Ultrastructure of neurons and large synaptic terminals in the lateral nucleus of the trapezoid body of the cat.

Neurons of the lateral nucleus of the trapezoid body (LNTB), the most prominent periolivary nucleus of the cat superior olivary complex, form an important component of the descending auditory pathways and also innervate the medial superior olive. Cells forming the posteroventral subnucleus (pvLNTB), when investigated by light microscopy, exhibit morphological similarities with globular bushy cells of the cochlear nucleus and principal cells of the medial nucleus of the trapezoid body. These latter two cell types are integral components of brainstem circuitry mediating the early stages of sound localization. In this report, ultrastructural features of LNTB neurons are described. pvLNTB cell bodies are characterized by a round to oval shape, smooth nuclear membrane, and the relative paucity of stacks of rough endoplasmic reticulum. In addition, pvLNTB cell bodies and proximal dendrites are contacted by large synaptic terminals which contain round synaptic vesicles and form multiple asymmetric synaptic junctions. These ultrastructural characteristics are similar to those previously described for globular and principal cells and distinguish pvLNTB cells from cells of the main subnucleus. Large terminals contacting pvLNTB cells contain a specialized organelle assembly, including an adherens plaque associated by filamentous strands with a mitochondrion. We name this organelle assembly the mitochondria-associated adherens complex (MAC) and note its proximity to synaptic junctions. Because high activity rates are characteristic of large terminals in the lower auditory system, the MAC may play a specialized role in membrane stabilization at synapses which generate high rates of vesicle membrane turnover.