Fig. 4. (A) Consistency among the gastropods in the occurrence of protraction-phase interneurones with axons ascending in the contralateral cerebro-buccal connective. Planorbis group 1 interneurone (Arshavsky et al., 1988a); Lymnaea N1 interneurone (Elliott and Benjamin, 1985a); Aplysia B34 interneurone (Hurwitz et al., 1997). (B) Variation between retraction-phase (N3) neurones in Lymnaea. The anatomical variation shown is accompanied by physiological and pharmacological variation: the tonic N3 fires many action potentials, each producing a short excitatory postsynaptic potential on the B3 motoneurone, while the phasic N3 and OC cells fire transiently at the start of this phase. The phasic N3 neurone produces a long-lasting excitation of N3 and the OC an inhibition of the B3 motoneurone. The OC (but not the other interneurones) uses octopamine as its transmitter. Modified from Elliott and Benjamin (1985a) and Vehovszky et al. (1998).