The mauthnerian apparatus in the Ichthyopsida; its nature and function and correlated problems of neurohistogenesis.

Abstract

A FTER the investigations, made in 1859 by Mauthner, which led to the discovery of the two giant fibres in the spinal cord of Esox lucius, we can consider the two giant cells, from which these fibres originate, as a true functional system of nervous activity. This functional system, which is peculiar to the majority of fishes and amphibians, is formed by a single pair of symmetrical neurons of extremely large size, the cell bodies of which are located in the tegmentum of the medulla at the level of the VIIIth nerve root, and axons of which reach the tip of the neural cord, each running along the medial longitudinal fasciculus of opposite sides. Since the first investigations, and especially since the wider comparative analysis made by Tagliani (1905), it has been clear that Mauthner's fibres are not present in all the species of Ichthyopsida. As we shall see later, this condition is very important for the functional interpretation of this bineuronal apparatus. The absence of a typical mauthlnerian apparatus was noted in the Cyclostomata and, among fishes, in all the selachians and in some teleosteans, e.g., the Anguilloidea and Murenoidea (Anguilla, Conger, Muraena, Congromuraena, Gymnotus, Ophyctlis), in many bottom-living fishes, e.g., Lophius, Uranoscopus, and Blennius, and in tailless species, e.g., Orthagoriscus. Many exceptions were also found among urodele Amphibians, e.g., Geotriton, Cryptobranchus, Siren, Proteus, Diemictilus, and Salamandra maculosa (according to observations by Mason, 1879, 1888; Osborn, 1888; Koelliker, 1896; Klaussner, 1883; Gage, 1893; Tagliani, 1905; Beccari, 1907), while the M. (Mauthner) neurons were considered to be absent in all the anurans (Reissner 1864; Stieda, 1870; Sala, 1892; Koelliker, 1893, 1896; Athias, 1897; Van Geuchten, 1898; Gaupp, 1899; Tagliani, 1905). Yet, in more recent years, with the help of better techniques, many of these statements had to be rectified. Thus Schmidt (1885) described the M. cells in Proteus, Tuerckheim (1903) and Kingsbury (1895) in Cryptobranchus and Diemyctilus (Triturus) and Szepsenwol (1935) in Geotriton. Also the belief in the absence of these neurons in the anurans was found to be erroneous, for Larsell (1935) found them in tadpoles of Rana fusca and Bombinator pachypus (their presence in Rana fusca having been recently confirmed by Willis, 1947) and Stefanelli described them in Rana esculenta, Rana agilis, and Hyla arborea at the larval stage (Stefaneli and Osti, 1942) and in tadpoles of Discoglossus pictus, Hyla savignii, and Xenopus laevis (1949). In tadpoles of Rana esculenta Stefanelli was also able to show a pair of premauthnerian giant cells, at the level of the Vth nerve root. On the other hand, the absence of a true mauthnerian apparatus in the Bufonida (Bufo vulgaris and Bufo viridis) was confirmed by Stefanelli (1949) and by Zacchei (1949). A revised knowledge of the condition prevailing in the cyclostomes and teleosts brought a modification of many statements. In petromyzonts there are ten pairs of reticular giant cells, only one of which is homologous, as to location and connections, with the M. cells. Observations by De Angelis (in press) show the presence of the M. cells also in Blennius and in other species which, though typical bottom-living fishes, show an active movement of the tail in swimming (Clinus, Gobius, Trachinus, Solea). The morphological and functional significance of the M. giant neurons was not easily understood, and for a long time, as we shall see later, many wrong ideas dominated the subject. Many anatomical, comparative, morpho-ecological and physiological researches were necessary to elucidate the enigmatic significance of these neurons, and only after the most recent results can we consider this question at last resolved. Today the interest in the M. neurons goes beyond their morphological and functional significance and reaches the wider fields of the general problems of neurohistogenesis, because of the size of these elements, their easy localization, and their