Action Potentials: Videos & Practice Problems

Action potentials are crucial for neuronal communication, and understanding their sequence is essential for grasping how neurons transmit signals. The process begins when a neuron is at resting potential, typically around -70 millivolts. This state is maintained until the neuron receives graded potentials, specifically excitatory postsynaptic potentials (EPSPs), which gradually depolarize the membrane. When the membrane potential reaches the threshold of -55 millivolts, voltage-gated sodium channels open, allowing sodium ions (Na⁺) to rush into the cell, driven by their electrochemical gradient. This influx of positive ions causes a rapid depolarization, pushing the membrane potential up to approximately +30 millivolts.

At this peak, the voltage-gated sodium channels close, and voltage-gated potassium channels open. Potassium ions (K⁺), which are more concentrated inside the cell, begin to exit, leading to repolarization as the cell loses positive charge. The membrane potential decreases, overshooting the resting level and resulting in hyperpolarization, which can drop to around -80 to -90 millivolts. This occurs because the potassium channels close slowly, allowing for a brief period where the membrane potential is more negative than the resting state.

Eventually, all channels close, and the sodium-potassium pump (Na⁺/K⁺ ATPase) restores the resting potential by moving sodium out and potassium back into the cell. This entire sequence illustrates the dynamic changes in membrane potential and the role of voltage-gated channels in action potentials, which are fundamental to neuronal signaling.

The action potential is a crucial process in neuronal communication, characterized by distinct phases that reflect changes in membrane potential. Understanding the behavior of voltage-gated sodium and potassium channels during these phases is essential for grasping how neurons transmit signals.

Initially, when the neuron is at rest, both voltage-gated sodium and potassium channels are closed. This resting state is vital for maintaining the membrane potential, typically around -70 mV. As the neuron receives a stimulus and depolarizes, the membrane potential reaches a threshold of approximately -55 mV, prompting the opening of voltage-gated sodium channels. This influx of sodium ions (Na⁺) causes the membrane potential to rise rapidly, leading to the depolarization phase, where the potential can reach around +30 mV.

Once the membrane potential peaks at +30 mV, the sodium channels close, and voltage-gated potassium channels open. This transition marks the beginning of repolarization, during which potassium ions (K⁺) exit the cell, causing the membrane potential to decrease and return towards the resting state.

However, the repolarization process often overshoots, resulting in hyperpolarization. During this phase, the sodium channels remain closed, while the potassium channels are in the process of closing. This gradual closure of potassium channels can lead to a temporary state where the membrane potential dips below the resting level, typically around -80 mV.

Eventually, all voltage-gated channels close, restoring the neuron to its resting state, where both sodium and potassium channels are again closed. This cycle of depolarization, repolarization, and hyperpolarization is fundamental to the propagation of action potentials along neurons, enabling effective communication within the nervous system.