Anatomy and physiology of principal cells of the medial nucleus of the trapezoid body (MNTB) of the cat.

We have recorded from principal cells of the medial nucleus of the trapezoid body (MNTB) in the cat's superior olivary complex using either glass micropipettes filled with Neurobiotin or horseradish peroxidase for intracellular recording and subsequent labeling or extracellular metal microelectrodes relying on prepotentials and electrode location. Labeled principal cells had cell bodies that usually gave rise to one or two primary dendrites, which branched profusely in the vicinity of the cell. At the electron microscopic (EM) level, there was a dense synaptic terminal distribution on the cell body and proximal dendrites. Up to half the measured cell surface could be covered with excitatory terminals, whereas inhibitory terminals consistently covered about one-fifth. The distal dendrites were very sparsely innervated. The thick myelinated axon originated from the cell body and innervated nuclei exclusively in the ipsilateral auditory brain stem. These include the lateral superior olive (LSO), ventral nucleus of the lateral lemniscus, medial superior olive, dorsomedial and ventromedial periolivary nuclei, and the MNTB itself. At the EM level the myelinated collaterals gave rise to terminals that contained nonround vesicles and, in the LSO, were seen terminating on cell bodies and primary dendrites. Responses of MNTB cells were similar to their primary excitatory input, the globular bushy cell (GBC), in a number of ways. The spontaneous spike rate of MNTB cells with low characteristic frequencies (CFs) was low, whereas it tended to be higher for higher CF units. In response to short tones, a low frequency MNTB cell showed enhanced phase-locking abilities, relative to auditory nerve fibers. For cells with CFs >1 kHz, the short tone response often resembled the primary-like with notch response seen in many globular bushy cells, with a well-timed onset component. Exceptions to and variations of this standard response were also noted. When compared with GBCs with comparable CFs, the latency of the MNTB cell response was delayed slightly, as would be expected given the synapse interposed between the two cell types. Our data thus confirm that, in the cat, the MNTB receives and converts synaptic inputs from globular bushy cells into a reasonably accurate reproduction of the bushy cell spike response. This MNTB cell output then becomes an important inhibitory input to a number of ipsilateral auditory brain stem nuclei.