Physiological and anatomical characteristics of primary vestibular afferent neurons in the bullfrog

Calretinin Immunoreactivity in the VIIIth Nerve and Inner Ear Endorgans of Ranid Frogs

Calcium-binding proteins are essential for buffering intracellular calcium concentrations, which are critical for regulating cellular processes involved in neuronal computations. One such calcium-binding protein, calretinin, is present in many neurons of the central nervous system as well as those which innervate cranial sensory organs, although often with differential distributions in adjacent cellular elements. Here, we determined the presence and distribution of calretinin-immunoreactivity in the peripheral vestibular and auditory system of ranid frogs. Calretinin-immunoreactivity was observed in ganglion cells innervating the basilar and amphibian papilla, and in a subpopulation of ganglion cells innervating the saccular epithelium. In contrast, none of the ganglion cells innervating the lagena, the utricle, or the three semicircular canals were calretinin-immunopositive, suggesting that this calcium-binding protein is a marker for auditory but not vestibular afferent fibers in the frog. The absence of calretinin in vestibular ganglion cells corresponds with the lack of type I hair cells in anamniote vertebrates, many of which in amniotes are contacted by the neurites of large, calyx-forming calretinin-immunopositive ganglion cells. In the sensory epithelia of all endorgans, the majority of hair cells were strongly calretinin-immunopositive. Weakly calretinin-immunopositive hair cells were distributed in the intermediate region of the semicircular canal cristae, the central part of the saccular macula, the utricular, and lagenar striola and the medial part of the amphibian papilla. The differential presence of calretinin in the frog vestibular and auditory sensory periphery might reflect a biochemical feature related to firing patterns and frequency bandwidths of self-motion versus acoustic stimulus encoding, respectively.

Models of vestibular semicircular canal afferent neuron firing activity

Semicircular canal afferent neurons transmit information about head rotation to the brain. Mathematical models of how they do this have coevolved with concepts of how brains perceive the world. A 19th-century "camera" metaphor, in which sensory neurons project an image of the world captured by sense organs into the brain, gave way to a 20th-century view of sensory nerves as communication channels providing inputs to dynamical control systems. Now, in the 21st century, brains are being modeled as Bayesian observers who infer what is happening in the world given noisy, incomplete, and distorted sense data. The semicircular canals of the vestibular apparatus provide an experimentally accessible, low-dimensional system for developing and testing dynamical Bayesian generative models of sense data. In this review, we summarize advances in mathematical modeling of information transmission by semicircular canal afferent sensory neurons since the first such model was proposed nearly a century ago. Models of information transmission by vestibular afferent neurons may provide a foundation for developing realistic models of how brains perceive the world by inferring the causes of sense data.

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.

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.

Determinants of spatial and temporal coding by semicircular canal afferents

The vestibular semicircular canals are internal sensors that signal the magnitude, direction, and temporal properties of angular head motion. Fluid mechanics within the 3-canal labyrinth code the direction of movement and integrate angular acceleration stimuli over time. Directional coding is accomplished by decomposition of complex angular accelerations into 3 biomechanical components-one component exciting each of the 3 ampullary organs and associated afferent nerve bundles separately. For low-frequency angular motion stimuli, fluid displacement within each canal is proportional to angular acceleration. At higher frequencies, above the lower corner frequency, real-time integration is accomplished by viscous forces arising from the movement of fluid within the slender lumen of each canal. This results in angular velocity sensitive fluid displacements. Reflecting this, a subset of afferent fibers indeed report angular acceleration to the brain for low frequencies of head movement and report angular velocity for higher frequencies. However, a substantial number of afferent fibers also report angular acceleration, or a signal between acceleration and velocity, even at frequencies where the endolymph displacement is known to follow angular head velocity. These non-velocity-sensitive afferent signals cannot be attributed to canal biomechanics alone. The responses of non-velocity-sensitive cells include a mathematical differentiation (first-order or fractional) imparted by hair-cell and/or afferent complexes. This mathematical differentiation from velocity to acceleration cannot be attributed to hair cell ionic currents, but occurs as a result of the dynamics of synaptic transmission between hair cells and their primary afferent fibers. The evidence for this conclusion is reviewed below.

Responses of irregularly discharging chinchilla semicircular canal vestibular-nerve afferents during high-frequency head rotations

Mammalian vestibular-nerve afferents innervating the semicircular canals have been divided into groups according to their discharge regularity, gain at 2-Hz rotational stimulation, and morphology. Low-gain irregular afferents terminate in calyx endings in the central crista, high-gain irregular afferents synapse more peripherally in dimorphic (bouton and calyx) endings, and regular afferents terminate in the peripheral zone as bouton-only and dimorphic endings. The response dynamics of these three groups have been described only up to 4 Hz in previous studies. Reported here are responses of chinchilla semicircular canal vestibular-nerve afferents to rotational stimuli at frequencies up to 16 Hz. The sensitivity of all afferents increased with increasing frequency with the sensitivity of low-gain irregular afferents increasing the most and matching the high-gain irregular afferents at 16 Hz. All afferents increased their phase lead with respect to stimulus velocity at higher frequencies with the highest leads in low-gain irregular afferents and the lowest in regular afferents. No attenuation of sensitivity or shift in phase consistent with the presence of a high-frequency pole over the range tested was noted. Responses were best fit with a torsion-pendulum model combined with a lead operator (tau(HF1)s + 1)(tau(HF2)s + 1). The discharge regularity of individual afferents was correlated to the value of each afferent's lead operator time constants. These findings suggest that low-gain irregular afferents are well suited for encoding the onset of rapid head movements, a property that would be advantageous for initiation of reflexes with short latency such as the vestibulo-ocular reflex.

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.

Afferent innervation of the utricular macula in pigeons

Biotinylated dextran amine (BDA) was used to retrogradely label afferents innervating the utricular macula in adult pigeons. The pigeon utriclar macula consists of a large rectangular-shaped neuroepithelium with a dorsally curved anterior edge and an extended medioposterior tail. The macula could be demarcated into several regions based on cytoarchitectural differences. The striola occupied 30% of the macula and contained a large density of type I hair cells with fewer type II hair cells. Medial and lateral extrastriola zones were located outside the striola and contained only type II hair cells. A six- to eight-cell-wide band of type II hair cells existed near the center of the striola. The reversal line marked by the morphological polarization of hair cells coursed throughout the epithelium, near the peripheral margin, and through the center of the type II band. Calyx afferents innervated type I hair cells with calyceal terminals that contained between 2 and 15 receptor cells. Calyx afferents were located only in the striola region, exclusive of the type II band, had small total fiber innervation areas and low innervation densities. Dimorph afferents innervated both type I and type II hair cells with calyceal and bouton terminals and were primarily located in the striola region. Dimorph afferents had smaller calyceal terminals with few type I hair cells, extended fiber branches with bouton terminals and larger innervation areas. Bouton afferents innervated only type II hair cells in the extrastriola and type II band regions. Bouton afferents innervating the type II band had smaller terminal fields with fewer bouton terminals and smaller innervation areas than fibers located in the extrastriolar zones. Bouton afferents had the most bouton terminals on the longest fibers, the largest innervation areas with the highest innervation densities of all afferents. Among all afferents, smaller terminal innervation fields were observed in the striola and large fields were located in the extrastriola. The cellular organization and innervation patterns of the utricular maculae in birds appear to represent an organ in adaptive evolution, different from that observed for amphibians or mammals.

Canal-specific excitation and inhibition of frog second-order vestibular neurons

Second-order vestibular neurons (secondary VNs) were identified in the in vitro frog brain by their monosynaptic excitation following electrical stimulation of the ipsilateral VIIIth nerve. Ipsilateral disynaptic inhibitory postsynaptic potentials were revealed by bath application of the glycine antagonist strychnine or of the gamma-aminobutyric acid-A (GABA(A)) antagonist bicuculline. Ipsilateral disynaptic excitatory postsynaptic potentials (EPSPs) were analyzed as well. The functional organization of convergent monosynaptic and disynaptic excitatory and inhibitory inputs onto secondary VNs was studied by separate electrical stimulation of individual semicircular canal nerves on the ipsilateral side. Most secondary VNs (88%) received a monosynaptic EPSP exclusively from one of the three semicircular canal nerves; fewer secondary VNs (10%) were monosynaptically excited from two semicircular canal nerves; and even fewer secondary VNs (2%) were monosynaptically excited from each of the three semicircular canal nerves. Disynaptic EPSPs were present in the majority of secondary VNs (68%) and originated from the same (homonymous) semicircular canal nerve that activated a monosynaptic EPSP in a given neuron (22%), from one or both of the other two (heteronymous) canal nerves (18%), or from all three canal nerves (28%). Homonymous activation of disynaptic EPSPs prevailed (74%) among those secondary VNs that exhibited disynaptic EPSPs. Disynaptic inhibitory postsynaptic potentials (IPSPs) were mediated in 90% of the tested secondary VNs by glycine, in 76% by GABA, and in 62% by GABA as well as by glycine. These IPSPs were activated almost exclusively from the same semicircular canal nerve that evoked the monosynaptic EPSP in a given secondary VN. Our results demonstrate a canal-specific, modular organization of vestibular nerve afferent fiber inputs onto secondary VNs that consists of a monosynaptic excitation from one semicircular canal nerve followed by disynaptic excitatory and inhibitory inputs originating from the homonymous canal nerve. Excitatory and inhibitory second-order (secondary) vestibular interneurons are envisaged to form side loops that mediate spatially similar but dynamically different signals to secondary vestibular projection neurons. These feedforward side loops are suited to adjust the dynamic response properties of secondary vestibular projection neurons by facilitating or disfacilitating phasic and tonic input components.

Intraotic administration of gentamicin: a new method to study ototoxicity in the crista ampullaris of the bullfrog

A new method of local gentamicin administration was tested in the bullfrog inner ear to achieve ototoxic-induced hair cell destruction. Gelfoam pledgets soaked with known amounts of gentamicin were inserted into the perilymphatic cisterna of the bullfrog through a ventral surgical approach. A dose of 1.20 mg gentamicin, consistent with a perilymphatic concentration of 65 microg/ml, resulted in the desired ototoxic-induced hair cell damage, that is, complete hair cell destruction with minimal disruption of other components of the sensory epithelium. This study demonstrates that this is a useful and simple method to investigate the process of vestibular ototoxicity and hair cell regeneration, including aspects of hair cell destruction and repair.

Size-related colocalization of glycine and glutamate immunoreactivity in frog and rat vestibular afferents

Presence and distribution of glutamate, glycine, GABA and beta-alanine in VIIIth nerves of frogs and rats were investigated with postembedding immunocytochemical methods on serial semithin sections. In Scarpa's ganglion of the frog, all cell bodies were glutamate immunoreactive. About 17% of the cells per section were also glycine immunoreactive, but none were GABA or beta-alanine immunoreactive. The mean diameter of glycine-positive cell bodies (26.7 +/- 6.9 microns; N = 130) was significantly (P < 0.0001) larger than that of glycine-negative cell bodies (15.7 +/- 5.4 microns; N = 272). The intensity of glutamate immunostaining decreased with cell diameter, whereas the intensity of glycine immunostaining increased with cell diameter. As a result, the staining intensities for glutamate and glycine in a given cell were negatively correlated. Glycine immunoreactivity was also present in a size-related manner in distal and proximal afferent fibers. The majority of thin fibers (< 4 microns) was glycine negative, whereas most of the thick fibers (> 10 microns) were glycine positive. Glycine-positive fibers were observed in the sensory epithelial of all end organs in the inner ear. The saccular macula and its nerve, however, contained only few glycine immunoreactive structures. In Scarpa's ganglion of the rat, all cells were immunoreactive for glutamate, about 12% for colocalized glycine, and none for GABA or beta-alanine. Glycine-positive cell bodies were significantly (P < 0.0001) larger (32.2 +/- 5.2 microns; N = 82) than glycine-negative cell bodies (25.1 +/- 5.3 microns; N = 274). Cell bodies in the spiral ganglion were only glutamate immunoreactive, whereas staining for glutamate, glycine, and GABA was dense in the ventral cochlear nucleus. These results demonstrate that thicker vestibular afferent fibers represent a particular subpopulation that differs from the majority of thinner afferents due to their glycine immunoreactivity.

The cholinergic pharmacology of the frog saccule

Stimulation of the efferent nerves to the vestibular organs of the frog's inner ear produces either facilitation or inhibition of afferent firing. Similarly, application of acetylcholine (ACH), the major transmitter of the efferents, can produce both facilitation and/or inhibition as previously reported [Guth et al. (1986) Acta Otolaryngol. 102, 194-204; Norris et al. (1988) Hear. Res. 32, 197-206]. The firing rates of afferent neurons of the semicircular canal (SCC) using multiunit recordings are generally facilitated by ACH. Conversely, the firing rates of afferent units innervating the saccule are generally inhibited by ACH. This latter inhibition is antagonized by strychnine more potently than by curare, which is more potent than atropine. When inhibition is antagonized by strychnine or curare an underlying facilitation is revealed. The inhibition of saccular afferents by ACH shows desensitization requiring about 20 min to recover. The ACH-induced inhibition is mimicked by nicotine at very high concentrations but not by dimethyl phenylpiperazinium or cytisine. The fact that multiunit afferent firing from the SCC is generally facilitated while that from the saccule is generally inhibited by ACH suggests a different distribution of ACH receptors and receptor types (i.e. muscarinic or nicotinic and their subtypes) in the two organs and demonstrates the usefulness of recording from multiple units simultaneously. The difference in distribution of ACH receptors may be important for understanding the physiology of vestibular efferents.

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.

Afferent innervation of the vestibular nuclei in the chinchilla. I. A method for labeling individual vestibular receptors with horseradish peroxidase

A new method was developed for specific labeling of primary vestibular afferent fibers from selected end-organs with horseradish peroxidase (HRP) applied extracellularly in the inner ear space. In 48 chinchillas, labeling was performed successfully in all animals by scratching the surface of the sensory end-organ of interest with an electrolytically sharpened needle and replacing the fluid in the vestibule with 30% HRP solution. Merely replacing the vestibular fluid (endo- and perilymph) with HRP did not label the ganglion cells or the afferent fibers in the brain stem. The specificity of labeling was verified by histological inspection of the ganglion cells and nerve fibers innervating the damaged and intact receptors. When the posterior semicircular canal and saccular receptors were scratched, labeled fibers and ganglion cells were found in the nerve and ganglion rostrodorsally and caudoventrally, respectively. Labeled ganglion cells from different superior vestibular nerve (SVN) receptors did not show as clear a segregation pattern as did labeled receptors from the inferior vestibular nerve (IVN). Once inside the brain stem, labeled fibers from the SVN receptors were rostral to those from the IVN receptors. The fibers of the vestibular root divided into an ascending and a descending branch which formed the vestibular tract. Labeled fibers from the SVN receptors divided rostrolaterally to those from the IVN receptors. In the vestibular tract, fibers coursed in different locations according to the receptor of origin. Fibers from the utriculus were lateral to those from the horizontal semicircular canal, which were lateral to those from the anterior semicircular canal. Fibers from the sacculus were lateral to those from the posterior semicircular canal.

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.

Hair cell tufts and afferent innervation of the bullfrog crista ampullaris

Within the bullfrog semicircular canal crista, hair cell tuft types were defined and mapped with the aid of scanning electron microscopy. Intracellular recording and Lucifer Yellow labeling techniques were used to study afferent responses and arborization patterns. Dye-filled planar afferent axons had mean distal axonal diameters of 1.6-4.9 microns, highly branched arbors, and contacted 11-24 hair cells. Dye-filled isthmus afferent axons had mean distal axonal diameters of 1.8-7.9 microns, with either small or large field arbors contacting 4-9 or 25-31 hair cells. The estimated mean number of contacts per innervated hair cell was 2.2 for planar and 1.3 for isthmus afferent neurons. Data on evoked afferent responses were available only for isthmus units that were observed to respond to our microrotational stimuli (less than 3 degrees/s peak rotational velocity). Of 21 such afferent neurons, 8 were successfully dye-filled. Within this small sample, high-gain units had large field arbors and lower-gain units had small field arbors. The sensitivity of each afferent neuron was analyzed in terms of noise equivalent input (NEI), the stimulus amplitude for which the afferent response amplitude is just equivalent to the RMS deviation of the instantaneous spike rate. NEI for isthmus units varied from 0.63 to 8.2 degrees/s; the mean was 3.2 degrees/s.

Differential effect of ischemia on spontaneous and sinusoidal-evoked activity in semicircular canal afferents in the bullfrog

Spontaneous and sinusoidal-evoked nerve activity in semicircular canal afferent fibers of the bullfrog was evaluated prior to and following the production of ischemia of the labyrinthine arterial supply by mechanical occlusion of the vestibular artery. Neuronal spontaneous firing rates were observed to diminish by up to 100% within 10 min following the onset of ischemia. In most neurons there was a substantial increase in firing rate during the first few minutes. The sensitivity of the fibers to natural stimulation as determined by the gain in their responses to sinusoidal motion also diminished by as much as 75% over the same period. No detectable changes in the membrane potentials of the neurons were observed. The changes in excitability were closely correlated with the changes in spontaneous firing rate, but not all the neurons whose responses changed showed changes in spontaneous activity. Likewise, the relative magnitude of change varied from neuron to neuron.

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.

Spontaneous activity of first-order horizontal canal neurons in the guinea pig

The spontaneous activity of the primary horizontal canal neurons was investigated in 12 alert albino guinea pigs. Seventy-one neurons were recorded for analyzing spontaneous activity. These neurons had an average resting rate of 46.6 +/- 20.9 spikes/sec and a range of 2.2 to 113.6 spikes/sec. This was significantly higher than previously reported data recorded from anesthetized guinea pigs. The neurons were classified into three groups from values of coefficient of variation; regular, intermediate, and irregular firing units. Of the 71 units measured, 42.3% were regular, 42.3% were intermediate, and 15.4% were irregular discharge units. The incidence of the irregular units was slightly lower than that in the anesthetized animals. The difference in resting activity of the primary neurons with or without general anesthesia is discussed.

Anatomical characteristics of the anterior vestibular nerve of the bullfrog

Study was made of the dimensions of the nerves to the receptor organs in the anterior branch of the bullfrog vestibular nerve. The number of fibers and their diameters and trajectories in the nerve and Scarpa's ganglion were studied. A correlation was made between the anatomical and physiological properties of neurons identified with intracellular injections of horseradish peroxidase. The data suggest a specific pattern of innervation of the organs, organization of fibers and cells in the vestibular nerve, and significant correlation between the anatomical and physiological properties of individual neurons. A comparison was made between the information obtained from the bullfrog and that from other animals, which suggests a similarity between species and supports the hypothesis of a differential physiological role of the neurons in each vestibular organ according to anatomical characteristics.

Design for a slender shaft glass micropipette

A method for the construction of long, slender shaft glass micropipettes is described. The diameter of the pipettes is comparable to metal microelectrodes that are used when multiple penetrations are required to record from neurons in relatively small targets located deep within brain tissue. The advantages of the slender micropipettes over metal microelectrodes are that: (1) the pipettes can be rapidly and reliably produced for consistent shape, size and impedance characteristics, (2) dyes can be used to define recording sites, (3) substances that carry a stable ionic charge such as soluble dyes or drugs can be ionophoreticaly applied through the pipette and, (4) both extra- and intracellular recordings are feasible.

Micromechanical properties of sensory hairs on receptor cells of the inner ear

The aim of the investigation was to obtain quantitative measures of mechanical filter properties of hair cell sensory hairs which were not restricted to move by auxiliary structures. The specimen used was the crista ampullaris of the frog, dissected free and positioned in a fluid-filled chamber where it could be viewed with differential interference contrast optics. The sensory hairs were displaced by a brief jet of frog Ringer's solution from a specially constructed microsyringe apparatus. The velocity of the jet could be stepwise controlled and was determined by measuring the speed of motion of 3 micron plastic beads propelled with the jet across the microscopic field. The stereocilia displacements were recorded on 16 min film, and both the angle of deflection and the time for the return to the resting position were measured on the film. It was found that after displacement the sensory hairs returned to the upright position by elastic properties in the hinge region at their insertion point in the cuticular plate. Sensory hairs differed in their speed of return so that some had fast time constants, others quite slow ones. This was correlated to a difference in structural development of the stereocilia, fast sensory hairs having thick and tall stereocilia, slow ones having thin and short stereocilia. The various bundle types were identified in the scanning electron microscope and their distribution on the crista was mapped. This was found to match the distribution of nerve fibres with different functional properties. It is concluded that sensory hair cells can differ in their mechanical filter properties as a result of the structural arrangement of their stereocilia and in accordance with functional demands.