AP Biology : Understanding Action Potentials

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understanding Action Potentials

Which of the following is true about cells at resting potential?

Possible Answers:

A sodium-potassium pump keeps the membrane polarized

There is a higher concentration of sodium inside of the cell

There is a higher concentration of potassium outside of the cell

They have a resting potential of –30mV relative to the outside of the cell

Correct answer:

A sodium-potassium pump keeps the membrane polarized

Explanation:

By pumping two positively-charged potassium molecules in for every three positively-charged sodium molecules that are pumped out of the cell, the sodium-potassium pump maintains a resting potential of –70mV relative to outside of the cell. This function is important for creating an electrochemical gradient along the neuron.

Remember that sodium flows down its gradient to enter the cell during depolarization, while potassium flows down its gradient to exit a cell after an action potential, causing hyperpolarization during the refractory period.

Example Question #1 : Understanding Action Potentials

The opening of a neuron's voltage-gated sodium channels is followed by all except which of the following actions?

Possible Answers:

Sodium continues to flood in due to a lower concentration of the molecule inside of the membrane

Depolarization, as the membrane potential climbs to +35mV

Opening of the potassium channel to allow for repolarization of the membrane

Action potential is propagated in both directions of the axon

Correct answer:

Action potential is propagated in both directions of the axon

Explanation:

After the sodium channel is opened, sodium rushes into the cell down its concentration gradient (as previously created by the sodium-potassium pump). This causes depolarization of the membrane as its potential reaches a value of +35mV, which is eventually lowered by the opening of the potassium channels. This leads to hyperpolarization, which prevents the signal from traveling backwards.

Example Question #3 : Understanding Action Potentials

Which of the following is characterized by having a membrane potential below –70mV?

Possible Answers:

Resting potential

Threshold

Refractory period

Action potential

Correct answer:

Refractory period

Explanation:

The refractory period, a phase in which action potentials cannot be fired, is the result of hyperpolarization, during which the membrane potential drops below –70mV. The membrane potential is at this –70mV level while the threshold, which needs to be reached to fire action potential, is slightly higher at –50mV. During the period of extreme hyperpolarization, an action potential will not form.

Example Question #4 : Understanding Action Potentials

An action potential will only be initiated if __________.

Possible Answers:

the neuron is depolarized to -60mV

the neuron reaches the threshold potential

the muscle contracts

a stimulus occurs

the muscle reaches the threshold potential

Correct answer:

the neuron reaches the threshold potential

Explanation:

Threshold potential is defined as the potential that must be reached in order for an action potential to be initiated by a neuron. Threshold potential is around -55mV in humans, which is slightly higher than the resting potential of -70mV. Once this threshold is reached, the electrical signal will propagate as the membrane depolarizes to a positive potential.

Sub-threshold stimuli, such as stimulus causing depolarization to -65mV, will not trigger action potentials. Muscle contractions can result from action potentials or provide sensory feedback, but the contractions themselves do not play a role in initiating action potentials.

Example Question #5 : Understanding Action Potentials

Electrical activity in the nervous system is transmitted by impulses known as action potentials. An action potential generally begins when a stimulus reaches the dendrites of a neuron, triggering a number of cell membrane conductivity changes. After the stimulus, there is a period of time during which no second stimulus, no matter how strong, can cause a second action potential. What is the name for this period of time?

Possible Answers:

Apoptosis

Absolute refractory period

Depolarization

Saltatory conduction

Threshold period

Correct answer:

Absolute refractory period

Explanation:

The refractory period is the span of time during which the neuron "recovers" and generally does not respond to a second stimulus as strongly as it did to the first. As the name implies, the absolute refractory period refers to the time when no stimulus, no matter how strong, can provoke a second action potential. This occurs because sodium channels, the opening of which causes depolarization, are sealed by a gating mechanism.

The relative refractory period follows the absolute refractory period. During the relative refractory period, the cell will not respond to normal stimuli, but can generate an action potential if an exceptionally large stimulus occurs. During this period the sodium channels are closed, but not sealed by the gating mechanism; they are essentially normal. The relative refractory period is caused by hyperpolarization as potassium rushes out of the cell after the action potential. Because the potential is lower than normal, only a very large stimulus can overcome the threshold.

Though threshold, saltatory conduction, and depolarization do relate to nervous system potentials, they do not refer to this specific period. Apoptosis is a completely unrelated process referring to a type of cell death.

Example Question #11 : Neural Physiology

During an action potential, why is there a hyperpolarization phase?

Possible Answers:

Negatively charged chloride ions enter the cell to end the action potential

Voltage-gated sodium channels remain open

Voltage-gated potassium channels remain open

The sodium-potassium pump is active

Correct answer:

Voltage-gated potassium channels remain open

Explanation:

At the end of an action potential, the voltage-gated potassium channels are slow to close. This allows both the normal "leaky" potassium channels and the voltage-gated potassium channels to be open simultaneously. Large amounts of potassium are able to flow down their concentration gradient, exiting the cell. The exiting of these positively charged ions results in the negative dip in cell membrane potential, known as hyperpolarization.

During hyperpolarization, voltage-gated potassium channels close and the sodium-potassium pump is activated to return the cell to the resting potential by moving potassium back into the cell, and sodium out of the cell.

Example Question #12 : Neural Physiology

What does "temporal summation" mean in regards to the generation of action potentials?

Possible Answers:

Increased firing rate of an individual neuron to generate an action potential

Adding together the total amount of action potentials produced over a length of time

Activating several unique motor units to generate an action potential 

Over time neurons will produce action potentials with no stimulus

Correct answer:

Increased firing rate of an individual neuron to generate an action potential

Explanation:

Temporal summation refers to the phenomenon that an individual neuron will fire with such a high frequency that previous changes in potential have not yet normalized before a new one begins. This summative effect can cause the generation of an action potential, once the threshold potential is surpassed.

Spatial summation refers to the simultaneous activation of several unique neurons to affect another. Numerous individual inputs sum together on the target neuron to stimulate an action potential.

Example Question #8 : Understanding Action Potentials

What ion is principally responsible for triggering the threshold membrane potential?

Possible Answers:

None of these

Sodium

Potassium

Calcium

Chloride

Correct answer:

Sodium

Explanation:

The resting membrane potential is approximately –70mV, while the threshold potential is roughly –55mV. When a neuron receives a stimulus, the binding of neurotransmitters elicits small, localized influxes of sodium known as postsynaptic potentials. These small potentials must sum together in order to raise the local region of the neuron to –55mV. Once this threshold potential is reached, an action potential is generated and the neuron perpetuates the signal.

Example Question #9 : Understanding Action Potentials

Action of which of the following is directly affected in a malnourished individual?

I. Sodium and potassium leak channels

II. Sodium-potassium pump

III. Voltage-gated sodium and potassium channels

Possible Answers:

III only

I and III

II and III

II only

Correct answer:

II only

Explanation:

The question states that the person is malnourished. This means that he/she is not getting enough nutrients and energy to fuel the body, which directly affects the production of ATP. The correct answer will be a protein that requires energy to transport the molecules (active transport).

Out of the three proteins presented in the question, only one uses ATP to transport molecules: the sodium-potassium pump. It requires ATP because the pump transports sodium (Na) and potassium (K) ions against their respective concentration gradients. The leak channels and the voltage-gated channels use facilitated diffusion and the electrochemical gradients of the ions as the driving force for transport.

Eventually, as ion concentrations fluctuate in the individual, all three types of proteins may be affected, but only as an indirect consequence. Malnourishment will directly affect the available ATP, reducing functionality of the sodium-potassium pump.

Example Question #10 : Understanding Action Potentials

The sodium-potassium pump is an example of which of the following?

Possible Answers:

Symporter because it transports sodium and potassium ions in the same direction

Antiporter because it transports sodium and potassium ions in the same direction

Symporter because it transports sodium and potassium ions in the opposite direction

Antiporter because it transports sodium and potassium ions in the opposite direction

Correct answer:

Antiporter because it transports sodium and potassium ions in the opposite direction

Explanation:

The sodium-potassium pump moves sodium to the outside of the cell and potassium to the inside of the cell. Since the pump moves the ions in opposite directions, the pump is classified as an antiporter. If the ions moved in the same direction it would be classified as a symporter.

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