Which finding best distinguishes acetylcholine (ACh) release via quantal hypothesis from non-quantal, continuous release of neurotransmitter?

Explanation

## Quantal vs. Non-Quantal Neurotransmitter Release ### The Quantal Hypothesis **Key Point:** The **quantal hypothesis** (proposed by Katz) states that neurotransmitter is released in discrete packets (quanta), each quantum representing one synaptic vesicle containing ~5,000–10,000 ACh molecules. This is the modern, accepted model of synaptic transmission. ### Distinguishing Evidence: Miniature End-Plate Potentials (mEPPs) | Aspect | Quantal Release | Non-Quantal (Hypothetical) | |--------|-----------------|---------------------------| | **Release Pattern** | Discrete packets (vesicles); integer multiples of quantal size | Continuous, random molecular diffusion | | **Spontaneous Activity** | mEPPs: small, uniform depolarizations (~0.5–1 mV) at irregular intervals | Smooth, graded, variable-amplitude noise | | **Statistical Distribution** | Binomial or Poisson; integer multiples of unit amplitude | Gaussian; continuous range | | **Evoked Response** | End-plate potential (EPP) = n × mEPP amplitude | Smooth, graded response | | **Biological Basis** | Exocytosis of intact vesicles | Leakage or diffusion of free molecules | ### Why mEPPs Are the Gold-Standard Discriminator **High-Yield:** mEPPs are **spontaneous, miniature versions of evoked EPPs**, occurring when a single vesicle releases spontaneously (even without an action potential). Their uniform amplitude and discrete nature prove that: 1. ACh is packaged in quanta (vesicles). 2. Each quantum produces a fixed-size postsynaptic response. 3. Larger EPPs are sums of multiple quanta (n × mEPP). **Mnemonic:** **QUANTA = Quantum + Amplitude** — each quantum (vesicle) produces a unit amplitude (mEPP). Multiple quanta sum to produce the full EPP. **Clinical Pearl:** Botulinum toxin blocks SNARE proteins, preventing vesicle fusion and abolishing both mEPPs and evoked EPPs. Curare (competitive antagonist) reduces EPP amplitude but does NOT change mEPP amplitude—it just requires more quanta to reach threshold. This confirms that the quantum (vesicle) is the fundamental unit of release. [cite:Kandel, Schwartz, Jessell Principles of Neural Science Ch 14]