[#1] POTENCIAL DE AÇÃO: Despolarização, Repolarização e Hiperpolarização | MK Fisiologia

Summary

This video explains the action potential in neurons, focusing on its generation, phases, and the processes involved. It begins by describing the resting potential and how ion channels maintain a negative membrane potential. The script then delves into how neurotransmitters trigger depolarization, leading to the opening of sodium channels and the start of the action potential. As the membrane becomes more positive, potassium channels open for repolarization. The process ends with hyperpolarization before returning to the resting potential. Key concepts like threshold potential, all-or-nothing principle, and the rapid sequence of events are emphasized throughout.

Takeaways

• 😀 The action potential in neurons is a complex process that involves several phases, including depolarization, repolarization, and hyperpolarization.
• 😀 Before action potentials can occur, the neuron must be in a resting state, where it maintains a negative membrane potential due to ion leakage channels, primarily for potassium and sodium.
• 😀 The resting membrane potential is negative because potassium is more concentrated inside the cell, and sodium is more concentrated outside the cell, with potassium channels being more permeable than sodium channels.
• 😀 The sodium-potassium pump helps maintain the resting potential by moving potassium into the cell and sodium out, counteracting the leakage of ions.
• 😀 Neurons can be stimulated by neurotransmitters that open ion channels, allowing sodium to enter and make the membrane potential less negative, leading to depolarization.
• 😀 If the depolarization reaches a specific threshold (the 'threshold potential'), typically -55 mV, it triggers the opening of voltage-gated sodium channels, initiating the action potential.
• 😀 The action potential is an 'all-or-nothing' response, meaning once triggered, it proceeds without stopping, regardless of the strength of the stimulus as long as it surpasses the threshold.
• 😀 Sodium influx causes a rapid depolarization, where the inside of the neuron becomes more positive, leading to a positive feedback loop that opens even more sodium channels.
• 😀 At the peak of depolarization, around +30 mV, sodium channels start to close, and voltage-gated potassium channels open, allowing potassium to exit and repolarize the membrane.
• 😀 Repolarization restores the membrane potential to a more negative value, but potassium channels are slow to close, leading to hyperpolarization, where the membrane potential becomes even more negative than the resting potential.
• 😀 The membrane potential eventually returns to its resting state after all ion channels close, and the sodium-potassium pump works to restore the proper ion concentrations across the membrane.

Q & A

What is the resting potential of a neuron?

The resting potential of a neuron is approximately -70mV. It is maintained due to the distribution of ions, with more potassium (K⁺) inside the cell and more sodium (Na⁺) outside, creating a negative internal charge.

How does the action potential begin in a neuron?

The action potential begins when a stimulus causes neurotransmitters to bind to receptors on the neuron's membrane, opening sodium channels and allowing sodium ions to enter the cell, causing depolarization.

What happens during the depolarization phase of the action potential?

During depolarization, sodium channels open, allowing a large influx of sodium ions into the neuron. This causes the membrane potential to become more positive, initiating a wave of depolarization that spreads along the neuron.

What is the threshold potential and why is it important?

The threshold potential is typically -55mV. It is the critical value that the membrane potential must reach for the action potential to be triggered. If the depolarization reaches this value, voltage-gated sodium channels open, starting the action potential.

What role do sodium channels play in the action potential?

Sodium channels, specifically voltage-gated sodium channels, are crucial for depolarization. When the membrane potential reaches the threshold, these channels open, allowing sodium ions to flood into the cell, causing the membrane potential to become positive.

What occurs during the repolarization phase of the action potential?

During repolarization, potassium channels open, allowing potassium ions to exit the neuron. This causes the membrane potential to become more negative, returning it toward the resting potential.

What causes hyperpolarization during the action potential?

Hyperpolarization occurs because potassium channels are slow to close, causing more potassium to leave the cell than necessary. This results in the membrane potential becoming more negative than the resting potential.

Why is the action potential considered an 'all-or-nothing' response?

The action potential is an 'all-or-nothing' response because once the threshold potential is reached, the neuron will fire an action potential with the same magnitude every time. If the threshold is not reached, no action potential occurs.

How does the sodium-potassium pump contribute to maintaining the resting potential?

The sodium-potassium pump helps maintain the resting potential by actively transporting sodium ions out of the cell and potassium ions into the cell, counteracting the natural ion leakage and ensuring the proper concentration gradients for each ion.

What is the role of neurotransmitters in the generation of the action potential?

Neurotransmitters bind to receptors on the neuron's membrane, which can open ion channels, such as sodium channels, allowing ions like sodium to enter the neuron. This leads to depolarization, which may trigger an action potential if the threshold is reached.