Recordings from cat trapezoid body and HRP labeling of globular bushy cell axons

Abstract

1. 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)

Response patterns to sound associated with labeled globular/bushy cells in cat

“…Therefore, the frequency of occurrence of a high-frequency inhibitory effect is unknown, as is whether inhibitory inputs play a role over the entire frequency range. However, inhibition was reported in nearly all fibers with a PLn response that were recorded in the trapezoid body (Spirou et al, 1990).…”

“…This was based on a scatter diagram comparing the spontaneous and maximum discharge rates of a relatively small number of PLn units, where clusters of points suggested that there could be 1-3 high-spon ANFs contacting the cell and that each ANF generated EPSPs which were above threshold (see discussion in Spirou et al, 1990). A comparison of the scatter diagram of maximum discharge rate versus SR for the population of GBCs in the present report showed that there are clusters of points, suggesting that individual endbulbs can discharge the postsynaptic element, i.e., all endbulbs are not created equal in terms of synaptic strength.…”

“…Cellular modeling studies have usually begun with a "perfectly spherical soma" and ignored the dendritic input where there are AN contacts (Spirou et al, 2005). Yet by varying the number and efficacy of each input on the soma alone, the range of PSTH patterns associated with GBCs could be duplicated, from PLn to OnL (Spirou et al, 1990;Spirou et al, 2005;Rothman et al, 1993). Though Spirou et al found that the inhibitory inputs covered a relatively small portion of the GBC soma (∼5%), some inhibitory inputs were necessary in the models to duplicate the entire range of PSTH patterns (Rothman et al, 1993).…”

“…OnI (octopus cells) units project to the contralateral ventral nucleus of the lateral lemniscus (VNLL) where they terminate in a calyx ending on cells that are glycinergic (e.g., Adams and Wenthold, 1987;Saint Marie et al, 1997;) and project to the ipsilateral inferior colliculus (IC). PLn/OnL cells have a major projection to the contralateral MNTB where they synapse on glycinergic inhibitory principal cells that project to the LSO (Bledsoe et al, 1990, Spirou et al, 1990. This connection plays a prominent role in creating an interaural level difference (ILD) coding observed in the response of LSO neurons that are used predominently to localize high frequency sounds.…”

“…In the latter instance, they are assumed to provide secure transmission of the afferent activity to the post-synaptic element. Modified endbulbs converge on GBCs with several to more than 60 endings (Liberman, 1993;Spirou et al, 1990;Spirou et al, 2005).…”