How does the gill-withdrawal reflex work in the marine snail Aplysia californica?

The gill-withdrawal reflex in the marine snail Aplysia californica is a model system for studying neuroplasticity and the cellular basis of learning. The reflex is mediated by a neural circuit where sensory neurons from the siphon form single-synapse junctions with motor neurons that control gill withdrawal. Additionally, these sensory neurons project onto interneurons that also synapse with the same motor neurons.1

When a tactile stimulus is applied, sensory neurons generate excitatory postsynaptic potentials, triggering action potentials in motor neurons that cause the gill to withdraw. This reflex demonstrates two fundamental forms of learning:1

• Habituation: Repeated stimulation leads to a decrease in the size of postsynaptic potentials, eventually causing the reflex to weaken or cease. This is often attributed to the closing of calcium channels, which reduces neurotransmitter release.1
• Sensitization: An increase in reflex strength following stimulation, driven by presynaptic facilitation. This process involves an increase in the second messenger cAMP within sensory neuron terminals.1

Research by Eric Kandel established that these synaptic changes are central to short-term memory and long-term memory.2