Abstract
Anatomical descriptions of the connections of the two giant Mauthner neurones in the central nervous system of teleost fishes 1 , 2 and amphibia 3 have strongly suggested a functional rôle in the equilibration of the organism during the performance of rapid and intricate movements.
The two perikarya lie in the medulla, at the level of entrance of the acoustic nerve roots. The axones decussate in the ventral commissure and course caudally through the spinal cord where synaptic connections are made with the motor horn cells. In addition to establishing a direct connection with the VIII root fibers, many additional dendritic connections have been described. 2 , 3
The nature of the synapse of this cell has been investigated by Bartelmez 2 and by Marui. 4 The latter author has also described certain effects of over-activity upon the morphological structure of the synapse. 5
The developmental specificity of this cell became apparent during experiments in which the cephalic end of the spinal cord was replaced by an extraneous medulla. In some animals with two medullas in series, four Mauthner's cells with axones coursing through the cord have been noted. 6 , 7
The experiments briefly reported here deal with the elimination of one of the two neurones normally present, with a view to studying the effect of such absence upon the activity of the organism. One ear vesicle of embryos in the tail-bud stage of development was excised, thus exposing the medulla. A very small piece of the lateral wall of the medulla was then excised, after which an otic vesicle from another embryo was grafted (with normal orientation) into the excavated ear region. For comparison, other experiments were carried out in which the ear vesicle alone was removed.
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