NEURONAL TYPES IN THE SPINAL DORSAL GRAY OF THE TURTLE CHRYSEMYS-DORBIGNY - A GOLGI-STUDY

At thoracic and lumbar levels the spinal dorsal gray of young specimen s of the turtle Chrysemys d'orbigny consists of a cell-free neuropil a nd an aggregation of perikarya termed here the lateral column of the d orsal horn (LCDH). Nerve cell clusters also occur in the dorsal commis sure. The main neuropil area can be divided into a thin superficial la yer containing some myelinated fibers (neuropil area Ib) and a compact core composed of unmyelinated axon terminals, dendritic branches, and thin glial processes (neuropil area II). A looser neuropil area is lo cated at the horn base (neuropil area III). The so-called marginal zon e of de Lange represents a fourth synaptic field termed here neuropil area Ia. The LCDH consists of neurons of different size and shape. Two peculiar nerve cell types have been recognized in the dorsal horn: gi ant and bitufted neurons. The former exhibits a large dendritic arbor, which after passing through neuropil areas II and Ib projects into ne uropil area Ia and the adjacent white matter. Most frequently Golgi-st ained giant neurons have perikarya and dendritic domains on the same s ide (ipsilateral giant neurons). There are also heterolateral giant ne urons whose dendritic branches invade the opposite horn. Bitufted neur ons are characterized by the presence of two main dendritic shafts con necting neuropil area II of both dorsal horns. At neuropil levels the major dendritic branches ramify profusely giving rise to short tortuou s terminal processes. Perikarya of bitufted neurons occur in the dorsa l commissure. The LCDH also contains many small and medium-sized neuro ns. These are oriented in two main directions: parallel or radial with respect to the dorsal horn surface. The population of horizontally or iented neurons comprises two subtypes termed here alpha and beta. Radi ally oriented neurons are pleomorphic, defying precise, unequivocal cl assification.