Neurophysiology and Synapses · USMLE Step 1

Which of the following is true about the probability of vesicle release at any given presynaptic action potential?

  1. Every vesicle always fuses with the membrane upon every action potential.
  2. Release is entirely random and independent of calcium.
  3. Only a fraction of vesicles fuse, so release is probabilistic and quantal.
  4. Release depends solely on the number of vesicles present, not on synaptic proteins.
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Correct answer: Only a fraction of vesicles fuse, so release is probabilistic and quantal.

Neurotransmitter release is probabilistic; only a fraction of vesicles fuse per action potential. This is consistent with the concept of quantal release, where each quantum represents one vesicle.

Difficulty: Medium Question 23 of 28

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