DEFENSE REACTION IN THE POND SNAIL PLANORBIS-CORNEUS .3. RESPONSE TO INPUT FROM STATOCYSTS

1. In the intact pond snail Planorbis corneus, a rapid tilt in any pla ne evoked a defense reaction consisting of a fast movement of the shel l towards the head, shortening of the foot, inhibition of locomotion a nd of rhythmical feeding movements. This reaction was similar to the f irst phase of the general defense reaction of Planorbis to cutaneous s timulation. 2. A method has been developed for inclination of the isol ated CNS in space (up to 900) and simultaneous intracellular recording s from different neurons. 3. The statocyst receptor cells (SRCs) respo nded both phasically and tonically to the tilt. The SRCs differ in the ir spatial zones of sensitivity. 4. Essential manifestations of the de fense reaction to the input from statocysts could be observed in the i n vitro preparation of the CNS isolated with statocysts. Both tilting of the CNS and electrical stimulation of individual SRCs elicited an e xcitatory response in numerous neurons from different ganglia, includi ng motor neurons (MNs) of the columellar muscle. This response was of ''all-or-none'' nature, and could be evoked by electrical stimulation of any SRC. The response was followed by a long( 10-20 s) period of re fractoriness. 5. Activation of SRCs resulted also in excitation of the giant dopaminergic cell in the left pedal ganglion (related to the co ntrol of respiration), in inhibition of the feeding rhythm generator, and in inhibition of the pedal neurons responsible for activation of t he ciliary locomotor system. 6. Combined stimulation of two inputs abl e to evoke a defense reaction, i.e., those from the statocyst and from cutaneous nerve, revealed a strong interdependence of their central e ffects. A defense response to a given input could not be evoked during the refractory period following the response to another input. This i nterdependence is most likely due to the fact that both inputs converg e on the same interneuron (DRN1) in the right pleural ganglion. The '' all-or-none'' character of the statocyst-driven defense reaction, in c ontrast to the graded response to cutaneous nerve stimulation, could b e explained if the input from statocysts activated only one DRN1.