The central projections of intracellularly labeled auditory nerve fibers in cats: an analysis of terminal morphology

Topographic organization of the central projections of the spiral ganglion in cats

The morphological organization of inputs from restricted sectors of the cat cochlear spiral ganglion into the cochlear nucleus was studied by making focal extracellular injections of horseradish peroxidase (HRP) into the spiral ganglion. Injections resulted in Golgi-like labeling of a small cluster of spiral ganglion cells and their peripheral and central axons. Large injections involved most of the cells within Rosenthal's canal in sectors of the spiral ganglion innervating greater than or equal to 1 mm of the basilar membrane and resulted in narrow, complete laminae of labeled axons and preterminal fields within each cochlear nucleus subdivision. The positions of these bands were consistent with the "isofrequency laminae" appropriate for the frequencies represented at the injection sites, with high frequency laminae situated more dorsally, and lower frequencies progressively more ventral. A discrete projection to the small cell cap area was observed that was discontinuous with the main projection laminae in the ventral cochlear nuclei (VCN). In the dorsal cochlear nucleus, projecting fibers and terminals were excluded from the molecular cell layer. No labeled fibers entered the granule cell areas. In contrast to larger injections, very small HRP deposits labeled only part of an isofrequency lamina. Specifically, injections restricted to the scala tympani aspect of the spiral ganglion labeled only the lateral part of VCN isofrequency laminae, whereas injections limited to the scala vestibuli aspect of the ganglion labeled the medial aspect of the isofrequency planes. Thus these data indicate a previously unrecognized topographic representation of the vertical dimension of the spiral ganglion across VCN isofrequency laminae. Some possible functional implications of this projection organization are discussed.

Endbulbs of held and spherical bushy cells in cats: morphological correlates with physiological properties

Single auditory nerve fibers of type I spiral ganglion cells in cats were electrophysiologically characterized by recording with micropipettes inserted into the axon and then labeled by intracellular injections of horseradish peroxidase (HRP) through the same pipettes. This method for staining and studying single neurons allowed us to describe structure-function relationships for labeled endbulbs of Held and the somata of their postsynaptic spherical bushy cells. The silhouette areas of terminal endbulbs and the corresponding somata of spherical bushy cells were determined by planimetry from drawings made with a light microscope and drawing tube. On the presynaptic side, endbulb area is related to fiber characteristic frequency (CF, the frequency to which a fiber is most sensitive) such that the largest endbulbs arise from fibers having CFs between 1 and 4 kHz; smaller endbulbs can arise from fibers of any CF. Endbulb area is not correlated with fiber spontaneous discharge rate (SR). Dividing the endbulb's silhouette area by its silhouette perimeter, however, yields a "form factor" that is a reliable indicator of fiber SR: Endbulbs from fibers of low-medium SR (less than or equal to 18 spikes/second) have form factor values less than 0.52, whereas endbulbs of high SR fibers (greater than 18 spikes/second) have values greater than 0.52. This form factor should therefore be predictive of SR groupings in auditory fibers for which physiological data are not available. On the postsynaptic side, the somata of spherical bushy cells receiving endbulbs from low-medium SR fibers are on average smaller than those receiving endbulbs from high SR fibers. In contrast, the nuclei of the spherical bushy cells are the same size regardless of presynaptic fiber SR. Some of the effects of low-medium SR fibers on their postsynaptic targets, when compared to those of high SR fibers, appear to be mimicked by effects of experimentally induced deprivation.

Central trajectories of type II spiral ganglion neurons

Previous attempts to trace the central pathways of the thin axons from type II spiral ganglion neurons have been hampered by technical difficulties such as fading of the reaction product as distance increases from the injection site (Ryugo et al.: Soc. Neurosci. Abstr. 12:779, '86; Brown: J. Comp. Neurol. 260:591-604, '87). By using small rodents (gerbils and mice), which have short auditory nerves, we have succeeded in filling the entire central axon and terminals of type II neurons after peripheral injections of horseradish peroxidase. The general course of the type II fibers within the auditory nerve and cochlear nucleus is similar to that of type I fibers except that terminals from type II neurons are often found in regions of the cochlear nucleus that have high densities of granule cells.

Central projections of intracellularly labeled auditory nerve fibers in cats: morphometric correlations with physiological properties

The central arborizations and endings of type I spiral ganglion neurons were labeled with intracellular injections of horseradish peroxidase (HRP) after their characteristic frequency (CF) and spontaneous discharge rate (SR) were physiologically determined. A fiber-by-fiber analysis was conducted and the morphological data compared with the fiber's response properties. The total number of branch points was correlated with total fiber length, a relationship that remained relatively constant when analyzing the ascending and descending branches together or separately. On the other hand, the ascending branches of four out of five fibers having CFs below 0.5 kHz bifurcated and gave rise to a pair of terminal endbulbs of Held. Low- and medium-SR fibers gave rise to more endings than did high-SR fibers, especially on the ascending branch. This difference was accounted for by small endings, a category composed of terminal boutons, string endings, and small complex endings. The categories of modified endbulbs, and endbulbs of Held did not vary in number with respect to fiber SR. The mean area of each ending type within the small ending category was statistically smaller for low- and medium-SR fibers than for high-SR fibers, whereas the mean area of modified endbulbs and endbulbs of Held was not correlated with fiber SR. Total ending area per fiber appeared independent of either CF or SR. These results are discussed in relation to issues of conservation of axon arborizations and terminals, and convergence of input from the different SR groups.

Characterization of HRP-labeled globular bushy cells in the cat anteroventral cochlear nucleus

We report on the anatomical and physiological features of globular bushy cells in the posterior division of the anteroventral cochlear nucleus based on the characteristics of 20 cells from this population. Each of these cells was recorded from and characterized intracellularly and/or extracellularly, injected with horseradish peroxidase, and studied at the light and/or electron microscopic level. Intracellular records from the vicinity of the globular bushy cell body displayed large, fast synaptic potentials, and in some instances a larger presumed action potential both in silence and during short tone stimulation. Intraaxonal recordings displayed large action potentials in addition to small, fast potentials, which evidence indicates may be the decrementally conducted subthreshold synaptic potentials. Both recording situations indicated that these auditory nerve inputs need not be suprathreshold. Bushy cells with high characteristic frequencies (CFs greater than 3 kHz) typically showed primary-like-with-notch responses to short tones at CF or on-type L responses if the sustained level of discharge after the notch was not as robust. Low-CF bushy cells phase-locked after a well-timed onset spike. Light microscopic anatomy revealed a typically oval cell body giving rise to one or two primary dendrites that branched profusely and an axon that gave off no collaterals within the cochlear nucleus before entering the trapezoid body. Electron microscopic analysis showed a high concentration of large, round, vesicle-containing terminals on the cell body and primary dendrite while the population of terminals on the initial segment and sparsely covered distal dendrites was made up mostly of flat and pleomorphic vesicle-containing terminals.

Projections to the inferior colliculus from the anteroventral cochlear nucleus in the cat: possible substrates for binaural interaction

The projections to the inferior colliculus of the cat are shown in autoradiographs after injections of 3H-amino acids into the anteroventral cochlear nucleus and anterograde axonal transport. Labeled bands of axons are seen in the central nucleus of the inferior colliculus, parallel to the fibrodendritic laminae, and in layers 3 and 4 of the dorsal cortex. A bilateral projection to the lateral, low-frequency part of the inferior colliculus is observed. In contrast, the more ventromedial, mid- and high-frequency parts receive only a contralateral input. The projections from the cochlear nucleus to both the contralateral midbrain and bilaterally to the superior olivary complex appear to be tonotopically organized. After HRP injections in the inferior colliculus, small numbers of stellate neurons are labeled in the lateral and ventral low-frequency parts of the anteroventral cochlear nucleus on the ipsilateral side. EM autoradiographs show labeled axonal endings from both sides of the anteroventral cochlear nuclei are present in the same proportion in pars lateralis. Axonal endings from either cochlear nucleus have small, round synaptic vesicles and make asymmetric synaptic contacts on dendrites. Axons from the contralateral side also make axosomatic contacts on large disc-shaped or stellate cells. Neurons from the ipsilateral anteroventral cochlear nucleus apparently make more synaptic endings per cell as compared to neurons from the contralateral side. Together, bilateral inputs from the anteroventral cochlear nucleus can account for a third of the endings with round synaptic vesicles in pars lateralis of the central nucleus. Morphological similarities among the ascending inputs to the inferior colliculus are discussed. Both direct circuits from the cochlear nucleus to the inferior colliculus and indirect circuits via the superior olivary complex or lateral lemniscus may display banding patterns, nucleotopic organization, or differential synaptic organization. The direct inputs from the anteroventral cochlear nucleus to the colliculus may influence binaural interactions which occur in the superior olivary complex. In addition, direct inputs may create new binaural responses in the inferior colliculus that are independent of lower centers.