Search

This study provides new information on brainstem areas, assumed to be auditory based on observations in other species, in the oscar, Astronotus ocellatus. The primary goal of the study was to explore the morphology of the dorsal descending octaval nucleus, which contains a population of neurons that receives acoustic afferents from the inner ear. Using cytoarchitectonic and connectional criteria, we revised the previously defined dorsal boundary of the descending octaval nucleus, such that the most dorsomedial neurons in this nucleus are positioned ventral to the cerebellar crest and medial to nucleus medialis. At some levels, these dorsomedial cells are continuous with another part of the dorsal descending nucleus that underlies nucleus medialis. The terminal fields of the saccule and lagena are located within this latter, more ventral part of the dorsal descending nucleus. However, the dorsomedial cells that are proximate to the cerebellar crest have long ventral dendrites that extend into these terminal fields, and therefore likely receive saccular and lagenar input. In contrast to a previous report, saccular afferents terminate more medially within the dorsal descending nucleus than do lagenar inputs. Injections of horseradish peroxidase in nucleus centralis of the torus semicircularis revealed that many descending nucleus neurons that lie within the saccular and lagenar terminal fields, including the dorsomedial neurons proximate to the cerebellar crest, project to this acoustic midbrain area. These injections also revealed a secondary octaval population like that described in otophysan fishes.

Extracellular recordings were made with microelectrodes from single neurons in the rat's dorsal nucleus of the lateral lemniscus (DNLL) and response characteristics were determined for monaural and binaural acoustic stimulation. The vast majority of DNLL neurons were narrowly tuned to sound frequency and their temporal responses to contralateral tone pulses fell into one of three broad categories: onset (57%), sustained (21%) or onset-pause-sustained (22%). Most DNLL neurons fired multiple action potentials to a single click delivered to the contralateral ear. The majority (77%) of DNLL neurons showed a monotonic increase in the number of spikes elicited by contralateral tone pulses of increasing sound pressure level; the remaining cells were weakly non-monotonic. No obvious tonotopic pattern was found in the distribution of characteristic frequency of neurons in DNLL. Most DNLL neurons exhibited either excitatory/inhibitory (74%) or excitatory/excitatory (9%) binaural response patterns. The remaining cells (17%) were monaural and driven exclusively by stimulation of the contralateral ear. The binaural neurons in DNLL were sensitive to both interaural intensity and interaural time differences as determined by presentation of dichotic tone bursts and clicks respectively. The responses of DNLL neurons could be distinguished on the basis of monaural and binaural response characteristics from those in surrounding areas including the sagulum, paralemniscal zone and the intermediate nucleus of the lateral lemniscus.

Acoustic signals are extensively used for guiding various behaviors in frogs such as vocalization and phonotaxis. While numerous studies have investigated the anatomy and physiology of the auditory system, our knowledge of intrinsic properties and connectivity of individual auditory neurons remains poor. Moreover, the neural basis of audiomotor integration still has to be elucidated. We determined basic response patterns, dendritic arborization and axonal projection patterns of auditory midbrain units with intracellular recording and staining techniques in an isolated brain preparation. The subnuclei of the torus semicircularis subserve different tasks. The principal nucleus, the main target of the ascending auditory input, has mostly intrinsic neurons, i.e., their dendrites and axons are restricted to the torus itself. In contrast, neurons of the magnocellular and the laminar nucleus project to various auditory and non-auditory processing centers. The projection targets include thalamus, tegmentum, periaqueductal gray, medulla oblongata, and in the case of laminar neurons--the spinal cord. Additionally, tegmental cells receive direct auditory input and project to various targets, including the spinal cord. Our data imply that both auditory and premotor functions are implemented in individual toral and tegmental neurons. Their axons constitute parallel descending pathways to several effector systems and might be part of the neural substrate for differential audiomotor integration.

A systematic study of the encoding properties of 146 auditory nerve fibers in the Tokay gecko (Gekko gecko, L) was conducted with respect to pure tones and two-tone rate suppression. Our aim was a comprehensive understanding of the peripheral encoding of simple tonal stimuli and their representation by temporal synchronization and spike rate codes as a prelude to subsequent studies of more complex signals. Auditory nerve fibers in the Tokay gecko have asymmetrical, V-shaped excitatory tuning curves with best excitatory frequencies that range from 200-5100 Hz and thresholds between 4-35 dB SPL. A low-frequency excitatory 'tail' extends far into the low-frequency range and two-tone suppression is present only on the high frequency side of the tuning curve. The response properties to pure tones at different loci within a tuning curve can differ greatly, due to evident interactions between the representations of temporal, spectral and intensity stimulus features. For frequencies below 1250 Hz, pure tones are encoded by both temporal synchronization and spike rate codes, whereas above this frequency a fiber's ability to encode the waveform periodicity is lost and only a rate code predominates. These complimentary representations within a tuning curve raise fundamental issues which need to be addressed in interpreting how more complex, bioacoustic communication signals are represented in the peripheral and central auditory system. And since auditory nerve fibers in the Tokay gecko exhibit tonal sensitivity, selective frequency tuning, and iso-intensity and iso-frequency contours that seem comparable to similar measures in birds and mammals, these issues likely apply to most higher vertebrates in general. The simpler wiring diagram of the reptilian auditory system, coupled with the Tokay gecko's remarkable vocalizations, make this animal a good evolutionary model in which to experimentally explore the encoding of more complex sounds of communicative significance.

The isolated, fixed cochlea of the mustached bat was studied with three dimensional magnetic resonance (MR) microscopy. The cochlea of this animal is about 4 mm in diameter and its entire volume was imaged. With the field of view and matrix size used, the volume elements (voxels) making up the volume data set were isotropic 25 x 25 x 25 micron cubes. Three dimensional (3D) MR microscopy based on isotropic voxels has many advantages over commonly used light microscopy: 1) it is non destructive; 2) it is much less time consuming; 3) no dehydration is required and shrinkage is minimized; 4) the data set can be used to create sections in any desired plane; 5) the proper alignment of sections is inherent in the 3D acquisition so that no reference points are required; 6) the entire data set can be viewed from any point of view in a volume rendered image; 7) the data is digital and features can be enhanced by computer image processing; and 8) the isotropic dimensions of the voxels make the data well-suited for structural reconstructions and measurements. Good images of the osseous spiral lamina, spiral ligament, scala tympani, scala vestibuli, and nerve bundles were obtained. The vestibular (Reissner's) membrane was easily identified in the mustached bat and it appears to bulge into the scala vestibuli. The visibility of this structure suggests that MR microscopy would be well-suited for studies of endolymphatic hydrops.

In the mammalian auditory nerve physiological recordings revealed that the spontaneous discharge rate of single auditory fibres correlates with the diversity of input-output functions which may be important for intensity discrimination (e.g., Sachs and Abbas, 1974, Liberman, 1978; Winter et al., 1990). In this study we determined if the spontaneous discharge rate of auditory nerve fibres in the guinea pig is correlated with an anatomical feature, namely the diameter of the respective fibres. The diameter of myelinated (Type I) guinea pig auditory nerve fibres was measured after staining with different techniques. Measurements were made on semithin sections using a video image analysis system. The diameters of fibres stained with toluidine blue from the portion of the auditory nerve containing fibres from the basal turn of the cochlea were found to have a normal distribution. Fibres were also labelled with horseradish peroxidase by bulk injection into the spiral ganglion. It was found that the presence of horseradish peroxidase within the fibres reduced the measured diameter in comparison to adjacent unlabelled fibres. A number of fibres were physiologically characterized with respect to spontaneous discharge rate and subsequently intracellularly labelled with horseradish peroxidase. Fibre diameter of a selected sample of intracellularly fibres was measured over a distance of 800 microns within the internal auditory meatus. At the positions nearest to the spiral ganglion fibres possessing low spontaneous rates were found to have smaller diameters than high spontaneous rate fibres. No difference in fibre diameter was found for the positions near the cochlear nucleus.

Horseradish peroxidase was injected into the torus semicircularis central nucleus of the red-eared turtle Chrysemys scripta elegans. The following observations were made. Axons originating in the central nucleus project ipsilaterally through the tectoreuniens tract to the ipsilateral nucleus reunions. A second tract leaves the central nucleus medioventrally, courses under the floor of the cerebral aqueduct and terminates as an axonal field in the contralateral central nucleus. Finally, there was a small pathway to the deep layers of the ipsilateral optic tectum.

The ability of auditory fibers in the anuran's VIIIth nerve to regenerate back into the central nervous system after their axons have been served was studied electrophysiologically and anatomically. Single unit recordings in the regenerated portion of the nerve indicate that: (1) fibers from both the amphibian and basilar papillae regenerate; (2) tuning curves of regenerated fibers, as in normal fibers, are 'V' shaped and retain their sharp frequency selectivity; (3) latency and threshold measurements of regenerated fibers are similar to values obtained in intact nerves. Filling the posterior branch of the regenerated VIIIth nerve with horseradish peroxidase indicates that fibers in this branch remain together within the nerve during regeneration and succeed in terminating in their correct target nuclei and no others.

The primary projections of the lateral line and eighth (octavolateralis) nerves in the teleost, Crenicichla lepidota, were found to conform to the pattern of projections of these nerves in non-teleost fishes. The lateral line nerves terminate in the dorsal portion of the octavolateralis area of the medulla, the eighth nerve terminates in the ventral portion of this area, and overlap is restricted to nucleus magnocellularis. An anastomotic connection between the posterior lateral line nerve and the eighth nerve, the lateralis branchlet, is composed of posterior lateral line nerve fibers which enter the medulla with the eighth nerve but terminate with lateral line inputs.

The height, width and cross-sectional area of the bullfrog amphibian papilla tectorial membrane were quantitatively analysed from frontal serial sections. The cross-sectional area (which is a measure of mass) of the tectorium does not appear to be linearly graded along the length of the papilla, but rather spatial gradations occur in more or less a step-wise manner. The spatially graded area (or mass) is well correlated with the width of the tectorium and their relationships with the tonotopic organization of the amphibian papilla are discussed.

The origin of efferent acoustic and vestibular fibers was determined in the turtle Terrapene ornata. After injection of an aqueous solution of horseradish peroxidase (HRP) into either the cochlear duct or into the ampullae of the horizontal and anterior semicircular canals, neurons in the medullary reticular formation were labeled by the reaction production of retrogradely transported HRP. These neurons were located bilaterally in the medial reticular nucleus. The majority were found ipsilateral to the injection site. There was no demonstrable difference in size, shape, and labeling pattern between efferent acoustic and efferent vestibular neurons. The crossed component component of efferent acoustic fibers, however, was rather sparsely developed.

In recent years several dye and cobalt iontophoresis techniques have been successfully used by invertebrate neurophysiologists for the localization of neuron somata and their processes. The cobalt iontophoresis technique has now been extended for use in the tracing of nerve fiber pathways and the localization of neuron somata in vertebrates. The brain and spinal cord of an animal are removed following perfusion with saline, and placed in a dish of cold saline. A suction electrode, filled with 300 mM cobalt chloride, is then placed over the cut end of the nerve trunk. Cobalt ions are then iontophoresed (by means of a voltage divider) within the nerve fibers, along their course. Following iontophoresis, the brain is bathed in an ammonium sulfide solution to precipitate the cobalt as black cobalt sulfide. The brain is then processed for histological procedures. A wide variety of vertebrates has been used, including amphibians, reptiles, aves and mammals, with uniform success. The cobalt iontophoresis technique presently in use has a wide range of applicability for neuroanatomical studies.

The inferior colliculus of the marmoset was studied in sections prepared by Nissl and Golgi techniques. As in other primates and subprimates it can be divided into a central nucleus (CN) and a cortex, further divisible into dorsal cortical and lateral zones. Afferent fibres enter CN from the lateral lemniscus and fan out from posteroventrolateral to anterodorsomedial, forming a set of laminae along which neuronal somata and dendrites tend to be aligned. Neurons are commonly bitufted or bipolar, and sometimes multipolar, with dendritic arbors oriented along the laminae. Other axons cut across the laminae and are probably efferent fibres destined for the brachium of the inferior colliculus en route to higher levels. The cortex is composed of a variety of bipolar and multipolar neurons and is not laminated. Injection of 2-deoxyglucose while exposing the marmoset to auditory stimulation demonstrates that tones at 4 and 30 kHz cause labelling along the laminae near, respectively, the dorsolateral and ventromedial limits of CN.

Ascending projections of the spinal cord in a teleost, Sebastiscus marmoratus, were studied by means of the horseradish peroxidase tracing method. Projecting fibers were observed in the reticular formation, vagal lobe, octaval nuclei, a dorsomedial portion of the descending nucleus of the trigeminal nerve, corpus cerebelli and nucleus ventromedialis thalami.

Projections of primary afferent fibers from the octavus nerve to the inferior reticular formation were determined by nerve degeneration and HRP labeling. Descending afferents from the horizontal ampullary nerve exit the ventral border of the nucleus octavus descendens via arcuate fiber tracts, and project to a group of neurons adjacent to the spinal lemniscus; to the inferior reticular formation; and to the nucleus funiculi lateralis. The possible influence of these afferent projections on directed swimming motion is discussed.

The C57BL/6J mouse organ of Corti was studied using transmission electron microscopy. The basal coil of the cochlea was examined in mice 1-50 days of age. At birth the cochlea was very immature but both types of hair cells were innervated. Inner hair cells (IHC) were connected to afferent and efferent processes, and efferent endings synapsed on IHC afferents. Outer hair cells (OHC) were innervated exclusively by afferents, which made well-defined synapses with the cell. Maturation at IHC took place rapidly and was essentially over by about 12 days of age. Changes were largely restricted to an increase in presynaptic specializations opposite afferents and a decrease in postsynaptic specializations (cisterns) opposite efferents. Ontogeny at OHC took place over a longer, 2.5 week, period. Afferent synapses lost their presynaptic specializations (synaptic bodies), and then efferent fibers arrived below OHC. The efferents subsequently made temporary axo-dendritic synapses with the afferents before replacing most of them at OHC. The first synapses between efferent endings and OHC were seen at 9 days of age, but it was not until about 20 days of age that mature synapses were seen. Some evidence of hair cell degeneration was seen in 30- and 50-day-old mice. The results are discussed in terms of sensory hair cell differentiation, the disappearance of OHC synaptic bodies, and age-related changes in auditory system function.

A new, improved surgical approach to the cochlear nucleus is developed in the gerbil. This new approach involves making a small hole in the lateral wall of the temporal bone located within the perimeter of the superior semicircular canal. Microelectrodes are passed through the intact parafloccular lobe of the cerebellum to the cochlear nucleus. One advantage of the new approach is that no removal of any CNS vasculature or neural tissue is necessary. Relations between the bulla, temporal bone and cochlear nucleus are presented in detail. The new approach is demonstrated by making single unit recordings from the cochlear nucleus and classifying response patterns as measured in PST histograms. All of the response types found in cat are found in the gerbil.

Scanning electron microscopy was used to examine the basilar papilla of the granite spiny lizard. The papilla contains three distinct hair cell populations: an apical and a basal population with free-standing cilia, and a central population with a tectorial membrane. In the free-standing populations, stereocilium length decreases towards the ends of the papilla. Ciliary tuft morphology differs in the free-standing and the tectorial membrane populations, except that several of the free-standing hair cells with the shortest stereocilia have a tuft morphology like the hair cells in the tectorial membrane population. On the basis of single-fiber physiology, auditory nerve fibers can be divided into a low characteristic frequency (CF) and a high CF population. Mappings of the tonotopic organization of the nerve demonstrated two groups of high CF fibers that correspond to the two free-standing hair cell populations. The low CF fibers are associated with the tectorial membrane hair cell population. Fiber CF correlated with hair cell cilium length, not position on basilar membrane, for hair cells with free-standing cilia. Tonotopic organization of high CF fibers could be predicted reasonably well from the histogram of fiber CFs.