MESENCEPHALIC PROJECTIONS OF THE COCHLEAR NUCLEUS IN THE FROG, RANA-ESCULENTA

The lectin Phaseolus vulgaris leucoagglutinin was injected into chochl ear nucleus in order to study the trajectory and projections of the au ditory pathway in the frog. A strong contralateral and a weak ipsilate ral fiber bundle representing the lemniscus lateralis could be followe d to the torus semicircularis. The following areas of termination coul d be discerned in the mesencephalon: (1) Each of the five subnuclei of the torus semicircularis received fibers from the cochlear nucleus, t he largest number of fibers terminated in the principal nucleus. (2) N uclei of the mesencephalic tegmentum were supplied by fibers from the cochlear nucleus on both sides. (3) The nucleus isthmi was innervated by a few small caliber fibers, and (4) the nucleus visceralis secundar ius was richly supplied by large caliber fibers. Two ill-defined neuro n groups in the mesencephalic tegmentum, the nucleus reticularis isthm i (5) and (6) the nucleus profundus mesencaphali were conspicuously ou tlined by the terminals of auditory fibers. Their positions suggested homology to the mammalian nucleus of the lemniscus lateralis. (7) A fe w fibers and boutons were labelled in the fifth and sixth layers of th e optic tectum. Retrogradely labelled cells were found in the nucleus laminaris of the torus semicircularis and in the nuclei of mesencephal ic tegmentum. Our results indicate that the frog auditory pathway is m ore complex at the level of the secondary fiber projections than has b een previously recognized. The retrogradely labelled neurons suggest a descending control of the cochlear nucleus from higher structures of the auditory pathway.