Contribution of polysynaptic pathways to the mediation and plasticity of ApZysia gill and siphon withdrawal reflex: evidence for differential modulation

The gill and siphon withdrawal (GSW) reflex of Aplysia is centrally mediated by a monosynaptic and a polysynaptic pathway between sensory and motor neurons. The first ob-jective of this article was to evaluate quantitatively the rel-ative importance of these two components in the mediation of the GSW reflex. We have used an artificial sea water (ASW) solution containing a high concentration of divalent cations to raise the action potential threshold of the inter-neurons without affecting the monosynaptic component of the reflex (2:l ASW). Compound EPSPs induced in gill or siphon motor neurons by direct stimulation of the siphon nerve or by tactile stimulation of the siphon skin were re-duced by more than 75 % in 2:l ASW. These results indicate that interneurons intercalated between sensory and motor neurons are responsible for a considerable proportion of th