Isthmotectal connections in plethodontid salamanders.

In the plethodontid salamander species Plethodon jordani and Hydromantes italicus, the morphology and connectivity of isthmic cells were investigated by means of intracellular staining with biocytin. Dendritic arborization, axonal pathways, and size and morphology of telodendritic structures in the fiber layers of the optic tectum were determined. The latter were studied electron microscopically. The majority of isthmic neurons project to both tectal hemispheres, each cell forming telodendritic structures of different extent in the ipsilateral and contralateral hemisphere. These structures are column-like with diameters of about 60 microm. The ipsilateral terminals extend through layers 1-3 of the tectal white matter and intermingle with terminals of contralateral retinal afferents in layers 1-3, as well as with ipsilateral retinal afferents in layer 3. The corresponding contralateral telodendritic structures are confined to layer 1 and are not in direct contact with ipsilateral retinal afferents in layer 3. Both telodendritic structures are located in the rostral two-thirds of the optic tectum, which are binocularly innervated. The rostrocaudal and mediolateral sites of isthmic telodendra are in register with the direct retinal map in each tectal hemisphere. Few isthmic cells project to the caudal one-third of the optic tectum, which is monocularly driven. These cells form only one axonal terminal in the ipsilateral tectal hemisphere. Size and structure of these ipsilateral terminals are similar to the ipsilateral telodendra of bilaterally projecting isthmic neurons.