Aplysia Gill-Withdrawal Reflex: Habituation & Sensitization

This simulator rebuilds the sea-slug synapse that founded modern learning neuroscience — Kandel's Aplysia gill-withdrawal reflex. Repeated siphon touches deplete the sensory neuron's vesicle pool, and the gill's withdrawal shrinks stimulus by stimulus: habituation, the simplest form of non-associative learning, playing out as literal presynaptic vesicle depletion and recovery. A tail shock instead recruits a facilitatory interneuron that bathes the same terminal in serotonin, boosting release probability through the cAMP–PKA pathway and producing sensitization — a bigger response even from an already-habituated synapse. Adjust the stimulus interval and vesicle recovery time constant to see habituation build and decay, then fire a sensitizing shock and watch the gill spring back to life.