Morphology of the cells of origin of descending pathways to the spinal cord in Rana esculenta. A tracing study using cobaltic-lysine complex.

The localization and morphological characteristics of neurons projecting to the spinal cord were studied with cobalt-filling technique. Cobaltic-lysine complex was iontophorized into descending pathways in the lumbar and cervical intumescence of the spinal cord and in some parts of the rhombencephalon. Well-filled cells located mainly in the rhombencephalon and telencephalon could be characterised as fusiform, triangular, multipolar and irregular neurons. Piriform and pyramidal cells predominated among projection neurons in the mesencephalon. Bilateral descending spinal pathways originate from the reticular nuclei of the rhombencephalon, nucleus vestibularis lateralis, nucl. anterodorsalis- and anteroventralis mesencephali, nucl. posterior thalami, nucl. ventralis hypothalami, area preoptica anterior and the striatum ventrale. Crossed pathways descend from the nucl. vestibularis descendens, nucl. tractus solitarii, nucl. cerebelli and the nucleus ruber. Uncrossed fibres originate from the nucl. tractus spinalis nervi trigemini, nucl. posteroventralis tegmenti, nucl. profundus mesencephali, nucl. fasciculi longitudinalis medialis and the nucleus ventrolateralis thalami. The organization of the frog's descending pathways is very similar to those in reptiles and in many ways to those in mammals. The possible synaptic connections of projection neurons have been discussed.