Organization of thalamic afferents to anterior dorsal ventricular ridge in turtles. II. Properties of the rotundo-dorsal map.

This study describes some properties of the map of nucleus rotundus onto dorsal area of anterior dorsal ventricular ridge (ADVR) in emydid turtles by correlating results of anterograde and retrograde tracing experiments with observations from Golgi- and myelin-stained brains. An earlier paper (Balaban and Ulinski, '81) demonstrated that this projections is restricted to zone 4 of dorsal area of ADVR. This paper indicates that the rotundal pathway is organized such that longitudinally aligned groups of neurons in nucleus rotundus project to longitudinal regions in zone 4 of dorsal area. The projections field spans the dorso-ventral (or concentric) dimensions of zone 4 at each transverse level. Comparisons of experimental and Golgi preparations suggest that each rotundal neuron projects, via collaterals, to the entire rostrocaudal extent of rotundorecipient zone 4. Individual terminal branches span the dorsoventral dimension of zone 4 and are confined with both sagittal and transverse planes. Lesion experiments suggest that collaterals of a single rotundal axon are also distributed over at least one-third to one-half of the superficial-deep dimension of zone 4. This is also reflected in the observation that neurons from disjoint dorsal, dorsolateral and medial rotundal loci project to overlapping, concentric regions of dorsal area. Both this prominent concentric component of terminal branches and the extensive overlap is projections of neurons at distinct rotundal loci preclude the possibility of a topographic representation of either dorsoventral or mediolateral rotundal axes in zone 4 of dorsal area.