Figure 1.

Partial circuit diagram for the Aplysia withdrawal reflex. The circuit can account for basic classical conditioning and a number of higher-order features of conditioning. Sensory neurons (SN) that are activated by the siphon conditioned stimulus (CS) make monosynaptic connections onto motor neurons (MN) that produce the gill- and siphon-withdrawal conditioned response (CR) and also onto several interneurons (not shown). The tail shock unconditioned stimulus (US) excites modulatory interneurons including the L29s. In addition, the L29s are excited by the siphon CS and excite the siphon motor neurons, so that they are also excitatory interneurons. Firing of the L29 neurons produces facilitation and activity-dependent facilitation at the SN–MN synapses, and also at the synapses from the SNs onto the L29 neurons themselves. Furthermore, the L29 neurons undergo spike accommodation during prolonged stimulation, caused in part by inhibitory feedback (horizontal bar) from L30 interneurons.