MCAT Biology : Action Potentials and Synapse Biology

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #22 : Neurons And Action Potential

Which of the following refers to the process by which action potentials jump from one node of Ranvier to another?

Possible Answers:

Saltatory conduction

Sodium-potassium pump

Potential distribution

Diffusion

Threshold stimulus

Correct answer:

Saltatory conduction

Explanation:

The answer is saltatory conduction. Saltatory conduction is the term used to define the process of action potential jumping described in the question. The other possbilities, while involved in the nervous system and its function, do not adaquately describe the process in question.

Example Question #31 : Nervous System And Nervous Tissue

Immediately after an action potential, there is a fraction of time when the neuron can only be stimulated if there is a stronger than normal stimulus. What is this fraction of time called?

Possible Answers:

Absolute refractory period

Relative refractory period

Depolarization

Action potential upstroke

Repolarization

Correct answer:

Relative refractory period

Explanation:

The relative refractory period is the moment directly after an action potential when the neuron can only be stimulated to fire another action potential if there is a larger than normal stimulus. During an action potential, voltage-gated sodium channels open. After the action potential, the channels are gated and cannot be re-stimulated. This period is the absolute refractory period. The secondary gating is released, making the sodium-channels functional again, but the neuron has not been fully restored to resting potential. Release of potassium through voltage-gated potassium channels leads to hyperpolarization until the sodium-potassium pump is able to restore ion balance. This restoration takes longer than the un-gating of sodium channels, creating a period when the cell is hyperpolarized, but the voltage-gated sodium channels are capable of stimulation. If a large enough stimulus overcomes the cell hyperpolarization and reaches threshold, and action potential can still occur. This period is the relative refractory period.

Example Question #31 : Nervous System And Nervous Tissue

Which of the following ions plays a direct role in the release of neurotransmitters from the pre-synaptic terminal?

Possible Answers:

Correct answer:

Explanation:

While sodium and potassium maintain important functions in the conduction of action potentials along the axon of the neuron, it is calcium that is responsible for the binding of vesicles containing neurotransmitters to the pre-synaptic membrane. A severe lack of calcium would inhibit the release of neurotransmitters into the synaptic cleft. When the action potential reaches the axon terminal, it stimulates the opening of voltage-gated calcium channels. The resulting influx of calcium binds to synaptic vesicles, initiating the process to release their neurotransmitter contents into the synaptic cleft.

Example Question #33 : Nervous System And Nervous Tissue

What feature makes the axon hillock the location for initiation of action potentials?

Possible Answers:

The nerve membrane is the thinnest at this region of a neuron

There is a very high density of voltage-gated sodium channels

None of these

Voltage-gated potassium channels are absent at this location

Sodium-potassium pumps are absent at this location

Correct answer:

There is a very high density of voltage-gated sodium channels

Explanation:

For an action potential to occur, voltage-gated sodium channels must open to cause a sharp depolarization (increase) in the membrane potential. Pairing that information with knowledge that action potentials originate at the axon hillock, no other answer choice makes sense. It is only logical, then, that a high density of voltage-gated channels be present at the location where action potentials are first initiated.

Example Question #102 : Biology

Saltatory conduction of action potentials requires which of the following?

Possible Answers:

Myelin

Chemical synapse

Electrical synapse

Thinner axon

None of these

Correct answer:

Myelin

Explanation:

Saltatory conduction is a process that propagates an action potential more quickly down the length of an axon in a "leapfrog" manner. This propagation occurs in the gaps between myelin on an axon, called nodes of Ranvier. Without myelin, these nodes would not exist, and the rate at which an action potential is transmitted would decrease. People suffering with multiple sclerosis (MS) have myelin degradation, and thus have decreased motor and other neurological processes.

Example Question #35 : Nervous System And Nervous Tissue

The transmission of electrical signals from one neuron to another __________.

Possible Answers:

is slower via electrical synapses than chemical synapses

is slower via chemical synapses than electrical synpases

involves saltatory conduction across the synapse

is bi-directional in chemical synapses

is uni-directional in electrical synapses

Correct answer:

is slower via chemical synapses than electrical synpases

Explanation:

Electrical synapses transmit signals faster than chemical synapses due to the physical connection of neural cells through gap junctions. Chemical synapses are slower due to the action potential needing to arrive in the terminal bud, causing calcium channels to open. This causes neurotransmitter vesicles to fuse to the presynaptic membrane, releasing neurotransmitters to diffuse across the synaptic cleft.

Electrical synapses can allow bi-directional transmission of signals, but chemical synapses cannot. Saltatory conduction involves action potential propagation along the axon via the nodes of Ranvier, and is not involved in the synapse.

Example Question #21 : Action Potentials And Synapse Biology

What mediates the docking and fusion of synaptic vesicles?

Possible Answers:

Binding of calcium to G-proteins in the vesicle membrane

Binding of calcium to T-snares

Binding of MAO to norepinephrine

Binding of V- and T-snares

Binding of acetylcholine molecules to nicotinic receptors

Correct answer:

Binding of V- and T-snares

Explanation:

During the docking and fusion of synaptic vesicles, the increased levels of calcium in the synaptic terminal will lead to calcium ions binding to synaptotagmin, which facilitates the binding of V- and T-snares to initiate fusion. None of the other answer choices make sense with respect to vesicle fusion at the presynaptic terminal.

Example Question #31 : Neurons And Action Potential

Which mode of synaptic transmission is generally faster?

Possible Answers:

Synapses using metabotropic receptors

Synapses using muscarinic receptors

Synapses using G-proteins

Synapses using saltatory receptors

Synapses using ionotropic receptors

Correct answer:

Synapses using ionotropic receptors

Explanation:

Metabotropic receptors involve the reception of a neurotransmitter via a G-protein signaling cascade. Muscarinic receptors are an example of metabotropic receptors.

Ionotropic receptors involve the binding of a neurotransmitter directly to an ion channel, and the ion channel subsequently opening and allowing its respective ion into or out of a cell.

As a result, ionotropic receptors elicit effects more quickly, as they do not involve intermediate steps.

Example Question #38 : Nervous System And Nervous Tissue

Tetrodotoxin TXX is a poison commonly found in pufferfish that blocks the voltage-gated Na+ channels. Which of the following is a most likely consequence of TXX ingestion?

Possible Answers:

Disruption of impulse propagation 

Rapid removal of K+ in the neuron to compensate for the Na+ flow blockage

Increased concentration of Na+ outside the neuron

The presynaptic neuron finding a different nearby postsynaptic neuron to transmit the impulse to, forming a new synapse between them

No physiological effect will result, since Na+ channels will find an alternative route to excite the postsynaptic neuron.

Correct answer:

Disruption of impulse propagation 

Explanation:

During normal impulse conduction, 3 Na+ ions move out of a neuron while 2 K+ ions move in. This results in a high concentration of Na+ outside the cell and low K+ outside the cell. TXX will disrupt the electrochemical gradient by blocking the Na+/K+ voltage-gated channel. A patient suffering from TXX intoxication usually dies from respiratory paralysis brought on by the disruption of neural conduction along nerve fibers and axons. The most appropriate response to the question is the disrupted conduction of nerve impulses.

Example Question #39 : Nervous System And Nervous Tissue

Which of the following does NOT correctly describe the action potential pattern of a neuron?

Possible Answers:

Resting potential is reached after repolarization.

Hyperpolarization occurs as the cell membrane potential decreases.

Depolarization leads up to action potential as the Na+ ions rush out of the neuron.

K+ ions move out of the cell during repolarization.

3 Na+ ions move into the cell via the Na+/K+ pump.

Correct answer:

Depolarization leads up to action potential as the Na+ ions rush out of the neuron.

Explanation:

Depolarization occurs as the Na+ ions rush into the neuron. During depolarization, 3 Na+ ions move in and 2 K+ ions move out of the cell via the Na+/K+ pump. Repolarization returns the cell potential to its resting value by rushing K+ ions out of the cell. Hyperpolarization further decreases the cell potential after repolarization.

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