Question 1

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

Accepted Answer

The threshold potential of a membrane refers to the minimum depolarization needed to activate voltage-gated sodium and potassium channels and trigger an action potential. This depolarization must be sufficient to overcome the resting membrane potential and reach the threshold potential. Therefore, choice C is the correct answer. Choices A, B, and D do not accurately describe the threshold potential of a membrane.

Question 2

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

Accepted Answer

When voltage-gated Na⁺ channels are opened in neurons, the Na⁺ concentration is much higher outside the cell than it is inside, and the Na⁺ ions are attracted to the negatively charged interior. This results in an influx of Na⁺ ions into the cell, causing depolarization and the initiation of an action potential. The sodium-potassium pump actively transports Na⁺ ions out of the cell and K⁺ ions into the cell, but this process is not responsible for the initial influx of Na⁺ ions during an action potential.

Question 3

Which of the following statements about action potentials is correct?

Accepted Answer

Action potentials are propagated down the length of the axon through the opening and closing of ion channels, allowing for the movement of ions (such as sodium and potassium) in and out of the neuron. Option A and B are incorrect as action potentials have a consistent magnitude and duration for a given neuron. Option D is incorrect as movement of ions during the action potential occurs mostly through ion channels, not solely through the sodium pump.

Question 4

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 5

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 because the myelin sheath acts as an insulator, preventing ion leakage and allowing the action potential to jump from node to node (saltatory conduction). Thus, the impulse travels faster along the axon. Thin, unmyelinated neurons have the slowest conduction velocity.

Question 6

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

Accepted Answer

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

Question 7

Action potentials move along axons ________.

Accepted Answer

Myelinated axons have a myelin sheath, which acts as an insulator, allowing for faster conduction of action potentials between nodes of Ranvier. Unmyelinated axons do not have this insulation, resulting in slower conduction. Additionally, action potentials move by depolarizing the membrane at one point, causing neighboring ion channels to open and depolarize the next segment of membrane, in a chain reaction. This does not involve the sodium-potassium "pump" (B) or reversing ion gradients (D). The size of the axon (A) does affect conduction speed, but myelination has a greater impact.

Question 8

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

Accepted Answer

After depolarization and the influx of sodium ions, the membrane potential becomes more positive. The restoration of the resting potential is mainly due to the opening of voltage-gated potassium channels, which allows potassium ions to leave the cell and return the membrane potential to its negative resting state. Additionally, the inactivation of sodium channels also contributes to the restoration of the resting potential.

Question 9

Why are action potentials usually conducted in one direction?

Accepted Answer

During the brief refractory period after an action potential, the voltage-gated sodium channels are temporarily inactivated and cannot be immediately reopened. This prevents backward flow of the depolarization wave and ensures that action potentials are conducted in one direction.

Question 10

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

Accepted Answer

The undershoot phase of hyperpolarization occurs after the peak of the action potential, and it is caused by the sustained opening of voltage-gated potassium channels. As these channels remain open, potassium ions continue to flow out of the cell, making the inside even more negative than the resting membrane potential. This undershoot phase is also known as afterhyperpolarization.

Question 11

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

Myelin is a fatty substance that wraps around axons and allows for faster conduction of action potentials. The fact that action potentials pass more rapidly along the fish's neurons suggests that the fish's axons are wrapped in myelin, which is not the case for the crab. Option A is incorrect because larger axons transmit action potentials more rapidly than small axons. Option C is incorrect because more ion channels would result in greater sensitivity to stimuli, but would not necessarily enhance the speed of action potential transmission. Option D is partially correct, but the explanation provided for option B is sufficient.

Question 12

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 would result in hyperpolarization of the neuron. This is because more positively charged potassium ions are leaving the cell, making the inside of the cell more negative and further away from the threshold needed to trigger an action potential.

Question 13

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 the cell will result in a more positive intracellular environment, as K⁺ has a positive charge. This will cause the membrane potential of the cell to become less negative as the driving force for K⁺ to leave the cell decreases. Therefore, the correct answer is B.

Question 14

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

Accepted Answer

Label B shows the phase of the action potential where the cell is not hyperpolarized but repolarization is in progress, as the sodium channels are inactivated or becoming inactivated, and many potassium channels have opened. This is the beginning of the falling phase of the action potential.

Question 15

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

Accepted Answer

By preventing inactivation of the voltage-gated sodium channels, pyrethroids prolong the depolarization phase of the action potential and prevent repolarization, which can lead to neuronal hyperexcitation and death. This means that neurons exposed to pyrethroids would not repolarize during an action potential.

Question 16

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

Accepted Answer

Increasing the membrane's permeability to K⁺ causes an efflux of K⁺ ions from the cell, leading to hyperpolarisation (increased negative charge inside the cell). Increasing permeability to Na⁺ would cause depolarisation (decreased negative charge inside the cell) and increasing permeability to Ca⁺⁺ or decreasing permeability to Cl- would not directly lead to hyperpolarisation.

Question 17

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 18

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

Accepted Answer

Conduction and refractory periods are characteristic of action potentials, which are brief electrical signals that travel along the axon of a neuron. Graded hyperpolarisations and excitatory postsynaptic potentials are changes in the membrane potential that do not involve conduction along the axon, whereas threshold potentials are the membrane potential required to initiate an action potential.

Question 19

Action potentials are normally carried in only one direction: from the axon hillock toward the axon terminals. If you experimentally depolarise 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 because the depolarization can spread both ways from the point of initiation.

Question 20

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

Nodes of Ranvier are essential for the propagation of the action potential in a myelinated neuron. They allow for the renewal of the action potential signal by opening sodium channels and depolarizing the membrane at regular intervals. Without the nodes of Ranvier, the signal would gradually fade as it moves down the axon, leading to a loss of signal strength at the synapse. Therefore, option C is the best choice.

Quiz 48: Neurons, Synapses, and Signalling