The neuronal architecture of the dorsal division of the medial geniculate body of the cat. A study with the rapid Golgi method.

The neurons in the nuclei of the dorsal division of the medial geniculate body were studied with the rapid Golgi method in kittens and young adult cats. The dorsal nuclei contain two principal cell types: a large stellate neuron with radiate dendrites and a bushy neuron with tufted dendrites, each with extensive dendritic fields. Two sizes of cells with locally arborizing axons (local circuit neurons) were found. The small and commonly observed one is stellate in shape, has a limited dendritic domain, and an axon with multiple, often profuse collaterals ending in the vicinity of the cell. A second, and much less common variety (large local circuit neuron) is somewhat larger and has fewer axon collaterals. Subtle, but distinct variations in these cell types distinguish the deep and superficial dorsal nuclei and also the anterior tier of nuclei (deep dorsal and superficial). These functional differences may correlate with the relative morphological homogeneity of the ventral nucleus compared to the extremely heterogeneous medial division. The dorsal division should be regarded as part of the pulvinar-lateralis posterior complex both structurally and functionally. In the suprageniculate nucleus, the principal neurons are stellate cells with large perikarya and numerous and extensive dendrites covered with appendages. The large axon is devoid of collaterals. A small local circuit cell with several axon collaterals, and sparse, restricted dendrites has also been observed. In the adjacent posterior limitans nucleus, the principal neuron has a medium-sized, piriform or somewhat elongated perikaryon, a few very long radiating dendrites, which may span the depth of the nucleus, and a long, poorly branched axon. Small neurons are also seen here. A comparison of the structure, connections, and function of the medial geniculate body suggests that the dorsal division is predominantly, but probably not exclusively auditory, while the ventral nucleus is entirely auditory and relatively homogeneous, and the medial division, polymodal and heterogeneous with respect to input.