MCAT Biology : Neurons and Action Potential

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #41 : Nervous System And Nervous Tissue

The brain is a very delicate structure with little room to move around. Surrounding the brain and the spinal cord are three protective layers in addition to the skull and the vertebral column. Directly surrounding the brain and spinal cord is the pia mater. Following the pia mater is the arachnoid mater. Between the pia mater and the arachnoid mater is the sub-arachnoid space where the cerebrospinal fluid circulates. Finally, the protective layer is the dura mater is loosely attached to the arachnoid mater but is strongly associated with the skull bone.

Depending on the type of injury, a certain type of vein and/or artery are more susceptible to injury. For example, the meningeal artery and vein run through the foramen spinosum and travel between the two layers making up the dura mater. As the artery and the vein are traveling in between the dura mater, there is a vulnerable region at the temple. A strike to the temple region could rupture these vessels and result in a epidural hematoma. 

Traveling from the cerebral cortex to the venous dural sinus (located at certain regions between the two layers of the dura mater) is the cerebral vein. When an injury results in the dura mater shifting away from the arachnoid mater, the cerebral vein could rupture and lead to a subdural hematoma.

A hematoma in the brain is a life-threatening condition. The brain needs constant supply of blood for nutrients, oxygen for metabolism and energy. Among other things, the brain uses energy to drive the sodium-potassium pump. What is the relationship between the pump and an action potential?  

Possible Answers:

The sodium-potassium pump is required propagate an action potential 

The sodium-potassium pump is required to drive potassium out of the cell

The sodium-potassium pump is required drive sodium into the cell 

The sodium-potassium pump is required to maintain the resting potential, which is about 

The sodium-potassium pump is required to maintain the resting potential, which is about 

The sodium-potassium pump is required to maintain the resting potential, which is about 

Correct answer:

The sodium-potassium pump is required to maintain the resting potential, which is about 

Explanation:

The sodium-potassium pump is required to maintain the resting potential of the cell. The pump pushes three sodium ions out of the cell for every two potassium ions into the cell. This odd number allows for the cell to stay in a negative resting potential.

Example Question #41 : Nervous System And Nervous Tissue

The central nervous system consists of the brain and the spinal cord. In general, tracts allow for the brain to communicate up and down with the spinal cord. The commissures allow for the two hemispheres of the brain to communicate with each other. One of the most important commissures is the corpus callosum. The association fibers allow for the anterior regions of the brain to communicate with the posterior regions. One of the evolved routes from the spinal cord to the brain is via the dorsal column pathway. This route allows for fine touch, vibration, proprioception and 2 points discrimination. This pathway is much faster than the pain route. From the lower limbs, the signal ascends to the brain via a region called the gracile fasciculus. From the upper limbs, the signal ascends via the cuneate fasciculus region in the spinal cord.

What allows for the dorsal column pathway to be faster than the pain pathway? 

Possible Answers:

Weaker action potential

Myelination 

Stronger action potential

Shorter distance 

Longer distance

Correct answer:

Myelination 

Explanation:

Fine touch, vibration, proprioception and 2 points discrimination all utilizes the dorsal column pathway. The upper region utilizes the cuneate fasciculus region in the spinal cord while the lower region depends on the gracile fasciculus. According to the passage, these sensations are part of the rapid pathway whereas other sensations such as pain is not as fast. The dorsal column pathway is heavily myelinated while the pain pathway is not as myelinated. Action potential is an all-or-nothing event and the amplitude is fixed.  

Example Question #31 : Action Potentials And Synapse Biology

The central nervous system consists of the brain and the spinal cord. In general, tracts allow for the brain to communicate up and down with the spinal cord. The commissures allow for the two hemispheres of the brain to communicate with each other. One of the most important commissures is the corpus callosum. The association fibers allow for the anterior regions of the brain to communicate with the posterior regions. One of the evolved routes from the spinal cord to the brain is via the dorsal column pathway. This route allows for fine touch, vibration, proprioception and 2 points discrimination. This pathway is much faster than the pain route. From the lower limbs, the signal ascends to the brain via a region called the gracile fasciculus. From the upper limbs, the signal ascends via the cuneate fasciculus region in the spinal cord.

Why is the pain of stepping on a nail not felt immediately?

Possible Answers:

There is little myelination in the pain pathway

There is excess myelination in the pain pathway

The action potential amplitude is too low

None of these

The action potential amplitude was too high

Correct answer:

There is little myelination in the pain pathway

Explanation:

Myelination allows for the action potential to travel at a faster rate via saltatory conduction of. The low amount of myelination in the pain pathway delays the pain signal to the brain. Note that myelination can increase the conduction speed of an action potential by 5-50 orders of magnitude.

Example Question #112 : Biology

The central nervous system consists of the brain and the spinal cord. In general, tracts allow for the brain to communicate up and down with the spinal cord. The commissures allow for the two hemispheres of the brain to communicate with each other. One of the most important commissures is the corpus callosum. The association fibers allow for the anterior regions of the brain to communicate with the posterior regions. One of the evolved routes from the spinal cord to the brain is via the dorsal column pathway. This route allows for fine touch, vibration, proprioception and 2 points discrimination. This pathway is much faster than the pain route. From the lower limbs, the signal ascends to the brain via a region called the gracile fasciculus. From the upper limbs, the signal ascends via the cuneate fasciculus region in the spinal cord.

One of the most utilized neurotransmitters is acetylcholine. Which of the following methods will increase the amount of the neurotransmitters in the synaptic cleft? 

I. Increasing the action potential frequency

II. Decrease the calcium concentration surrounding the neuron

III. Inhibit acetylcholine esterase

Possible Answers:

III only

I only

II only

I and III

II and III 

Correct answer:

I and III

Explanation:

The presynaptic neuron require an action potential in order to open the voltage gated calcium channel. The opening of this calcium channel will allow the influx of calcium and trigger the release of vesicles with the neurotransmitter inside. The exocytosis of acetylcholine from the presynaptic cleft will then bind to the receptor on the postsynaptic cleft. Inhibiting acetylcholinesterase will prevent the breakdown of the neurotransmitter and allow for it to bind to the receptor longer.  

Example Question #41 : Nervous System And Nervous Tissue

Which of the following are potential fates of neurotransmitters that have been released into the synaptic cleft?

I. Reuptake

II. Degradation

III. Passive diffusion away from synaptic cleft

IV. Bind receptors

Possible Answers:

I and IV

I, II, III, and IV

I, II, and III

IV only

Correct answer:

I, II, III, and IV

Explanation:

Every choice listed is a potential fate of neurotransmitters that have been released into the synaptic cleft. They can passively diffuse away from the synaptic cleft due to normal chemical principles. Neurotransmitters can also bind their target receptors and stimulate the post-synaptic neuron. There may also be special enzymes that inactivate or degrade neurotransmitters in the synaptic cleft, as acetylcholinesterase does with acetylcholine. It is also possible for reuptake to occur (the neurotransmitters to be taken back into the pre-synaptic neuron).

Example Question #42 : Nervous System And Nervous Tissue

Which of the following neurons would have vesicles of norepinephrine in its axon terminal? 

Possible Answers:

Preganglionic neurons in the parasympathetic nervous system

None of these

Postganglionic neurons in the parasympathetic nervous system

Preganglionic neurons in the sympathetic nervous system

Postganglionic neurons of the sympathetic nervous system

Correct answer:

Postganglionic neurons of the sympathetic nervous system

Explanation:

Norepinephrine (also called noradrenaline), is associated with the sympathetic nervous system. Acetylcholine is the most commonly used neurotransmitter in the autonomic nervous system, however norepinephrine is the neurotransmitter released from sympathetic postganglionic neurons to elicit sympathetic responses from target tissues.

The parasympathetic nervous system uses only acetylcholine.

Example Question #43 : Nervous System And Nervous Tissue

Sarin gas is a potent nerve agent that quickly causes serious physiological effects if ingested, even in very small quantities. It inhibits acetylcholinesterase, an enzyme that degrades acetylcholine. Acetylcholinesterase generally acts at the neuromuscular junction.

Sarin gas may cause which of the following?

Possible Answers:

Dilated pupils

Increased production of white blood cells

Reduced blood pH (acidosis)

Loss of control of respiratory muscles

Inhibition of peristalsis

Correct answer:

Loss of control of respiratory muscles

Explanation:

Acetylcholine is the neurotransmitter that acts at neuromuscular junctions. Acetylcholinesterase degrades acetylcholine at the synaptic cleft, allowing the muscle to relax. If acetylcholinesterase is inhibited, acetylcholine will remain in the synaptic cleft and continuously stimulate the muscle. Breathing requires the ability to contract and relax respiratory muscles. Without rapid administration of an antidote, sarin gas usually results in death from asphyxiation. Acetylcholine causes pupil constriction and gastrointestinal motility. It is not associated with a rapid increase in leukocyte production.

Example Question #4 : Neurotransmitters

Which of the following neurotransmitters functions to stimulate muscle cells to contract?

Possible Answers:

Acetylcholine

Endorphins

Norepinephrine

Serotonin

Dopamine

Correct answer:

Acetylcholine

Explanation:

The neurotransmitter acetylcholine is the only neurotransmitter released at the neuromuscular junctions between neurons and skeletal muscles, where it stimulates the muscles to contract.

The effects of norepinephrine prepare the body to respond to short-term threats and stressful situations. Serotonin is believed to affect mood and sleep. Serotonin imbalances have been linked to depression, and a classification of antidepressants is termed selective seratonin re-uptake inhibitors. The absence of dopamine is associated with Parkinson's disease. Endorphins produce analgesia by binding to the opiate receptor sites involved in pain perception. 

Example Question #5 : Neurotransmitters

What neurotransmitter is released by the postganglionic neurons of the sympathetic nervous system?

Possible Answers:

Norepinephrine

GABA

Serotonin

Dopamine

Acetylcholine

Correct answer:

Norepinephrine

Explanation:

Norepinephrine is an excitatory neurotransmitter that readies the body for the "fight-or-flight" response. The sympathetic nervous system releases epinephrine and norepinephrine from postanglionic neurons to stimulate this response from targeted organs.

Acetylcholine is released from preganglionic sympathetic neurons. It is also released from both preganglionic and postganglionic neurons of the parasympathetic nervous system. It is also the primary neurotransmitter involved in neuromuscular junctions.

Serotonin affects mood and social behavior, while dopamine is involved in mood and focus. GABA, unlike acetylcholine and norepinephrine, is an inhibitory neurotransmitter.

Example Question #6 : Neurotransmitters

Which of the following best discriminates between small-molecule neurotransmitters and peptide neurotransmitters?

Possible Answers:

Peptide neurotransmitter precursors and their enzymes are axonally transported in vesicles

Peptide neurotransmitters are synthesized in the nucleus of the neuron

Small-molecule neurotransmitters are synthesized in the soma

Peptide neurotransmitters are synthesized at the synaptic terminals

Small-molecule neurotransmitters are stored in large dense-core vesicles

Correct answer:

Peptide neurotransmitter precursors and their enzymes are axonally transported in vesicles

Explanation:

Peptide neurotransmitters cannot be synthesized at the synaptic terminals. Since these molecules are proteins by nature, they must be constructed by ribosomes found in the soma near the nucleus. Specifically, ribosomes bound to the rough endoplasmic reticulum will synthesize peptide neurotransmitters, in order for them to be properly packaged for transmission. From the ER, the proteins iare sent to the Golgi apparatus where they are modified and packaged into vesicles, which then are transported along microtubules much like in a normal exocytosis process.

Small-molecule neurotransmitters are not stored in large dense-core vesicles, and are instead synthesized in the synaptic terminals.

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