Question

1. A scientist stimulates an axon two times within fifteen milliseconds. When measuring voltage changes on the other end of the axon, they only see one action potential. Why does this happen?

a. The membrane does not have enough time to return to its resting potential, and so no action potential can be initiated.

b. Voltage-gated Na+ channels are still inactivated after the first action potential, and so a second action potential can’t be initiated.

c. Na+/K+ pumps are actively pumping Na+ out of the axon, which prevents the axon from reaching threshold.

d. Voltage-gated K+ channels are opposing the flow of current from the electrode, preventing the axon from reaching threshold.

Answer 1

ANSWER :-

CORRECT ANSWER

The most appropriate answer is option 'B' which indicates that the sodium ion channels remain inactivated due to which the second action potential is not being generated. It should be noted that there are two types of sodium channels out of which one can undergo the process of depolarization and the second one is associated with inactivation or in other words it closes and do not take in sodium ions into the cell leading to depolarization. After the threshold membrane potential is reached, there will be process of depolarization which occurs during which sodium ion channels gets activated and there will be entry of sodium ions into the cell with the subsequent removal of potassium ions. After some time, the second type of sodium channel that has been mentioned above becomes inactivated which is actually associated with the effects that one can see with respect to depolarization. If the cell becomes hyperpolarized again followed by removal of potassium ions out of cell, followed by subsequent action potential, the ion channel still tend to remain in the inactivated state suggesting that the depolarization event will not take place until the inactivated ion channel again becomes activated. This is called as Absolute Refractive Period (ARP). Hence, option 'B' is the most appropriate answer.

INCORRECT OPTIONS

1. The action potential can be maintained for around 1-2 ms and then returns back to the resting membrane potential which indicates that although, the hyperpolarization event takes in a timely manner, there is no dependence of lengthening of the action potential by means of not reverting back to resting membrane potential taking into consideration the types of channels that are actively involved in the regulatory process. Hence, option 'A' is not the most appropriate answer.
2. There are ion channels that pump ions out of the neuron but in a limited manner. The functioning of the neuron in terms of action potential is mainly dependent upon the depolarization and the repolarization event mediated in most of the cases by the two ions namely the sodium and the potassium ions. If sodium ions are getting pumped out of the neuron, there will be no more action potential which will be generated but this indicates an error in the processing of the ion channels. This is not the case in the information provided above and hence option 'C' is not the most appropriate answer.
3. Potassium ions cannot interact with the charges associated with the electrodes used for generation into the axon of the squid. This cannot act as a barrier but a mediator of hyperpolarization and the intensity of the charge determines the amount of stimulation of the neuron. This indicates that the barrier is not created by potassium ion but depolarization event meinly depends upon intensity of stimulation. Hence, option 'D' is not the most appropriate answer.