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We examined the morphological peculiarities of Mauthner neurons, MNs, in goldfishes with a phenotypically different or an experimentally modified preference to perform rightward vs leftward turnings in the course of motor behavior; this preference was characterized by values of the motor asymmetry coefficient (MAC). 3D reconstruction of MNs was performed based on several histological sections; volumes of the soma, lateral and ventral dendrites (LD and VD, respectively), initial segment of the axon, as well as full volumes of the right and left neurons, were calculated. Differences between the above parameters were expressed as structural asymmetry coefficients (SACs). It was shown that clear orientation asymmetry of motor behavior of the fish is accompanied by differences in the dimensions of MNs and their compartments; MNs localized contralaterally with respect to the preferred turning side were considerably bigger than ipsilateral neurons. Experimental influences inducing inversion of the motor asymmetry of fishes inverted structural asymmetry of their MNs. In fishes with no phenotypical preference of the turning side and in individuals whose motor asymmetry was smoothed due to experimental influences (rotational stimulations), structural asymmetry of the MNs was also smoothed. Changes of the structural proportions developed, as a rule, due to decreases in the dimensions of one or both MNs and their compartments. The MAC value was in direct correlation with the value of SAC of the MNs and with values of this coefficient for the soma and the sum soma + LD. At the same time, reciprocal relations were found for the MAC and structural asymmetry of the VD; the decrease in the volume of VD was related to an increase in the preference of the contralateral turning side by the fish, and vice versa. In general, the results of our study demonstrate that both morphological and functional peculiarities of MNs correlate to a significant extent with such a form of motor behavior of fishes as realization of spontaneous turnings.

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Investigation of the influence of cAMP on neuronal electric activity suggests that nerve cells can solve problems using an intraneuronal calculating medium based on the cytoskeleton. When a new problem is posed, this structure has to be disassembled and assembled by the neuronal molecular computer according to the program recorded in DNA. If DNA lacks an appropriate program, the cytoskeleton will not be assembled. In our experiments, fishes which were rotated simultaneously around two mutually perpendicular axes lost their swimming ability, and some dramatic changes were observed in the cytoskeleton of their Mauthner neurons. These changes disappeared after a long-term rest: the cytoskeleton was restored simultaneously with the ability for normal swimming.

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