GRE Subject Test: Biology : Understanding Action Potentials and Ion Channels

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Action Potentials And Ion Channels

Which of the following is most closely linked to neuron hyperpolarization?

Possible Answers:

A membrane potential of –65mV

Absolute refractory period

Potassium efflux

Sodium influx

Correct answer:

Potassium efflux

Explanation:

The main stages of an action potential are depolarization, hyperpolarization, and repolarization. The resting membrane potential of the cell is roughly –65mV. During depolarization the neuron initiates the action potential by opening voltage-gated sodium channels. This allows an influx of sodium ions, which raises to membrane potential to roughly 50mV. Sodium channels are quick to react to the action potential stimulus, but voltage-gated potassium channels are slower. After the depolarization, the potassium channels open, allowing for a rapid efflux of potassium ions. This causes the membrane potential to rapidly drop, so much so that it becomes more negative than the resting potential. This drop below resting potential is known as hyperpolarization. Repolarization then occurs by action of the sodium-potassium pump, which uses ATP to reestablish the resting potential by removing sodium and importing potassium.

The absolute refractory period occurs when the initial gating mechanism of the sodium channels is activated, making them impervious to stimuli. In contract, the relative refractory period is closely linked to hyperpolarization and describes the period during which the cell can be stimulated, but only if the stimulus is large enough to overcome the hyperpolarized environment and reach threshold.

Example Question #62 : Animal Biology

Which of the following events is correlated with the repolarization of the neuron?

Possible Answers:

The increased rate of the sodium-potassium pump

The closing of voltage-gated potassium channels

The opening of voltage-gated sodium channels

The opening of voltage-gated potassium channels

Correct answer:

The opening of voltage-gated potassium channels

Explanation:

The action potential is composed of key changes in voltage for the neuronal cell body. The resting voltage in the cell is negative, due to the action of the sodium-potassium pump. When an action potential reaches the cell, voltage-gated sodium channels open, and sodium ions rush into the cell. This raises the voltage inside the cell in a process called depolarization.

As the voltage in the cell rises, the sodium channels begin to close, and voltage-gated potassium channels begin to open. As potassium ions exit the cell, the voltage drops back down to negative once again. This process is called repolarization. It takes a bit longer for the potassium channels to close, which causes a temporary hyperpolarization of the cell; however, once they close and the cell will eventually return to the initial negative resting potential by action of the sodium-potassium pump.

Example Question #63 : Animal Biology

Which of the following ions causes the release of neurotransmitters into the synaptic cleft?

Possible Answers:

Sodium ions

Calcium ions

Potassium ions

Chlorine ions

Correct answer:

Calcium ions

Explanation:

There is a larger number of voltage-gated calcium channels near the synaptic cleft on the pre-synaptic neuron. As an action potential approaches the synaptic cleft, these voltage-gated calcium channels open and allow for a rapid influx of calcium. The sudden influx of calcium ions into the cell causes a release of neurotransmitters into the synaptic cleft.

Sodium and potassium play key roles in establishing the resting membrane potential and propagating the action potential, but do not actually stimulate the release of the neurotransmitter.

Example Question #2 : Understanding Action Potentials And Ion Channels

Which of the following compounds works by crossing the synaptic cleft and is involved in triggering contraction of muscles, stimulating the excretion of hormones, and exciting the nervous system?

Possible Answers:

Cortisol

Acetylcholine

GABA

Serotonin

Epinephrine

Correct answer:

Acetylcholine

Explanation:

Neurotransmitters cross the synaptic cleft to cause a change in the excitability of the downstream neuron. Acetylcholine is an excitatory neurotransmitter matching the description in the question stem. GABA is an inhibitory neurotransmitter and serotonin is responsible for feelings of happiness. Cortisol and epinephrine are hormones, not neurotransmitters, thus they are released into the bloodstream, not the synaptic cleft.

Example Question #3 : Understanding Action Potentials And Ion Channels

Repolarization of the neuron is associated with what event in the action potential?

Possible Answers:

Sodium exiting the cell through voltage-gated channels.

Sodium entering the cell through voltage-gated channels.

Potassium entering the cell through voltage-gated channels.

Potassium leaving the cell through voltage-gated channels.

Correct answer:

Potassium leaving the cell through voltage-gated channels.

Explanation:

Repolarization is one of the last steps of an action potential, where the cell potential of the neuron is made to be negative in value once again. This step is accomplished by the opening of voltage-gated potassium channels, which allows for potassium to exit the neuron.

Example Question #4 : Understanding Action Potentials And Ion Channels

Which of the following is true of action potentials in neurons?

Possible Answers:

There is no refractory period for action potentials.

Multiple action potentials can be summed up over time.

The amplitude of the action potential is proportional to the size of the stimulus which caused it.

They are considered an "all or nothing" event.

Correct answer:

They are considered an "all or nothing" event.

Explanation:

Action potentials are unique in that they are a one-way transmission of impulses throughout the nervous system. Action potentials will always be the same amplitude for a given neuron, regardless of the stimulus which caused it; however, the stimulus must be sufficient enough to cross the threshold, or the action potential will not occur. It is because of this feature that action potentials are said to be  "all or nothing" in nature.

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