Aplysia Gill-Withdrawal Reflex: Quantal Vesicle Release (2D)

This is the discrete, single-vesicle counterpart to the 3D Aplysia gill-withdrawal simulator. Rather than a smooth vesicle-pool fraction decaying under an ODE, twelve individual presynaptic docking sites are tracked one by one — each independently empty or occupied, each releasing stochastically on a stimulus. Habituation is literally fewer occupied sites firing fewer quanta after repeated touches; sensitization is serotonin from a facilitatory interneuron raising each site's release probability. Every released vesicle becomes a neurotransmitter quantum that must physically random-walk across the synaptic cleft before it can contribute to the postsynaptic EPSP — Katz's quantal hypothesis, animated. Adjust the stimulus interval, site-recovery time constant and serotonin gain, then fire a train of touches followed by a tail shock to watch the gill withdrawal habituate and then snap back to life.