Neuromuscular Junction: Quantal Transmission

Stimulation

Presynaptic Conditions

Extracellular Ca²⁺ 2.0 mM

Readily-releasable pool size N 50 vesicles

Postsynaptic receptor blockade 0%

This Event

Press "Fire Action Potential" to release a quantum of transmitter.

0 mV30 mV

Quantal content (k)

–

EPP amplitude

–

Pool remaining

50 / 50

Mean release p(Ca)

–

How it works

Each nerve impulse releases transmitter in discrete quanta — one synaptic vesicle's worth of acetylcholine — not as a continuous trickle (del Castillo & Katz, 1954). With N vesicles docked in the readily-releasable pool, each firing with an independent release probability p, the number k that actually fuse on a given impulse is a binomial random variable:

P(k) = C(N,k) · p^k · (1-p)^(N-k)
mean quantal content:  m = N·p

Release probability itself rises steeply with extracellular Ca²⁺ because four Ca²⁺ ions must bind cooperatively to trigger a vesicle's fusion sensor (synaptotagmin) — the classic 4th-power Ca²⁺ dependence (Dodge & Rahamimoff, 1967):

p(Ca) = p0 · (Ca / Ca_ref)^4,  clamped to [0, 0.95]

Each fused vesicle contributes one quantum of depolarization q to the end-plate potential (EPP); receptor blockade (e.g. curare, or autoimmune loss as in myasthenia gravis) scales q down:

EPP = k · q · (1 − blockade)

A healthy neuromuscular junction has a huge safety factor: EPP normally overshoots the ~threshold needed to fire the muscle fiber by 3–5×. This model exposes that margin — push Ca²⁺ down, shrink the pool, or block receptors far enough and the EPP falls below threshold and the muscle fails to fire, exactly as it does in fatigue, Lambert–Eaton syndrome (low release probability) or myasthenia gravis (receptor loss).

The 50 Hz train button repeatedly depletes the readily-releasable pool faster than it refills (pool recovers with time constant τ ≈ 0.8 s), reproducing short-term synaptic depression: quantal content — and EPP — visibly shrink pulse over pulse.

Synaptic vesicle (ACh) Released neurotransmitter AChR receptor (bound)

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Neuromuscular Junction: Quantal Synaptic Transmission