A Golgi study of the medial geniculate body in the tree shrew (Tupaia glis).

The subdivisions of the medial geniculate body in Tupaia recognized in previous connectional and cytoarchitectural studies are identified in Golgi-impregnated material. They may be distinguished by the organization of the neuropil, especially the dendrites, and, in many cases, by differences in the neurons. The ventral nucleus contains tufted cells with disc-shaped dendritic fields which are aligned to form laminae. The caudomarginal and deep dorsal nuclei have less tufted, less precisely arranged cells with longer, thin appendages. Neurons in the suprageniculate and dorsal nuclei are similar except that they apparently are arranged more randomly and tend to have more spherically shaped dendritic fields. The medial division is characterized rostrally by the presence of stellate cells and caudally by large cells which appear to be the neurons, observed in previous studies to have widespread connections. The results of this Golgi study suggest that the subdivisions of the medial geniculate body might be grouped differently than in previous reports. When combined with data from previous studies of connections, the results show that the medial geniculate body of even distantly related species may follow a common plan. The ventral nucleus is the medial geniculate component of the central pathway which extends from the central nucleus of the inferior colliculus to the primary auditory cortex. Most of the other medial geniculate subdivisions participate in either pericentral pathways originating in the cortex and other nuclei which surround the central nucleus of the inferior colliculus or in the pathways of the lateral midbrain tegmentum. Pericentral and lateral tegmental pathways terminate in non-primary auditory cortex. The widespread pathway involves only the caudal nucleus of the medial division. It receives afferents from most, if not all, of the midbrain regions that give rise to the other pathways and distributes to all parts of the auditory cortex where it terminated in layers other than layer III-IV.