C2.2.8 Depolarization and repolarization during action potentials (HL) Notes

Depolarization and Repolarization Drive Signal Transmission in Neurons

1. An action potential is a temporary change in the electrical charge across a neuron’s membrane.
2. It allows the neuron to transmit a signal along its length.
3. This process involves depolarization (the inside of the cell becomes more positive) and repolarization (the return to a negative resting potential).
4. These changes are mediated by voltage-gated sodium (Na⁺) and potassium (K⁺) channels and require a threshold potential to initiate.

Steps in Depolarization and Repolarization

Resting Potential

1. Before the action potential, the neuron is at its resting potential (around -70 mV), where the inside of the cell is negatively charged compared to the outside.
2. The resting potential is maintained by the sodium-potassium pump and the selective permeability of the membrane.

Threshold Potential

1. The action potential begins when the membrane potential reaches a critical value called the threshold potential (usually around -55 mV).
2. This is the point at which voltage-gated sodium channels open.
3. If the threshold is not reached, no action potential will occur, this is called the all-or-nothing response.

Depolarization

1. Voltage-gated sodium channels open in response to the membrane reaching the threshold.
2. This allows Na⁺ ions to rush into the cell, making the inside of the neuron more positive.
3. This rapid influx of sodium ions causes the membrane potential to rise sharply from around -70 mV to a peak of about +30 mV, which is known as the depolarization phase.

Repolarization

1. Once the membrane potential reaches the peak, the voltage-gated sodium channels close, and the voltage-gated potassium channels open.
2. Potassium ions (K⁺) move out of the neuron, restoring the negative charge inside the cell, which is the repolarization phase.
3. The exit of K⁺ ions causes the membrane potential to return to a more negative value (around -70 mV).

Hyperpolarization

1. After repolarization, the membrane potential temporarily becomes more negative than the resting potential, which is called hyperpolarization (about -80 mV).
2. This is due to the continued outflow of K⁺ ions through potassium channels.
3. Eventually, the sodium-potassium pump restores the resting potential by pumping Na⁺ out and K⁺ in, returning the neuron to its resting state.

Return to Resting Potential

1. The neuron returns to its resting potential of around -70 mV.
2. The cell is now ready to transmit another action potential if it receives a sufficient stimulus.

Why Sodium Ions Enter the Neuron

1. Concentration Gradient: There are more $Na^+$ ions outside the neuron than inside.
2. Electrical Gradient: The inside of the neuron is initially negative, attracting the positive $Na^+$ ions.

The combined force of these two gradients is referred to as the electrochemical gradient, which drives Na⁺ ions into the neuron when sodium channels open.

Why Potassium Ions Exit the Neuron

1. Concentration Gradient: There are more $K^+$ ions inside the neuron than outside.
2. Electrical Gradient: The positive charge inside the neuron pushes $K^+$ ions out.

The Role of Voltage-Gated Channels

Voltage-Gated Sodium Channels

1. These channels are sensitive to changes in membrane potential.
2. When the membrane depolarizes to the threshold potential, these channels open, allowing sodium ions (Na⁺) to rush into the neuron.
3. The channels close when the membrane potential reaches its peak, halting sodium influx.

Voltage-Gated Potassium Channels

1. These channels open in response to membrane depolarization, but they are slower than sodium channels.
2. The opening of potassium channels allows potassium ions (K⁺) to flow out of the cell, leading to repolarization.
3. These channels close as the membrane potential approaches the resting potential.

Why is the Threshold Potential Important?

1. The threshold potential is the minimum voltage needed to trigger an action potential.
2. Once this potential is reached, voltage-gated sodium channels open, triggering the rapid depolarization that constitutes the action potential.
3. If the threshold isn’t reached, the neuron remains at rest. This is known as the all-or-nothing principle of action potentials.
4. This ensures that only significant stimuli generate signals, preventing random firing.