The medial division of the medial geniculate body of the cat: implications for thalamic organization.

The structure of neurons and axons was studied in the medial division of the medial geniculate body of the cat with the Golgi methods. The results show that the medial division consists of morphologically heterogeneous neurons. The main types, in descending order of frequency, are medium-sized neurons with (1) radiate, (2) tufted, or (3) elongate dendrites; (4) small stellate or radiate neurons, including Golgi type II cells with a locally arborizing, sparsely branching axon collateral system; (5) large neurons, which are weakly tufted. A variety of afferent axons impose a reticulate appearance on the fiber architecture of the medial division. The dominant element in the neuropil consists of axons terminating in the medial division as well as a collateral system of fibers traveling to the adjacent ventral and dorsal divisions of the medial geniculate body. Four types of extrinsic axons are described, including two kinds of thin axons with collateral systems, thick fibers with restricted branches, and large axons with elaborate, serpentine collaterals. Compared to the dorsal and ventral divisions of the medial geniculate body, where, respectively, radiate and tufted neurons are more frequent, the medial division is intermediate in a sense-not that the degree of radiate or tufted dendritic branching is less well developed, but neither type of cell predominates. Moreover, all of the cell types are overlapping in distribution, although the large ones tend to be more common rostrally in the medial division. Likewise, there is no clear-cut regional segregation of axonal types. The ascending projections to the medial division originate in many different structures, involving purely auditory tracts from the inferior colliculus, spinal inputs, and pathways from polysensory regions, such as the midbrain tegmentum and deep layers of the superior colliculus. The spinothalamic tract projects most heavily to the rostromedial region of the medial division bordering the ventrobasal complex. The auditory input from the central nucleus of the inferior colliculus tends to project more heavily to the caudolateral region next to the ventral nucleus of the medial geniculate body. However, these inputs overlap in the medial division. Moreover, the pathways from the tegmentum and the superior colliculus have a broad distribution in the medial division. Still other inputs are known, not to mention those from the cerebral cortex and a widespread and complicated pattern of thalamocortical projections.(ABSTRACT TRUNCATED AT 400 WORDS)