Question

A neuron has reached a threshold potential when it has depolarized to the point where a. the voltage-gated Na \(^{+}\) channels have become inactivated. b. sufficient numbers of voltage-gated Na \(^{+}\) channels open to initiate a positive feedback cycle, contributing to further depolarization. c. voltage-gated \(\mathrm{K}^{+}\) channels close. d. voltage-gated Na \(^{+}\) channels close. e. both b and c occur.

Short answer

The correct answer is option b. When the threshold potential of a neuron is reached, a sufficient number of voltage-gated Na\(^{+}\) channels open, initiating a positive feedback cycle that contributes to further depolarization.

Step-by-step solution

Understanding the role of Na\(^{+}\) and K\(^{+}\) during an action potential

During the resting state, the neuron's interior has a negative charge relative to the outside. When an action potential is triggered, voltage-gated sodium channels (Na\(^{+}\)) open, allowing Na\(^{+}\) ions to rush in due to the concentration gradient, causing a depolarization. When the membrane potential reaches about +40 mV, the Na\(^{+}\) channels close, becoming inactivated. At the same time, voltage-gated potassium (K\(^{+}\)) channels open, allowing K\(^{+}\) to rush out of the cell, causing repolarization.

Analyzing multiple choices

Now, let's analyze the available choices based on what we know about action potentials. Choice a is incorrect because the inactivation of Na\(^{+}\) channels occurs after the threshold potential has been reached, not when it is reached. Choice c and d are also incorrect because these events do not occur when the threshold potential is reached; K\(^{+}\) channels open (not close) and Na\(^{+}\) channels open (not close) at the threshold. Regarding choice e, as we've dismissed choice c as false, obviously e cannot be correct either.

Identifying the correct choice

The correct answer is choice b—it correctly describes the sequence of events during an action potential. When the threshold potential is reached, sufficient numbers of voltage-gated Na\(^{+}\) channels open, initiating a positive feedback cycle contributing to further depolarization.

Key concepts

Action Potential

An action potential is a rapid electrical impulse that travels along the membrane of a neuron. Before diving into the complexities of an action potential, it's important to understand the resting state of a neuron. The inside of a neuron has a negatively charged environment, while the outside is positively charged, a difference known as the resting membrane potential. An action potential occurs when a neuron sends information down an axon, away from the cell body. A stimulus causes the resting potential to move towards 0 mV. When the depolarization reaches about -55 mV, a level known as the threshold potential, an action potential will fire.

This leads to the opening of voltage-gated Na⁺ channels and the influx of Na⁺ ions, which causes further depolarization. The inside of the neuron briefly becomes positively charged, reversing the polarity. This process propagates down the axon, leading to the transmission of the nerve impulse. After the peak is reached, repolarization occurs. This is the part of the action potential when the internal charge of the neuron returns to a more negative value, eventually returning to the resting membrane potential.

Voltage-Gated Sodium Channels

Voltage-gated sodium (Na⁺) channels are integral proteins in the neuron's cell membrane. They are responsible for the initial phase of the action potential. These channels are sensitive to changes in membrane potential. In their closed state during the resting potential period, they prevent Na⁺ ions from entering the neuron.

Once the membrane potential depolarizes to the threshold level, these channels open rapidly, allowing Na⁺ ions to flow into the cell. This influx causes further depolarization, leading to the rising phase of the action potential. After a brief opening, the channels become inactivated, which prevents more Na⁺ ions from entering and thus contributes to the eventual repolarization of the neuron as the action potential progresses.

Depolarization

Depolarization is a critical process in the function of neurons. It refers to a change in the neuron's membrane potential, making it less negative in comparison to the resting potential. This usually results from the opening of voltage-gated Na⁺ channels and the subsequent influx of Na⁺ ions.

In neurophysiology, depolarization is the first phase of the action potential. The entry of positively charged ions makes the inside of the neuron more positive and propels the membrane potential towards the positive direction. This change in voltage triggers adjacent channels to open, resulting in a wave-like effect of depolarization that moves along the axon.

Repolarization

Repolarization is the process by which the membrane potential of a neuron returns to a negative value after the depolarization phase of the action potential. It occurs primarily due to the opening of voltage-gated K⁺ channels and the subsequent outflow of K⁺ ions from the neuron.

This outflow begins when the membrane potential reaches its peak positive value, usually at around +40 mV, as the inactivated Na⁺ channels close. K⁺ moving out of the cell helps restore the membrane potential back to its resting state. Notably, sometimes the repolarization can overshoot, leading to a temporarily more negative membrane potential than the normal resting potential, a phase called hyperpolarization.

Neurophysiology

Neurophysiology is the branch of neuroscience that studies the function of the nervous system. It often involves exploring how electrical and chemical signals are processed and transferred by neurons. Understanding neurophysiology is key to grasping concepts like the action potential, depolarization, and repolarization.

Through neurophysiology, one learns about the dynamics of the neurons' resting membrane potential and how changes in this potential lead to neural communication. It elucidates the role of different ion channels, synaptic transmission, and the way nerves interact with muscles and organs. Knowledge of neurophysiology provides the foundational understanding needed to explore more complex neurological functions and disorders.