Refractory Periods | Action potentials in neurons

Summary

This video explains the process of action potentials in neurons, focusing on the concepts of absolute and relative refractory periods. It discusses how neurons respond to stimuli, including the opening and closing of sodium and potassium channels. The video also explains the significance of these refractory periods, emphasizing how they prevent excessive firing of neurons. Additionally, it highlights the use of lidocaine in blocking pain by keeping sodium channels inactive, preventing action potentials from being triggered, and stopping pain signals from reaching the brain. This insight into neural physiology is both informative and practical.

Takeaways

• 😀 Neurons start at a resting membrane potential of about -70 millivolts, with all voltage-gated sodium and potassium channels closed.
• 😀 Sodium channels have two gates: an activation gate and an inactivation gate, while potassium channels have only an activation gate.
• 😀 When the membrane potential reaches -55 millivolts (threshold), the sodium channels open, allowing sodium to enter and depolarize the cell.
• 😀 Once sodium channels are open and fully depolarized, they cannot reopen during the action potential due to their inactivation gate, marking the beginning of the absolute refractory period.
• 😀 The absolute refractory period occurs when the sodium channels are either open or inactivated, and a new action potential cannot be initiated no matter how strong the stimulus.
• 😀 After depolarization, potassium channels open, allowing potassium to leave the cell, leading to repolarization of the membrane.
• 😀 Sodium channels, once inactivated, are locked and cannot be used to start another action potential until they reset to their original shape, ending the absolute refractory period.
• 😀 Once the membrane becomes hyperpolarized (more negative than the resting potential), the relative refractory period begins, making it harder, but not impossible, to initiate a new action potential.
• 😀 The relative refractory period requires a stronger stimulus to reach the -55 millivolt threshold due to the cell's hyperpolarized state.
• 😀 Lidocaine, a local anesthetic, works by binding to voltage-gated sodium channels and keeping them in their inactive state, preventing action potentials from firing and blocking pain signals to the brain.

Q & A

What is the resting membrane potential of a typical neuron?

The resting membrane potential of a typical neuron is around -70 millivolts.

What role do voltage-gated sodium channels play in an action potential?

Voltage-gated sodium channels open when the membrane reaches a threshold of about -55 millivolts, allowing sodium ions to enter the neuron, depolarizing the membrane and starting the action potential.

What happens during the depolarization phase of an action potential?

During depolarization, the sodium activation gates open, allowing sodium ions to enter the neuron, making the inside of the cell more positive and triggering the action potential.

What is the absolute refractory period?

The absolute refractory period is the phase during which the neuron cannot start another action potential, regardless of the stimulus strength. This occurs when the sodium channels are either fully open or locked in an inactivated state.

How does the inactivation gate of the sodium channel contribute to the refractory period?

The inactivation gate of the sodium channel blocks the channel after it has been opened, preventing sodium from entering the cell. This results in the sodium channel being inactive and unable to contribute to another action potential during the absolute refractory period.

What happens during the repolarization phase of an action potential?

During repolarization, the sodium channels become inactivated, and potassium channels open, allowing potassium to exit the neuron. This makes the membrane potential more negative again, bringing the neuron back toward its resting state.

What is the relative refractory period?

The relative refractory period occurs after the absolute refractory period, when the neuron is hyperpolarized (more negative than the resting state). During this time, it is possible to trigger another action potential, but it requires a stronger stimulus than usual.

Why is it harder to trigger an action potential during the relative refractory period?

During the relative refractory period, the neuron is hyperpolarized, meaning it is more negative than its resting potential. To trigger another action potential, a stronger stimulus is required to bring the membrane potential to the threshold of -55 millivolts.

How does lidocaine work as a pain reliever?

Lidocaine works by binding to the voltage-gated sodium channels and keeping them in an inactive state. This prevents action potentials from being generated, effectively blocking the pain signals from reaching the brain.

What is the importance of refractory periods in the context of lidocaine's effect?

Refractory periods are crucial for understanding how lidocaine works. By keeping sodium channels in their inactive state, lidocaine makes the neuron refractory to further stimulation, ensuring that pain signals are not transmitted to the brain during painful procedures like dental work.