Axons of the dorsal division of the medial geniculate body of the cat: a study with the rapid Golgi method.

The arrangement of eight groups of axons afferent to the nuclei of the dorsal division of the medial geniculate body is described in rapid Golgi impregnations from young cats. Three kinds of axons travel predominantly in the brachium of the inferior colliculus and enter the medial geniculate body ventromedially: group I, thin axons resembling ivy tendrils ending along dendrites; group II, thicker axons with a sinuous course and few branches; group IV, coarse thick axons with grumous collaterals and massive peridendritic terminals near principal cells and interneurons. Three kinds of axons enter from the parabrachial region and pass laterally: group III, very thin axons with many collaterals forming dense terminal nests; group V, runcinate axons with sparse, thin collaterals; group VI, either medium-sized (group VIa) or thin (group VIb) smooth axons, perhaps corticofugal, and ending near principal neuron dendrites; group VII, thick axons, entering from the auditory radiation, with large, grapelike terminal arbors; and group VIII, thin and forming peridendritic festoons on principal cells after entering from the brachium of the superior colliculus. There appears to be some, though not complete, segregation of axons in the dorsal division nuclei. Thus axons of groups I, III, IV, and VI are found in each nucleus, although group VI axons are conspicuous in the superficial dorsal nucleus, and group IV endings are much more elaborate in the dorsal and deep dorsal nuclei than in the superficial dorsal nucleus. Each axon type has a specific pattern of terminal branches, which contributes to the texture of the neuropil in each nucleus. Golgi type II axons accentuate these textural differences. Thus each nucleus has a specific pattern of neuropil by virtue of the relative proportions of the different groups of axons ending there and the density and architecture of the axonal plexus. For example, both the dorsal nucleus and the deep dorsal nucleus receive the same groups of afferent axons, but the axonal plexus is more diffusely and evenly distributed in the dorsal nucleus, whereas the neuropil of the deep dorsal nucleus is highlighted by aggregates of grumous endings, more irregularities in the distribution of the axonal plexus, and many more fibers of passage. The extrinsic axons in the dorsal division come from the inferior and superior colliculi, the lateral tegmental system of the midbrain, and the cerebral cortex.(ABSTRACT TRUNCATED AT 400 WORDS)