The bushy cells in the anteroventral cochlear nucleus of the cat. A study with the electron microscope

Abstract The bushy cells in the anterior division of the anteroventral cochlear nucleus of the cat were studied with the electron microscope. In the anterior part of the anterior division, profiles of bushy cells and their processes are easily identified, since few cells of other types are found in this region. In the posterior and posterodorsal parts of the anterior division, the bushy cells are intermingled with stellate and small cells but can be identified on the basis of light-microscopic descriptions and comparisons with the results from the anterior part. Bushy cells are large, spherical cells with a centrally located nucleus enveloped by sheets of rough endoplasmic reticulum. Thin proximal dendrites jut abruptly from the cell body and contain a relatively pale cytoplasm. The distal dendrites contain few organelles other than numerous, very large mitochondria. The cell soma and proximal dendrites, as well as the axon hillock, receive numerous synaptic terminals, but the distal dendritic processes are contacted by relatively few endings. At least four types of terminals form synaptic contacts with the bushy cells. Very large terminals, containing large, spherical synaptic vesicles and forming multiple asymmetrical contacts, correspond to the end-bulbs of Held from the cochlea. These terminals disappear after cochlear ablations, but the other three types remain. The most numerous of these is a large terminal that contains flattened synaptic vesicles and forms long, nearly symmetrical contacts with the soma and dendrites of bushy cells. The second type of non-cochlear terminal is smaller and contains small, pleiomorphic synaptic vesicles that are not flattened. The third type occurs mainly on bushy cell dendrites, contains small, spherical synaptic vesicles, and forms moderately asymmetrical contacts. The bushy cells probably correspond to the primarylike units described in electrophysiological studies of the anterior division. Primarylike units respond to activity in auditory nerve fibers in a one-to-one manner, a finding compatible with the observation that much of the surface of the soma and dendrites of the bushy cells is contacted by auditory nerve terminals (end-bulbs of Held). Neither the origins nor the functions of the several types of non-cochlear inputs to the bushy cells are known. Further analysis of these inputs and of the other neuronal types in the anterior division, when correlated with physiological and biochemical data from the same cell types, could clarify the functional significance of the observed patterns of synaptic organization.