Understanding the Depolarizing Phase of Membrane Potential

The question describes a critical event in the electrical activity of cells, particularly neurons and muscle cells. This event involves a change in the cell membrane's electrical charge.

What is Membrane Potential?

The membrane potential is the difference in electrical potential (voltage) between the interior and the exterior of a biological cell. In most cells, this resting potential is negative on the inside relative to the outside, typically around -70 millivolts (mV) for neurons.

Phases of an Action Potential

When a neuron or muscle cell is stimulated sufficiently, it undergoes an action potential, which is a rapid rise and subsequent fall in membrane potential. This involves several phases:

• Resting Potential: The stable, negative charge of the membrane when the cell is not stimulated (e.g., -70 mV).
• Depolarization: The membrane potential starts becoming less negative, moves towards zero, and eventually becomes positive. This is the phase described in the question.
• Repolarization: The membrane potential rapidly shifts back towards the negative resting potential after reaching its peak positive value.
• Hyperpolarization: Sometimes, the membrane potential briefly becomes even more negative than the resting potential before returning to normal.

Analyzing the Description

The question states: "During the_________negative membrane becomes less negative, reaches zero and then becomes positive." Let's break this down:

• "negative membrane becomes less negative": This signifies the initial shift away from the resting potential towards zero.
• "reaches zero": The potential crosses the neutral point.
• "then becomes positive": The interior of the cell becomes electrically positive relative to the exterior.

This sequence of changes—from negative, towards zero, and then becoming positive—is the hallmark of the Depolarizing phase. This occurs due to the rapid influx of positive ions (usually sodium, Na⁺) into the cell through voltage-gated ion channels.

Why Other Options Are Incorrect

• Hyperpolarizing phase: This phase involves the membrane potential becoming *more* negative than the resting potential.
• Repolarizing phase: This phase involves the membrane potential returning from positive values back towards negative values.
• Action potential: This is the entire electrical event, encompassing all phases, not just the specific phase described.

Therefore, the correct term for the phase where the negative membrane potential becomes less negative, crosses zero, and turns positive is the Depolarizing phase.