MCAT Biology : Neurons and Action Potential

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #21 : Action Potentials And Synapse Biology

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

Repolarization

Action potential upstroke

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 #32 : 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:

None of these

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

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

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 #21 : Action Potentials And Synapse Biology

Saltatory conduction of action potentials requires which of the following?

Possible Answers:

Chemical synapse

Electrical synapse

Myelin

None of these

Thinner axon

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 bi-directional in chemical synapses

is slower via chemical synapses than electrical synpases

is uni-directional in electrical synapses

is slower via electrical synapses than chemical synapses

involves saltatory conduction across the synapse

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 MAO to norepinephrine

Binding of V- and T-snares

Binding of acetylcholine molecules to nicotinic receptors

Binding of calcium to G-proteins in the vesicle membrane

Binding of calcium to T-snares

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 #22 : Action Potentials And Synapse Biology

Which mode of synaptic transmission is generally faster?

Possible Answers:

Synapses using metabotropic receptors

Synapses using saltatory receptors

Synapses using ionotropic receptors

Synapses using muscarinic receptors

Synapses using G-proteins

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 

Increased concentration of Na+ outside the neuron

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

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:

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

K+ ions move out of the cell during repolarization.

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

Hyperpolarization occurs as the cell membrane potential decreases.

Resting potential is reached after repolarization.

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.

Example Question #31 : Action Potentials And Synapse Biology

The optic nerve is formed from the axons of all retinal ganglion cells. The optic nerves from each eye join at the optic chiasm and eventually enter either the left or right optic tract. The optic tract projects to three subcortical areas. One is the lateral geniculate nucleus, which is responsible for processing visual information. One is the pretectal area, which produces pupillary reflexes based on information from the retina. Finally, the superior colliculus uses the information from the retina to generate eye movement.

When light is shone upon one eye, it causes constriction of the pupil in both eyes. Constriction of the eye in which the light is shone is the direct response while constriction of the other is known as the consensual response. The pupillary reflexes are mediated through retinal ganglion neurons that project to the pretectal area which lies anterior to the superior colliculus. The cells in the pretectal area project bilaterally to preganglionic parasympathetic neurons in the Edinger-Westphal nucleus. This is also known as the accessory oculomotor nucleus. The preganglionic parasympathetic neurons in the Edinger-Westphal nucleus send axons through the oculomotor nerve to innervate the ciliary ganglion. The ciliary ganglion's postganglionic neuron innervates the smooth muscle of the pupillary sphincter.

It has been determined that the frequency of action potentials increases dramatically in axons once they have left the optic nerve. The most likely explanation for this increase is __________.

Possible Answers:

a lower density of sodium channels are found in the axons leaving the optic disc

the axons are myelinated by Schwann cells

the axons are myelinated by oligodendrocytes

a higher density of sodium channels are found in the axons leaving the optic disc

these axons are made up of more thickly myelinated "A" class nerve fibers

Correct answer:

the axons are myelinated by oligodendrocytes

Explanation:

The axons are myelinated by oligodendrocytes. This question calls on our knowledge of the nervous system outside of what is stated in the passage. We are looking for the most likely explanation for the increase in the frequency of the action potential. Myelinated nerves have the ability to increase the frequency of action potential conduction. Therefore, we can narrow the options down to either myelinated by Schwann cells or oligodendrocytes. The question then becomes: Which cells are responsible for the myelination? In both cases, glial cells are responsible for laying down the myelin sheath. In the central nervous system (CNS), these cells are called oligodendrocytes, while in the peripheral nervous system they are called Schwann cells. Since we are talking about nerves located in the CNS, the correct answer is oligodendrocytes.

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