Structure of the network mediating siphon-elicited siphon withdrawal in Aplysia.

1. The network mediating siphon-elicited siphon withdrawal in Aplysia is a useful model system for cellular studies of simple forms of learning and memory. Here we describe three new cells in this circuit, L33, L34, and L35, and several new connections among the following network neurons: LE, L16, L29, L30, L32, L33, L34, and L35. On the basis of these findings we present an updated diagram of the network. Altogether, 100 neurons have now been identified in the abdominal ganglion that can participate in both siphon-elicited and spontaneous respiratory pumping siphon withdrawals. 2. Two features of the interneuronal population may have important behavioral functions. First, the L29 interneurons make fast and slow excitatory connections onto the LFS cells, which may be important for transforming brief sensory neuron discharges into the long-lasting motor neuron firing that underlies withdrawal duration. Second, inhibitory interneurons are prominent in the network. The specific connectivity of certain of these interneurons is appropriate to block potentially interfering inhibitory inputs from other networks during execution of the behavior. 3. Deliberate searches have so far revealed very few excitatory interneuronal inputs to the network interneurons and motor neurons within the abdominal ganglion. These results, together with intracellular studies by others, are more consistent at present with a relatively dedicated rather than a highly distributed organizational scheme for the siphon-elicited siphon withdrawal circuitry.