MCAT Biology : Action Potentials and Synapse Biology

Study concepts, example questions & explanations for MCAT Biology

varsity tutors app store varsity tutors android store

Example Questions

Example Question #11 : Nervous System And Nervous Tissue

In humans, nerve impulses are transmitted with the coordinated action of sodium and potassium ion channels. These channels open in a specific sequence, to allow for membrane potential changes to take place in a directional manner along the length of an axon. 

Figure 1 depicts a single phospholipid layer of a cell membrane, and three transmembrane channels important to action potential propagation.
 

Untitled

The protein labeled "2" in the diagram facilitates repolarization following the peak of an action potential. What ion is most likely to be controlled by this protein channel?

Possible Answers:

Potassium

Sodium

Chloride

Magnesium

Calcium

Correct answer:

Potassium

Explanation:

Potassium is the major species that repolarizes a neuron following depolarization. After sodium has entered the cell to create depolarization, repolarization is driven by potassium ion efflux.

Example Question #92 : Biology

In humans, nerve impulses are transmitted with the coordinated action of sodium and potassium ion channels. These channels open in a specific sequence, to allow for membrane potential changes to take place in a directional manner along the length of an axon. 

Figure 1 depicts a single phospholipid layer of a cell membrane, and three transmembrane channels important to action potential propagation.

 

Untitled

Before any of the voltage-sensitive channels in a neuron open in response to adjacent depolarization, what is true of the the resting membrane potential?

Possible Answers:

It is closest to the sodium equilibrium potential, because of the presence of potassium leak channels.

It is closest to the potassium equilibrium potential, because of the presence of sodium leak channels.

It is exactly midway between the sodium and potassium equilibrium potentials.

It is closest to the sodium equilibrium potential, because of the presence of sodium leak channels.

It is closest to the potassium equillibrium potential, because of the presence of potassium leak channels.

Correct answer:

It is closest to the potassium equillibrium potential, because of the presence of potassium leak channels.

Explanation:

The presence of potassium leak channels in the membrane allows potassium to drive the resting cell membrane potential nearer to its equilibrium potential than to sodium's.

The equilibrium potential is the electric potential that would exaclty balance the competing forces of concentration and electrical gradients. High potassium concentration in the cytosol drives potassium out of leak channels in the membrane, toward the extracellular space, but the inside develops a negative charge as a result. When this negative charge pulling positive potassium ions back in is enough to exactly cancel the concentration forces pushing potassium out, the equilibrium potential has been reached.

Example Question #11 : Action Potentials And Synapse Biology

In humans, nerve impulses are transmitted with the coordinated action of sodium and potassium ion channels. These channels open in a specific sequence, to allow for membrane potential changes to take place in a directional manner along the length of an axon. 

Figure 1 depicts a single phospholipid layer of a cell membrane, and three transmembrane channels important to action potential propagation.

 

Untitled

The protein labeled "3" is an active transport pump that restores the normal balance of sodium and potassium every time an action potential travels through the region of the axon. What is this pump most likely to transport?

Possible Answers:

Two sodium out of the cell and two potassium into the cell

Two sodium out of the cell and three potassium into the cell

Three sodium into the cell and two potassium out of the cell

Two sodium into the cell and three potassium out of the cell

Three sodium out of the cell and two potassium into the cell

Correct answer:

Three sodium out of the cell and two potassium into the cell

Explanation:

The sodium-potassium pump, or Na/K ATPase, is what restores ionic concentrations back to normal after an action potential. This pump is electrogenic, and active, using ATP to pump three sodium out of the cell, and two potassium into the cell. Along wtih the potassium leak channels, this keeps the potassium concentration in a cell high, and sodium concentration low.

Example Question #11 : Action Potentials And Synapse Biology

In humans, nerve impulses are transmitted with the coordinated action of sodium and potassium ion channels. These channels open in a specific sequence, to allow for membrane potential changes to take place in a directional manner along the length of an axon. 

Figure 1 depicts a single phospholipid layer of a cell membrane, and three transmembrane channels important to action potential propagation.
 

Untitled

The refractory period is the period of time after action potential that a neuron is unable to "refire" if another stimulus is present.

If protein 1 is a voltage-gated sodium channel, protein 2 is a voltage-gated potassium channel, and protein 3 is a leak channel, which channel contributes most to the absolute refractory period?

Possible Answers:

Protein 3, due to its lack of an inactivation gate

Potassium channel, because its lacks an activation gate

Protein 1, due to its inactivation gate

Leak channel, due to its inactivation gate

Protein 2, due to its activation gate

Correct answer:

Protein 1, due to its inactivation gate

Explanation:

The sodium channel being inactivated, via its inactivation gate, prevents a stimulus from initiating an action potential immediately after a previous stimulus.

During the absolute refractory period, this is a fact regardless of how strong the stimulus is. During the relative refractory period the neuron can be stimulated, but only by a very large stimulus.

Example Question #95 : Biology

In humans, nerve impulses are transmitted with the coordinated action of sodium and potassium ion channels. These channels open in a specific sequence, to allow for membrane potential changes to take place in a directional manner along the length of an axon. 

Figure 1 depicts a single phospholipid layer of a cell membrane, and three transmembrane channels important to action potential propagation.

 

Untitled

The cell body associated with the axon in Figure 1 is actively taking in electrical inputs from neighboring cells. Which of the following properties is the major difference between post-synaptic potentials from neighboring neurons and pre-synaptic action potentials?

Possible Answers:

Post-synaptic potentials are "all-or-nothing;" action potentials are graded

Post-synaptic potentials are always excitatory; action potentials are always inhibitory

Post-synaptic potentials are driven by calcium depolarization; action potentials are driven by sodium depolorization

Post-synaptic potentials are always inhibitory; action potentials are always excitatory

Post-synaptic potentials are graded; action potentials are "all-or-nothing"

Correct answer:

Post-synaptic potentials are graded; action potentials are "all-or-nothing"

Explanation:

Post-synaptic potentials are graded, while action potentials are "all-or-nothing". This means that the farther from the point of integration in a nerve cell an electrical input enters, the weaker its corresponding post-synaptic potential will be when it reaches the distant integration site.

In this way, post-synaptic potentials can be summed as a function of intensity and distance, while action potentials are always the same amplitude no matter from how far they travel.

Example Question #21 : Neurons And Action Potential

The heart contains autorhythmic cells, which can generate an action potential on their own. These cells then spread the action potential throughout the heart, resulting in a contraction. Which of the following mechanisms is an explanation for why these cells can spontaneously generate action potentials?

Possible Answers:

These cells have no resting potential

Specialized channels allow sodium to exit the cell, which leads to depolarization

Specialized channels allow sodium to enter the cell, which leads to depolarization

These cells do not have sodium-potassium pumps, which allows for quicker depolarization

Correct answer:

Specialized channels allow sodium to enter the cell, which leads to depolarization

Explanation:

Remember that an action potential starts with the diffusion of sodium into the cell. As more sodium enters the cell, more voltage gated sodium channels open up. This leads to depolarization of the cell. With a steady diffusion of sodium into the cell, the threshold stimulus will eventually be attained, and an action potential will be generated. It is the steady diffusion of sodium into the autorhythmic cells which results in an action potential.

Example Question #22 : Neurons And Action Potential

In humans, nerve impulses are transmitted with the coordinated action of sodium and potassium ion channels. These channels open in a specific sequence, to allow for membrane potential changes to take place in a directional manner along the length of an axon. 

Figure 1 depicts a single phospholipid layer of a cell membrane, and three transmembrane channels important to action potential propagation.

 

Untitled

A dendrite carries an electrical signal to the nerve cell body associated with the axon in Figure 1. If this signal is inhibitory (an inhibitory post synaptic potential), which of the following is likely true of the impact of this signal on the cell?

Possible Answers:

It will hyperpolarize the cell, and make it more difficult to fire

It will hyperpolarize the cell, and make it less difficult to fire

It will not change the cell membrane potential, but will make it more difficult to fire

It will depolarize the cell, and make it more difficult to fire

It will depolarize the cell, and make it less difficult to fire

Correct answer:

It will hyperpolarize the cell, and make it more difficult to fire

Explanation:

An inhibitory post synaptic potential (IPSP) drives the post synaptic cell membrane toward hyperpolarization, and thus away from the threshold necessary to fire an action potential. As a result, the axon requires more stimuli in order to fire an action potential.

Example Question #23 : Neurons And Action Potential

An action potential travels down a neuronal axon. Which of the following is occurring during depolarization of the neuron?

Possible Answers:

Sodium flows into of the cell down its electrical gradient, but against its chemical gradient

Potassium flows into the neuron down its electrical and chemical gradients

Sodium flows out of the neuron down its electrical and chemical gradients

Sodium flows into the neuron down its electrical and chemical gradients

Correct answer:

Sodium flows into the neuron down its electrical and chemical gradients

Explanation:

It is important to recognize that sodium is flowing into the neuron during depolarization. The area outside of the neuron is electrically positive relative to the area inside of the neuron, resulting in the negative resting membrane potential of the cell. This potential allows positively charged sodium ions to flow from a high concentration of positive charge, towards the negative charge in the cell interior. Because the sodium travels from a region of relatively positive charge to a region of relatively negative charge, it is flowing down its electrical gradient.

Due to action by the sodium-potassium pump, there is also a large concentration of sodium ions outside of the cell, relative to the small sodium ion concentration inside the cell. This imbalance creates a chemical gradient across the axon membrane. The opening of the voltage-gated sodium channels during depolarization allows sodium to flow down chemical gradient from high ion concentration to low ion concentration.

Example Question #24 : Neurons And Action Potential

Which of the following correctly pairs neuron structure with function?

Possible Answers:

The potassium-calcium pump reestablishes the membrane resting potential of -70 mV

Sodium leaky channels allow the passive release of sodium from the cell

Voltage-gated calcium channels cause depolarization

Voltage-gated potassium channels actively export potassium out of the cell

Voltage-gated sodium channels allow the influx of sodium into the cell

Correct answer:

Voltage-gated sodium channels allow the influx of sodium into the cell

Explanation:

Voltage-gated calcium channels do not cause depolarization in neurons, but are integral to depolarization in muscle. Voltage-gated sodium channels are responsible for neural depolarization; there are no sodium leaky channels in neurons, as these would disrupt the resting potential. Voltage-gated potassium channels actively import potassium, whereas the sodium-potassium pump actively exports potassium. There is no such thing a potassium-calcium pump.

Example Question #121 : Mcat Biological Sciences

During an action potential, depolarization is associated with which of the following?

Possible Answers:

The closing of voltage-gated sodium ion channels

Decreased membrane potential

Endocytosis of neurotransmitters

An influx of sodium ions

Correct answer:

An influx of sodium ions

Explanation:

During depolarization, voltage-gated sodium channels open and allow a rapid influx of sodium ions. The membrane voltage rises from its resting potential of -70 mV to 35 mV. Depolarization is not associated with endocytosis of neurotransmitters. 

Learning Tools by Varsity Tutors