of origin, that voltage change doesn’t spread very far.

Liken it to dropping a little pebble in a pond, which causes waves to form that go a short distance before fading out due to the resistance of the water. Action potentials, on the other hand, are self-propagating. As one triggers off one ahead of it, it, in turn, triggers another, and so on, like a series of dominoes falling. As a result, a signal can travel far from the stimulation site.

Can A Heart Be Depolarized?

Depolarization is the process through which the internal voltage of the cells decreases to zero. It is known that each cardiac cell is polarized. This has been demonstrated by placing tiny electrodes inside individual cells and connecting them to a measuring tool, such as an oscilloscope. The cells’ interiors are then discovered to be approximately 90 mV negative compared to their exteriors.

The -90mV quickly advances towards zero if a cell gets a signal from a nearby muscle cell that makes up the heart’s signaling system. The cells begin to repolarize after about 200 ms. By roughly 300 ms, their interiors are once more at -90 mV compared to the exterior.

The calcium released inside the cells due to depolarization causes the cells to contract. The decreased volume of the ventricles and atria due to the heart’s cells contracting helps pump out the blood that had accumulated in them when the heart was in diastole, which is the period when the heart chambers aren’t contracting.

The sino-atrial node, located in the right atrium, contains modified, specialized muscle tissue that depolarizes on its own. Depolarization of that tissue then extends to the atrial muscle and the atrioventricular node, which are located where the right atrium connects to the ventricles.

The ventricles’ unique conduction tissue quickly carries the depolarization signal to their inner surface. The ventricles then contract due to the ventricle muscles depolarizing from the inside to the exterior. The depolarization signal can be observed by inserting electrodes through the veins into the heart.

Depolarization and the repolarization that follows the cardiac muscle cells produce the signal that gets detected. An electrocardiograph, also known as an EKG, a sensitive voltage indicator, can detect the movement of the sites of depolarization. It is possible to track the pattern of depolarization traveling through the chest by positioning electrodes at various locations on the chest.

The voltage that arises from each muscle cell’s depolarization added up in a particular order is what is referred to as depolarization of the heart. Wondering if depolarization happens in just cardiac and neuron cells? Depolarization happens in many other areas of the body, such as the ear.

The flow of ions entering the hair cell across the membrane increases along with the tension. This influx of ions depolarizes the cell, creating an electrical potential that eventually results in a signal for the brain and the auditory nerve.

Why Is Depolarization Important?

Depolarization is crucial because it enables communication across the cell membrane and initiates the opening and closing of membrane potential-sensitive channels. Voltage-gated channels are the name for these channels.

Depolarization allows neurons to quickly transmit messages from one end of the cell to the other. The fastest way to send messages down a very long membrane is through depolarization, which is why it is utilized. Some neurons are known to be several feet long, so depolarization comes in handy.

Reproduction serves as an illustration. After being fertilized by a sperm, an egg must act fast to block the entry of other sperm. In reaction to the first sperm, it will depolarize its membrane to accomplish this. This gives the ovum’s whole membrane a rapid reaction, preventing subsequent sperm from penetrating.

Depolarization Vs. Repolarization Vs. Hyperpolarization

Although all three terms have the same suffix, they mean the complete opposite of each other. Keeping two sets of facts in mind is vital to understand the differences between these terms. These are that the outside of a cell has more sodium than the inside, which happens to have more potassium.

The other fact is that the outside of a cell membrane is more positively charged. Whereas the inside of a cell membrane is more negatively charged. With that in mind, even though there is positive potassium inside a cell, other things inside the cell membrane make it more negatively charged.

You are dealing with membrane potential when it comes to depolarization and repolarization. This is the charge difference between the inside and outside of a cell. At rest, the cell membrane is said to be polarized. This is because there is a difference between the charges. Therefore at this point, all the channels are closed.

Depolarization happens when the sodium channels open. Since there is more sodium on the outside of the cell, as mentioned above, that positive sodium starts to make its way into the cell once those channels open. This leads to the inside of the cell becoming more positive. Therefore, it is depolarizing because there is less of a difference between the inside and the outside of a cell.

Repolarization, on the other hand, happens when the sodium channels close and the potassium channels open. Since the potassium channels are now open, all the positive potassium inside the cell will start flowing out of it. This leads to the charges flipping back, meaning that the outside becomes more positive, and the interior is more negative.

The other thing that occurs is the potassium channels stay open much longer than they need to to ensure that regular resting membrane is achieved. This is where hyperpolarization comes in. It occurs when the inside of the cell becomes more negative than it had been in the beginning because the positive potassium keeps exiting.

The difference between the inside and the outside of the cell membrane is re-established. Anything below regular resting membrane potential is hyperpolarization. Normal resting is re-established by those sodium-potassium pumps.

To sum it up, depolarization is when the sodium channels open and sodium flows into the cell, making the inside of the membrane more positive than the outside. Repolarization is where the sodium channels close, the potassium channels open, and potassium flows out of the cell, making the inside return to being negative.

Hyperpolarization is when the inside of the cell membrane becomes more negative than it had been at resting because of the potassium leaving the cell during repolarization.

Conclusion

Depolarization is when a cell experiences a shift in the distribution of its electric charges, leading to a far less negative charge in the cell. Because of a change in permeability and the flow of sodium ions into the interior, it results in the loss of the imbalance in charge between the inside and the outside of the membrane of a muscle or nerve cell.

References

https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/depolarization-hyperpolarization-and-action-potentials

https://www.sciencedirect.com/topics/neuroscience/membrane-depolarization

Theodore T.

Theodore is a professional psychology educator with over 10 years of experience creating educational content on the internet. PracticalPsychology started as a helpful collection of psychological articles to help other students, which has expanded to a Youtube channel with over 2,000,000 subscribers and an online website with 500+ posts.