Figure 1: A partial circuit for sensitization and classical conditioning in Aplysia. In sensitization, a shock to the tail acts on siphon sensory neurons to bring about presynaptic facilitation. In classical conditioning, presynaptic facilitation is dependent upon the temporal pairing of CS (siphon) pathway activity and US (tail) pathway activity. The CS pathway is active just prior to activation of the US pathway, thereby enhancing presynaptic facilitation of the siphon sensory neuron. (Adapted from Hawkins and Kandel 1984.)