Recordings were made from single nerve fibers in barbiturate-anesthetized cats in the midline trapezoid body, a location that permits selective sampling of efferent cells of the ventral cochlear nucleus. Single units were localized to either the dorsal or ventral components of the trapezoid body. The fibers were physiologically classified on the basis of their peristimulus time histograms (PSTH) and receptive-field properties. In addition, low characteristic frequency (CF) units were probed for rapid rate and phase shifts with increases in intensity. The projection patterns of some fibers were traced by iontophoresing horseradish peroxidase (HRP) into their axons. 2. HRP-labeled fibers most likely originated from globular bushy cells of the ventral cochlear nucleus in that they sent a large branch into the contralateral medial nucleus of the trapezoid body which terminated in a calyceal ending and an ipsilateral branch into the lateral nucleus of the trapezoid body. A thin branch, usually starting from the large branch, wound its way through the medial nucleus of the trapezoid body to its termination in the ventral nucleus of the trapezoid body. Additional branches from the parent axon could pass through medial periolivary groups throughout the rostrocaudal extent of the superior olivary complex. The parent fiber was traced as far as the ventral lateral lemniscus where it faded before reaching its termination. 3. The majority of units were recorded in the ventral component of the trapezoid body. Although the ventral component is comprised of both large and small diameter fibers, our sample was biased to the larger diameter fibers representing the activity of axons originating from globular bushy cells in the ventral cochlear nucleus. Ventral component units were not tonotopically arrayed and had CFs that spanned the audible range for cats. HRP labeling of ventral component axons revealed that the section of the axon traveling through the midline shifted its dorsal-ventral location. This pattern was compatible with the lack of tonotopy found in the ventral component. Recordings were also made from the dorsal component of the trapezoid body, which contained medium diameter axons. These axons originated from spherical bushy cells in the ventral cochlear nucleus. Dorsal component units were tonotopically arrayed and had CFs less than 7 kHz. 4. Cells were characterized by their PSTH at CF. Primary-like and phase-locked units constituted most of the dorsal component units.(ABSTRACT TRUNCATED AT 400 WORDS)
