Anatomy and Physiology of Principal Cells of the Medial Nucleus of the Trapezoid Body (MNTB) of the Cat

Abstract

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.

The Medial Nucleus of the Trapezoid Body in the Gerbil Is More Than a Relay: Comparison of Pre- and Postsynaptic Activity

“…Displaying these responses using small bin widths (Fig. 10 C, F insets) indicates that these responses had the same primarylike notch patterns that have previously been described for MNTB units and globular bushy cells in the VCN (Smith et al 1991(Smith et al , 1998Tsuchitani 1997).…”

“…The tonotopic organization of the medial nucleus of the trapezoid body of the gerbil is comparable to that of other mammals (cat: Guinan 1972b;rat: Sommer et al 1993;Smith et al 1998). Differentiating the MNTB from other nuclei within the SOC during recording is facilitated by the fact that the MNTB contains only monaural units, i.e., they are all acoustically driven only by the contralateral ear.…”

“…In addition, the latency of the initial spike discharge was highly variable. To our knowledge, such a discharge pattern has never been described for MNTB neurons or globular bushy cells, which typically show primarylike responses or primarylike notch responses (e.g., Smith et al 1991Smith et al , 1998. In addition, during two-tone stimulation, IPPs and SPPs exhibited temporally correlated changes in spike number that cannot easily be accounted for if these signals are generated by different neural elements rather than by FIG.…”

“…The superior paraolivary nucleus, in which most neurons label for GABA, might provide another source of GABAergic input to the MNTB (Helfert et al 1989). The glycinergic terminals arise, in part, from recurrent projections of MNTB neurons Smith et al 1998). To date, the physiological effect of inhibitory neurotransmitters and of inhibitory input to the MNTB has been studied exclusively using slice preparations (Banks and Smith 1992;Forsythe and Barnes-Davies 1993b;Wu and Kelly 1995;Turecek and Trussell 2001).…”

“…The innervation of MNTB neurons by extrinsic and intrinsic inhibitory afferents, the effect of inhibitory neurotransmitters, and the fact that the MNTB projects to several other brainstem auditory nuclei in addition to the LSO (Kuwabara and Zook 1991;Schofield and Cant 1991;Banks and Smith 1992;Sommer et al 1993;Schofield 1994;Smith et al 1998) suggest that the MNTB does not serve only as a sign-converting relay to the LSO. To test this hypothesis, the excitatory afferent input to the MNTB neuron and the firing of the MNTB neuron itself were simultaneously monitored with the same electrode during in vivo extracellular recordings in the gerbil.…”