The fine structure of two types of stellate cells in the anterior division of the anteroventral cochlear nucleus of the cat

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The stellate cells in the anterior division of the anteroventral cochlear nucleus of the cat were studied with the electron microscope. Although only one type of stellate cell has been identified at the light-microscopic level, two types can be recognized in electron micrographs. Both can be distinguished from the bushy cells that are also present in the anterior division, since they lack the nuclear cap of granular endoplasmic reticulum characteristic of the bushy cells. The somas of the type I stellate cells receive very few synaptic contacts, but the number of synaptic terminals increases markedly along the proximal dendrites. In contrast, both the soma and proximal dendrites of the type II stellate cells receive numerous synaptic contacts. Both neuronal types receive synaptic endings that contain large, spherical vesicles and that disappear after cochlear ablation. Both types of stellate cells are also contacted by synaptic terminals with small vesicles similar to those that contact bushy cells. In addition, the type II stellate cells receive a type of synaptic ending unlike those previously described. This is a relatively large terminal, containing large, flattened or disk-shaped vesicles, and forming slightly asymmetric synaptic complexes with the postsynaptic cell. These terminals as well as those with small synaptic vesicles survive cochlear ablation. The sources of the non-cochlear terminals are not known. The results indicate that the anterior division of the anteroventral cochlear nucleus of the cat contains at least three types of large neurons, each of which receives synaptic input from the cochlea as well as from other sources. The organization of the synaptic endings on the surface of each type is different. Since distinctive arrangements of cochlear and non-cochlear synaptic terminals could result in different response patterns to acoustic stimuli, each of these neuronal types may correspond to a different type of single unit, defined physiologically.