Question

When the action potential begins, sodium gates open, allowing \(\mathrm{Na}^{+}\) to cross the membrane. Now the polarity changes to a. negative outside and positive inside. b. positive outside and negative inside. c. neutral outside and positive inside. d. All of these are correct.

Short answer

a. negative outside and positive inside.

Step-by-step solution

Recall the resting membrane potential

Before an action potential occurs, the inside of a neuron is more negative compared to the outside. This is due to the unequal distribution of ions across the membrane, with more positive charges (such as Na+ and K+) outside and more negative charges (such as Cl- and negatively charged proteins) inside.

Understand the role of sodium channels in action potential

When an action potential begins, voltage-gated sodium channels open in response to a stimulus that causes depolarization. As these channels open, Na+ ions flow into the cell, down their electrochemical gradient. The influx of positive charges causes the inside of the cell to become more positive.

Determine the polarity change

As the Na+ ions enter the cell, the polarity of the membrane changes. The inside of the cell becomes more positive, while the outside becomes more negative, due to the loss of positive charges (Na+ ions). Therefore, the correct answer is: a. negative outside and positive inside.

Key concepts

Membrane Potential

Imagine a neuron as a tiny battery, with a difference in electric charge across its membrane creating what we call the membrane potential. This difference is crucial for the neuron's ability to send signals.

Typically, the membrane potential is negative, due to a higher concentration of positively charged ions outside the cell compared to the inside. Various ions, such as sodium ( a^{+} ) and potassium ( K^{+} ), play a key role in establishing this charge difference. Proteins structures called ion channels and pumps, working within the cell's plasma membrane, actively transport these ions in and out to maintain the membrane potential.

It's important to note that this potential is dynamic and changes in response to neuronal activity, leading to the initiation of an action potential, which is a rapid and temporary change in the membrane potential.

Voltage-Gated Sodium Channels

When we talk about voltage-gated sodium channels, we're discussing the gatekeepers that manage the flow of sodium ions into the neuron.

These channels are sensitive to changes in the membrane potential. At rest, they are closed, but once the membrane potential reaches a certain threshold, they open rapidly. This mechanism is like a triggered trapdoor that only opens when the electrical conditions are just right. The opening of these channels is the starting gun for the action potential, as it allows sodium ions to rush into the neuron, following their electrochemical gradient. The flow of positively charged sodium ions into the cell is the primary event that leads to the neuron depolarization, which is the next step in transmitting a nerve signal.

Neuron Depolarization

Following along the action potential track, the next phase is neuron depolarization. Imagine this like a wave of positive charge sweeping along the neuron.

Depolarization is when the inside of the neuron becomes less negative compared to the outside, due to the influx of sodium ions. This shift in charge is a bit like flipping the switch from 'off' to 'on' for the neuron's signaling state. It's this dramatic change in charge that enables the nerve cell to pass its message down the line and communicate with the next cell. This depolarization process is fast and spreads like a wildfire, allowing the neuron to transmit an electrical signal along its length quickly and efficiently.

Electrochemical Gradient

The term electrochemical gradient refers to the combined forces of the concentration gradient and the electric charge gradient that drive ions like sodium across the cell membrane.

Think of it as a hill with sodium ions at the top. The concentration gradient is the slope of the hill, encouraging sodium ions to roll downhill into the cell, where there are fewer of them. The electric charge gradient is like an electric field across the hill, drawing positively charged sodium ions toward the negatively charged inside of the neuron. Together, these gradients form an 'electrochemical slope' that sodium ions 'slide down' during an action potential, ultimately leading to the changes we observe as the signal passes along the neuron.