Aplysia Gill-Withdrawal Reflex: Habituation & Sensitization

Stimulate

Parameters

Stimulus interval (ISI) 1.5 s

Vesicle recovery time τ 8 s

Serotonin facilitation gain 2.5×

Live Readouts

Vesicle pool

100%

EPSP amplitude

0.0 mV

Gill withdrawal

5-HT facilitation

off

How it works

This models the sensory-to-motor synapse in Aplysia's gill-withdrawal reflex — the circuit Eric Kandel used to win the 2000 Nobel Prize for showing that simple learning is a change in synaptic strength, not new wiring.

Habituation (non-associative, homosynaptic): each siphon touch releases a fraction of the readily-releasable vesicle pool V from the sensory neuron's terminal. Repeated touches faster than the pool can refill deplete V, so the postsynaptic EPSP — and the gill withdrawal it drives — shrinks with each stimulus.

dV/dt = (1 − V)/τ − p·δ(stim)
EPSP  ∝  p · V              (release probability × available pool)
Gill withdrawal ∝ EPSP

Sensitization (heterosynaptic facilitation): a noxious tail shock activates a facilitatory interneuron that releases serotonin (5-HT) onto the sensory-neuron terminal. Via the cAMP–PKA pathway this broadens the presynaptic action potential and boosts Ca²⁺ influx, multiplying the release probability p for several seconds — so even a depleted, habituated synapse suddenly fires a large EPSP again.

p(t) = p₀ · [1 + (G−1)·e^(−t/τ_5HT)]   after a tail shock

Touch Siphon — fires one test pulse without starting the train, so you can probe synaptic strength at any moment.
Start Habituating Train — fires repeated touches at the chosen ISI; watch the vesicle pool (orange particles at the terminal) run down and the gill's withdrawal shrink.
ISI slider — shorter intervals outrun vesicle recovery and habituate faster; long intervals let the pool refill between touches.
Recovery time τ — how quickly depleted vesicles are replenished between stimuli.
Sensitizing Tail Shock — releases serotonin (yellow burst) that facilitates release for ~6 s, temporarily overriding habituation — dishabituation and sensitization share this same mechanism.

Sensory neuron Motor neuron → gill Facilitatory interneuron (5-HT)

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.