Interneurons involved in mediation and modulation of gill-withdrawal reflex in Aplysia. I. Identification and characterization.

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Abstract
The gill-withdrawal reflex of Aplysia undergoes both habituation and sensitization and has been used as a model system for analyzing the neural bases of these simple, nonassociative forms of learning. Previous work has concentrated on the monosynaptic component of the reflex. Identified and characterized are 6 types of interneurons involved in the polysynaptic component. Of these, 4 (L22, L23, L28 and L29) are excitatory interneurons that receive excitatory postsynaptic potentials (EPSP) from LE mechanoreceptor neurons and produce EPSP in gill or siphon motor neurons. L22 and L29 each refer to groups of similar neurons. Two (L16 and L30) are inhibitory interneurons. L16 inhibits L22 and L23 and may mediate recurrent inhibition of the LE neurons. The L30 group of neurons mediates recurrent inhibition of the L29 neurons. The neuronal circuit diagram for the reflex is brought up to date, and the role of the interneurons in habituation of the reflex is discussed. Decrement of the postsynaptic potentials (PSP) from the mechanoreceptor neurons to the excitatory interneurons and also of PSP from excitatory interneurons to gill and siphon motor neurons probably contributes to habituation.

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