O.W. Henson

Cochlear fluid space dimensions for six species derived from reconstructions of three-dimensional magnetic resonance images

Objectives: To establish the dimensions and volumes of the cochlear fluid spaces.

Echo intensity compensation by echolocating bats.

When mounted on a swinging pendulum, mustache bats, Pteronotus p. parnellii, emit ultrasonic pulses as they move toward and away from fixed targets. During forward swings they systematically decrease the intensity of their emitted pulses and during backward swings they increase the intensity. In this way, echo strength is continuously adjusted and apparently optimized for signal analysis. We have called this behavior echo intensity compensation. Pteronotus simultaneously Doppler and echo intensity compensate during forward swings of the pendulum but during backward swings they only echo intensity compensate. Pteronotus can regulate the intensity of both the constant frequency and frequency modulated components of their pulses; this regulation is independent of vestibular cues, pulse repetition rates, pulse durations and pulse-echo intervals.

Tension fibroblasts and the connective tissue matrix of the spiral ligament

Fibroblasts with stress fibers (tension fibroblasts) have previously been described in the marginal region of the spiral ligament of bats and mice (Henson et al., 1984, 1985). The location, structure and attachments of these cells and the fact that they contain contraction associated proteins, have suggested a role in the generation of tension within the basilar membrane-spiral ligament complex. In this study the structure of these fibroblasts and their relationships to different types of connective tissue matrices were examined in representative Marsupalia, Insectivora, Chiroptera, Rodentia, Lagomorpha, Carnivora and Primates. Tension fibroblasts occur in all species but they are remarkably different in their actin filament content, their structure and distribution, and in their association with the extracellular matrix and otic capsule. Six types of matrices are described (dense filamentous, bundled, laminated, honeycombed, trabeculated and loose filamentous). The configurations of the cells and fibers indicate that tension on the basilar membrane may be accomplished in different ways and to different degrees in different mammals. The arrangement of the matrix and cells in some animals is markedly different in the parts of the cochlea that respond to high, middle and low frequencies.

The efferent cochlear projections of the superior olivary complex in the mustached bat

Following the placement of horseradish peroxidase in the scala tympani, labeled neurons were found in the ipsilateral interstitial nucleus (INT) and throughout the ipsilateral and contralateral dorsomedial periolivary nuclei (DMPO). The neurons in the INT were morphologically distinct from those in the DMPO. The INT neurons formed a thin shell over the lateral superior olivary nucleus (LSO) and their dendrites extended into the body and hilar region. The DMPO neurons had long, tapering dendrites that extended in every direction. Data indicate that the crossed fibers in the floor of the ventricle arise entirely from the DMPO while uncrossed olivocochlear fibers originate in the INT and DMPO. It was estimated that 75% of the efferent fibers arise from the INT and 25% from the DMPO. Approximately 70% of the efferent neurons in each DMPO project to the contralateral cochlea via the crossed olivocochlear bundle. The number of olivocochlear neurons associated with each ear was determined to be approximately 1585. This number is similar to that found in cats and guinea pigs, but the number of neurons per unit length of the basilar membrane is considerably higher in the mustached bat than in other species examined to date. The compact, restricted locations of the neurons in the INT and DMPO in the mustached bat are different from those described for most other mammals and the arrangement in the mustached bat offers advantages over other species for future anatomical and physiological studies.

Detection and quantification of endolymphatic hydrops in the guinea pig cochlea by magnetic resonance microscopy

Three-dimensional magnetic resonance microscopy (MRM) was used to study normal and hydropic cochleae of the guinea pig. With this technique consecutive serial slices representing the entire volume of isolated, fixed cochleae were obtained. The voxels (volume elements) making up the contiguous slices were isotropic (25 microns 3) and in each slice the boundaries of scala media, including the position of Reissners membrane, were clearly delineated. Three-dimensional reconstructions of the endolymphatic and perilymphatic scale were generated. Custom software was developed to quantify cross-sectional area (CSA) of all scalae. In the normal cochlea all 3 scalae, including scala media, showed a gradual decrease in CSA from base to apex. Marked differences existed between our findings and previously reported cochlear dimensions, especially for the perilymphatic scalae in the basal turn. In hydropic cochleae the scala media was enlarged to a varying extent in different turns and marked changes in the degree of distension of Reissners membrane occurred along the cochlea. MRM and subsequent computer analysis of the isotropic data provide excellent methods for imaging and quantifying the fluid spaces of normal and hydropic cochleae.

Course and distribution of efferent fibers in the cochlea of the mouse

The course, distribution and termination of single efferent fibers to the cochlea has been described in only a few animals and relatively few fibers have been studied with knowledge of their ipsilateral or contralateral origin. In order to examine the efferent fibers in the mouse, the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was iontophoretically injected into one side of the brain stem near the location of known efferent nuclei. Examination of surface preparations of the cochlea revealed detailed information for both the lateral olivocochlear (LOC) and medial olivocochlear (MOC) systems. Many, but not all, fibers entered the cochlea within the intraganglionic spiral bundle (IGSB). The LOC fibers were restricted to the ipsilateral cochlea and rarely branched within the IGSB and osseous spiral lamina (OSL). In the organ of Corti, they traveled either basally or apically in the region of the inner hair cells (IHCs), spanning lengths up to 130 microns (basally) and 890 microns (apically). Terminal swellings of these fibers were ca 3.0 microns in diameter. Numerous en passant swellings were present where the fibers formed a plexus in the area of the IHCs. The MOC fibers followed a similar course in the IGSB and OSL, and within the OSL the fibers had few branches. Within the organ of Corti they traveled apically (up to 70 microns) in the nerve bundles located in the IHC area before they crossed the tunnel of Corti. In the region of the OHCs, 9% of the traceable fibers branched to innervate two to three OHCs while 91% appeared to innervate only one OHC. There was no discernible difference in the distribution of contralateral and ipsilateral MOC projections in terms of cochlear region or outer hair cell rows.

Imaging the cochlea by magnetic resonance microscopy

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 (Reissners) 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.

Specializations for sharp tuning in the mustached bat: The tectorial membrane and spiral limbus

The sense of hearing in the mustached bat, Pteronotus parnellii, is specialized for fine frequency analysis in three narrow bands that correspond to approx 30, 60 and 90 kHz constant frequency harmonics in the biosonar signals used for Doppler-shift compensation and acoustic imaging of the environment. Previous studies have identified anatomical specializations in and around the area of the cochlea that processes the dominant second harmonic component, but similar features have not been found in areas related to sharp tuning and high sensitivity for the first or third harmonics. In this report we call attention to the large size of the tectorial membrane and spiral limbus in all three areas that appear to process the harmonically related constant frequency components. These structures are especially pronounced in the regions of the cochlea that respond to the approx 61 kHz, second harmonic and 91.5 kHz, third harmonic bands; they correspond specifically to areas where the density of afferent nerve fibers is high and where very sharply tuned neurons occur. These data for cochleae with multiple specializations lend strong support to the idea that the mass of the tectorial membrane can be an important factor in establishing the response properties of the cochlea.

Cochlear and CNS tonotopy: Normal physiological shifts in the mustached bat

The ear of the mustached bat (Pteronotus parnellii) shows marked cochlear resonance near 60 kHz and many sharply tuned neurons throughout the brain have best frequencies (BF) near the cochlear resonance frequency (CRF). Controlled changes in the normal physiological range of body temperature (approx 37-42 degrees C) were used to change the CRF and to study the tuning properties of neurons in the cochlear nucleus (CN) and inferior colliculus (IC). In all cases there were concomitant shifts in the CRF and the BFs. Results were the same for single and multi-units, and for CN and IC units. Although the BF reliably changed with shifts in the CRF, the majority of the units showed no change in minimum threshold or the sharpness (Q10 dB) of tuning. The temperature-induced effects on cochlear tuning were similar to those previously described in nonmammalian vertebrates. The physiological data reveal that, within a narrow frequency band, cochlear and CNS tonotopy are labile in the mustached bat. The lability of tuning is further substantiated by adaptations of biosonar emission behavior with shifts in CRF (Henson et al., 1990).

The presence and arrangement of type II collagen in the basilar membrane

Previous studies demonstrating the presence of collagen II in the basilar membrane have used a biochemical approach or have used immunohistochemistry at the light microscopic level. In this investigation both the presence and arrangement of collagen II were demonstrated at the ultrastructural level using pre- and post-embedding immunoelectron microscopy. Labeling was dependent on the development of protocols to expose epitopes while maintaining identifiable ultrastructure. Both positive and negative controls indicate that the labeling was specific for collagen II. Collagen II was detected in the fibrous sheet of the pars tecta and in the two fibrous layers of the pars pectinata. It was detected in situ and on isolated individual 10-12 nm fibrils. The presence of collagen II in all the fibrous layers of the basilar membrane places constraints on the biomechanical properties of this important structure.