Questions: Neurotransmitter Synthesis and Storage

Question 1

What is the primary functional reason neurotransmitters are packaged into synaptic vesicles immediately after synthesis?

• A. To prevent them from diffusing out of the axon terminal through the plasma membrane
• B. To protect them from cytoplasmic degradative enzymes and enable precise quantal release
• C. To allow them to be recycled without resynthesis after exocytosis
• D. To maintain the alkaline pH required for enzymatic activity

Answer: (B)

Cytoplasmic enzymes (e.g., MAO) would rapidly degrade free neurotransmitters. Vesicular packaging sequesters the transmitter from these enzymes and creates discrete 'quanta' — each vesicle holds a fixed amount, so release of one vesicle produces a stereotyped postsynaptic response. This quantal organization is what allows graded control of signaling strength.

Question 2

Small-molecule neurotransmitters (such as dopamine and acetylcholine) are synthesized in the cell body and transported down the axon in finished form to the presynaptic terminal.

• T. True
• F. False

Answer: False

Small-molecule neurotransmitters are synthesized locally in the presynaptic terminal from precursors. The enzymes that catalyze synthesis are made in the cell body and transported down the axon, but the final synthesis steps happen at the terminal. Neuropeptide transmitters, by contrast, are synthesized on ribosomes in the cell body and transported as finished peptides.

Question 3

How does vesicular packaging of neurotransmitters support the precision and reliability of synaptic transmission?

Model answer: Each vesicle contains a fixed, quantized amount of neurotransmitter. Releasing one vesicle produces a miniature postsynaptic potential of predictable size (a quantum). The synapse controls signal strength by varying how many vesicles are released per action potential, not by varying the concentration per vesicle. This quantal organization gives the synapse reliable, graded control over postsynaptic responses.

Quantal release was discovered by Fatt and Katz studying miniature end-plate potentials. The fixed packet size means biological noise in single-vesicle release does not translate to unpredictable postsynaptic responses — it just determines the unit size of the signal.