Question

Describe how the \(\mathrm{Na}^{+} / \mathrm{K}^{+}\)pumps contribute to the resting membrane potential. Also, describe how the membrane potential would be affected if (1) gated \(\mathrm{Na}^{+}\)channels were to open, and ( 2 ) gated \(\mathrm{K}^{+}\) channels were to open.

Short answer

The \(\mathrm{Na}^{+} / \mathrm{K}^{+}\) pump contributes to the resting membrane potential by actively transporting three \(\mathrm{Na}^{+}\) ions out of the cell and two \(\mathrm{K}^{+}\) ions into the cell per ATP molecule hydrolyzed, creating an electrochemical gradient across the cell membrane. If gated \(\mathrm{Na}^{+}\) channels were to open, \(\mathrm{Na}^{+}\) ions would influx, causing depolarization and potentially triggering an action potential. If gated \(\mathrm{K}^{+}\) channels were to open, \(\mathrm{K}^{+}\) ions would efflux, causing hyperpolarization and making it more difficult to generate an action potential.

Step-by-step solution

Understanding the Na+/K+ Pump's Contribution to Resting Membrane Potential

The \(\mathrm{Na}^{+} / \mathrm{K}^{+}\) pump, or sodium-potassium pump, is an essential component in establishing and maintaining the resting membrane potential in neurons. The pump actively transports three \(\mathrm{Na}^{+}\) ions out of the cell and two \(\mathrm{K}^{+}\) ions into the cell for each ATP molecule hydrolyzed. This process results in a higher concentration of \(\mathrm{K}^{+}\) ions inside the cell, and a higher concentration of \(\mathrm{Na}^{+}\) ions outside the cell, maintaining an electrochemical gradient across the cell membrane.

Effect of Opening Gated Na+ Channels on Membrane Potential

If gated \(\mathrm{Na}^{+}\) channels were to open, the electrochemical gradient established by the pump would cause an influx of \(\mathrm{Na}^{+}\) ions into the cell. This movement would make the interior of the cell more positive compared to the extracellular space, leading to a depolarization of the membrane potential. The change in voltage inside the cell would eventually trigger an action potential if it reaches the threshold voltage.

Effect of Opening Gated K+ Channels on Membrane Potential

If gated \(\mathrm{K}^{+}\) channels were to open, the electrochemical gradient established by the pump would cause an efflux of \(\mathrm{K}^{+}\) ions out of the cell. This movement would make the interior of the cell more negative compared to the extracellular space, resulting in a hyperpolarization of the membrane potential. This can make it more difficult for the neuron to reach the threshold voltage necessary to generate an action potential.

Key concepts

Na+/K+ pump

The sodium-potassium pump is a fundamental element in maintaining the neuron's resting membrane potential. Imagine it as a little engine on the neuron's membrane. Its main job is to transport ions, specifically it moves three sodium (Na\(^+\)) ions out of the neuron and brings two potassium (K\(^+\)) ions in, using energy derived from ATP.
This action results in a higher concentration of Na\(^+\) outside the cell, and more K\(^+\) ions inside the cell. This movement creates an imbalance, known as the electrochemical gradient, which generates the resting membrane potential essential for nerve impulses.
The resting membrane potential is incredibly important because it keeps the neuron ready to fire a message at any moment. It's like a spring-loaded toy, ready to jump when triggered.

Gated Na+ Channels

Gated sodium channels are like special gates on the cell membrane that respond to certain signals. When these gates open, Na\(^+\) ions rush into the neuron due to the electrochemical gradient. This sudden influx causes the inner side of the cell membrane to become more positive.
This change in charge is called depolarization. It is crucial because if enough Na\(^+\) enters the cell, it can bring the membrane potential to a level known as the threshold. When the threshold is reached, an action potential is triggered, sending a signal down the neuron's axon.
In simple terms, think of gated Na\(^+\) channels as doors that when opened, allow a crowd of ions to rush in, causing an excitement, leading to the neuron firing an action potential.

Gated K+ Channels

Gated potassium channels are another type of membrane gate, but they work in opposition to sodium channels. Their primary function is to release K\(^+\) ions out of the neuron. When these channels open, K\(^+\) flows out due to the electrochemical gradient.
This causes the membrane to become more negative, a process called hyperpolarization. This makes the inside of the cell more negative compared to the outside, which is the opposite of what happens when Na\(^+\) channels open.
In practical terms, this means the neuron is less likely to fire an action potential, as moving further away from the threshold makes triggering one more difficult. Imagine these channels as exits that when open, help calm the neuron down, making it less excitable.

Neuron Electrochemical Gradient

The electrochemical gradient is akin to the neuron's battery power which depends on the distribution of ions inside and outside the cell.
This gradient is created primarily by the Na+/K+ pump and is crucial for neuron excitability. It represents two types of gradients: a concentration gradient and an electric gradient.

• **Concentration Gradient**: More Na\(^+\) outside and more K\(^+\) inside due to the pumps.
• **Electric Gradient**: The charge difference across the membrane with a more negative inside.

This potential energy is stored across the neuron's membrane and allows the neuron to be primed—ready to send rapid signals by turning this stored energy into an action potential.

Action Potential

An action potential is a thrilling electrical event when a neuron sends a signal along its axon to communicate with other neurons, muscles, or glands.
It starts when there is a significant influx of Na\(^+\) , leading to depolarization, and if the threshold potential is reached, the action potential is fired.
The sequence continues with a brief repolarization phase, often involving the exit of K\(^+\) to restore the resting state. Then, the neuron can be triggered again.

• **Depolarization**: Na\(^+\) influx after Na\(^+\) channels open.
• **Repolarization**: Return to resting potential through K\(^+\) exit.

In easier terms, think of an action potential as a brief burst of electricity that travels along a neuron's wire, sending important messages throughout the body in a flash.