MCAT Biology : Neurotransmitters

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1 : Neurotransmitters

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, and III

IV only

I, II, III, and IV

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 #1 : Neurotransmitters

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

Possible Answers:

None of these

Preganglionic neurons in the parasympathetic nervous system

Preganglionic neurons in the sympathetic nervous system

Postganglionic neurons of the sympathetic nervous system

Postganglionic neurons in the parasympathetic 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 #1 : Neurotransmitters

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:

Reduced blood pH (acidosis)

Increased production of white blood cells

Inhibition of peristalsis

Loss of control of respiratory muscles

Dilated pupils

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:

Dopamine

Norepinephrine

Acetylcholine

Endorphins

Serotonin

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 #44 : Neurons And Action Potential

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

Possible Answers:

Acetylcholine

Dopamine

GABA

Norepinephrine

Serotonin

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 #2 : Neurotransmitters

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

Possible Answers:

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

Peptide neurotransmitters are synthesized at the synaptic terminals

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

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.

Example Question #3 : Neurotransmitters

Which of the following are major inhibitory neurotransmitters, causing inhibitory postsynaptic potentials (IPSPs)?

Possible Answers:

Glycine and norepinephrine

Synaptobrevin and glutamate

GABA and glycine

Glycine and acetylcholine

Glycine and epinephrine

Correct answer:

GABA and glycine

Explanation:

GABA and glycine are the two major inhibitory neurotransmitters.

Norepinephrine, epinephrine, glutamate, and acetylcholine cause excitatory responses. Synaptobrevin is a snare protein involved in vesicle docking and fusion; it has no effect on whether or not a neurotransmitter is or is not inhibitory.

Example Question #3 : Neurotransmitters

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.

The neurotransmitter released by the axons in the Edinger-Westphal neurons is most likely __________.

Possible Answers:

dopamine

acetylcholine

glutamine

norepinephrine

epinephrine

Correct answer:

acetylcholine

Explanation:

Acetylcholine is correct. We are told from the passage that the neurons which make up the Edinger-Westphal nucleus are parasympathetic neurons. Therefore, this question is really testing one's knowledge of the neurotransmitter used by parasympathetic neurons. We cannot be expected to know from the question alone which neurotransmitter these neurons use. However, we are supposed to be aware that neurons that are parasympathetic use the neurotransmitter acetylcholine. 

Example Question #4 : Neurotransmitters

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 common neurotransmitters is acetylcholine.  Which of the following methods will decrease the amount of the neurotransmitters in the synaptic cleft? 

I. Increase the action potential frequency

II. Decrease the calcium concentration surrounding the neuron

III. Inhibit acetylcholine esterase 

Possible Answers:

I and II only

III only

I only

I and III only

II only

Correct answer:

II only

Explanation:

The presynaptic neuron require an action potential in order to open the calcium voltage 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. Decreasing the surrounding concentration of calcium will inhibit the release of the acetylcholine filled vesicles into the synaptic cleft.

Example Question #5 : Neurotransmitters

The cellular membrane is a very important structure. The lipid bilayer is both hydrophilic and hydrophobic. The hydrophilic layer faces the extracellular fluid and the cytosol of the cell. The hydrophobic portion of the lipid bilayer stays in between the hydrophobic regions like a sandwich. This bilayer separation allows for communication, protection, and homeostasis. 

One of the most utilized signaling transduction pathways is the G protein-coupled receptor pathway. The hydrophobic and hydrophilic properties of the cellular membrane allows for the peptide and other hydrophilic hormones to bind to the receptor on the cellular surface but to not enter the cell. This regulation allows for activation despite the hormone’s short half-life. On the other hand, hydrophobic hormones must have longer half-lives to allow for these ligands to cross the lipid bilayer, travel through the cell’s cytosol and eventually reach the nucleus. 

Cholesterol allows the lipid bilayer to maintain its fluidity despite the fluctuation in the body’s temperature due to events such as increasing metabolism. Cholesterol binds to the hydrophobic tails of the lipid bilayer. When the temperature is low, the cholesterol molecules prevent the hydrophobic tails from compacting and solidifying. When the temperature is high, the hydrophobic tails will be excited and will move excessively. This excess movement will bring instability to the bilayer. Cholesterol will prevent excessive movement.

Which of the following molecules can be found inside of a cell? 

I. Cyclic adenosine monophosphate (cAMP) 

II. Protein kinase A

III. Acetylcholine

Possible Answers:

I only

I and II

II only

I, II, and III

III only

Correct answer:

I and II

Explanation:

Cyclic adenosine monophosphate and protein kinase A are both second messengers in the G protein-coupled receptor pathway. Since they are second messengers, they amplify and transmit the signal inside of the cell. Acetylcholine, however is a hydrophillic neurotransmitter and binds to the receptor located on the surface of the cell, thereby inducing intracellular signaling.  

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