CONTROL OF LOCOMOTION IN MARINE MOLLUSK CLIONE-LIMACINA .8. CEREBROPEDAL NEURONS

1. The pteropod mollusk Clione limacina swims by rhythmical oscillatio ns of two wings, and its spatial orientation during locomotion is dete rmined by tail movements. The majority of neurons responsible for gene ration of the wing and tail movements are located in the pedal ganglia . On the other hand, the majority of sensory inputs that affect wing a nd tail movements project to the cerebral ganglia. The goal of the pre sent study was to identify and characterize cerebropedal neurons invol ved in the control of the swimming central generator or motor neurons of wing and tail muscles. Cerebropedal neurons affecting locomotion-co ntrolling mechanisms are located in the rostromedial (CPA neurons), ca udomedial (CPB neurons), and central (CPC neurons) zones of the cerebr al ganglia. According to their morphology and effects on pedal mechani sms, 10 groups of the cerebropedal neurons can be distinguished. 2. CP A1 neurons project through the ipsilateral cerebropedal connective to both pedal ganglia. Activation of a CPA1 by current injection resulted in speeding up of the locomotor rhythm and intensification of the fir ing of the locomotor motor neurons. 3. CPA2 neurons send numerous thin fibers into the ipsi- and contralateral pedal and pleural ganglia thr ough the cerebropedal and cerebropleural connectives. They strongly in hibit the wing muscle motor neurons and, to a lesser extent, slow down the locomotor rhythm. 4. CPB1 neurons project through the contralater al cerebropedal connective to both pedal ganglia. They activate the lo comotor generator. 5. CPB2 neurons also project, through the contralat eral cerebropedal connective, to both pedal ganglia. They affect wing muscle motor neurons. 6. CPB3 neurons have diverse morphology: they pr oject to the pedal ganglia either through the ipsilateral cerebropedal connective, or through the contralateral one, or through both of them . They affect putative motor neurons of the tail muscles. 7. CPC1, CPC 2, and CPC3 neurons project through the ipsilateral cerebropedal conne ctive to both pedal ganglia. They activate the locomotor generator. 8. CPC4 and CPC5 neurons project through the contralateral cerebropedal connective to the contralateral pedal ganglia. They activate the locom otor generator. 9. Serotonergic neurons were mapped in the CNS of Clio ne by immunohistochemical methods. Location and size of cells in two g roups of serotonin-immunoreactive neurons in the cerebral ganglia appe ared to be similar to those of CPA1 and CPB1 neurons. This finding sug gests a possible mechanism for serotonin's ability to exert a strong e xcitatory action on the locomotor generator of Clione. 10. The role of different groups of cerebropedal neurons is discussed in relation to different forms of Clione's behavior in which locomotor activity is in volved.