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1. The correlations between anatomic and physiological characteristics of primary afferent neurons innervating the anterior semicircular canal in the bullfrog were investigated. These characteristics were examined separately in large groups of neurons, and the direct correlations between them were established in a subset of neurons by means of intraaxonal recording and labeling. 2. Anatomic features of the anterior canalicular nerve that were related with fiber diameter were studied. This nerve was composed of an average of 1,142 fibers (standard deviation of 171 in 5 samples), of which 42% were less than 2 microns in diameter and 8% were greater than 7 microns. The nerve branched into 6 clearly defined bundles, whose fiber diameter-dependent composition could be determined in 5 samples. In the 2 center bundles, 32% of the fibers had diameters greater than 7 microns. In contrast, these thick fibers comprised only 4% of the fiber population in the 2 lateralmost bundles, in which 44% of the fibers had diameters less than 2 microns. The projections of labeled afferent fibers were traced into the neuroepithelium, and it was demonstrated that all thick fibers, even those of the lateral bundles, turned toward more central regions of the crista. Consequently, in the bullfrog, there is a clear predominance of thick afferent fibers innervating the anterior crista's central region and thin fibers in the peripheral region. 3. The dendritic morphology of the broad classes of afferent fibers (i.e., thick and thin) was elucidated. Individually labeled thick afferents possessed dendrites forming short, thick, clawlike extensions to contact a few hair cells. The thinnest afferents were labeled through extracellular horseradish peroxidase (HRP) injections. In contrast to the thick fibers, thin afferents were characterized by an unbranched trajectory with serially located bouton-like structures that were apposed to successive hair cells. 4. The characteristics of spontaneous firing and the responses to rotational stimuli were determined for 138 anterior canalicular neurons. Spontaneous firing rates ranged from 0 to 95 spikes.s-1. The coefficient of variation (CV) of spontaneous firing ranged from 0.12 to 2.5. Response gains to high- (0.5 and 0.4 Hz) and medium- (0.05 Hz) frequency sinusoidal acceleration stimuli were positively correlated with CV (P less than 0.001) for neurons with a CV value less than or equal to 0.5. The gain of neurons characterized by more irregular spontaneous firing (CV values greater than 0.5) was uncorrelated with CV.(ABSTRACT TRUNCATED AT 400 WORDS)

The cellular and cytoarchitectural features of the lateral superior olive, the medial superior olive, the superior paraolivary nucleus and the medial, lateral and ventral nuclei of the trapezoid body are described in C57BL/6 mice using Nissl, Bodian and Golgi techniques. Principal, spindle and marginal cells are present in a well-defined lateral superior olive. The dendrites of these cells run primarily within rostrocaudal sheets as in the cat. The principal cells of the medial nucleus of the trapezoid body are similar to the principal cells in the cat. Large multipolar cells characterize the lateral nucleus of the trapezoid body and bipolar cells with a medial-lateral orientation are found in the medial superior olive. The largest neurons are found in the superior paraolivary nucleus and the lateral superior olive, and the medial and ventral nuclei of the trapezoid body. While brain weight and neuronal packing density change with development, the characteristic location of cell groups and the shape and Nissl-staining pattern of neurons in the youngest brains examined were essentially unchanged in the adult mice, although dendritic maturation had occurred. The homologies of the C57BL/6 superior olivary complex nuclei with the same areas described in other mouse strains, rat and cat are discussed. This study expands our understanding of the organization of the superior olivary complex in an inbred strain of Mus musculus and relates it to other species. The data about changes occurring during postnatal maturation may aid in the interpretation of behavioral and physiological studies of neonatal plasticity of the auditory system.

Two populations of superior olivary neurons which project to different sensory cell regions in the cochlea also give off collateral projections to the ventral cochlear nucleus (VCN) and adjacent brainstem. To determine whether these VCN projections also have different targets they were characterized by selective retrograde amino acid transport. Retrograde transport of 3H-d-aspartate (D-ASP) selectively labeled the unmyelinated fibers and neurons of the lateral olivocochlear (OC) system including a dense collateral projection to the central VCN. Retrograde transport of 3H-nipecotic acid (NIP) labeled the myelinated fibers and neurons of the medial OC system, including collateral projections to the peripheral VCN, subpeduncular granule cells, and nucleus Y. Medial and lateral OC efferent collaterals thus innervate different regions of the CN. Lateral system collaterals overlap extensively with Type I spiral ganglion cell afferent input. They are well positioned to play a role in modulating afferent input to the central auditory system, as is the primary projection of these efferents to the cochlea. The medial system collaterals project near the recently described afferent projections of Type II spiral ganglion cells. The medial system collaterals may therefore be related to the function of outer hair cells, as the medial system primary axons appear to be in the cochlea.

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