C2.2.2 Generation of the resting potential by pumping Notes

Resting Potential is Established by Sodium-Potassium Pumps Maintaining Ion Gradients

1. Imagine a neuron as a tightly secured vault, with its doors carefully controlling the flow of ions.
2. This control creates a resting potential, a stable electrical state essential for transmitting nerve impulses.
3. But how is this resting potential established and maintained?

Resting Potential is the Neuron's Stable Charge Difference, Powered by the Sodium-Potassium Pump

1. Resting potential is the electrical charge difference across a neuron’s plasma membrane when it is not transmitting a signal.
2. This difference is typically around −70 mV, with the inside of the neuron being more negative than the outside.
3. It is established and maintained by the sodium-potassium pump (Na⁺/K⁺ pump), a membrane protein that uses ATP to drive the movement of ions, and the selective permeability of the membrane to certain ions.
4. This creates the conditions necessary for generating and propagating nerve impulses.

The Sodium-Potassium Pump Maintains Resting Potential by Unequal Ion Transport

1. The sodium-potassium pump is a transmembrane protein that actively transports ions against their concentration gradients.
2. It plays a central role in establishing the resting potential by:
   1. Pumping Ions Unequally
      1. For every three sodium ions ($Na^+$) pumped out of the neuron, two potassium ions ($K^+$) are pumped in.
      2. This creates a net loss of positive charge inside the neuron, contributing to its negative resting potential.
   2. Using Energy from ATP
      1. The pump relies on ATP to move ions against their concentration gradients, making this an active transport process.

Why Is the Resting Potential Negative?

Three main factors contribute to the negative resting potential:

1. Unequal Ion Exchange: The sodium-potassium pump moves more positive ions out (3 $Na^+$) than it brings in (2 $K^+$), creating a charge imbalance.
2. Selective Permeability of the Membrane: The neuron’s membrane is more permeable to $K^+$ than $Na^+$. Potassium ions tend to leak out of the cell through potassium leak channels, further increasing the negative charge inside.
3. Presence of Negatively Charged Proteins: Large, negatively charged proteins inside the neuron cannot cross the membrane, adding to the internal negativity.

The Sodium-Potassium Pump Creates Ion Gradients in Five Key Steps

1. The sodium-potassium pump operates in a cycle with distinct steps:
   1. Step 1: The pump binds 3 Na⁺ ions from inside the neuron.
   2. Step 2: ATP is hydrolyzed, transferring a phosphate group to the pump, causing it to change shape.
   3. Step 3: The pump releases the 3 Na⁺ ions to the outside of the cell.
   4. Step 4: The pump binds 2 K⁺ ions from the extracellular space.
   5. Step 5: The phosphate group is released, restoring the pump's original shape and moving the 2 K⁺ ions into the neuron.
2. A high concentration of Na⁺ is maintained outside the cell, and a high concentration of K⁺ is maintained inside the cell.
3. This creates a concentration gradient and a negative resting potential

Action Potential and Resting Potential

1. The resting potential is a stable, negative voltage that exists when the neuron is not actively firing an action potential.
2. However, when a neuron is stimulated, changes in the membrane potential occur, leading to the generation of an action potential (nerve impulse).
   1. When a stimulus triggers the neuron, sodium channels open allowing sodium ions (Na⁺) to flow into the cell, which depolarizes the membrane.
   2. After depolarization, the potassium channels open to let potassium ions (K⁺) flow out, leading to repolarization.
3. This change in membrane potential is what transmits the nerve impulse along the length of the axon.

Resting Potential Prepares Neurons for Rapid Signaling and Maintains Ion Gradients

1. The resting potential is critical for neuron function because it:
   1. Prepares the Neuron for Action: The negative resting potential sets the stage for rapid changes in charge during an action potential.
   2. Maintains Ion Gradients: The sodium-potassium pump ensures that $Na^+$ and $K^+$ gradients are preserved, enabling the neuron to respond quickly to stimuli.