Central influence on peripherally mediated habituation of an Aplysia gill withdrawal response

The parieto-visceral ganglion (PVG) modulates gill habituation, which in turn is mediated by a peripheral neural plexus. 1. A central influence accelerates the rate of habituation and depresses the responsiveness to waterdrops applied to a gill lobe (Figs, 2, 3). The branchial nerve is the major pathway for this influence (Figs. 4, 5). The ctenidio-genital nerve, on the other hand, elicits facilitatory effects in response to gill stimulation (Fig. 4). 2. Complete recovery, with and without the PVG connected to the gill, occurs in 3 hours, indicating that the gill plexus is responsible for the recovery (Fig. 4). 3. PVG influence increases with repeated stimulation, and it is suggested that the influence evokes inhibition in the gill neural plexus (Figs. 3C, 4). 4. Waterdrops evoked spike and PSP activity in L7; EPSP's decremented to repeated application of the stimulus (Fig. 6). L7 cannot be responsible for the observed central effects, however, because they were not suppressed when the cell was hyperpolarized. 5. The PVG is a more efficient analyzer of repeated stimulation to the gill than is the neural plexus. A model is presented for the interaction between the CNS and the gill neural plexus during habituation (Fig. 7).