What is depolarization and how does it happen?

As a neuroscientist with a focus on cellular and molecular neuroscience, I can explain the process of depolarization and how it occurs within the context of neuronal signaling. Depolarization is the process by which the membrane potential of a neuron becomes less negative, moving towards a less polarized state. This is a critical step in the generation of an action potential, which is the electrical signal that neurons use to communicate with each other. Here's a step-by-step explanation of how depolarization happens: 1. Resting Potential: At rest, a neuron has a membrane potential that is negative inside relative to the outside, typically around -70 millivolts (mV). This is due to the unequal distribution of ions across the cell membrane and the operation of the sodium-potassium pump. 2. Stimulus: When a stimulus is applied to a neuron, it can cause the opening of ion channels, particularly sodium (Na⁺) channels. 3. Sodium Influx: Sodium ions, which are more concentrated outside the cell, rush into the neuron through these open channels. Since sodium has a positive charge, their influx causes the inside of the cell to become more positive. 4. Threshold: If the stimulus is strong enough to cause a significant influx of sodium ions, the membrane potential can reach a critical value known as the threshold potential (usually around -55 mV). At this point, more sodium channels open, leading to a rapid and self-sustaining depolarization. 5. Action Potential: The rapid depolarization that follows the threshold crossing is known as an action potential. It's characterized by a rapid rise in the membrane potential to positive values as more sodium channels open, followed by a repolarization phase where potassium (K⁺) channels open and sodium channels close, allowing K⁺ ions to leave the cell and restoring the negative membrane potential. 6. Refractory Period: After an action potential, there is a brief period during which the neuron cannot generate another action potential. This is known as the refractory period and it ensures that the action potential travels in one direction along the neuron.

A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized.