Functional characterization of primary vestibular afferents in the frog

The functional operation of the vestibulo-ocular reflex

The biophysical properties of the labyrinthine semicircular canals, and the electrophysiological properties of peripheral vestibular afferent neurons over a range of stimulus frequencies, are reviewed. Resting discharge activity and adaptive properties of vestibular neurons are discussed. Central processing of vestibular signals is then examined, including push-pull organization and the velocity storage mechanism. A detailed treatment of the final common neural integrator for oculomotor signals follows with consideration of its neural substrate and how distributed networks of neurons can overcome several problems posed by conventional control-systems models, such as why neural signals, but not background discharge, are integrated. Next, the behavior of the vestibulo-ocular reflex in darkness is compared with how it satisfies visual demands during natural activities. Finally, the reflex's performance at high frequencies of head rotation is discussed.

Differences in the Structure and Function of the Vestibular Efferent System Among Vertebrates

The role of the mammalian vestibular efferent system in everyday life has been a long-standing mystery. In contrast to what has been reported in lower vertebrate classes, the mammalian vestibular efferent system does not appear to relay inputs from other sensory modalities to the vestibular periphery. Furthermore, to date, the available evidence indicates that the mammalian vestibular efferent system does not relay motor-related signals to the vestibular periphery to modulate sensory coding of the voluntary self-motion generated during natural behaviors. Indeed, our recent neurophysiological studies have provided insight into how the peripheral vestibular system transmits head movement-related information to the brain in a context independent manner. The integration of vestibular and extra-vestibular information instead only occurs at next stage of the mammalian vestibular system, at the level of the vestibular nuclei. The question thus arises: what is the physiological role of the vestibular efferent system in mammals? We suggest that the mammalian vestibular efferent system does not play a significant role in short-term modulation of afferent coding, but instead plays a vital role over a longer time course, for example in calibrating and protecting the functional efficacy of vestibular circuits during development and aging in a role analogous the auditory efferent system.

Acute consequences of a unilateral VIIIth nerve transection on vestibulo-ocular and optokinetic reflexes in Xenopus laevis tadpoles

Loss of peripheral vestibular function provokes severe impairments of gaze and posture stabilization in humans and animals. However, relatively little is known about the extent of the instantaneous deficits. This is mostly due to the fact that in humans a spontaneous loss often goes unnoticed initially and targeted lesions in animals are performed under deep anesthesia, which prevents immediate evaluation of behavioral deficits. Here, we use isolated preparations of Xenopus laevis tadpoles with functionally intact vestibulo-ocular (VOR) and optokinetic reflexes (OKR) to evaluate the acute consequences of unilateral VIIIth nerve sections. Such in vitro preparations allow lesions to be performed in the absence of anesthetics with the advantage to instantly evaluate behavioral deficits. Eye movements, evoked by horizontal sinusoidal head/table rotation in darkness and in light, became reduced by 30% immediately after the lesion and were diminished by 50% at 1.5 h postlesion. In contrast, the sinusoidal horizontal OKR, evoked by large-field visual scene motion, remained unaltered instantaneously but was reduced by more than 50% from 1.5 h postlesion onwards. The further impairment of the VOR beyond the instantaneous effect, along with the delayed decrease of OKR performance, suggests that the immediate impact of the sensory loss is superseded by secondary consequences. These potentially involve homeostatic neuronal plasticity among shared VOR-OKR neuronal elements that are triggered by the ongoing asymmetric activity. Provided that this assumption is correct, a rehabilitative reduction of the vestibular asymmetry might restrict the extent of the secondary detrimental effect evoked by the principal peripheral impairment.

Inertial Sensing and Encoding of Self-Motion: Structural and Functional Similarities across Metazoan Taxa

To properly orient and navigate, moving animals must obtain information about the position and motion of their bodies. Animals detect inertial signals resulting from body accelerations and rotations using a variety of sensory systems. In this review, we briefly summarize current knowledge on inertial sensing across widely disparate animal taxa with an emphasis on neuronal coding and sensory transduction. We outline systems built around mechanosensory hair cells, including the chordate vestibular complex and the statocysts seen in many marine invertebrates. We next compare these to schemes employed by flying insects for managing inherently unstable aspects of flapping flight, built around comparable mechanosensory cells but taking unique advantage of the physics of rotating systems to facilitate motion encoding. Finally, we highlight fundamental similarities across taxa with respect to the partnering of inertial senses with visual senses and conclude with a discussion of the functional utility of maintaining a multiplicity of encoding schemes for self-motion information.

Semicircular canal biomechanics in health and disease

The semicircular canals are responsible for sensing angular head motion in three-dimensional space and for providing neural inputs to the central nervous system (CNS) essential for agile mobility, stable vision, and autonomic control of the cardiovascular and other gravity-sensitive systems. Sensation relies on fluid mechanics within the labyrinth to selectively convert angular head acceleration into sensory hair bundle displacements in each of three inner ear sensory organs. Canal afferent neurons encode the direction and time course of head movements over a broad range of movement frequencies and amplitudes. Disorders altering canal mechanics result in pathological inputs to the CNS, often leading to debilitating symptoms. Vestibular disorders and conditions with mechanical substrates include benign paroxysmal positional nystagmus, direction-changing positional nystagmus, alcohol positional nystagmus, caloric nystagmus, Tullio phenomena, and others. Here, the mechanics of angular motion transduction and how it contributes to neural encoding by the semicircular canals is reviewed in both health and disease.

Wave Mechanics of the Vestibular Semicircular Canals

The semicircular canals are biomechanical sensors responsible for detecting and encoding angular motion of the head in 3D space. Canal afferent neurons provide essential inputs to neural circuits responsible for representation of self-position/orientation in space, and to compensatory circuits including the vestibulo-ocular and vestibulo-collic reflex arcs. In this work we derive, to our knowledge, a new 1D mathematical model quantifying canal biomechanics based on the morphology, dynamics of the inner ear fluids, and membranous labyrinth deformability. The model takes the form of a dispersive wave equation and predicts canal responses to angular motion, sound, and mechanical stimulation. Numerical simulations were carried out for the morphology of the human lateral canal using known physical properties of the endolymph and perilymph in three diverse conditions: surgical plugging, rotation, and mechanical indentation. The model reproduces frequency-dependent attenuation and phase shift in cases of canal plugging. During rotation, duct deformability extends the frequency bandwidth and enhances the high frequency gain. Mechanical indentation of the membranous duct at high frequencies evokes traveling waves that move away from the location of indentation and at low frequencies compels endolymph displacement along the canal. These results demonstrate the importance of the conformal perilymph-filled bony labyrinth to pressure changes and to high frequency sound and vibration.

Reviewing the Role of the Efferent Vestibular System in Motor and Vestibular Circuits

Efferent circuits within the nervous system carry nerve impulses from the central nervous system to sensory end organs. Vestibular efferents originate in the brainstem and terminate on hair cells and primary afferent fibers in the semicircular canals and otolith organs within the inner ear. The function of this efferent vestibular system (EVS) in vestibular and motor coordination though, has proven difficult to determine, and remains under debate. We consider current literature that implicate corollary discharge from the spinal cord through the efferent vestibular nucleus (EVN), and hint at a potential role in overall vestibular plasticity and compensation. Hypotheses range from differentiating between passive and active movements at the level of vestibular afferents, to EVS activation under specific behavioral and environmental contexts such as arousal, predation, and locomotion. In this review, we summarize current knowledge of EVS circuitry, its effects on vestibular hair cell and primary afferent activity, and discuss its potential functional roles.

Transformation of spatiotemporal dynamics in the macaque vestibular system from otolith afferents to cortex

Sensory signals undergo substantial recoding when neural activity is relayed from sensors through pre-thalamic and thalamic nuclei to cortex. To explore how temporal dynamics and directional tuning are sculpted in hierarchical vestibular circuits, we compared responses of macaque otolith afferents with neurons in the vestibular and cerebellar nuclei, as well as five cortical areas, to identical three-dimensional translational motion. We demonstrate a remarkable spatio-temporal transformation: otolith afferents carry spatially aligned cosine-tuned translational acceleration and jerk signals. In contrast, brainstem and cerebellar neurons exhibit non-linear, mixed selectivity for translational velocity, acceleration, jerk and position. Furthermore, these components often show dissimilar spatial tuning. Moderate further transformation of translation signals occurs in the cortex, such that similar spatio-temporal properties are found in multiple cortical areas. These results suggest that the first synapse represents a key processing element in vestibular pathways, robustly shaping how self-motion is represented in central vestibular circuits and cortical areas.

Efferent Vestibular Neurons Show Homogenous Discharge Output But Heterogeneous Synaptic Input Profile In Vitro

Despite the importance of our sense of balance we still know remarkably little about the central control of the peripheral balance system. While previous work has shown that activation of the efferent vestibular system results in modulation of afferent vestibular neuron discharge, the intrinsic and synaptic properties of efferent neurons themselves are largely unknown. Here we substantiate the location of the efferent vestibular nucleus (EVN) in the mouse, before characterizing the input and output properties of EVN neurons in vitro. We made transverse serial sections through the brainstem of 4-week-old mice, and performed immunohistochemistry for calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT), both expressed in the EVN of other species. We also injected fluorogold into the posterior canal and retrogradely labelled neurons in the EVN of ChAT:: tdTomato mice expressing tdTomato in all cholinergic neurons. As expected the EVN lies dorsolateral to the genu of the facial nerve (CNVII). We then made whole-cell current-, and voltage-clamp recordings from visually identified EVN neurons. In current-clamp, EVN neurons display a homogeneous discharge pattern. This is characterized by a high frequency burst of action potentials at the onset of a depolarizing stimulus and the offset of a hyperpolarizing stimulus that is mediated by T-type calcium channels. In voltage-clamp, EVN neurons receive either exclusively excitatory or inhibitory inputs, or a combination of both. Despite this heterogeneous mixture of inputs, we show that synaptic inputs onto EVN neurons are predominantly excitatory. Together these findings suggest that the inputs onto EVN neurons, and more specifically the origin of these inputs may underlie EVN neuron function.

The frog vestibular system as a model for lesion-induced plasticity: basic neural principles and implications for posture control

Studies of behavioral consequences after unilateral labyrinthectomy have a long tradition in the quest of determining rules and limitations of the central nervous system (CNS) to exert plastic changes that assist the recuperation from the loss of sensory inputs. Frogs were among the first animal models to illustrate general principles of regenerative capacity and reorganizational neural flexibility after a vestibular lesion. The continuous successful use of the latter animals is in part based on the easy access and identifiability of nerve branches to inner ear organs for surgical intervention, the possibility to employ whole brain preparations for in vitro studies and the limited degree of freedom of postural reflexes for quantification of behavioral impairments and subsequent improvements. Major discoveries that increased the knowledge of post-lesional reactive mechanisms in the CNS include alterations in vestibular commissural signal processing and activation of cooperative changes in excitatory and inhibitory inputs to disfacilitated neurons. Moreover, the observed increase of synaptic efficacy in propriospinal circuits illustrates the importance of limb proprioceptive inputs for postural recovery. Accumulated evidence suggests that the lesion-induced neural plasticity is not a goal-directed process that aims toward a meaningful restoration of vestibular reflexes but rather attempts a survival of those neurons that have lost their excitatory inputs. Accordingly, the reaction mechanism causes an improvement of some components but also a deterioration of other aspects as seen by spatio-temporally inappropriate vestibulo-motor responses, similar to the consequences of plasticity processes in various sensory systems and species. The generality of the findings indicate that frogs continue to form a highly amenable vertebrate model system for exploring molecular and physiological events during cellular and network reorganization after a loss of vestibular function.

Morphology and innervation of the vestibular lagena in pigeons

The morphological characteristics of the pigeon lagena were examined using histology, scanning electron microscopy, and biotinylated dextran amine (BDA) neural tracers. The lagena epithelium was observed to lie partially in a parasagittal plane, but was also U-shaped with orthogonal (lateral) directed tips. Hair cell planar polarities were oriented away from a central reversal line that ran nearly the length of the epithelium. Similar to the vertebrate utricle and saccule, three afferent classes were observed based upon their terminal innervation pattern, which include calyx, dimorph, and bouton fibers. Calyx and dimorph afferents innervated the striola region of the lagena, whereas bouton afferents innervated the extrastriola and a small region of the central striola known as the type II band. Calyx units had large calyceal terminal structures that innervated only type I hair cells. Dimorph afferents innervated both type I and II hair cells, with calyx and bouton terminals. Bouton afferents had the largest most complex innervation patterns and the greatest terminal areas contacting many hair cells.

Cellular and network contributions to vestibular signal processing: impact of ion conductances, synaptic inhibition, and noise

Head motion-related sensory signals are transformed by second-order vestibular neurons (2°VNs) into appropriate commands for retinal image stabilization during body motion. In frogs, these 2°VNs form two distinct subpopulations that have either tonic or highly phasic intrinsic properties, essentially compatible with low-pass and bandpass filter characteristics, respectively. In the present study, physiological data on cellular properties of 2°VNs of the grass frog (Rana temporaria) have been used to construct conductance-based spiking cellular models that were fine-tuned by fitting to recorded spike-frequency data. The results of this approach suggest that low-threshold, voltage-dependent potassium channels in phasic and spike-dependent potassium channels in tonic 2°VNs are important contributors to the differential, yet complementary response characteristics of the two vestibular subtypes. Extension of the cellular model with conductance-based synapses allowed simulation of afferent excitation and evaluation of the emerging properties of local feedforward inhibitory circuits. This approach revealed the relative contributions of intrinsic and synaptic factors on afferent signal processing in phasic 2°VNs. Additional extension of the single-cell model to a population model allowed testing under more natural conditions including asynchronous afferent labyrinthine input and synaptic noise. This latter approach indicated that the feedforward inhibition from the local inhibitory network acts as a high-pass filter, which reinforces the impact of the intrinsic membrane properties of phasic 2°VNs on peak response amplitude and timing. Thus, the combination of cellular and network properties enables phasic 2°VNs to work as a noise-resistant detector, suitable for central processing of short-duration vestibular signals.

Variation in response dynamics of regular and irregular vestibular-nerve afferents during sinusoidal head rotations and currents in the chinchilla

In mammals, vestibular-nerve afferents that innervate only type I hair cells (calyx-only afferents) respond nearly in phase with head acceleration for high-frequency motion, whereas afferents that innervate both type I and type II (dimorphic) or only type II (bouton-only) hair cells respond more in phase with head velocity. Afferents that exhibit irregular background discharge rates have a larger phase lead re-head velocity than those that fire more regularly. The goal of this study was to investigate the cause of the variation in phase lead between regular and irregular afferents at high-frequency head rotations. Under the assumption that externally applied galvanic currents act directly on the nerve, we derived a transfer function describing the dynamics of a semicircular canal and its hair cells through comparison of responses to sinusoidally modulated head velocity and currents. Responses of all afferents were fit well with a transfer function with one zero (lead term). Best-fit lead terms describing responses to current for each group of afferents were similar to the lead term describing responses to head velocity for regular afferents (0.006 s + 1). This finding indicated that the pre-synaptic and synaptic inputs to regular afferents were likely to be pure velocity transducers. However, the variation in phase lead between regular and irregular afferents could not be explained solely by the ratio of type I to II hair cells (Baird et al 1988), suggesting that the variation was caused by a combination of pre- (type of hair cell) and post-synaptic properties.

Efferent-mediated responses in vestibular nerve afferents of the alert macaque

The peripheral vestibular organs have long been known to receive a bilateral efferent innervation from the brain stem. However, the functional role of the efferent vestibular system has remained elusive. In this study, we investigated efferent-mediated responses in vestibular afferents of alert behaving primates (macaque monkey). We found that efferent-mediated rotational responses could be obtained from vestibular nerve fibers innervating the semicircular canals after conventional afferent responses were nulled by placing the corresponding canal plane orthogonal to the plane of motion. Responses were type III, i.e., excitatory for rotational velocity trapezoids (peak velocity, 320 degrees/s) in both directions of rotation, consistent with those previously reported in the decerebrate chinchilla. Responses consisted of both fast and slow components and were larger in irregular (approximately 10 spikes/s) than in regular afferents (approximately 2 spikes/s). Following unilateral labyrinthectomy (UL) on the side opposite the recording site, similar responses were obtained. To confirm the vestibular source of the efferent-mediated responses, the ipsilateral horizontal and posterior canals were plugged following the UL. Responses to high-velocity rotations were drastically reduced when the superior canal (SC), the only intact canal, was in its null position, compared with when the SC was pitched 50 degrees upward from the null position. Our findings show that vestibular afferents in alert primates show efferent-mediated responses that are related to the discharge regularity of the afferent, are of vestibular origin, and can be the result of both afferent excitation and inhibition.

Differential dynamic processing of afferent signals in frog tonic and phasic second-order vestibular neurons

The sensory-motor transformation of the large dynamic spectrum of head-motion-related signals occurs in separate vestibulo-ocular pathways. Synaptic responses of tonic and phasic second-order vestibular neurons were recorded in isolated frog brains after stimulation of individual labyrinthine nerve branches with trains of single electrical pulses. The timing of the single pulses was adapted from spike discharge patterns of frog semicircular canal nerve afferents during sinusoidal head rotation. Because each electrical pulse evoked a single spike in afferent fibers, the resulting sequences with sinusoidally modulated intervals and peak frequencies up to 100 Hz allowed studying the processing of presynaptic afferent inputs with in vivo characteristics in second-order vestibular neurons recorded in vitro in an isolated whole brain. Variation of pulse-train parameters showed that the postsynaptic compound response dynamics differ in the two types of frog vestibular neurons. In tonic neurons, subthreshold compound responses and evoked discharge patterns exhibited relatively linear dynamics and were generally aligned with pulse frequency modulation. In contrast, compound responses of phasic neurons were asymmetric with large leads of subthreshold response peaks and evoked spike discharge relative to stimulus waveform. These nonlinearities were caused by the particular intrinsic properties of phasic vestibular neurons and were facilitated by GABAergic and glycinergic inhibitory inputs from tonic type vestibular interneurons and by cerebellar circuits. Coadapted intrinsic filter and emerging network properties thus form dynamically different neuronal elements that provide the appropriate cellular basis for a parallel processing of linear, tonic, and nonlinear phasic vestibulo-ocular response components in central vestibular neurons.

Differential intrinsic response dynamics determine synaptic signal processing in frog vestibular neurons

Central vestibular neurons process head movement-related sensory signals over a wide dynamic range. In the isolated frog whole brain, second-order vestibular neurons were identified by monosynaptic responses after electrical stimulation of individual semicircular canal nerve branches. Neurons were classified as tonic or phasic vestibular neurons based on their different discharge patterns in response to positive current steps. With increasing frequency of sinusoidally modulated current injections, up to 100 Hz, there was a concomitant decrease in the impedance of tonic vestibular neurons. Subthreshold responses as well as spike discharge showed classical low-pass filter-like characteristics with corner frequencies ranging from 5 to 20 Hz. In contrast, the impedance of phasic vestibular neurons was relatively constant over a wider range of frequencies or showed a resonance at approximately 40 Hz. Above spike threshold, single spikes of phasic neurons were synchronized with the sinusoidal stimulation between approximately 20 and 50 Hz, thus showing characteristic bandpass filter-like properties. Both the subthreshold resonance and bandpass filter-like discharge pattern depend on the activation of an I(D) potassium conductance. External current or synaptic stimulation that produced impedance increases (i.e., depolarization in tonic or hyperpolarization in phasic neurons) had opposite and complementary effects on the responses of the two types of neurons. Thus, membrane depolarization by current steps or repetitive synaptic excitation amplified synaptic inputs in tonic vestibular neurons and reduced them in phasic neurons. These differential, opposite membrane response properties render the two neuronal types particularly suitable for either integration (tonic neurons) or signal detection (phasic neurons), respectively, and dampens variations of the resting membrane potential in the latter.

Retrograde neuron tracing with microspheres reveals projection of CGRP-immunolabeled vestibular afferent neurons to the vestibular efferent nucleus in the brainstem of rats

OBJECTIVE

A new retrograde neuron-tracing technique with microspheres was used to explore the possible innervation of calcitonin gene-related peptide (CGRP)-immunolabeled vestibular afferent neurons in the vestibular efferent immunolabeled nucleus in the brainstem.

METHODS

0.1 microl of 5% microfluorospheres was injected into the area of the vestibular efferent nucleus, which is located lateral to the genu of the facial nerve. CGRP immunohistochemistry was processed in serial sections of the brainstem at the facial nerve genu level. Double-labeled neurons with both CGRP immunoreactivity and microfluorospheres were examined with fluorescence and confocal laser microscopy.

RESULTS

Three types of labeled neurons were observed: (1) neurons only retrogradely microfluorosphere-labeled that were mainly located in the medial vestibular nucleus, lateral vestibular nucleus, superior vestibular nucleus and parvicellular reticular nucleus on the ipsilateral side of the injection; (2) neurons that were both immunolabeled with CGRP and also retrogradedly labeled with microfluorospheres, indicating that they are CGRP cells projecting to the area of vestibular efferent nucleus, these cells were mainly distributed in the superior vestibular nucleus and dorsal vestibular nucleus, and (3) cells only immunolabeled for CGRP that were scattered extensively in the brainstem.

CONCLUSION

The presented methodical contribution demonstrates the suitability of fluorescein-labeled microspheres for retrograde neuronal tracing. The vestibular nuclei contain numerous afferent neurons that send projections to the vestibular efferent nucleus, some of which are CGRP cells. This afferent innervation provides morphological evidence that the vestibular efferent neurons receive input from the vestibular afferent neurons including CGRP cells. These vestibular primary CGRP afferent neurons may have an influence on vestibular efferent neurons. CGRP acts as an important co-transmitter or modulator in the afferent-mediated activity of vestibular efferent neurons, which in turn affect afferents in the vestibular end organs.

Precerebellar Hindbrain Neurons Encoding Eye Velocity During Vestibular and Optokinetic Behavior in the Goldfish

Elucidating the causal role of head and eye movement signaling during cerebellar-dependent oculomotor behavior and plasticity is contingent on knowledge of precerebellar structure and function. To address this question, single-unit extracellular recordings were made from hindbrain Area II neurons that provide a major mossy fiber projection to the goldfish vestibulolateral cerebellum. During spontaneous behavior, Area II neurons exhibited minimal eye position and saccadic sensitivity. Sinusoidal visual and vestibular stimulation over a broad frequency range (0.1–4.0 Hz) demonstrated that firing rate mirrored the amplitude and phase of eye or head velocity, respectively. Table frequencies >1.0 Hz resulted in decreased firing rate relative to eye velocity gain, while phase was unchanged. During visual steps, neuronal discharge paralleled eye velocity latency (∼90 ms) and matched both the build-up and the time course of the decay (∼19 s) in eye velocity storage. Latency of neuronal discharge to table steps (40 ms) was significantly longer than for eye movement (17 ms), but firing rate rose faster than eye velocity to steady-state levels. The velocity sensitivity of Area II neurons was shown to equal (±10%) the sum of eye- and head-velocity firing rates as has been observed in cerebellar Purkinje cells. These results demonstrate that Area II neuronal firing closely emulates oculomotor performance. Conjoint signaling of head and eye velocity together with the termination pattern of each Area II neuron in the vestibulolateral lobe presents a unique eye-velocity brain stem-cerebellar pathway, eliminating the conceptual requirement of motor error signaling.

Bone conducted vibration selectively activates irregular primary otolithic vestibular neurons in the guinea pig

The main objective of this study was to determine whether bone-conducted vibration (BCV) is equally effective in activating both semicircular canal and otolith afferents in the guinea pig or whether there is preferential activation of one of these classes of vestibular afferents. To answer this question a large number (346) of single primary vestibular neurons were recorded extracellularly in anesthetized guinea pigs and were identified by their location in the vestibular nerve and classed as regular or irregular on the basis of the variability of their spontaneous discharge. If a neuron responded to angular acceleration it was classed as a semicircular canal neuron, if it responded to maintained roll or pitch tilts it was classified as an otolith neuron. Each neuron was then tested by BCV stimuli-either clicks, continuous pure tones (200-1,500 Hz) or short tone bursts (500 Hz lasting 7 ms)-delivered by a B-71 clinical bone-conduction oscillator cemented to the guinea pig's skull. All stimulus intensities were referred to that animal's own auditory brainstem response (ABR) threshold to BCV clicks, and the maximum intensity used was within the animal's physiological range and was usually around 70 dB above BCV threshold. In addition two sensitive single axis linear accelerometers cemented to the skull gave absolute values of the stimulus acceleration in the rostro-caudal direction. The criterion for a neuron being classed as activated was an audible, stimulus-locked increase in firing rate (a 10% change was easily detectable) in response to the BCV stimulus. At the stimulus levels used in this study, semicircular canal neurons, both regular and irregular, were insensitive to BCV stimuli and very few responded: only nine of 189 semicircular canal neurons tested (4.7%) showed a detectable increase in firing in response to BCV stimuli up to the maximum 2 V peak-to-peak level we delivered to the B-71 oscillator (which produced a peak-to-peak skull acceleration of around 6-8 g and was usually around 60-70 dB above the animal's own ABR threshold for BCV clicks). Regular otolithic afferents likewise had a poor response; only 14 of 99 tested (14.1%) showed any increase in firing rate up to the maximum BCV stimulus level. However, most irregular otolithic afferents (82.8%) showed a clear increase in firing rate in response to BCV stimuli: of the 58 irregular otolith neurons tested, 48 were activated, with some being activated at very low intensities (only about 10 dB above the animal's ABR threshold to BCV clicks). Most of the activated otolith afferents were in the superior division of the vestibular nerve and were probably utricular afferents. That was confirmed by evidence using juxtacellular injection of neurobiotin near BCV activated neurons to trace their site of origin to the utricular macula. We conclude there is a very clear preference for irregular otolith afferents to be activated selectively by BCV stimuli at low stimulus levels and that BCV stimuli activate some utricular irregular afferent neurons. The BCV generates compressional and shear waves, which travel through the skull and constitute head accelerations, which are sufficient to stimulate the most sensitive otolithic receptor cells.

Semicircular canal geometry, afferent sensitivity, and animal behavior

The geometry of the semicircular canals has been used in evolutionary studies to predict the behaviors of extinct animals. These predictions have relied on an assumption that the responses of the canals can be determined from their dimensions, and that an organism's behavior can be determined from these responses. However, the relationship between a canal's sensitivity and its size is not well known. An intraspecies comparison among canal responses in each of three species (cat, squirrel monkey, and pigeon) was undertaken to evaluate various models of canal function and determine how their dimensions may be related to afferent physiology. All models predicted the responses of the cat afferents, but the models performed less well for squirrel monkey and pigeon. Possible causes for this discrepancy include incorrectly assuming that afferent responses accurately represent canal function or errors in current biophysical models of the canals. These findings leave open the question as to how reliably canal anatomy can be used to estimate afferent responses and how closely afferent responses are related to behavior. Other labyrinthine features, such as orientation of the horizontal canal, which is reliably held near earth-horizontal across many species, may be better to use when extrapolating the posture and related behavior of extinct animals from labyrinthine morphology.

Efferent-mediated fluctuations in vestibular nerve discharge: a novel, positive-feedback mechanism of efferent control

We compared the background discharge of vestibular nerve afferents in barbiturate-anesthetized and unanesthetized, decerebrate chinchillas. Based on their interspike-interval statistics, units were categorized as regular, intermediate, or irregular. Background discharge rates were higher in irregular units from decerebrates compared to anesthetized preparations; no such difference was observed for regular or intermediate units. Large fluctuations in discharge rate were confined to intermediate and irregular units in decerebrates, but were not seen at all in anesthetized animals. The most prominent examples of fluctuations consisted of oscillations with periods exceeding 500 s and peak-to-peak amplitudes as large as 300 spikes/s. Several observations show that the fluctuations are mediated by the efferent vestibular system (EVS): (1) they are abolished when the vestibular nerve is cut proximal to the recording electrode; (2) their amplitude is correlated with the size of efferent-mediated rotational responses in individual units; and (3) they occur even when vital signs are stable. Previous studies had provided evidence that the EVS involves positive feedback: vestibular nerve afferents and EVS neurons excite one another. To study how oscillations could be produced, we developed a nonlinear model of positive feedback in which afferent feed-forward discharge was nonlinearly related to its inputs from hair cells and the EVS, while these inputs declined (adapted) as discharge was prolonged. Provided that the gain of the efferent feedback loop was sufficiently large, the model showed oscillations similar to those observed experimentally. Although large fluctuations in afferent discharge are unlikely to occur under physiological circumstances, positive feedback may be a normal feature that can amplify the influence of the EVS.

Organization of dye-coupled cerebellar granule cells labeled from afferent vestibular and dorsal root fibers in the frogRana esculenta

Application of neurobiotin to the nerves of individual labyrinthine organs and dorsal root fibers of limb-innervating segments of the frog resulted in labeling of granule cells in the cerebellum showing a significant overlap with a partial segregation in the related areas of termination. In different parts of the cerebellum, various combinations of different canal and otolith organ-related granule cells have been discerned. The difference in the extension of territories of vertical canals vs. horizontal canals may reflect their different involvement in the vestibuloocular and vestibulospinal reflex. Dye-coupled cells related to the lagenar and saccular neurons were localized in more rostral parts of the cerebellum, whereas cells of the utricle were represented only in its caudal half. This separation is supportive of the dual function of the lagena and the saccule. The territories of granule cells related to the cervical and lumbar segments of the spinal cord were almost completely separated along the rostrocaudal axis of cerebellum, whereas their territories were almost entirely overlapping in the mediolateral and ventrodorsal directions. The partial overlap of labyrinthine organ-related and dorsal root fiber-related granule cells are suggestive of a convergence of sensory modalities involved in the sense of balance. We propose that the afferent input of vestibular and proprioceptive fibers mediated by gap junctions to the cerebellar granule cells subserve one of the possible morphological correlates of a very rapid modification of the motor activity in the vestibulocerebellospinal neuronal circuit.

Intrinsic membrane properties of vertebrate vestibular neurons: function, development and plasticity

Central vestibular neurons play an important role in the processing of body motion-related multisensory signals and their transformation into motor commands for gaze and posture control. Over recent years, medial vestibular nucleus (MVN) neurons and to a lesser extent other vestibular neurons have been extensively studied in vivo and in vitro, in a range of species. These studies have begun to reveal how their intrinsic electrophysiological properties may relate to their response patterns, discharge dynamics and computational capabilities. In vitro studies indicate that MVN neurons are of two major subtypes (A and B), which differ in their spike shape and after-hyperpolarizations. This reflects differences in particular K(+) conductances present in the two subtypes, which also affect their response dynamics with type A cells having relatively low-frequency dynamics (resembling "tonic" MVN cells in vivo) and type B cells having relatively high-frequency dynamics (resembling "kinetic" cells in vivo). The presence of more than one functional subtype of vestibular neuron seems to be a ubiquitous feature since vestibular neurons in the chick and frog also subdivide into populations with different, analogous electrophysiological properties. The ratio of type A to type B neurons appears to be plastic, and may be determined by the signal processing requirements of the vestibular system, which are species-variant. The membrane properties and discharge pattern of type A and type B MVN neurons develop largely post-natally, through the expression of the underlying ion channel conductances. The membrane properties of MVN neurons show rapid and long-lasting plastic changes after deafferentation (unilateral labyrinthectomy), which may serve to maintain their level of activity and excitability after the loss of afferent inputs.

Assessing vestibular contributions during changes in gait trajectory

Displacing prisms and galvanic stimulation were used to examine visual-vestibular interactions during target-directed gait. Participants walked towards a wall 6 m away. After taking four steps, a target on the wall, located directly in front or to the right of the participant, was illuminated. Participants continued walking towards the target. Galvanic vestibular stimulation was triggered at either gait initiation, a step before the potential turn, or at target illumination. Although the visual and vestibular perturbations significantly altered gait trajectory, the greatest interaction occurred when galvanic stimulation was triggered one step before the target appeared. This implies an increase in the weighting of vestibular inputs just before turning to prepare for the potential change in direction.

Second-Order Vestibular Neurons Form Separate Populations With Different Membrane and Discharge Properties

Membrane and discharge properties were determined in second-order vestibular neurons (2°VN) in the isolated brain of grass frogs. 2°VN were identified by monosynaptic excitatory postsynaptic potentials after separate electrical stimulation of the utricular nerve, the lagenar nerve, or individual semicircular canal nerves. 2°VN were classified as vestibulo-ocular or -spinal neurons by the presence of antidromic spikes evoked by electrical stimulation of the spinal cord or the oculomotor nuclei. Differences in passive membrane properties, spike shape, and discharge pattern in response to current steps and ramp-like currents allowed a differentiation of frog 2°VN into two separate, nonoverlapping types of vestibular neurons. A larger subgroup of 2°VN (78%) was characterized by brief, high-frequency bursts of up to five spikes and the absence of a subsequent continuous discharge in response to positive current steps. In contrast, the smaller subgroup of 2°VN (22%) exhibited a continuous discharge with moderate adaptation in response to positive current steps. The differences in the evoked spike discharge pattern were paralleled by differences in passive membrane properties and spike shapes. Despite these differences in membrane properties, both types, i.e., phasic and tonic 2°VN, occupied similar anatomical locations and displayed similar afferent and efferent connectivities. Differences in response dynamics of the two types of 2°VN match those of their pre- and postsynaptic neurons. The existence of distinct populations of 2°VN that differ in response dynamics but not in the spatial organization of their afferent inputs and efferent connectivity to motor targets suggests that frog 2°VN form one part of parallel vestibulomotor pathways.

Postlesional vestibular reorganization improves the gain but impairs the spatial tuning of the maculo-ocular reflex in frogs

The ramus anterior (RA) of N.VIII was sectioned unilaterally. Two months later we analyzed in vivo responses of the ipsi- and of the contralesional abducens nerve during horizontal and vertical linear acceleration in darkness. The contralesional abducens nerve had become responsive again to linear acceleration either because of a synaptic reorganization in the vestibular nuclei on the operated side and/or because of a reinnervation of the utricular macula by regenerating afferent nerve fibers. Significant differences in the onset latencies and in the acceleration sensitivities allowed a separation of RA frogs in a group without and in a group with functional utricular reinnervation. Most important, the vector orientation for maximal abducens nerve responses was clearly altered: postlesional synaptic reorganization resulted in the emergence of abducens nerve responses to vertical linear acceleration, a response component that was barely detectable in RA frogs with utricular reinnervation and that was absent in controls. The ipsilesional abducens nerve, however, exhibited unaltered responses in either group of RA frogs. The altered spatial tuning properties of contralesional abducens nerve responses are a direct consequence of the postlesional expansion of signals from intact afferent nerve and excitatory commissural fibers onto disfacilitated 2nd-order vestibular neurons on the operated side. These results corroborate the notion that postlesional vestibular reorganization activates a basic neural reaction pattern with more beneficial results at the cellular than at the network level. However, given that the underlying mechanism is activity-related, rehabilitative training after vestibular nerve lesion can be expected to shape the ongoing reorganization.

Differential spatial organization of otolith signals in frog vestibular nuclei

Activation maps of pre- and postsynaptic field potential components evoked by separate electrical stimulation of utricular, lagenar, and saccular nerve branches in the isolated frog hindbrain were recorded within a stereotactic outline of the vestibular nuclei. Utricular and lagenar nerve-evoked activation maps overlapped strongly in the lateral and descending vestibular nuclei, whereas lagenar amplitudes were greater in the superior vestibular nucleus. In contrast, the saccular nerve-evoked activation map coincided largely with the dorsal nucleus and the adjacent dorsal part of the lateral vestibular nucleus, corroborating a major auditory and lesser vestibular function of the frog saccule. The stereotactic position of individual second-order otolith neurons matched the distribution of the corresponding otolith nerve-evoked activation maps. Furthermore, particular types of second-order utricular and lagenar neurons were clustered with particular types of second-order canal neurons in a topology that anatomically mirrored the preferred convergence pattern of afferent otolith and canal signals in second-order vestibular neurons. Similarities in the spatial organization of functionally equivalent types of second-order otolith and canal neurons between frog and other vertebrates indicated conservation of a common topographical organization principle. However, the absence of a precise afferent sensory topography combined with the presence of spatially segregated groups of particular second-order vestibular neurons suggests that the vestibular circuitry is organized as a premotor map rather than an organotypical sensory map. Moreover, the conserved segmental location of individual vestibular neuronal phenotypes shows linkage of individual components of vestibulomotor pathways with the underlying genetically specified rhombomeric framework.

Patterns of canal and otolith afferent input convergence in frog second-order vestibular neurons

Second-order vestibular neurons (2 degrees VN) were identified in the isolated frog brain by the presence of monosynaptic excitatory postsynaptic potentials (EPSPs) after separate electrical stimulation of individual vestibular nerve branches. Combinations of one macular and the three semicircular canal nerve branches or combinations of two macular nerve branches were stimulated separately in different sets of experiments. Monosynaptic EPSPs evoked from the utricle or from the lagena converged with monosynaptic EPSPs from one of the three semicircular canal organs in ~30% of 2 degrees VN. Utricular afferent signals converged predominantly with horizontal canal afferent signals (74%), and lagenar afferent signals converged with anterior vertical (63%) or posterior vertical (37%) but not with horizontal canal afferent signals. This convergence pattern correlates with the coactivation of particular combinations of canal and otolith organs during natural head movements. A convergence of afferent saccular and canal signals was restricted to very few 2 degrees VN (3%). In contrast to the considerable number of 2 degrees VN that received an afferent input from the utricle or the lagena as well as from one of the three canal nerves (~30%), smaller numbers of 2 degrees VN (14% of each type of 2 degrees otolith or 2 degrees canal neuron) received an afferent input from only one particular otolith organ or from only one particular semicircular canal organ. Even fewer 2 degrees VN received an afferent input from more than one semicircular canal or from more than one otolith nerve (~7% each). Among 2 degrees VN with afferent inputs from more than one otolith nerve, an afferent saccular nerve input was particularly rare (4-5%). The restricted convergence of afferent saccular inputs with other afferent otolith or canal inputs as well as the termination pattern of saccular afferent fibers are compatible with a substrate vibration sensitivity of this otolith organ in frog. The ascending and/or descending projections of identified 2 degrees VN were determined by the presence of antidromic spikes. 2 degrees VN mediating afferent utricular and/or semicircular canal nerve signals had ascending and/or descending axons. 2 degrees VN mediating afferent lagenar or saccular nerve signals had descending but no ascending axons. The latter result is consistent with the absence of short-latency macular signals on extraocular motoneurons during vertical linear acceleration. Comparison of data from frog and cat demonstrated the presence of a similar organization pattern of maculo- and canal-ocular reflexes in both species.

Reflections of efferent activity in rotational responses of chinchilla vestibular afferents

To study presumed efferent-mediated responses, we determined if afferents responded to head rotations that stimulated semicircular canals other than the organ being innervated. To minimize stimulation of an afferent's own canal, its plane was placed nearly orthogonal to the rotation plane. Otolith units were tested in a horizontal head position with the ear placed near the rotation axis to minimize linear forces. Under these circumstances, angular-velocity trapezoids (2-s ramps, 2-s plateau) evoked excitatory responses for both rotation directions. These type III responses were considerably larger in decerebrate than in anesthetized preparations. In addition to their being exclusively excitatory, the responses resembled those obtained with electrical stimulation of efferent pathways in including per-stimulus and more prolonged post-stimulus components and in being larger in irregularly discharging than in regularly discharging units. Responses, which were not seen for rotations <80 degrees/s, grew as velocity increased between 80 and 500 degrees/s but were seldom larger than 20 spikes/s. Complete section of the VIIIth nerve abolished type III responses, leaving conventional afferent responses intact. To study the separate contributions of canals on the two sides, responses were compared when the labyrinths were intact and when the ipsilateral or contralateral horizontal canal was mechanically inactivated. Both sides contributed to the efferent-mediated responses. That afferents could be influenced from the contralateral labyrinth was confirmed with the use of unilateral galvanic currents. Following inactivation, excitatory responses were produced by rotations exciting or inhibiting the intact horizontal canal with the responses resulting from excitatory rotations being much larger. Such a response asymmetry is consistent with a semicircular-canal origin for the type III responses. A similar asymmetry was seen in the post-stimulus responses to contralateral cathodal (excitatory) and anodal (inhibitory) galvanic currents. We conclude that the efferent system receives a sufficiently powerful vestibular input from both the ipsilateral and contralateral labyrinths to affect afferent discharge.

The receptor potential in type I and type II vestibular system hair cells: a model analysis

Several studies have shown that type I hair cells present a large outward rectifying potassium current (g(K,L)) that is substantially activated at the resting potential, greatly reducing cell input resistance and voltage gain. In fact, mechanoelectrical transducer currents seem not to be large enough to depolarize type I hair cells to produce neurotransmitter release. Also, the strongly nonlinear transducer currents and the limited voltage oscillations found in some hair cells did not account for the bidirectionality of response in hair cell systems. We developed a model based in the analysis of nonlinear Goldman-Hodgkin-Katz equations to calculate the hair cell receptor potential and ionic movements produced by transducer current activation. Type I hair cells displaying the large g(K,L) current were found to produce small receptor potentials (3-13.8 mV) in response to mechanoelectrical transducer current input. In contrast, type II cells that lack g(K,L) produced receptor potentials of about 30 mV. Properties of basolateral ionic conductances in type II hair cells will linearize hair bundle displacement to receptor potential relationship. The voltage to obtain the half maximal activation of g(K,L) significantly affects the resting membrane potential, the amplitude, and the linearity of the receptor potential. Electrodiffusion equations were also used to analyze ionic changes in the intercellular space between type I hair cell and calyx endings. Significant K(+) accumulation could take place at the intercellular space depending on calyx structure.

Principles of linear and angular vestibuloocular reflex organization in the frog

We compared the spatial organization patterns of linear and angular vestibuloocular reflexes in frogs by recording the multiunit spike activity from cranial nerve branches innervating the lateral rectus, the inferior rectus, or the inferior obliquus eye muscles. Responses were evoked by linear horizontal and/or vertical accelerations on a sled or by angular accelerations about an earth-vertical axis on a turntable. Before each sinusoidal oscillation test in darkness, the static head position was systematically altered to determine those directions of horizontal linear acceleration and those planes of angular head oscillation that were associated with minimal response amplitudes. Inhibitory response components during angular accelerations were clearly present, whereas inhibitory response components during linear accelerations were absent. Likewise was no contribution from the vertical otolith organs (i.e., lagena and saccule) observed during vertical linear acceleration. Horizontal linear acceleration evoked responses that originated from eye muscle-specific sectors on the contralateral utricular macula. The sectors of the inferior obliquus and lateral rectus muscles on the utricle had an opening angle of 45 and 60 degrees, respectively and overlapped to a large extent in the laterorostral part of the utricle. Both sectors were coplanar with the horizontal semicircular canals. The sector of the inferior rectus muscle was narrow (opening 5 degrees), laterocaudally oriented, and slightly pitched up by 6 degrees. Angular acceleration evoked maximal responses in the inferior obliquus muscle nerve that originated from the ipsilateral horizontal and the contralateral anterior vertical canals in a ratio of 50:50. Lateral rectus excitation originated from the contralateral horizontal and anterior vertical semicircular canals in a ratio of 80:20. The excitatory responses of the inferior rectus muscle nerve originated exclusively from the contralateral posterior vertical canal. Measured data and known semicircular canal plane vectors were used to calculate the spatial orientation of maximum sensitivity vectors for the investigated eye muscle nerves in semicircular canal coordinates. Comparison of the directions of maximal sensitivity vectors of responses evoked by linear or angular accelerations in a given eye muscle nerve showed that the two vector directions were oriented about orthogonally with respect to each other. With this arrangement the linear and the angular vestibuloocular reflex can support each other dynamically whenever they are co-activated without a change in the spatial response characteristics. The mutual adaptation of angular and linear vestibuloocular reflexes as well as the differences in their organization described here for frogs may represent a basic feature common for vertebrates in general.

Responses to efferent activation and excitatory response-intensity relations of turtle posterior-crista afferents

Multivariate statistical formulas were used to infer the morphological type and longitudinal position of extracellularly recorded afferents. Efferent fibers were stimulated electrically in the nerve branch interconnecting the anterior and posterior VIIIth nerves. Responses of bouton (B) units depended on their inferred position: BP units (near the planum semilunatum) showed small excitatory responses; BT units (near the torus) were inhibited; BM units (in an intermediate position) had a mixed response, including an initial inhibition and a delayed excitation. Calyx-bearing (CD-high) units with an appreciable background discharge showed large per-train excitatory responses followed by smaller post-train responses that could outlast the shock train by 100 s. Excitatory responses were smaller in calyx-bearing (CD-low) units having little or no background activity than in CD-high units. Excitatory response-intensity functions, derived from the discharge during 2-s angular-velocity ramps varying in intensity, were fit by empirical functions that gave estimates of the maximal response (r(MAX)), a threshold velocity (v(T)), and the velocity producing a half-maximal response (v(1/2)). Linear gain is equal to r(MAX)/v(S), v(S) = v(1/2) - v(T). v(S) provides a measure of the velocity range over which the response is nearly linear. For B units, r(MAX) declines by as much as twofold over the 2-s ramp, whereas for CD units, r(MAX) increases by 15% during the same time period. At the end of the ramp, r(MAX) is on average twice as high in CD as in B units. Thresholds are negligible in most spontaneously active units, including almost all B and CD-high units. Silent CD-low units typically have thresholds of 10-100 deg/s. BT units have very high linear gains and v(S) < 10 deg/s. Linear gains are considerably lower in BP units and v(S) > 150 deg/s. CD-high units have intermediate gains and near 100 deg/s v(S) values. CD-low units have low gains and v(S) values ranging from 150 to more than 300 deg/s. The results suggest that BT units are designed to measure the small head movements involved in postural control, whereas BP and CD units are more appropriate for monitoring large volitional head movements. The former units are silenced by efferent activation, whereas the latter units are excited. This suggests that the efferent system switches the turtle posterior crista from a "postural" to a "volitional" mode.

Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity

Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity. J. Neurophysiol. 80: 3297-3311, 1998. The electrophysiological activity of horizontal semicircular canal primary afferents (HSCPA) was investigated in vivo in the barbiturate-anesthetized pigeon by means of extracellular single-fiber vestibular nerve action potential recordings. The spontaneous and driven discharges to pulse (step/trapezoid waveform, peak velocity = 120 degrees/s) and sum-of-sines (0.03, 0.09, 0.21, 0.39, 0.93, 1.83 Hz, peak velocity = 30 degrees/s for each frequency) rotations were measured both in normal control animals and a group of animals at 30, 40, 50, 60, 71, and 150 days postinjection sequence (PIS) of streptomycin sulfate. Prior to 30 days PIS, the activity in the nerve was not appropriately modulated during and after rotation. At 30 days PIS and thereafter, the responses resembled those observed in control animals but with systematic changes in parameters of fitted pulse responses and fitted Bode plots as days PIS increased. The return of parameters characterizing the neural dynamics of the semicircular canals were monotonic and could be best described by either linear or exponential functions. After 30 days PIS, the mechanical cupula-endolymph system, the function of which can be inferred from the cupula long time constant (tauL) following step velocity, did not change systematically (tauL = 6.92 +/- 3.96, 8.64 +/- 5.52, 8.35 +/- 4.21, 10.00 +/- 2.79, 9.05 +/- 3.67, 7.05 +/- 2.72; means +/- SD). However, the mean gain (G) of the HSCPA response to pulse rotation nearly doubled between 30 and 150 days PIS (from 1.31 +/- 0. 39 to 2.40 +/- 1.04) and returned linearly to control values (G = 2. 39 +/- 0.77) over this time period [G = 1.33 + 0.009(PIS-30), R2 = 0. 92, P < 0.05]. Meanwhile, neural adaptation as quantitated using a fractional operator, k, decayed exponentially (single exponential) to an asymptote. The time constant of this exponential was approximately 55 days [k = 0.034 + 0.33e-(PIS-30)/55.4, R2 = 0.99, P < 0.01]. Features of the spontaneous discharge previously shown to be correlated with k changed appropriately. That is, the coefficient of variation (CV) and frequency of firing (FF) decayed and grew asymptotically, respectively. These parameters also exhibited an exponential time course of return to control values from 30 to 150 days PIS [CV = 0.44 + 0.65e-(PIS-30)/21.5, R2 = 0.96, P < 0.01, and FF = 39.97 + 101.42(1 - e-(PIS-30)/32.6), R2 = 0.97, P < 0.01]. The trends of recovery for G, k, and tauL derived from analysis of the pulse response were confirmed by strong positive correlations with best fitted parameters obtained from analysis of the sum-of-sines frequency domain response of HSCPAs. There were statistically significant correlations (r = 0.90, P < 0.05 and r = 0.93, P < 0.05) between parameters (G, k) derived from pulse responses and those (G', k') from sum-of-sines responses, respectively. The cupula time constant based on sum-of-sines' data (tau'L) showed no statistically significant change between 30 and 150 days PIS (P > 0.05, analysis of variance). Thus the results in present study indicate that both the spontaneous discharge and the driven response to rotation of pigeon HSCPAs recovered their normal physiological status between 30 and 150 days PIS after hair cell death due to aminoglycoside ototoxicity. The recovery was systematic for the parameters chosen to be tested with the exception of the cupula long time constant, tauL. The mechanisms (changes in ciliary dynamics, changes in hair cell ionic currents, changes in bouton terminals, etc.) underlying these changes await further morphophysiological studies.

Three-dimensional reconstruction of the membranous vestibular labyrinth in the toadfish, Opsanus tau

Membranous vestibular labyrinths from the oyster toadfish, Opsanus tau, were fixed, dissected from the animal, stained, and embedded in rectangular blocks of clear histological resin. Photomicrographs of complete embedded labyrinths were taken from six orthogonal directions and used to construct three-dimensional (3D) geometrical models of the semicircular canals, ampullae, utricular vestibule and common crus. Membraneous ducts and ampullae were modeled using a set of cross-sectional elliptical curves laced together to generate curved tubular models of each structure. The ensemble of these curved tubes was used to generate a complete 3D reconstruction of the outside surface of the membranous labyrinth. When viewed from six orthogonal directions, reconstructions closely matched the embedded tissue. Dimensions of the reconstruction and histological sections were compared to measurements of fresh tissue taken from the same animals prior to fixation and used to correct the reconstructions for tissue shrinkage. Results provide estimates of the endolymphatic volumes, local cross-sectional areas and elliptical eccentricities as well as 3D orientations of the geometric canal planes relative to the skull. Ten micrometer histological sections of the material were also prepared to measure wall thickness in various regions of the labyrinth.

The papilla neglecta of turtles: a detector of head rotations with unique sensory coding properties

The turtle papilla neglecta (PN) is a small organ lying in the ventrolateral utricular wall between the posterior crista (PC) and the utriculosaccular foramen. Innervated by a branch of the posterior ampullary nerve, the organ is covered by a cupula extending only a small distance into the endolymphatic space. Although most rotation-sensitive units in the posterior division of the eighth nerve have sensory coding properties expected of PC fibers, a few have unique properties. Intra-axonal labeling studies show that the former are PC units and the latter are PN units. PC units are maximally responsive to head rotations in the posterior canal plane and are sensitive to a combination of angular velocity and angular acceleration. PN units respond maximally to pitch rotations and are sensitive to a combination of angular acceleration and angular jerk. A maximal response to pitches can be related to the location of the PN, which allows it to sample endolymph flow from both vertical semicircular canals. Differences in response dynamics may reflect macromechanics. Because the cupula of each vertical canal occludes the endolymphatic space, its displacement should be proportional to endolymph displacement. In contrast, the PN cupula is probably coupled to endolymph flow by viscous forces, in which case its displacement should be proportional to endolymph velocity. In many vertebrates, the PN is similar to that seen in turtles in its location and in the size and shape of its cupula, which suggests that its function in these other species is also similar.

A comparative study of the responses of isolated first-order semicircular canal afferents to angular and linear acceleration, analysed in the time and frequency domains

The experiments reported here represent a study of the responses of semicircular canals in the isolated surviving labyrinth of elasmobranch fishes, with special reference to their sensitivity to linear acceleration, including positional stimuli, in comparison with their 'classical ’ function as inertial angular accelerometers. Responses to angular velocity steps, to step changes in position (linear acceleration steps), and responses to sinusoidal stimuli were analysed in the time domain or in the frequency domain respectively. In this investigation the existence of peripheral adaptation and the associated time constants were taken into account. It was found that the time constants and phase aspects of the res­ponses to angular and linear acceleration stimuli are consistently com­parable. This is believed to point to similarities in the basic mechanism of both types of canal response. It is suggested, therefore, that the sensitivity of semicircular canals to linear acceleration, referred to in this paper as the ‘Ledoux effect’, originates within the cupula-endolymph system in a manner fundamentally similar to that governing the inertial responses to angular acceleration. Although a considerable proportion of the canal-controlled first-order neurons studied responded to positional stimuli, no predictable and directionally fixed relation was found to exist between head positions and response levels. This suggests that differences in density as between cupula and endolymph, considered to be the cause of the positional responses, must be assumed to be subject to fluctuation both in sign and magnitude. We conclude that it is, therefore, doubtful whether the Ledoux effect falls within the range of ‘normal’ canal function in the control of posture and movement. Nevertheless, its existence cannot be discounted in the qualitative and quantitative analysis of vestibular responses, and, especially, in the differential diagnosis of malfunction in the field of clinical otology. Work along similar lines on higher vertebrates is discussed in the context of the reported results.

Peripheral innervation patterns of vestibular nerve afferents in the bullfrog utriculus

Vestibular nerve afferents innervating the bullfrog utriculus differ in their response dynamics and sensitivity to natural stimulation. They also supply hair cells that differ markedly in hair bundle morphology. To examine the peripheral innervation patterns of individual utricular afferents more closely, afferent fibers were labeled by the extracellular injection of horseradish peroxidase (HRP) into the vestibular nerve after sectioning the vestibular nerve medial to Scarpa's ganglion to allow the degeneration of sympathetic and efferent fibers. The peripheral arborizations of individual afferents were then correlated with the diameters of their parent axons, the regions of the macula they innervate, and the number and type of hair cells they supply. The utriculus is divided by the striola, a narrow zone of distinctive morphology, into medial and lateral parts. Utricular afferents were classified as striolar or extrastriolar according to the epithelial entrance of their parent axons and the location of their terminal fields. In general, striolar afferents had thicker parent axons, fewer subepithelial bifurcations, larger terminal fields, and more synaptic endings than afferents in extrastriolar regions. Afferents in a juxtastriolar zone, immediately adjacent to the medial striola, had innervation patterns transitional between those in the striola and more peripheral parts of the medial extrastriola. Most afferents innervated only a single macular zone. The terminal fields of striolar afferents, with the notable exception of a few afferents with thin parent axons, were generally confined to one side of the striola. Hair cells in the bullfrog utriculus have previously been classified into four types based on hair bundle morphology (Lewis and Li: Brain Res. 83:35-50, 1975). Afferents in the extrastriolar and juxtastriolar zones largely or exclusively innervated Type B hair cells, the predominant hair cell type in the utricular macula. Striolar afferents supplied a mixture of four hair cell types, but largely contacted Type B and Type C hair cells, particularly on the outer rows of the medial striola. Afferents supplying more central striolar regions innervated fewer Type B and large numbers of Type E and Type F hair cells. Striolar afferents with thin parent axons largely supplied Type E hair cells with bulbed kinocilia in the innermost striolar rows.

Dynamic polarization vector of spatially tuned neurons

The study of the dynamic properties of otolith neurons has been difficult previously because of the differing response sensitivities of individual cells to specific stimulus directions and the lack of a general mathematical scheme that could explain and account for all their response features. The present paper describes a method for estimating both the spatial and temporal properties of neurons like the otolith neurons that are spatially tuned to different stimulus directions. At each stimulus frequency, a response elipse can be constructed from the neural responses elicited by stimulation along three linear independent axes. The semimajor axis of the ellipse will specify the neuron's direction of maximum sensitivity (polarization vector), whereas the semiminor axis will provide its sensitivity in the perpendicular direction. The predictions of the method for nonzero length of the semiminor axis are qualitatively the same as the experimentally observed dependance of response phase on stimulus orientation.

Response properties of gerbil otolith afferents to small angle pitch and roll tilts

The responses from isolated single otolith afferent fibers were obtained to small angle sinusoidal pitch and roll tilts in anesthetized gerbils. The stimulus directions that produced the maximum (response vector) and minimum response sensitivities were determined for each otolith afferent, with response vectors for the units being spread throughout the horizontal plane, similar to those reported for other species. A breadth of tuning measure was derived, with narrowly tuned neurons responding maximally to stimulation in one direction and minimally along an orthogonal ('null') direction. Most (approximately 80%) otolith afferents are narrowly tuned, however, some fibers were broadly tuned responding significantly to stimulations in any direction in the horizontal plane. The number of broadly tuned otolith afferents (approximately 20%) differs significantly from the more substantial number of broadly tuned vestibular nuclei neurons (88%) recently reported in rats.

Vestibular afferent responses to microrotational stimuli

Intracellular microelectrode recording/labelling techniques were used to investigate vestibular afferent responses in the bullfrog, to very small amplitude (less than 0.5 degree p-p) sinusoidal rotations in the vertical plane over the frequency range of 0.063-4 Hz. The axis of rotation was congruent with the axis of the anterior semicircular canal. Robust responses to peak accelerations as low as 0.031 degree/S2 were obtained from units subsequently traced to either the central portion of the anterior canal crista or the striolar region of the utricle. All of these microrotationally sensitive afferent neurons had irregular resting discharge rates and the majority had transfer ratios (relative to rotational velocity) of 1-40 spikes/s per degree/s. Individual utricular afferent velocity transfer ratios were nearly constant over the frequency range of 0.125-4 Hz. Canal units generally displayed decreasing response transfer ratios as stimulus frequencies increased. These findings indicate that although utricular striolar and central crista afferent velocity transfer ratios to microrotations were very similar, utricular striolar afferent neurons were more faithful sensors of very small amplitude rotational velocity in the vertical plane.

Responses of frog vestibular neurons to combined temperature microstimulation of the semicircular canals

The temperature microstimulation of the semicircular canal (heat bursts lasting about 2 sec with a peak amplitude of 0.5-5.0 degrees C) can be regarded as an analog of angular acceleration acting within the cavity of the given canal; the combination of temperature microstimulations targeted simultaneously on several canals can serve as a physical model of accelerated rotation having complex spatial characteristics. The recording of the activity of neurons of the vestibular nuclei of the frog (n = 278) in response to temperature microstimulation of the canals showed that 80% of the neurons have inputs from one to two canals and only 20% from three to six canals. The distribution of laterality was characterized by the predominance of ipsilateral inputs: 201 neurons (72.3%) had only ipsilateral inputs; 14 neurons (5%) had only contralateral inputs; 63 neurons (22.7%) had both inputs. The ipsilateral horizontal (67.6%) and the posterior (61.9%) were the most effective inputs; while among the contralateral, the posterior (21.2%) canals were the most effective; the least effective were the contralateral horizontal (8.6%) and the anterior (5.0%) canals. The presence of latent canal inputs (both excitatory and inhibitory) which showed up in combination with effective inputs was demonstrated.

Orientation of the semicircular canals in rat

The orientation of the rat semicircular canals was determined using one of two techniques. Null point analysis was used to define physiologically the planar equations of the anterior (n = 15) and posterior canals (n = 15); equations for the horizontal canal (n = 19) were determined using an anatomical dissection technique. Canal orientation was defined with respect to stereotaxic coordinate system and, for comparison, relative to head position during freeze (startle) behavior. Results show that ipsilateral canal planes are orthogonal within 4-8 degrees, and pairs of right-left synergistic pairs are essentially co-planar. The horizontal canals are inclined upwards 35 degrees with respect to the horizontal plane, but a head position of 43 degrees nose-down was determined to produce near optimal horizontal canal and minimal vertical canal activation with horizontal rotation. Finally, a loud or unexpected auditory stimulus initiates a freeze (startle) response in rat characterized by an transient followed by a sustained head position lasting several seconds. Transients are complete within 300-400 ms. Thereafter, the head becomes momentarily stabilized in the startle position which averaged 14 +/- 8 degrees (nose-down with respect to horizontal stereotaxic zero) across the population (n = 14). The response habituated only slightly, but the final position was sufficiently variable so as to limit the usefulness of the freeze (startle) position as a reference of semicircular canal position in the rat.

The role of compensatory eye and head movements for gaze stabilization in the unrestrained frog

Compensatory eye, head and gaze movements of unrestrained frogs were recorded simultaneously in response to table movements in the light. Passive displacement was compensated with a gain between 0.55 and 0.85, depending on stimulus amplitude. At small stimulus amplitudes gaze was stabilized exclusively by compensatory eye movements. At larger stimulus amplitudes compensatory head movements contributed up to 80% gaze stabilization. The contribution of compensatory eye movements became increasingly more restricted to those brief transient periods, at which head velocity changed only slowly in response to a change in stimulus direction or velocity. The wave forms of both eye and head movements exhibited characteristic and complementary distortions. Their combination, the gaze wave form compensated almost exactly in phase for the imposed passive displacement in space. Head saccades of small amplitude were rather well compensated by fast eye movements in the opposite direction, with the result that the combined gaze movement was smooth. The occurrence of these compensatory fast eye movements depended neither upon the function of the labyrinthine organs nor upon retinal image slip.

Some hydrodynamic properties of the posterior canal in the frog labyrinth related to neuronal responses

Measurements of the posterior-canal radius of curvature (R), the semicircular-duct radius (r) and the cupula radius (rC) were performed in the frog labyrinth. The Steinhausen-van Egmond equation led to an estimate of the canal endolymph flow, cupula deflection and sensory hair bending during constant angular accelerations (0.2-64 degrees/s2) of opposite directions and increasing duration (1.2-12 s). This analysis suggests that the non-linear canal afferent discharge behaviour exhibited by some units may not arise at the presynaptic level but rather postsynaptically, at the encoder site.

Effect of tilt on the response of neuronal activity within the cat vestibular nuclei during slow and constant velocity rotation

The responses of neurons sensitive to static tilt in the vestibular nuclei were examined in decerebrate cats during slow and constant velocity rotations about an axis tilted at various angles from the vertical. During any 360 degrees rotation, each unit showed a modulation of their firing rate, with a position-dependent maximum and minimum. Changes in amplitude of head displacement from 5 degrees to 25 degrees decreased the response gain of the units without affecting the locations of the discharge maxima and minima.