Problem 9 - Biology of Humans: Concepts, Applications, and Issues

Question

What is the initial (first) event of an action potential? a. depolarization caused by potassium ions rushing to the inside b. repolarization due to rushing of potassium ions to the inside c. depolarization due to rushing of sodium ions to the inside d. repolarization due to departure of potassium ions from the axon

Step-by-Step Solution

Answer

c. depolarization due to rushing of sodium ions to the inside

Step 1: Understand the process of action potential

An action potential is a rapid rise and subsequent fall in voltage or membrane potential across a cellular membrane with a characteristic pattern.

Step 2: Identify the key events

The key events include depolarization, peak, and repolarization. The initial event involves the movement of ions across the membrane.

Step 3: Determine the initial ion movement

The initial event is the depolarization of the membrane. This occurs when sodium ions (Na+) rush into the cell.

Step 4: Match to the correct option

From the given options, the correct initial event is 'depolarization due to rushing of sodium ions to the inside'. This matches option c.

Key Concepts

depolarization

Depolarization is a crucial step in the process of an action potential. It is the initial phase where the membrane potential becomes less negative. This means that the inside of the cell temporarily becomes more positive compared to the outside.
The main trigger for depolarization is the opening of sodium (Na+) channels, which allows sodium ions to flow into the cell. When these channels open, there is a rapid influx of sodium ions, causing the membrane potential to shift from a negative value towards a positive one.
In summary, depolarization happens due to the entry of sodium ions into the cell, making it more positive inside. This sets the stage for the subsequent phases of the action potential.

sodium ions

Sodium ions (Na+) play a pivotal role in generating an action potential. These positively charged ions are essential in changing the membrane potential of the cell during depolarization.
When an action potential begins, sodium channels in the cell membrane open up. This allows sodium ions, which are more concentrated outside the cell, to rush in.
This influx of sodium ions leads to depolarization as the inside of the cell becomes less negative. Moreover, this movement of ions is an all-or-nothing response. Once the threshold is reached, an action potential will fully occur.
In the context of the exercise, it's important to note that the initial event in an action potential is triggered by sodium ions entering the cell.

repolarization

Repolarization is the phase following depolarization in an action potential. During repolarization, the cell's membrane potential returns to a negative value.
This process mainly involves the opening of potassium (K+) channels. Potassium ions, which are more concentrated inside the cell, move out to the extracellular space.
As potassium exits the cell, the inside of the cell becomes more negative again, moving the membrane potential back towards its resting state. This phase ensures that the cell can go through another action potential after a refractory period.
Unlike depolarization, repolarization is driven by the efflux of potassium ions, helping reset the cell's membrane potential.