Desan, Pant Henri THE ORGANIZATION OF THE CEREBRAL CORTEX OF THE POND TURTLE, PSEUDEMYS SCRIPTA ELEGANS

The cerebral cortex of the turtle Pseudemys scripta elegans consists of five areas, L, D2, D1, DM and M, forming narrow strips curving in parallel around the hemisphere. The cell types of the cortex were studied with the Golgi method in areas D1 and D2. The principle cell type possesses a soma in the cellular layer, spiny apical and basal dendrites, and a local axon aborization as well as long-projecting collaterals. Cells with few or no spines are rarer, have local axon arborizations only and vary greatly is morphology of dendritic tree and location of somas at least one specific subtype exists. The connections of the cortex were analyzed with retrograde and anterograde tracers. The olfactory bulb projects to the superficial third of the molecular layer of area L, the thalamus to the superficial third of the molecular layer of areas D2 and D1, and to a deeper zone in areas DM and M. Nucleus lgn of the thalamus projects to the rostral part of area D2, nucleus na to the caudal part of area D2, and nucleus dla to areas D1, DM and M. Nucleus dma and scattered perirotundal cells project densely to the striatum and diffusely to the cortex. The cortical areas also receive a topographic projection from basal forebrain structures shown to contain cholinergic neurons by immunohistochemical methods. Cortical areas L and D1 project to the superficial and middle third of the molecular layer of areas DM and M; these areas in turn project back to the deep third of the molecular layer of L and D1. Area D2 projects to D1. Each cortical area is interconnected with itself by a plexus of axons, which run in the middle third of the molecular layer in areas L, D1 and L and in the deep third in areas DM and M. Some other afferent and efferent connections of the cortex are described, and a model of the homology of turtle and mammalian cortex is suggested. Figure 1. Major structures of the forebrain. This is a coronal section, stained with cresyl violet, through the middle of the left cerebral hemisphere. The right border of the photograph is on the plane of the midline. Lateral is to the left, dorsal is up, ventral is down. Unless noted otherwise, all of the figures in the thesis are of the left hemisphere in this orientation. Most of the large cell groups of the telencephalon are seen at this level. The areas of the cerebral cortex form the surface of the dorsal telencephalon: from lateral to medial these are L, D2, D1, DM and M. Small arrows mark the boundaries between these areas. The dorsal ventricular ridge bulges into the lateral ventricle and has been divided into dorsal (dDVR), medial (mDVR) and ventral (vDVR) sectors. The basal teleacephalon at this level contains three areas which are parts of the striatum in turtles: the paleostriatum (PA), the globus pallidus (GP) and area d. The central lateral nucleus (cl) is a. small nucleus between the cortex and the striatum. The lateral forebrain bundle (1fb) is the major component of the cerebral peduncle. At this level it is seen crossing from the diecephalon to the teleacephalon. Medial to it is the cell plate of the hypothalamus (hy). The optic tract (ot) is also shown. The scale bar is one millimeter long.