Organization of the cat trapezoid body and the discharge characteristics of its fibers.

The laminae of the midline cat trapezoid body (TB) are delimited and named on the basis of their characteristic fiber diameters. Proceeding ventrally from the dorsal boundary of the TB are found the medium, mixed, large, and small diameter populations (the MDP, XDP, LDP, and SDP respectively). Specific cell types of the cochlear nuclear complex give rise to the fibers of the TB. Medium diameter fibers originate from spherical cells while globular cells are the source of the large diameter fibers. Bilateral cochlear nuclei ablations demonstrate that, withe the possible exception of a few thin fibers, all midline TB fibers are second order. The discharge characteristics of single units recorded near the midline in the MDP, XDP, and LDP are presented. The MDP displays a dorso-ventral progression of best frequencies, from low to high. Very high best frequencies (greater than 12 kHz) do not seem to be represented in the MDP but are found in the XDP and LDP. Except for latent periods, the properties of units in the MDP are the same as those of spherical cells and eight nerve fibers for all parameters where comparisons are available. The excitatory response characteristics of units in the XDP and LDP are similar to those of the MDP. They differ from MDP fibers in having tuning curves with inhibitory sidebands as well as a lower mean spontaneous rate of discharge. The results confirm previous observations on the spherical cells and define the discharge characteristics of the globular cells.