Resting discharge patterns of macular primary afferents in otoconia-deficient mice.

Vestibular primary afferents in the normal mammal are spontaneously active. The consensus hypothesis states that such discharge patterns are independent of stimulation and depend instead on excitation by vestibular hair cells due to background release of synaptic neurotransmitter. In the case of otoconial sensory receptors, it is difficult to test the independence of resting discharge from natural tonic stimulation by gravity. We examined this question by studying discharge patterns of single vestibular primary afferent neurons in the absence of gravity stimulation using two mutant strains of mice that lack otoconia (OTO-; head tilt, het-Nox3, and tilted, tlt-Otop1). Our findings demonstrated that macular primary afferent neurons exhibit robust resting discharge activity in OTO- mice. Spike interval coefficient of variation (CV = SD/mean spike interval) values reflected both regular and irregular discharge patterns in OTO- mice, and the range of values for rate-normalized CV was similar to mice and other mammals with intact otoconia although there were proportionately fewer irregular fibers. Mean discharge rates were slightly higher in otoconia-deficient strains even after accounting for proportionately fewer irregular fibers [OTO- = 75.4 +/- 31.1(113) vs OTO+ = 68.1 +/- 28.5(143) in sp/s]. These results confirm the hypothesis that resting activity in macular primary afferents occurs in the absence of ambient stimulation. The robust discharge rates are interesting in that they may reflect the presence of a functionally 'up-regulated' tonic excitatory process in the absence of natural sensory stimulation.

A place principle for vertigo.

OBJECTIVE: To provide a road map of the vestibular labyrinth and its innervation leading to a place principle for different forms of vertigo. METHOD: The literature describing the anatomy and physiology of the vestibular system was reviewed. RESULTS: Different forms of vertigo may be determined by the type of sense organ, type of ganglion cell and location in the vestibular nerve. CONCLUSION: Partial lesions (viral) of the vestibular ganglion are manifested as various forms of vertigo.

Age-related differences in parameters of vestibular evoked myogenic potentials.

CONCLUSIONS: The statistically significant correlations between vestibular evoked myogenic potential (VEMP) parameters and age may be due to hair cell loss of the otolith organ and/or to degenerative changes of the vestibular neural pathway. These findings indicate that age should be taken into account when interpreting VEMP results. It is also important to determine a standard method for performing VEMP and a universal index for comparison among laboratories. OBJECTIVES: VEMP, which measures the surface electric potential from the cervical muscle evoked by sufficiently loud sounds, is a useful tool to evaluate vestibule-colic reflex function. We have assayed the effect of age on VEMP results. SUBJECTS AND METHODS: After excluding subjects with a previous history of dizziness, middle ear pathology, or other inner ear symptoms, a total of 97 healthy volunteers (194 ears) were included. All VEMP parameters were analyzed to find differences related to side and gender, as well as the relationship between age and each VEMP parameter. RESULTS: Age was correlated with all VEMP parameters. Latency of p13, n23 showed a negative correlation and amplitude of p13-n23 showed a positive correlation with age. Differences between the right and left sides were not significant.

Regeneration of vestibular otolith afferents after ototoxic damage.

Regeneration of receptor cells and subsequent functional recovery after damage in the auditory and vestibular systems of many vertebrates is well known. Spontaneous regeneration of mammalian hair cells does not occur. However, recent approaches provide hope for similar restoration of hearing and balance in humans after loss. Newly regenerated hair cells receive afferent terminal contacts, yet nothing is known about how reinnervation progresses or whether regenerated afferents finally develop normal termination fields. We hypothesized that neural regeneration in the vestibular otolith system would recapitulate the topographic phenotype of afferent innervation so characteristic of normal development. We used an ototoxic agent to produce complete vestibular receptor cell loss and epithelial denervation, and then quantitatively examined afferent regeneration at discrete periods up to 1 year in otolith maculas. Here, we report that bouton, dimorph, and calyx afferents all regenerate slowly at different time epochs, through a progressive temporal sequence. Furthermore, our data suggest that both the hair cells and their innervating afferents transdifferentiate from an early form into more advanced forms during regeneration. Finally, we show that regeneration remarkably recapitulates the topographic organization of afferent macular innervation, comparable with that developed through normative morphogenesis. However, we also show that regenerated terminal morphologies were significantly less complex than normal fibers. Whether these structural fiber changes lead to alterations in afferent responsiveness is unknown. If true, adaptive plasticity in the central neural processing of motion information would be necessitated, because it is known that many vestibular-related behaviors fully recover during regeneration.

Central projections of the utricular nerve in the gerbil.

The central projections of primary afferent fibers in the utricular nerve, which convey linear head acceleration signals to neurons in the brainstem and cerebellum, are not completely defined. The purpose of this investigation was twofold: 1) to define the central projections of the gerbil utricular afferents by injecting horseradish peroxidase (HRP) and biotinylated dextran amine (BDA) into the utricular macula; and 2) to investigate the projections of individual utricular afferents by injecting HRP intracellularly into functionally identified utricular neurons. We found that utricular afferents in the gerbil projected to all divisions of the vestibular nuclear complex, except the dorsal lateral vestibular nucleus. In addition, terminals were observed in the interstitial nucleus of the eighth nerve, nucleus Y, external cuneate nucleus, and lobules I, IV, V, IX, and X of the cerebellar vermis. No projections appeared in the flocculus or paraflocculus. Fibers traversed the medial and intermediate cerebellar nuclei, but terminals appeared only occasionally. Individual utricular afferents collateralize extensively, projecting to much of the brainstem area innervated by the whole of the utricular nerve. This study did not produce complete filling of individual afferent collateral projections into the cerebellar cortex.

Ultrastructural localization of ChAT-like immunoreactivity in the human vestibular periphery.

Acetylcholine (ACh) has long been considered a neurotransmitter candidate in the efferent vestibular system of mammals. Recently, choline acetyltransferase (ChAT), the synthesizing enzyme for ACh, was immunocytochemically localized in all five end-organs of the rat vestibule (Kong et al. (1994) Hear. Res. 75, 192-200). However, there is little information in the literature concerning the cholinergic innervation in the vestibular periphery of man. In the present study the ultrastructural localization of the ChAT-like immunoreactivity in the human vestibular periphery was investigated in order to reveal the cholinergic innervation in the human vestibular end-organs. A modified method of pre-embedding immunoelectron microscopy was applied. It was found that the ChAT-like immunoreactivity was located in the bouton-type vesiculated nerve terminals in the vestibular neurosensory epithelia of man. These ChAT-like immunostained nerve terminals make synaptic contacts either with afferent chalices surrounding type I vestibular sensory hair cells, or with type II vestibular sensory hair cells. These results show that the ChAT-like immunoreactivity in the human vestibular periphery is confined to the efferent vestibular system. The ChAT-containing efferents innervate both type I hair cells and type II hair cells, making postsynaptic and presynaptic contacts, respectively. This study presents evidence that ACh is a neurotransmitter candidate in the efferent vestibular system of man.

Ultrastructural localization of GABA-like immunoreactivity in the human utricular macula.

In the vertebrate vestibular periphery, gamma-aminobutyric acid (GABA) has long been presumed to be a neurotransmitter candidate. However, experimental reports about the localization and function of GABA in the vestibular systems of vertebrates are contradictory. In addition, there is no information in the literature concerning the localization of GABA in the human vestibular periphery. The present study investigates the ultrastructural localization of GABA-like immunoreactivity in the human utricular macula. A modified pre-embedding immunostaining electron microscopy technique was applied using two different commercially available polyclonal antibodies to GABA. GABA-like immunoreactivity is confined to the vesiculated nerve fibers and terminals of the human vestibular neurosensory epithelia. The GABA-containing nerve terminals make asymmetrical axo-dendritic synapses with the afferent chalices surrounding the type I sensory hair cells. Type I and type II hair cells as well as afferent chalices are devoid of GABA-like immunoreactive staining. The present study demonstrates that GABA exists in the human vestibular periphery, and that GABA is a neurotransmitter candidate of the human efferent vestibular system.

A comparison of the linear tuning properties of two classes of axons in the bullfrog lagena.

Various vertebrate inner-ear end organs appear to have switched their sensory function between equilibrium sensing and acoustic sensing over the courses of various lines of evolution. It is possible that all that is required to make this transition is to provide an end organ with access to the appropriate stimulus mode and frequency range. If, as we believe, however, the adaptive advantage of an acoustic sensory system lies in its ability to sort the total acoustic input into components that correspond to individual acoustic sources, and the adaptive advantage of an equilibrium sensory system lies in its ability to compute the total orientation and motion of the head without regard to the individual sources contributing to that orientation and motion, then it is easy to argue that the differences between acoustic and equilibrium sensors should be more profound than simply access to the appropriate stimuli. Effective signal-sorting requires high resolution in both time and frequency; to achieve this resolution, a peripheral tuning structure must be one of high dynamic order (i.e., constructed from multiple independent energy storage elements). If the peripheral tuning structure simply converts head acceleration to head displacement, velocity, or jerk (i.e., provides one or two steps of integration or differentiation with respect to time, where one energy storage element per step is required), then high dynamic order is inappropriate. Because the bullfrog lagena possesses both acoustic and equilibrium sensitive regions, it is especially suited for comparing these two sensor types and addressing the question of dynamic order of tuning. In this paper we report observations of the linear tuning properties of bullfrog lagenar primary afferent nerve fibers obtained by stimulating the lagena with random, dorsoventral micromotion over the frequency range from 10 Hz to 1.0 kHz. Tuning curves obtained by reverse correlation analysis and discrete Fourier transformation were used to estimate the dynamic order of each fiber's associated peripheral tuning structure. We found two classes of lagenar afferent axons--those with lowpass amplitude tuning characteristics (44 units) and those with bandpass amplitude tuning characteristics (73 units). Lowpass units were found to originate at the equilibrium region of the macula, and they exhibited low dynamic order--summed low- and high-frequency slopes (absolute values) ranged from 10 dB/decade to 64 dB/decade, implying dynamic orders of less than one to three (the modal value was equal to one). Bandpass units were found to originate at the acoustic region of the macula, and they exhibited higher dynamic order than lowpass units--summed low- and high-frequency slopes (absolute values) ranged from 53 dB/decade to 185 dB/decade, implying dynamic orders of three to nine (the modal value was equal to five). It appears that while lagenar equilibrium and acoustic sensors both possess access to signals in the acoustic frequency range, lagenar acoustic sensors are tuned by means of peripheral structures with markedly greater dynamic order and consequently markedly greater physical complexity. These results suggest that steep-sloped (high-dynamic-order) tuning properties reflect special adaptations in acoustic sensors not found in equilibrium sensors, and that any evolutionary transition between the two sensor types must have involved profound structural changes.

Evidence of a sensory processing unit in the mammalian macula.

We cut serial sections through the medial part of the rat vestibular macula for transmission electron microscopic (TEM) examination, computer-assisted 3-D reconstruction, and compartmental modeling. The ultrastructural research showed that many primary vestibular neurons have an unmyelinated segment, often branched, that extends between the heminode (putative site of the spike initiation zone) and the expanded terminal(s) (calyx, calyces). These segments, termed the neuron branches, and the calyces frequently have spine-like processes of various dimensions with bouton endings that morphologically are afferent, efferent, or reciprocal to other macular neural elements. The major questions posed by this study were whether small details of morphology, such as the size and location of neuronal processes or synapses, could influence the output of a vestibular afferent, and whether a knowledge of morphological details could guide the selection of values for simulation parameters. The conclusions from our simulations are (1) values of 5.0 k omega cm2 for membrane resistivity and 1.0 nS for synaptic conductance yield simulations that best match published physiological results; (2) process morphology has little effect on orthodromic spread of depolarization from the head (bouton) to the spike initiation zone (SIZ); (3) process morphology has no effect on antidromic spread of depolarization to the process head; (4) synapses do not sum linearly; (5) synapses are electrically close to the SIZ; and (6) all whole-cell simulations should be run with an active SIZ.

Compartmental modeling of rat macular primary afferents from three-dimensional reconstructions of transmission electron micrographs of serial sections.

1. We cut serial sections through the medial part of the rat vestibular macula for transmission electron microscopic (TEM) examination, computer-assisted three-dimensional (3-D) reconstruction, and compartmental modeling. The ultrastructural research showed that many primary vestibular neurons have an unmyelinated segment, often branched, that extends between the heminode [putative site of the spike initiation zone (SIZ)] and the expanded terminal(s) (calyx, calyces). These segments, termed the neuron branches, and the calyces frequently have spinelike processes of various dimensions that morphologically are afferent, efferent, or reciprocal to other macular neural elements. The purpose of this research was to determine whether morphometric data obtained ultrastructurally were essential to compartmental models [i.e., they influenced action potential (AP) generation, latency, or amplitude] or whether afferent parts could be collapsed into more simple units without markedly affecting results. We used the compartmental modeling program NEURON for this research. 2. In the first set of simulations we studied the relative importance of small variations in process morphology on distant depolarization. A process was placed midway along an isolated piece of a passive neuron branch. The dimensions of the four processes corresponded to actual processes in the serial sections. A synapse, placed on the head of each process, was activated and depolarization was recorded at the end of the neuron branch. When we used 5 nS synaptic conductance, depolarization varied by 3 mV. In a systematic study over a representative range of stem dimensions, depolarization varied by 15.7 mV. Smaller conductances produced smaller effects. Increasing membrane resistivity from 5,000 to 50,000 omega cm2 had no significant effect. 3. In a second series of simulations, using whole primary afferents, we examined the combined effects of process location and afferent morphology on depolarization magnitude and latency, and the effect of activating synapses individually or simultaneously. Process location affects peak latency and voltage recorded at the heminode. A synapse on a calyceal process produced < or = 8% more depolarization and a 23% increase in peak latency compared with a synapse on a process of a neuron branch. For whole primary afferents, depolarization decreased 40% between simulations of the smallest and largest afferents. Simulations in which membrane resistivity and synaptic conductance were varied while afferent geometry was kept constant indicated that use of 5,000 omega cm2 and 1.0 nS produced results that best fit electrophysiological findings. Synaptic inputs activated simultaneously did not sum linearly at the heminode. Total depolarization was approximately 14% less than a simple summation of responses of synapses activated one at a time.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative transduction mechanisms of hair cells in the bullfrog utriculus. II. Sensitivity and response dynamics to hair bundle displacement.

1. Hair cells in whole-mount in vitro preparations of the utricular macula of the bullfrog (Rana catesbeiana) were selected according to their macular location and hair bundle morphology. The sensitivity and response dynamics of selected hair cells to natural stimulation were examined by recording their voltage responses to step and sinusoidal hair bundle displacements applied to their longest stereocilia. 2. The voltage responses of 31 hair cells to sinusoidal hair bundle displacements were characterized by their gains and phases, taken with respect to peak hair bundle displacement. The gains of Type B and Type C cells at both 0.5 and 5.0 Hz were markedly lower than those of Type F and Type E cells. Phases, with the exception of Type C cells, lagged hair bundle displacement at 0.5 Hz. Type C cells had phase leads of 25-40 degrees. At 5.0 Hz, response phases in all cells were phase lagged with respect to those at 0.5 Hz. Type C cells had larger gains and smaller phase leads at 5.0 Hz than at 0.5 Hz, suggesting the presence of low-frequency adaptation. 3. Displacement-response curves, derived from the voltage responses to 5.0-Hz sinusoids, were sigmoidal in shape and asymmetrical, with the depolarizing response having a greater magnitude and saturating less abruptly than the hyperpolarizing response. When normalized to their largest displacement the linear ranges of these curves varied from < 0.5 to 1.25 microns and were largest in Type B and smallest in Type F and Type E cells. Sensitivity, defined as the slope of the normalized displacement-response curve, was inversely correlated with linear range. 4. The contribution of geometric factors associated with the hair bundle to linear range and sensitivity were predicted from realistic models of utricular hair bundles created using morphological data obtained from light and electron microscopy. Three factors, including 1) the inverse ratio of the lengths of the kinocilium and longest stereocilia, representing the lever arm between kinociliary and stereociliary displacement; 2) tip link extension/linear displacement, largely a function of stereociliary height and separation; and 3) stereociliary number, an estimate of the number of transduction channels, were considered in this analysis. The first of these factors was quantitatively more important than the latter two factors and their total contribution was largest in Type B and Type C cells. Theoretical models were also used to calculate the relation between rotary and linear displacement.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative transduction mechanisms of hair cells in the bullfrog utriculus. I. Responses to intracellular current.

1. Hair cells in whole-mount in vitro preparations of the utricular macula of the bullfrog (Rana catesbeiana) were selected according to their macular location and hair bundle morphology. The voltage responses of selected hair cells to intracellular current steps and sinusoids in the frequency range of 0.5-200 Hz were studied with conventional intracellular recordings. 2. The utricular macula is divided into medial and lateral parts by the striola, a 75- to 100-microns zone that runs for nearly the entire length of the sensory macula near its lateral border. The striola is distinguished from flanking extrastriolar regions by the elevated height of its apical surface and the wider spacing of its hair cells. A line dividing hair cells of opposing polarities, located near the lateral border of the striola, separates it into medial and lateral parts. On average, the striola consists of five to seven medial and two to three lateral rows of hair cells. 3. Utricular hair cells were classified into four types on the basis of hair bundle morphology. Type B cells, the predominant hair cell type in the utricular macula, are small cells with short sterocilia and kinocilia 2-6 times as long as their longest stereocilia. These hair cells were found throughout the extrastriola and, more rarely, in the striolar region. Three other hair cell types were restricted to the striolar region. Type C cells, found primarily in the outer striolar rows, resemble enlarged versions of Type B hair cells. Type F cells have kinocilia approximately equal in length to their longest stereocilia and are restricted to the middle striolar rows. Type E cells, found only in the innermost striolar rows, have short kinocilia with prominent kinociliary bulbs. 4. The resting potential of 99 hair cells was -58.0 +/- 7.6 (SD) mV and did not vary significantly for hair cells in differing macular locations or with differing hair bundle morphology. The RN of hair cells, measured from the voltage response to current steps, varied from 200 to > 2,000 M omega and was not well correlated with cell size. On average, Type B cells had the highest RN, followed by Type F, Type E, and Type C cells. When normalized to their surface area, the membrane resistance of hair cells ranged from < 1,000 to > 10,000 k omega.cm2. The input capacitance of hair cells ranged from < 3 to > 15 pA, corresponding on average to a membrane capacitance of 0.8 +/- 0.2 pA/cm2.(ABSTRACT TRUNCATED AT 400 WORDS)

A spaceflight study of synaptic plasticity in adult rat vestibular maculas.

Behavioral signs of vestibular perturbation in altered gravity have not been well correlated with structural modifications in neurovestibular centers. This ultrastructural research investigated synaptic plasticity in hair cells of adult rat utricular maculas exposed to microgravity for nine days on a space shuttle. The hypothesis was that synaptic plasticity would be more evident in type II hair cells because they are part of a distributed modifying macular circuitry. All rats were shared with other investigators and were subjected to treatments unrelated to this experiment. Maculas were obtained from flight and control rats after shuttle return (R + 0) and nine days post-flight (R + 9). R + 9 rats had chromodacryorrhea, a sign of acute stress. Tissues were prepared for ultrastructural study by conventional methods. Ribbon synapses were counted in fifty serial sections from medial utricular macular regions of three rats of each flight and control group. Counts in fifty additional consecutive sections from one sample in each group established method reliability. All synapses were photographed and located to specific cells on mosaics of entire sections. Pooled data were analyzed statistically. Flown rats showed abnormal posture and movement at R + 0. They had statistically significant increases in total ribbon synapses and in sphere-like ribbons in both kinds of hair cells; in type II cells, pairs of synapses nearly doubled and clusters of 3 to 6 synapses increased twelve-fold. At R + 9, behavioral signs were normal. However, synapse counts remained high in both kinds of hair cells of flight maculas and were elevated in control type II cells. Only counts in type I cells showed statistically significant differences at R + 9. High synaptic counts at R + 9 may have resulted from stress due to experimental treatments. The results nevertheless demonstrate that adult maculas retain the potential for synaptic plasticity. Type II cells exhibited more synaptic plasticity, but space flight induced synaptic plasticity in type I cells.

Computer-assisted three-dimensional reconstruction and simulations of vestibular macular neural connectivities.

The macular neuroepithelium is morphologically organized as a weighted neural network for parallel distributed processing of information. The network is continuous across the striola, where some type II hair cells synapse with calyces containing type I cells with tufts of opposite directional polarities. Whether other hair cell to calyx appositions that lack synapses interact because of intercellular potassium accumulation remains an open question. A functionally important inference of macular organization is that just as arrays of hair cells communicate an entire piece of information to a nerve fiber, so do macular subarrays of nerve fibers (not single units) carry the whole coded message to the brain stem. Moreover, the size of the network subarray can expand or become more limited depending upon the strength and/or duration of the input. It is the functioning of the network and its subarrays that must be understood if we are to learn how maculas carry out their work and adapt to new environments. Simulations of functioning maculas, or subparts, based on precise morphology and known physiology are useful tools to gain insights into macular information processing. The current simulations of afferent collateral electrical activity are a prelude to development of a 3-D model. The simulations demonstrate a relationship between geometry and function, with the diameter of the stem apparently being a major determinant of electrical activity transmitted to the base in the case of collaterals with short stems. Thus, while changes in synaptic number and/or size may be an important adaptive mechanism in an altered g environment, changes in diameter of the stem is another means of altering outflow. Research on the effects of microgravity should be extremely useful in examining the validity of this and other concepts of neural adaptation, since maculas are biological linear accelerometers ideally suited to the task. Maculas are also extremely interesting to study in detail because of the richness of connectivities and submicroscopic organization they present. Many of their features are common with more complex parts of the brain. It seems possible that knowledge of the three-dimensional geometric relationships operative in a functioning macula will contribute much to the understanding of the dynamics underlying more complex behavior. Computerized approaches greatly facilitate this task and provide an objective method of analysis. It is likely that, in the end, simple rules will be found to govern optimal neural architectural organization, even at higher cognitive levels. The architecture only appears complex because we do not yet grasp its meaning.(ABSTRACT TRUNCATED AT 400 WORDS)

Innervation patterns and spontaneous activity of afferent fibres to the lagenar macula and apical basilar papilla of the chick's cochlea.

To investigate the origin of non-auditory fibres in the apical area of the avian cochlear ganglion, we recorded from nerve fibres in the young chick (87% of animals were aged between 5 and 10 days post-hatching). After characterization of their spontaneous activity patterns and, if present, their responses to sound, some fibres were stained with cobalt-ion injections and traced to their peripheral terminals. All stained fibres which were traced to the lagenar macula (N = 13) were non-auditory. They did not increase firing rate or phase-couple to sound stimuli. Their spontaneous activity was either regular (12 cases) or irregular (1 case). Regularly-firing cells all innervated several to very many hair cells, whereby there was no great difference in the pattern of spontaneous activity between those making calyx endings on relatively few hair cells in the striola region and those making small bouton endings on up to 80 hair cells outside the striola. All fibres that responded in any way to sound were irregularly spontaneously active. Three fibres, two of which only responded to sound with phase-coupling, innervated several hair cells in the apical, abneural region of the basilar papilla. Two other fibres traced to the basilar papilla are of previously undescribed types.

Ultrastructural and cytochemical evidence for single impulse initiation zones in vestibular macular nerve fibers of rat.

Cupric ion-ferricyanide labeling methods and related ferrocyanide-stained tissues were used to locate and characterize, at the ultrastructural level, presumptive impulse initiation zones in the three types of vestibular macular nerve fibers. Large-diameter, M-type vestibular nerve fibers terminate in a calyx at the heminode, and labeling is coextensive with the base of the calyx. Intermediate, M/U-type nerve fibers have short, unmyelinated preterminal segments that sometimes bifurcate intramacularly, and small-diameter, U-type nerve fibers have long, unmyelinated preterminal axons and up to three branches. Preterminals of these nerve fibers display ultrastructural heterogeneity that is correlated with labeling patterns for sodium channels and/or associated polyanionic sites. They have a nodelike ultrastructure and label heavily from near the heminode to the base of the macula. Their intramacular branches, less organized ultrastructurally, label only slightly. Results indicate that vestibular nerve fibers have one impulse initiation zone, located near the heminode, that varies in length according to nerve fiber type. Structural heterogeneity may favor impulse conduction in the central direction, and length of the impulse initiation zone could influence nerve discharge patterns.

3-D components of a biological neural network visualized in computer generated imagery. II. Macular neural network organization.

Computer-assisted reconstructions of small parts of the macular neural network show how the nerve terminals and receptive fields are organized in 3-dimensional space. This biological neural network is anatomically organized for parallel distributed processing of information. Processing appears to be more complex than in computer-based neural networks, because spatiotemporal factors figure into synaptic weighting. Serial reconstruction data show anatomical arrangements which suggest that 1) assemblies of cells analyse and distribute information with inbuilt redundancy, to improve reliability; 2) feedforward/feedback loops provide the capacity for presynaptic modulation of output during processing; 3) constrained randomness in connectivities contributes to adaptability; and 4) local variations in network complexity permit differing analyses of incoming signals to take place simultaneously. The last inference suggests that there may be segregation of information flow to central stations subserving particular functions.

Localization and origins of calcitonin gene-related peptide containing fibres in the vestibular end-organs of the rat.

The distribution, origin and fine structure of nerve terminals immunoreactive to calcitonin gene-related peptide (CGRP) were investigated in the vestibular end-organs of the rat by means of immunocytochemistry. Dense plexus of CGRP-like immunoreactive (CGRPI) fibres were observed just beneath the sensory epithelial layers of the ampullary crista of the semicircular canal, and utricular and saccular maculae of the otolith organ. Some CGRPI fibres left the plexus to enter the sensory epithelial layer. Parasagittal transection of the brain just medial to the cochlear nucleus revealed the presence of a few CGPRI fibres in the vestibular end-organs ipsilaterally, indicating that these fibres originate in the central nervous system. A combination of retrograde tracing and immunocytochemistry was then used to identify the origins of CGRPI fibres found in the vestibular end-organs. After injection of fast blue dye (FB) into the vestibular cistern, CGRPI neurons in the area dorsolateral to the genu of the facial nerve were labelled by FB bilaterally. The present ultrastructural study has revealed that most of the CGRPI fibres in the vestibular end-organs form direct contacts with afferent nerve terminals which surround type I vestibular sensory cells.

Correspondences between afferent innervation patterns and response dynamics in the bullfrog utricle and lagena.

Otoconial afferents in the bullfrog were characterized as gravity or vibratory sensitive by their resting activity and their responses to head tilt and vibration. The responses of gravity afferents to head tilt were tonic, phasic-tonic, or phasic. A few afferents, termed vibratory/gravity afferents, had gravity as well as vibratory sensitivity. Functionally identified otoconial afferents were injected with Lucifer Yellow and subsequently traced to their peripheral arborizations. Morphological maps, previously constructed with the scanning electron microscope, were used to identify microstructural features of the sensory maculae associated with the peripheral arborizations of dye-filled afferents. The utricular and lagenar macula each is composed of a specialized central band surrounded by a peripheral field. The central bands are composed of densely packed medial rows and more sparsely packed lateral rows of hair cells. Hair cells exhibit a variety of surface topographies which correspond with their macular location. The response dynamics of afferents in the utricle and lagena correspond with the macular locations of their peripheral arborizations. Tonic afferents were traced to hair cells in the peripheral field. Phasic-tonic and phasic afferents innervated hair cells in the lateral rows of the central band, the former innervating hair cells at the edges of the central band and the latter innervating hair cells located more medially. Afferents with vibratory sensitivity were traced to hair cells in the medial rows of the lagenar central band. The response dynamics of afferents corresponded with the surface topography of their innervated hair cells. Tonic and phasic-tonic gravity afferents innervated hair cells with stereociliary arrays markedly shorter than their kinocilium (Lewis and Li types B and C) while phasic gravity and vibratory afferents innervated hair cells with stereociliary arrays nearly equal to their kinocilium (Lewis and Li types E and F). Vibratory sensitivity was uniquely associated with hair cells possessing bulbed kinocilium (Lewis and Li type E) while afferents sensitive to both gravity and vibration innervated hair cells from both of the above groups. We argue that afferent response dynamics are determined, at least in part, at the level of the sensory hair bundle and that morphological variations of the kinocilium and the otoconial membrane are dictated by specialization of sensitivity. We propose that morphological variations of the kinocilium reflect variations in its viscoelastic properties and that these properties determine the nature of the mechanical couple between the stereociliary array and the otoconial membrane.