Question

In a step-by-step manner, explain how the voltage-regulated channels produce an action potential.

Short answer

In a step-by-step manner, voltage-regulated channels produce an action potential as follows: 1. A neuron is at its resting potential (-70 mV), maintained by an uneven distribution of ions. 2. When a stimulus arrives and reaches the threshold potential (-55 mV), voltage-gated sodium channels open, allowing Na+ ions to enter the cell and depolarize the membrane. 3. During the depolarization phase, the membrane potential rises to around +40 mV. 4. At the peak, voltage-gated sodium channels close while voltage-gated potassium channels open, allowing K+ ions to leave the cell and initiating repolarization. 5. The membrane potential drops, briefly overshooting the resting potential during hyperpolarization. 6. Voltage-gated potassium channels close and the sodium-potassium pump restores the resting potential at -70 mV. 7. The neuron experiences a refractory period, ensuring the action potential propagates in one direction. Overall, voltage-regulated channels maintain the resting potential, facilitate depolarization and repolarization, and restore the resting membrane potential.

Step-by-step solution

Understanding Neuron Resting Potential

Before an action potential occurs, a neuron is at its resting potential, which is typically around -70 mV. The neuron membrane is more permeable to potassium (K+) ions than sodium (Na+) ions, and the concentration of K+ ions is higher inside the cell than outside. The concentration of Na+ ions is higher outside the cell. This uneven distribution of ions maintains the resting potential.

Voltage-Gated Sodium Channels Opening

When a stimulus arrives, the membrane potential becomes less negative (depolarizes), reaching the threshold potential, which is usually around -55 mV. At this point, voltage-gated sodium channels open, allowing Na+ ions to rush into the cell, further depolarizing the membrane. This opening of sodium channels is known as the "activation" of the channels.

Depolarization Phase

As more sodium ions enter the cell through the open sodium channels, the membrane potential continues to rise, reaching its peak potential at around +40 mV during the depolarization phase. The positive membrane potential indicates that the inside of the cell is positively charged relative to the outside.

Voltage-Gated Sodium Channels Closing (Inactivation) and Voltage-Gated Potassium Channels Opening

When the membrane potential reaches around +40 mV, the voltage-gated sodium channels begin to close due to a process known as "inactivation." Simultaneously, voltage-gated potassium channels open, allowing K+ ions to flow out of the cell. This outflow of K+ ions causes the membrane potential to become more negative, initiating the repolarization phase.

Repolarization Phase

During the repolarization phase, the membrane potential drops as K+ ions leave the cell through the open potassium channels. The membrane potential gradually returns towards the resting potential, but slightly overshoots it, causing a brief period of hyperpolarization.

Hyperpolarization and Restoration of Resting Potential

During the hyperpolarization phase, the membrane potential is more negative than the resting potential. Voltage-gated potassium channels close, and the sodium-potassium pump (an ATPase enzyme) actively transports Na+ ions out and K+ ions back into the cell to restore the resting membrane potential at -70 mV.

Refractory Period

During the action potential, the neuron experiences a refractory period where it is unable to generate another action potential. This period ensures the propagation of the action potential in one direction along the neuron and prevents the signal from traveling backward. In summary, voltage-regulated channels (specifically voltage-gated sodium and potassium channels) play a crucial role in generating and propagating action potentials by opening and closing in response to changes in membrane potential. These channels help maintain the resting potential, facilitate depolarization and repolarization phases, and allow for the restoration of the resting membrane potential.

Key concepts

Neuron Resting Potential

The resting potential of a neuron is a critical starting point for understanding how neural signals are transmitted. It refers to the electrical potential difference across the neuron's cell membrane when it is not actively sending a signal. The inside of the neuron is typically kept at about -70 millivolts (mV) compared to the outside. This difference is due to the higher concentration of potassium ions (K+) inside the cell and the higher concentration of sodium ions (Na+) outside the cell. This balance is maintained by ion pumps and channels, ensuring that the neuron is ready to 'fire' when needed.

The neuron's membrane contains sodium-potassium pumps that actively transport K+ ions in and Na+ ions out, against their concentration gradients, using ATP as an energy source. This activity contributes to the maintenance of the resting potential. If the neuron's membrane were equally permeable to all ions, the ions would simply diffuse down their concentration gradients, and there would be no resting potential. However, at rest, the membrane is much more permeable to K+ ions due to more open potassium channels, allowing K+ ions to flow out of the neuron more freely. Thus, the resting potential is largely set by the movement of K+ ions, contributing to a negative charge inside the neuron.

Voltage-Gated Sodium Channels

Voltage-gated sodium channels are essential proteins in the neuron's membrane that are sensitive to changes in voltage across the membrane. These channels are usually closed at the resting potential. However, when a stimulus causes the membrane potential to become less negative, reaching a threshold of around -55 mV, these channels open rapidly. This opening allows Na+ ions to enter the neuron, which is crucial for initiating the action potential.

As Na+ ions rush into the neuron, they carry positive charge with them, causing the membrane potential to become more positive, a process called depolarization. This is the 'activation' phase of the sodium channels. But these channels are not just simple on/off switches. They have a kind of built-in timer that causes them to become inactive shortly after opening, so they close again as the membrane potential nears its peak during an action potential. This inactivation is crucial because it marks the end of the depolarization phase and paves the way for repolarization, ensuring the one-way propagation of the action potential.

Depolarization and Repolarization Phases

The depolarization and repolarization phases are two critical segments of the action potential. During depolarization, after the opening of the voltage-gated sodium channels, Na+ ions flood the cell interior, causing the membrane potential to quickly rise from the negative resting potential toward a positive value, peaking at around +40 mV. This rapid influx changes the polarization of the neuron's membrane, hence the term 'depolarization'.

Following depolarization, the neuron must reset its membrane potential to be ready for the next signal, which is achieved during the repolarization phase. The voltage-gated sodium channels close (inactivate), and voltage-gated potassium channels open, allowing K+ ions to flow out of the neuron. This movement of K+ out of the cell returns the membrane potential back towards the negative resting value. It's a coordinated dance of ions moving in and out through their respective channels, driven by changes in membrane potential.

Neural Signal Propagation

Neural signal propagation refers to how the action potential moves down the length of a neuron, from the cell body to the axon terminals. Because the action potential involves a localized change in membrane potential, it triggers nearby voltage-gated channels to open as well. The action potential thus propagates like a wave along the neuron.

Importantly, neurons have a 'refractory period' after an action potential during which they are less likely or unable to fire again. This period ensures the wave-like signal moves in one direction only—toward the axon terminals—and doesn't double back on itself. The combined activity of voltage-gated sodium and potassium channels, and the subsequent flow of Na+ and K+ ions, is the basis for action potential generation. This well-orchestrated sequence of events is what allows neurons to transmit signals rapidly and efficiently across long distances within the nervous system, resulting in the complex choreography of brain activity and nervous system function.