Test: Neuron Membrane Potentials - 1 - MCAT MCQ

A cation is an ion that has a positive charge. It is formed when an atom or molecule loses one or more electrons. The loss of electrons creates an imbalance between the number of protons (positively charged particles) and electrons (negatively charged particles), resulting in a net positive charge on the ion.

The term that describes the cell membrane potential of a neuron at rest is polarized. When a neuron is at rest, the inside of the neuron has a negative charge relative to the outside. This difference in charge across the cell membrane is known as the resting membrane potential, and it is maintained through the activity of ion channels and ion pumps.

At rest, the concentration of potassium ions (K+) is higher inside the neuron compared to the extracellular environment. This concentration gradient drives the passive movement of potassium ions out of the cell through specific potassium channels. This movement helps to maintain the resting membrane potential by balancing the tendency of potassium ions to diffuse out of the cell.

The sodium-potassium pump (option B) is responsible for actively transporting potassium ions back into the cell and sodium ions out of the cell, but it is not the primary mechanism for moving potassium ions out of the cell when the membrane is at rest. I apologize for the confusion in my previous response.

During an action potential, the neuron undergoes a rapid and transient depolarization phase. This depolarization is caused by the opening of voltage-gated sodium channels, allowing the influx of sodium ions into the cell. This rapid depolarization is the key characteristic of an action potential.

Therefore, the correct answer is option C: Rapid depolarization.

Absolute and relative refractory periods are important aspects of action potentials.

During an action potential, there are specific periods of time where the neuron is unresponsive or less responsive to further depolarization and the generation of another action potential. These periods are known as refractory periods.

The absolute refractory period is the initial phase of the refractory period where the neuron is completely unresponsive to further stimulation. This period coincides with the depolarization and repolarization phases of the action potential.

The relative refractory period follows the absolute refractory period. During this phase, the neuron is still in a refractory state but can be stimulated with a stronger-than-normal stimulus to generate another action potential.

Therefore, the correct answer is option B: Action potentials.

Graded potentials can be either depolarizing or hyperpolarizing, depending on the specific circumstances and the type of ion channels involved. They can result in either a decrease (hyperpolarization) or an increase (depolarization) in the membrane potential.

Therefore, the correct answer is option A: Graded potentials are always hyperpolarizing, whereas action potentials are always depolarizing.

During the peak of an action potential, the membrane potential is not exactly equal to the Na⁺ equilibrium potential but rather slightly less positive. This is because the voltage-gated Na⁺ channels begin to close, leading to a decrease in the influx of Na⁺ ions and a subsequent repolarization of the membrane. The membrane potential returns to a negative value during the repolarization phase. Thank you for pointing out the error.

If the nodes of Ranvier are far apart in a myelinated axon, the distance between the sites where action potentials are generated is increased. This can lead to a phenomenon known as "saltatory conduction," where the action potential "jumps" from one node to another. If the distance between nodes is too large, the action potential may fail to propagate along the axon, resulting in a loss of signal transmission and a potential stoppage of action potentials. 

An action potential is a rapid and brief electrical signal that travels along the axon of a neuron. The axon is the long, slender extension of a neuron that carries the action potential away from the cell body (soma) towards the axon terminals, where it can transmit the signal to other neurons or target cells. The specialized properties of the axon, including its myelin sheath and nodes of Ranvier, enable efficient and rapid transmission of action potentials along its length.

Saltatory conduction refers to the rapid conduction of action potentials in myelinated axons. In a myelinated axon, the myelin sheath acts as an insulating layer, preventing the leakage of electrical charge across the membrane. The action potential "jumps" from one node of Ranvier to the next, where the myelin sheath is interrupted, rather than propagating continuously along the entire length of the axon. This saltatory conduction allows for faster and more efficient transmission of the action potential, as it skips the depolarization and repolarization steps in the myelinated regions.