Mesencephalic projections of the cochlear nucleus in the frog, Rana esculenta

The lectin Phaseolus vulgaris leucoagglutinin was injected into chochlear nucleus in order to study the trajectory and projections of the auditory pathway in the frog. A strong contralateral and a weak ipsilateral fiber bundle representing the lemniscus lateralis could be followed to the torus semicircularis. The following areas of termination could be discerned in the mesencephalon: (1) Each of the five subnuclei of the torus semicircularis received fibers from the cochlear nucleus, the largest number of fibers terminated in the principal nucleus. (2) Nuclei 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 secundarius was richly supplied by large caliber fibers. Two ill-defined neuron groups in the mesencephalic tegmentum, the nucleus reticularis isthmi (5) and (6) the nucleus profundus mesencaphali were conspicuously outlined by the terminals of auditory fibers. Their positions suggested homology to the mammalian nucleus of the lemniscus lateralis. (7) A few fibers and boutons were labelled in the fifth and sixth layers of the optic tectum. Retrogradely labelled cells were found in the nucleus laminaris of the torus semicircularis and in the nuclei of mesencephalic tegmentum. Our results indicate that the frog auditory pathway is more complex at the level of the secondary fiber projections than has been previously recognized. The retrogradely labelled neurons suggest a descending control of the cochlear nucleus from higher structures of the auditory pathway.