Question

An action potential occurs when ______. a. membrane potential increases to threshold level b. gated sodium channels close c. neurotransmitter is reabsorbed d. gated potassium channels open

Short answer

An action potential occurs when the membrane potential increases to the threshold level (Option A).

Step-by-step solution

Identify Key Terms

Understand that an action potential is a process where a neuron's membrane potential changes. Membrane potential refers to the voltage difference across the neuronal membrane.

Understand Action Potential Conditions

Action potentials are triggered when the membrane potential reaches a certain critical level known as the threshold potential. This is primarily caused by the opening of gated sodium channels, which leads to an influx of sodium ions.

Review Each Option

Evaluate each option based on the knowledge of action potentials: - Option A: Membrane potential increases to threshold level. This is when the action potential is initiated. - Option B: Gated sodium channels close. This occurs after the action potential peak. - Option C: Neurotransmitter is reabsorbed. This is part of synaptic transmission, not action potential generation. - Option D: Gated potassium channels open. This happens to repolarize the cell after the action potential peak.

Conclusion

From these evaluations, only option A accurately describes the condition under which an action potential is initiated.

Key concepts

Membrane Potential

The membrane potential is a fundamental concept in biology, particularly crucial for understanding how nerve cells transmit signals. Simply put, it's the electrical potential difference across the membrane of a cell.
The inside of a neuron is negatively charged relative to the outside. This difference is maintained by ion pumps and ion channels that regulate ionic gradients.
Neurons maintain a resting membrane potential of about -70 millivolts (mV). It's the result of different concentrations of ions, primarily sodium (Na+) and potassium (K+), on either side of the cell membrane.

• A high concentration of Na+ ions outside the cell and K+ ions inside creates this potential.
• Active transport proteins, such as the sodium-potassium pump, help maintain this balance by pumping Na+ out and K+ into the cell, using energy.

Understanding membrane potential is crucial because it sets the stage for neurons to send electrical impulses throughout the body, known as action potentials.

Threshold Potential

The threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential. In neurons, this threshold is typically around -55 millivolts (mV). Once reached, it triggers a rapid rise and fall in the electrical potential across the neuron's membrane.
Reaching the threshold potential is essential for neuron activation and subsequent signal transmission. If the depolarization does not reach this threshold, the neuron will not fire an action potential.
Here's how it works:

• If the membrane potential becomes more positive and reaches the threshold, it triggers the opening of sodium channels, leading to the influx of Na+ ions.
• This drastic change propels the membrane potential in a positive direction, initiating the action potential.

In summary, reaching the threshold potential acts as a crucial switch that turns neuronal communication on.

Gated Sodium Channels

Gated sodium channels are specific proteins embedded in the cell membrane, playing a pivotal role in the initiation and propagation of action potentials. These channels open or close in response to changes in membrane potential.
When a membrane potential reaches the threshold due to a stimulus, these channels open rapidly, allowing Na+ ions to rush into the neuron.
This creates a massive change in the cell's electrical state, which rapidly depolarizes the membrane and initiates the action potential.

• They are voltage-gated, meaning that their opening is dependent on the membrane reaching the threshold potential.
• Once open, they allow a sudden influx of Na+ ions, reversing the membrane potential from negative to positive.
• This positive shift is what constitutes the depolarization phase of the action potential.

Immediately following this, the sodium channels close and cannot reopen until the neuron's membrane returns to its resting potential, allowing the action potential to propagate only in one direction.