Why does Aplysia californica respond less strongly after a mild stimulus is repeated?

In Aplysia californica, the weakening of the gill withdrawal reflex following repeated mild tactile stimulation is a form of nonassociative learning known as habituation.12

This behavioral decline occurs due to changes in the activity of central neurons rather than sensory adaptation or effector fatigue. Specifically, the process involves neuroplasticity at the synapse, where the excitatory postsynaptic potential—the electrical signal required to transmit an impulse between neurons—decreases in size.12

Mechanistically, this reduction in synaptic efficacy is often attributed to the closing of calcium channels, which limits calcium influx into the presynaptic terminals and subsequently decreases neurotransmitter release. Additionally, research suggests that long-term habituation may involve a reduction in the number of active zones at nerve terminals. By ignoring familiar, non-threatening stimuli, the snail conserves energy for more vital activities, demonstrating the adaptive significance of this neural process.21