MCAT Biology : Action Potentials and Synapse Biology

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1 : 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

At the distal end of the axon shown in Figure 1, what process directly drives the fusion of synaptic vesicles to discharge neurotransmitter into the synaptic cleft?

Possible Answers:

The influx of potassium at the synaptic terminal

The influx of chloride at the synaptic terminal

The influx of magnesium at the synaptic terminal

The influx of calcium at the synaptic terminal

The influx of sodium at the synaptic terminal

Correct answer:

The influx of calcium at the synaptic terminal

Explanation:

Calcium is a very common vehicle that drives membrane fusion, including the fusion of synaptic vesicles with the synaptic cell membrane. This allows the ejection of neurotransmitter into the synaptic cleft.

Example Question #11 : Nervous System And Nervous Tissue

Which of the following is false regarding synapses?

Possible Answers:

In a chemical synapse, neurotransmitters bind to receptors on the postsynaptic neuron

The gap between a presynaptic and a postsynaptic neuron is called a synaptic cleft

The most common type of synapse found in cardiac cells are electrical synapses

In a chemical synapse, there is an influx of calcium ions into the postsynaptic neuron upon neurotransmitter binding

Correct answer:

In a chemical synapse, there is an influx of calcium ions into the postsynaptic neuron upon neurotransmitter binding

Explanation:

There are two types of synapses: the chemical synapse and the electrical synapse. Chemical synapses are more common than electrical synapses, and use neurotransmitters (chemicals) to propagate action potentials. Electrical synapses have tunnels between cells, called gap junctions, that quickly transmit signals from one cell to the other. Electrical synapses are found extensively in the heart, since it is essential to have quick signal transmission between cardiac cells.

In chemical synapses an action potential reaches the end of a presynaptic neuron, which causes neurotransmitters to be released from the presynaptic neuron. These neurotransmitters bind to receptors on the postsynaptic neuron and initiate a signal transduction pathway in the postsynaptic neuron. The space between the presynaptic and postsynaptic neuron is called the synaptic cleft. Synaptic clefts are important regions where neurotransmitters are released and regulated.

Calcium ions play an important role in chemical synapses. Once an action potential arrives at the presynaptic neuron terminal, calcium ion channels on the presynaptic neuron become permeable to calcium ions. This facilitates the movement of calcium ions into the presynaptic neuron. Influx of calcium ions signals the presynaptic neuron to release neurotransmitters into the synaptic cleft, which eventually bind to receptors on the postsynaptic neuron. The calcium ions do not enter the postsynaptic neuron at the synapse.

Example Question #12 : Nervous System And Nervous Tissue

A/An __________ synapse uses gap junctions to transmit signals, and a/an __________ synapse transmits the signals the fastest.

Possible Answers:

chemical . . . chemical

electrical . . . chemical

electrical . . . electrical

chemical . . . electrical

Correct answer:

electrical . . . electrical

Explanation:

Electrical synapses have two main characteristics. First, they transmit signals through specialized tunnels between cells called gap junctions. Gap junctions facilitate movement of molecules and ions between cells. This movement enables transmission of signals between adjacent cells. Second, electrical synapses have a much higher speed of signal transmission than chemical synapses. This occurs because the signals in electrical synapses are transmitted directly from one cell to the next via gap junctions. In chemical synapses, however, signals are transmitted indirectly via neurotransmitters, which decreases the speed of signal transmission.

Example Question #13 : Nervous System And Nervous Tissue

Which of the following is characteristic of a chemical synapse?

I. Signal transmission involves neurotransmitters

II. They are bidirectional

III. They are more common than electrical synapses

Possible Answers:

I, II, and III

II only

I only

I and III

Correct answer:

I and III

Explanation:

A chemical synapse is a type of synapse that uses neurotransmitters to transmit signals. A presynaptic neuron receives an action potential, which prompts neurotransmitters to be released into the synaptic cleft. These neurotransmitters traverse across the synaptic cleft and bind to receptors on the postsynaptic neuron. Binding of neurotransmitters initiates a signal pathway in the postsynaptic neuron.

One of the characteristics of a chemical synapse is that it is unidirectional. This means that the signal can only be propagated in one direction. The signal is always transmitted from the presynaptic to the postsynaptic neuron, never the other way around.

Chemical synapses are indeed more common than electrical synapses. Most nerves, neuromuscular junctions, and major organs in the body use chemical synapses to transmit action potentials. The only major exception is the heart; cardiac cells in the heart use electrical synapses to transmit signal from one cell to the other.

Example Question #14 : Nervous System And Nervous Tissue

In a chemical synapse, which structure contains the neurotransmitter receptors?

Possible Answers:

Dendrites of the postsynaptic neuron

Dendrites of the presynaptic neuron

Axon of the presynaptic neuron

Axon of the postsynaptic neuron

Correct answer:

Dendrites of the postsynaptic neuron

Explanation:

In a chemical synapse, the presynaptic neuron transmits a signal to the adjacent postsynaptic neuron. The postsynaptic neuron receives this signal via neurotransmitters. Recall that a neuron has directionality, with dendrites on one end and an axon on the other end. Dendrites receive an outside signal (signal enters neuron), whereas an axon transmits the signal to an adjacent neuron or muscle (signal exits neuron). This means that the axon end of the presynaptic neuron transmits the signal to the dendrite end of the postsynaptic neuron via a chemical synapse.

Example Question #15 : Nervous System And Nervous Tissue

When a neuron is unable to produce another action potential no matter how much stimulation is provided, what period is the neuron said to be in?

Possible Answers:

The relative refractory period

The absolute refractory period

The hyperpolarization period

The impulse propagation period

The depolarization period

Correct answer:

The absolute refractory period

Explanation:

During the absolute refractory period, no action potential can occur. In the relative refractory period, an action potential can occur with more stimulation than is normally required.

Example Question #16 : Nervous System And Nervous Tissue

What is the process by which action potentials are able to "jump" from one node of Ranvier to the next?

Possible Answers:

Depolarization

Saltatory conduction

Relative refractory period

Repolarization

Correct answer:

Saltatory conduction

Explanation:

Saltatory conduction is the property that allows an action potential to jump from one node to the next along a neural axon. This is accomplished by the presence of myelin, a non-conducting sheath around the axon. Myelin interrupts the flow of current down the membrane, forcing it to skip from one region of membrane to the next, rather than fluidly traveling down the entire axon length.

Depolarization is the stage of action potential transmission in which sodium channels are opened, and sodium rushes into the cell down its concentration gradient. The resting potential of the neural membrane is roughly . The rapid influx of positive sodium ions causes this potential in increase to at the action potential peak.

Repolarization is the stage of action potential transmission in which potassium channels of a cell are opened, and potassium moves out of the cell. This event re-establishes the negative resting membrane potential. 

The refractory period is the obligatory temporal gap between action potentials. After an action potential, the primary gating mechanism of the voltage-gated sodium channels causes the channels to close and deactivate. This constitutes the absolute refractory period, during which no stimulus is capable of producing an action potential. The relative refractory period follows, during the cell repolarization, when potassium efflux causes the membrane potential to drop below the resting potential. This state of hyperpolarization means that a greater stimulus is required to reach threshold, and constitutes the relative refractory period.

Example Question #1 : 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

A scientist is studying the nerve cell depicted in the above figure. He notices that proteins like 1, 2, and 3 are only located a certain regions along the length of the axon. What are these regions most likely to be called?

Possible Answers:

Nodes of Ranvier

Nissl bodies

Dendrites

Axon hillocks

Synaptic clefts

Correct answer:

Nodes of Ranvier

Explanation:

The proteins responsible for allowing ionic flow into and out of axons are most likely to be found at Nodes of Ranvier, where there is no myelin and ions can move freely. Action potentials travel via saltatory conduction, meaning that the ion channels are only stimulated a certain points on the membrane. The majority of the impulse is conducted through the interior of the axon without further external stimulation.

Example Question #2 : Action Potentials And Synapse Biology

In saltatory conduction displayed by neurons containing myelinated axons, ion flow takes place at which region of the axon?

Possible Answers:

Schwann cells

Ion flow does not take place during saltatory conduction

Nodes of Ranvier

Myelinated portion of the axon

Correct answer:

Nodes of Ranvier

Explanation:

Saltatory conduction is defined as the method by which action potentials are propagated along axons in myelinated neurons. The method by which they do this is by the generation of action potentials at each node of Ranvier. The only places along the myelinated axon that display ion flow are the nodes of Ranvier. The myelinated portions do not display ion flow, allowing the electrical stimulus to rapidly jump down the axon from one node to the next rather than slowly flow down the full axon length.

Schwann cells are types of cell that make up the myelin coated sheath for select neurons.

Example Question #19 : 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

A scientist shows that the protein labeled "1" has a voltage gate, as well as an inactivation gate, while proteins 2 and 3 lack this dual gate architecture. What ion is most likely to be controlled by protein 1?

Possible Answers:

Potassium

Chloride

Magnesium

Calcium

Sodium

Correct answer:

Sodium

Explanation:

Sodium channels have an inactivation gate, as well as a voltage gate. This allows the sodium channels to be turned off, even while voltage changes persist, thereby facilitating repolarization. This dual gate structure also causes the refractory period.

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