Question 1

Opening all of the sodium channels on an otherwise typical neuron, with all other ion channels closed (which is an admittedly artificial setting), should move its membrane potential to ________.

Accepted Answer

Opening all of the sodium channels on a neuron would move the membrane potential to the equilibrium potential for sodium, which is around +50 mV. This is because the concentration gradient for sodium would drive sodium ions into the cell until the electrical gradient, which is opposing the movement of positively charged ions into the cell, is equal and opposite to the concentration gradient. At this point, there would be no net movement of sodium ions and the membrane potential would be at the equilibrium potential for sodium.

Question 2

Action potentials are normally carried in only one direction: from the axon hillock toward the axon terminals. If you experimentally depolarize the middle of the axon to threshold, using an electronic probe, then ________.

Accepted Answer

Depolarizing the middle of the axon to threshold will initiate action potentials in both directions: toward the axon terminal and back toward the axon hillock, as there is no refractory period preventing backward propagation.

Question 3

The fastest possible conduction velocity of action potentials is observed in ________.

Accepted Answer

Thick, myelinated neurons have the fastest possible conduction velocity of action potentials due to their ability to conduct saltatory conduction, where the action potential jumps from one node of Ranvier to the next, rather than traveling the entire length of the axon. This increases the speed of conduction compared to unmyelinated neurons and thin myelinated neurons.

Question 4

If you experimentally increase the concentration of K⁺ inside a cell while maintaining other ion concentrations as they were, what would happen to the cell's membrane potential?

Accepted Answer

Increasing the concentration of K⁺ inside a cell would result in a higher concentration of positively charged ions inside the cell, causing the membrane potential to become less negative. This is because K⁺ ions would be attracted to the negative charge of the cell's interior, leading to more positive charges inside the cell and decreasing the potential difference across the membrane.

Question 5

Why are action potentials usually conducted in one direction?

Accepted Answer

During the brief refractory period, the voltage-gated sodium channels cannot be reopened, which prevents the backward propagation of action potentials. Thus, action potentials can only be conducted in one direction.

Question 6

Conduction and refractory periods (states) are typical of ________.

Accepted Answer

Conduction and refractory periods are both characteristics of action potentials, which are the main form of electrical signaling in neurons. Graded hyperpolarizations and excitatory postsynaptic potentials (EPSPs) do not have distinct conduction or refractory periods like action potentials do. Threshold potentials are important for generating action potentials but are not the same as conduction or refractory periods.

Question 7

After the depolarization phase of an action potential, the resting potential is restored by ________.

Accepted Answer

During depolarization, sodium channels open, allowing sodium ions to rush into the cell, and the membrane potential becomes more positive. To restore the resting potential, voltage-gated potassium channels open, allowing potassium ions to move out of the cell, while voltage-gated sodium channels become inactivated. This movement of ions helps to re-establish the negative resting potential. Therefore, the opening of voltage-gated potassium channels and the inactivation of sodium channels is required for the restoration of the resting potential. Choices B, C, and D are incorrect because they do not describe the process involved in repolarization.

Question 8

The "undershoot" phase of hyperpolarization is due to ________.

Accepted Answer

The undershoot phase of hyperpolarization is due to sustained opening of voltage-gated potassium channels. This allows for an efflux of potassium ions, leading to a more negative membrane potential.

Question 9

Refer to the following graph of an action potential to answer the question.

The neuronal membrane is at its resting potential at label ________.

Accepted Answer

The answer of Refer to the following graph of an...

Question 10

Which of the following statements about action potentials is correct?

Accepted Answer

Action potentials are propagated down the length of the axon. They do not vary in magnitude or duration within a given neuron, as they are all-or-nothing events. Movement of ions during the action potential occurs through ion channels, not the sodium pump.

Question 11

A graded hyperpolarization of a membrane can be induced by ________.

Accepted Answer

Increasing membrane permeability to K⁺ allows more K⁺ ions to leave the cell, creating a negative charge inside the cell and resulting in a graded hyperpolarization. Increasing permeability to Na⁺ or Ca⁺⁺ would result in depolarization, and decreasing permeability to Cl⁻ would have little effect on membrane potential.

Question 12

Refer to the following graph of an action potential to answer the question.

At label ________, the cell is not hyperpolarized; however, repolarization is in progress, as the sodium channels are inactivated or becoming inactivated, and many potassium channels have opened.

Accepted Answer

Label B represents the period of repolarization where the cell is not hyperpolarized yet and sodium channels are inactivated or becoming inactivated while many potassium channels have opened.

Question 13

A neurophysiologist is investigating nerve reflexes in two different animals: a crab and a fish. Action potentials are found to pass more rapidly along the fish's neurons. What is the most likely explanation?

Accepted Answer

The presence of myelin around axons provides insulation and increases the speed of action potential propagation. This is likely the reason for the faster nerve reflexes in fish compared to crabs.

Question 14

Refer to the following graph of an action potential to answer the question.

The minimum graded depolarization needed to operate the voltage-gated sodium and potassium channels is indicated by the label ________.

Accepted Answer

Label A indicates the threshold potential, which is the minimum graded depolarization needed to activate the voltage-gated sodium channels and trigger an action potential.

Question 15

For a neuron with an initial membrane potential at -70 mV, an increase in the movement of potassium ions out of that neuron's cytoplasm would result in the ________.

Accepted Answer

An increase in the movement of potassium ions out of the neuron's cytoplasm will result in more negative charge leaving the neuron, causing the membrane potential to become more negative. This is known as hyperpolarization.

Question 16

Why do Na⁺ ions enter the cell when voltage-gated Na⁺ channels are opened in neurons?

Accepted Answer

Voltage-gated Na⁺ channels open in response to a depolarization of the cell membrane, which means the inside of the cell becomes less negative. Because the Na⁺ concentration is much higher outside the cell than it is inside, the Na⁺ ions are attracted to the negatively charged interior and move into the cell through the open channels. The sodium-potassium pump is not involved in this process as it actively transports Na⁺ ions out of the cell and K⁺ ions into the cell.

Question 17

Action potentials move along axons ________.

Accepted Answer

Myelination allows for saltatory conduction, where action potentials jump between the nodes of Ranvier, increasing the speed of propagation along the axon. Thus, action potentials move more rapidly in myelinated than in unmyelinated axons.

Note: A is incorrect, as larger diameter axons actually conduct action potentials more rapidly. B is incorrect, as the sodium-potassium pump is not involved in generating action potentials but rather in restoring the resting membrane potential after an action potential. D is incorrect, as action potentials move along axons by opening and closing voltage-gated ion channels, not by reversing concentration gradients.

Question 18

What would probably happen if a long neuron had one continuous myelin sheath down the length of the axon with no nodes of Ranvier?

Accepted Answer

Without nodes of Ranvier, the signal would fade because it is not renewed by the opening of more sodium channels. The myelin sheath decreases resistance to current flow across the membrane, so the signal would travel faster, but without the regeneration of the signal at the nodes of Ranvier, the signal would dissipate along the length of the axon.

Question 19

Pyrethroid insecticides prevent the voltage-gated sodium channels of insects from inactivating. Neurons that were exposed to pyrethroids would ________.

Accepted Answer

Pyrethroid insecticides prevent the voltage-gated sodium channels of insects from inactivating. This means that neurons exposed to pyrethroids will not repolarize during an action potential.

Question 20

The "threshold" potential of a membrane is the ________.

Accepted Answer

The threshold potential is the minimum depolarization needed to operate the voltage-gated sodium and potassium channels, leading to the initiation of an action potential. Choice A is incorrect because the threshold potential does not determine the frequency of action potentials, but rather whether they will occur at all. Choice B is incorrect because hyperpolarization, not depolarization, is required to prevent action potentials. Choice D is incorrect because the threshold potential is not a measure of peak depolarization, but rather the minimum amount of depolarization needed to initiate an action potential.

Quiz 48: Neurons, Synapses, and Signaling

Opening all of the sodium channels on an otherwise typical neuron, with all other ion channels closed (which is an admittedly artificial setting) , should move its membrane potential to ________.

Action potentials are normally carried in only one direction: from the axon hillock toward the axon terminals. If you experimentally depolarize the middle of the axon to threshold, using an electronic probe, then ________.

The fastest possible conduction velocity of action potentials is observed in ________.

If you experimentally increase the concentration of K⁺ inside a cell while maintaining other ion concentrations as they were, what would happen to the cell's membrane potential?

Why are action potentials usually conducted in one direction?

Conduction and refractory periods (states) are typical of ________.

After the depolarization phase of an action potential, the resting potential is restored by ________.

The "undershoot" phase of hyperpolarization is due to ________.

Refer to the following graph of an action potential to answer the question. The neuronal membrane is at its resting potential at label ________.

Which of the following statements about action potentials is correct?

A graded hyperpolarization of a membrane can be induced by ________.

Refer to the following graph of an action potential to answer the question. At label ________, the cell is not hyperpolarized; however, repolarization is in progress, as the sodium channels are inactivated or becoming inactivated, and many potassium channels have opened.

A neurophysiologist is investigating nerve reflexes in two different animals: a crab and a fish. Action potentials are found to pass more rapidly along the fish's neurons. What is the most likely explanation?

Refer to the following graph of an action potential to answer the question. The minimum graded depolarization needed to operate the voltage-gated sodium and potassium channels is indicated by the label ________.

For a neuron with an initial membrane potential at -70 mV, an increase in the movement of potassium ions out of that neuron's cytoplasm would result in the ________.

Why do Na⁺ ions enter the cell when voltage-gated Na⁺ channels are opened in neurons?

Action potentials move along axons ________.

What would probably happen if a long neuron had one continuous myelin sheath down the length of the axon with no nodes of Ranvier?

Pyrethroid insecticides prevent the voltage-gated sodium channels of insects from inactivating. Neurons that were exposed to pyrethroids would ________.

The "threshold" potential of a membrane is the ________.