Biochemistry : Synapse Biochemistry

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Synapse Biochemistry

The release of which ion triggers release of neurotransmitters at the axon terminal of a presynaptic cell? 

Possible Answers:

Cobalt

Sodium

Chloride

Potassium

Calcium

Correct answer:

Calcium

Explanation:

The release of calcium ions at the axon terminal is responsible for the exocytosis of vesicles carrying neurotransmitters. 

Example Question #2 : Synapse Biochemistry

Acetylcholine transferase is an enzyme involved in the synthesis of acetylcholine. Which of the following molecules are involved in this reaction?

I. Choline

II. Acetyl-CoA

III. Acetic acid

Possible Answers:

I and II

I, II, and III

II and III

I and III

Correct answer:

I and II

Explanation:

When an action potential reaches the synapse, choline enters the neuron. Once inside, the choline molecule binds to acetyl-CoA and forms acetylcholine, which is then packaged into vesicles. Upon calcium influx, the acetylcholine vesicles fuse with the synaptic membrane and release acetylcholine into the synaptic cleft. The acetylcholine molecules can now bind to receptors on the postsynaptic membrane and initiate an action potential in the postsynaptic neuron.

Example Question #3 : Synapse Biochemistry

__________ muscle contains electrical synapses and __________ muscle contains chemical synapses.

Possible Answers:

Cardiac . . . skeletal

Skeletal . . . skeletal

Cardiac . . . cardiac

Skeletal . . . cardiac

Correct answer:

Cardiac . . . skeletal

Explanation:

There are two types of synapses: electrical and chemical. Electrical synapses have gap junctions between adjacent cells and are usually found between cardiac muscle cells. Chemical synapses are more abundant and utilize neurotransmitters (such as acetylcholine) to transmit signals between adjacent cells. They are typically found in neuromuscular junctions of skeletal muscle cells.

Example Question #4 : Synapse Biochemistry

Myasthenia gravis is an autoimmune disease that decreases muscle contraction. Circulating antibodies bind to acetylcholine receptors and prevent acetylcholine from binding to the receptors. Which of the following could alleviate the symptoms of Myasthenia gravis?

Possible Answers:

Increasing acetylcholine half-life

Decreasing calcium influx in presynaptic neuron

Decreasing the activity of acetylcholinesterase

More than one of these are correct

Correct answer:

More than one of these are correct

Explanation:

Acetylcholine is a neurotransmitter found in neuromuscular junctions. Release of acetylcholine into the synaptic cleft allows acetylcholine to bind to its receptors on the muscle membrane. Once bound, acetylcholine activates a signaling cascade that eventually leads to muscle contraction. Circulating antibodies in Myasthenia gravis patients prevent this interaction between acetylcholine and its receptor, thus decreasing muscle contraction. One way to treat this disease is by administering drugs that increase the half-life of each acetylcholine molecule. The most common way to do this is by administering an acetylcholinesterase inhibitor. Acetylcholinesterase is an enzyme that breaks down acetylcholine molecules in the synaptic cleft; decreasing or inhibiting this enzyme will lead to increased acetylcholine concentration. This increased concentration will compete with the antibodies and facilitate muscle contraction. Decreasing calcium influx in the presynaptic neuron will decrease the release of acetylcholine into the synaptic cleft. This will make Myasthenia gravis symptoms worse.

Example Question #5 : Synapse Biochemistry

Which of the following neurotransmitters is not a catecholamine?

Possible Answers:

Epinephrine

All of these are catecholamines

Norepinephrine

Dopamine

Serotonin

Correct answer:

Serotonin

Explanation:

Out of all the neurotransmitters listed, the only one that isn't a catecholamine is serotonin. This neurotransmitter is initially derived from the amino acid tryptophan, whereas the catecholamines are derived from the amino acid tyrosine.

Dopamine, norepinephrine, and epinephrine are all catecholamine neurotransmitters. In fact, in the metabolic pathway that produces these compounds, dopamine is an intermediate that can be converted into norepinephrine, which can subsequently be converted into epinephrine.

Example Question #6 : Synapse Biochemistry

What category of neurotransmitters are synthesized in the endoplasmic reticulum, and are typically packaged in dense-core vesicles when examined via electron microscopy?

Possible Answers:

Small molecule neurotransmitters

Catecholamines

Transferases

Gasotransmitters

Neuropeptides

Correct answer:

Neuropeptides

Explanation:

Transferases are not neurotransmitters and can be omitted from selection. Catecholamines and small-molecule transmitters are overlapping categories and are typically packed in small, clear core vesicles. Gasotransmitters are membrane permeable and do not require vesicles for release. Neuropeptides are unique in that they are large and are synthesized at the ER, and are packed in large, dense vesicles, and thus this is the correct answer. 

Example Question #7 : Synapse Biochemistry

Mutations in ion channels can often cause defects in synaptic transmission since propagation of an electrical signal is crucial to proper transmission at the synaptic cleft. You examine mutant mice and identify that the step in synaptic transmission that is defective is at the vesicle release step; that is, the presynaptic cell undergoes a massive depolarization, vesicles in the presynaptic cell dock at the membrane, but the vesicles do not fuse and therefore neurotransmitter is not released into the cleft. Which ion channel is most likely mutated in these animals?

Possible Answers:

Voltage-gated sodium channels

Voltage-gated potassium channels

Ligand-gated sodium channels

Voltage-gated calcium channels

Ligand-gated calcium channels

Correct answer:

Voltage-gated calcium channels

Explanation:

An influx of calcium at the presynaptic terminal is absolutely required to activate fusion of vesicles with the membrane, and therefore release of their contents into the presynaptic cleft. Given that the specific deficit in these mutants is at the final stage of fusion, we know that the action potential propagated (so it's likely not sodium or potassium) and the presynaptic membrane is not responding to the voltage change to permit an influx of calcium. Therefore, voltage-gated calcium channels are the likely cause of this deficit. 

Example Question #8 : Synapse Biochemistry

Which of the following is false about endocrine cells and nerve cells?

Possible Answers:

The electrical impulses of nerve cells travel at speeds of up to

Endocrine signaling is relatively slow compared to synaptic signaling.

Neurotransmitters are delivered through the bloodstream, whereas hormones are found primarily in the synaptic cleft.

Neurotransmitters are locally concentrated; hormones are diffused.

Neurotransmitter receptors have a relatively low affinity for their ligand, compared to hormone receptors.

Correct answer:

Neurotransmitters are delivered through the bloodstream, whereas hormones are found primarily in the synaptic cleft.

Explanation:

Neurotransmitters are found in the synaptic cleft; hormones travel through the bloodstream. Endocrine signaling is much slower than synaptic signaling, but hormone receptors have a much higher affinity for their ligand, than neurotransmitter receptors do. The highest speed of nerve cell electrical impulses is somewhere around . Neurotransmitters are localized around the synaptic cleft; hormones are dispersed in the blood.

Example Question #9 : Synapse Biochemistry

The absolute refractory period during depolarization is the result of which of these?

Possible Answers:

Closing of the voltage-gated sodium channels

Inactivation of both the voltage-gated sodium and voltage-gated potassium channels

Closing of the voltage-gated potassium channels

Inactivation of the voltage-gated potassium channels 

Inactivation of the voltage-gated sodium channels

Correct answer:

Inactivation of the voltage-gated sodium channels

Explanation:

The absolute refractory period during depolarization is the period in which it is impossible for another depolarization to occur.  The reason that another depolarization can not occur is that the voltage-gated sodium channels are inactivated.  This renders them unable to function, and also unable to receive any signal to activate again.  If the channels were closed rather than inactivated, they could still receive electrical input to open again.

Example Question #10 : Synapse Biochemistry

How is acetylcholine removed from the synaptic space after acting on its receptors in the postsynaptic membrane?

Possible Answers:

Acetylcholine is taken up into the presynaptic neuron by reuptake channels

Acetylcholine's effect simply wears off and it is not necessary to remove it from the synapse

Choline acetyltransferase moves acetylcholine from the synaptic space back into the presynaptic neuron

Acetylcholinesterase breaks down the acetylcholine and returns it to the presynaptic neuron

Acetylcholine is absorbed through its receptor on the postsynaptic membrane 

Correct answer:

Acetylcholinesterase breaks down the acetylcholine and returns it to the presynaptic neuron

Explanation:

After acetylcholine is excised from the presynaptic neuron to act on its receptors in the postsynaptic neuron, it is removed from the synaptic space by the enzyme, acetylcholinesterase.  Acetylcholinesterase breaks it down into acetate and choline which can then be removed.

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