Fine structure and distribution of axon terminals from the cochlear nucleus on neurons in the medial superior olivary nucleus of the cat.

The morphology and distribution of axon terminals on central column and marginal neurons of the cat medial superior olivary nucleus (MSO) were analyzed by electron microscopy. Individual neurons or groups of cells oriented such that substantial lengths of their dendrites were within a 5-7 mu thich section were selected for detailed study. Thin sections were cut from remounted thick sections. Boutons with spherical vesicles arise directly from myelinated axons; more than one synaptic region of an axon, each separated by a myelinated segment, may contact a given dendrite. Boutons with flattened and occasionally dense core vesicles arise from both myelinated and unmyelinated portions of axons; these axons may also have more than one synaptic region. Both kinds of synaptic profiles are found on the somata and dendrites of all MSO neurons. To determine which nerve endings are from the cochlear nucleus (CN) lesions were made to produce orthograde degeneration. Following unilateral CN lesions degenerating spherical vesicle terminals were observed on the lateral dendrites and somata of ipsilateral central column cells and the medial dendrites and somata of contralateral neurons. Degenerating terminals were rarely seen on the opposite dendrite (three of 48 cells). In six of seven instances where medial and lateral dendrites of two cells overlapped degeneration was limited to one oriented toward the lesion. Marginal cells examined received virtually all spherical vesicle terminals from only one CN. Terminals with flattened vesicles persisted on the somata and dendrites of all neurons studied including cells from cats with bilateral lesions.