CONTROL OF LOCOMOTION IN MARINE MOLLUSK CLIONE-LIMACINA .9. NEURONAL MECHANISMS OF SPATIAL ORIENTATION

1. When swimming freely, the pteropod mollusk Clione limacina actively maintains a vertical orientation, with its head up. Any deflection fr om the vertical position causes a correcting motor response, i.e., ben ding of the tail in the opposite direction, and an additional activati on of the locomotor system. Clione can stabilize not only the vertical orientation with its head up, but also the posture with its head down . The latter is observed at higher water temperature, as well as at a certain stage of hunting behavior, The postural control is absent in s ome forms of behavior (vertical migrations, defensive reactions, ''loo ping'' when hunting). The postural reflexes are driven by input from t he statocysts. After removal of the statocysts, Clione was unable to m aintain any definite spatial orientation. 2. Activity of the neuronal mechanisms controlling spatial orientation of Clione was studied in in vitro experiments, with the use of a preparation consisting of the CN S and statocysts. Natural stimulation (tilt of the preparation up to 9 0 degrees) was used to characterize responses in the statocyst recepto r cells (SRCs). It was found that the SRCs depolarized and fired (10-2 0 Hz) when, during a tilt, they were in a position on the bottom part of the statocyst, under the statolith. Intracellular staining has show n that the SRC axons terminate in the medial area of the cerebral gang lia. Electrical connections have been found between some of the symmet rical SRCs of the left and right statocysts.3. Gravistatic reflexes we re studied by using both natural stimulation (tilt of the preparation) and electrical stimulation of SRCs. The reflex consisted of three com ponents: 1) activation of the locomotor rhythm generator located in th e pedal ganglia; this effect of SRCs is mediated by previously identif ied CPA1 and CPB1 interneurons that are located in the cerebral gangli a and send axons to the pedal ganglia; 2) bending the tail evoked by d ifferential excitation and inhibition of different groups of tail musc le motor neurons; this effect is mediated by CPB3 interneurons; and 3) modification of wing movements by differential excitation and inhibit ion of different groups of wing motor neurons; this effect is mediated by CPB2 interneurons. 4. Gravistatic reflexes in the tail motor neuro ns were inhibited or reversed at a higher water temperature. 5. The SR Cs are not ''pure'' gravitation sensory organs because they are subjec ted to strong influences from the CNS. In particular, CPC1 interneuron s, participating in coordination of different aspects of the hunting b ehavior, exert an excitatory action on some of the SRCs, and inhibitor y actions on others. It seems likely that, via these central influence s on SRCs, the spatial orientation of Clione can be changed in relatio n to different forms of behavior.