Question

Which one of the following statements best describes the differences between classical neurotransmitters and neuropeptides? (A) neuropeptides are synthesized in the cell body (B) classical neurotransmitters have a longer duration of action (C) neuropeptides undergo rapid reuptake into the presynaptic terminal (D) classical neurotransmitters are packaged into vesicles (E) neuropeptides are degraded by acetylcholinesterase in the synapse

Short answer

The correct answer is (A): Neuropeptides are synthesized in the cell body.

Step-by-step solution

Understanding Classical Neurotransmitters

Classical neurotransmitters are small molecule transmitters such as dopamine, serotonin, and acetylcholine. These neurotransmitters are synthesized in the axon terminal and usually have shorter action durations. They are often reabsorbed by reuptake into the presynaptic terminal or degraded by enzymes.

Understanding Neuropeptides

Neuropeptides are larger and more complex molecules than classical neurotransmitters. They are synthesized in the cell body and transported to the terminal. Neuropeptides typically have longer action durations and are often degraded by extracellular peptidases, not by reuptake.

Analyzing the Options

Let's break down each option: - (A) Neuropeptides are synthesized in the cell body: This is a correct statement as neuropeptides are made in the cell body. - (B) Classical neurotransmitters have a longer duration of action: This is incorrect; classical neurotransmitters usually have shorter action durations compared to neuropeptides. - (C) Neuropeptides undergo rapid reuptake into the presynaptic terminal: This is incorrect; neuropeptides are generally not reuptaken. - (D) Classical neurotransmitters are packaged into vesicles: This is a true statement but doesn't highlight a difference with neuropeptides. - (E) Neuropeptides are degraded by acetylcholinesterase in the synapse: This is incorrect; acetylcholinesterase acts on acetylcholine, not neuropeptides.

Selecting the Correct Answer

The best statement that describes the difference based on the given options is (A): Neuropeptides are synthesized in the cell body. This is a key distinction between how neuropeptides and classical neurotransmitters are processed in neurons.

Key concepts

Classical Neurotransmitters

Classical neurotransmitters are often referred to as small molecule neurotransmitters. Some well-known examples include dopamine, serotonin, and acetylcholine. These substances are typically synthesized at the axon terminal, which is a structure at the end of neurons.
Once released into the synaptic cleft, they have shorter durations of action compared to other types of neurotransmitters, such as neuropeptides.
Here’s how they work:

• Synthesis: Occurs at the axon terminal, rather than the cell body.
• Action Duration: Generally quick but shorter-lived effects.
• Reuptake: Often reabsorbed into the presynaptic neuron for reuse or degraded by specific enzymes.
• Vesicle Packaging: Stored in synaptic vesicles ready for release.

Neuropeptide Synthesis

Neuropeptides are a different class of signaling molecules characterized by their larger size and complex structure when compared to classical neurotransmitters. The synthesis of neuropeptides is distinct from that of classical neurotransmitters.
Generally, neuropeptides are synthesized in the neuron’s cell body and not at the axon terminal. Here’s what differentiates them:

• Synthesis Location: These molecules are created in the cell body. This is due to their complex structures requiring more substantial machinery and resources than what’s available in the axon terminal.
• Transportation: After they are synthesized, neuropeptides are transported to the axon terminal, where they can be released upon stimulation.
• Action Duration: Typically exert longer-lasting effects on their target cells, contributing to prolonged physiological responses.
• Degradation: Degraded by extracellular enzymes after their release and function, not typically reabsorbed like classical neurotransmitters.

Neurotransmitter Reuptake

Neurotransmitter reuptake is a key mechanism that neurons use to recycle neurotransmitters. This process mostly applies to classical neurotransmitters.
After these neurotransmitters have been released into the synaptic cleft and have bound to their receptors on the postsynaptic neuron, they may be taken back into the presynaptic neuron. Let's look at the details:

• Efficiency: Reuptake allows for the efficient reuse of neurotransmitters, conserving cellular resources and helping terminate the neurotransmitter action quickly.
• Example: In serotonin reuptake, transporters on the presynaptic neuron capture serotonin molecules for reuse.
• Lack of Reuptake in Neuropeptides: Unlike classical neurotransmitters, neuropeptides are not typically taken back up by the presynaptic neuron.

This distinction is crucial for understanding different neural communication modalities.

Synaptic Vesicle Packaging

Synaptic vesicle packaging is an essential process that ensures neurotransmitters are effectively stored and released by neurons. This is particularly relevant for classical neurotransmitters. Here’s how it works:

• Storage: Classical neurotransmitters are packaged into small vesicles within the axon terminal. These vesicles serve as storage containers until the neuron is activated to release their contents.
• Release Mechanism: Upon receiving a signal, these vesicles fuse with the axon terminal’s membrane to release neurotransmitters into the synaptic cleft.
• Efficiency: This packaging ensures a rapid response when neurotransmitters are needed to convey a signal across a synapse.
• Contrast with Neuropeptides: While neuropeptides are also packaged in vesicles, they undergo different processing, as they originate in the cell body.

This method of packaging is crucial for rapid and efficient signal transmission between neurons.