Reciprocal synapses between hair cells and first order afferent dendrites in the crista ampullaris of the bullfrog

Reciprocal synapses were found between afferent dendrites and hair cells of the crista ampullaris in the vestibular system of the bullfrog. The reciprocal synapse consisted of two components forming a dual synaptic complex between a single hair cell and a single dendritic process such that the receptor cell was both presynaptic and postsynaptic to the afferent dendrite. The first component, a typical afferent synaptic complex, consisted of a synaptic sphere surrounded by a single layer of clear vesicles, and was located within the hair cell. In addition, 3-4 arciform structures were located between the sphere and the plasma membrane of the hair cell. The second component of the reciprocal synapse, located approximately 1.5 micron from the first, was recognized by a localized accumulation of clear membrane vesicles within the neuronal process and a short membrane profile within the cytoplasm of the adjacent hair cell. Additional characteristic features included continuity of the arciform structures with the plasma membrane of the hair cell, and within the neuronal process, a greatly reduced number of vesicles when compared with the efferent axonal terminal. This latter was important in distinguishing a first-order afferent dendritic process from an efferent neuronal process. The possible functional significance of these reciprocal synapses has been discussed in terms of the two components having an opposite effect on the membrane polarization of the hair cell.

New dimensions of benign paroxysmal positional vertigo

Twenty-nine patients who demonstrated the classic nystagmus of benign paroxysmal positional vertigo in the provocative, ear-down position had a high incidence of concurrent symptoms. These included vertigo provoked by arising, bending over, head rotation, linear acceleration, and vertical oscillation. Some have not been reported previously in relation to this syndrome. Elimination of both concurrent and classic symptoms via singular neurectomy in nine patients indicates a common pathophysiologic mechanism, probably involving cuplolithiasis in the posterior semicircular canal. These concurrent symptoms should be considered part of the syndrome.

The cochlear nuclei of snakes

The cochlear nuclei of three burrowing snakes (Xenopeltis unicolor, Cylindrophis rufus, and Eryx johni) and three non-burrowing snakes (Epicrates cenchris, Natrix sipedon, and Pituophis catenifer) were studied. The posterior branch of the statoacoustic nerve and its posterior ganglion were destroyed and the degenerated nerve fibers and terminals traced to primary cochlear nuclei in 13 specimens of Pituophis catenifer. All these snake species possess three primary and one secondary cochlear nuclei. The primary cochlear nuclei consist of a small nucleus angularis located at the cerebello-medullary junction and a fairly large nucleus magnocellularis forming a dorsal cap over the cephalic end of the alar eminence. Nucleus magnocellularis may be subdivided into a medially placed group of rounder cells, nucleus magnocellularis medialis, and a laterally placed group of more ovate and paler-staining cells, nucleus magnocellularis lateralis. A small but well-defined secondary nucleus which showed no degenerated nerve terminals after nerve root section, nucleus laminaris, underlies the cephalic part of both nucleus magnocellularis medialis and nucleus magnocellularis lateralis. Larger and better-developed cochlear nuclei were found in burrowing species than in non-burrowing species of snakes. Of the three burrowing species studied, Xenopeltis showed the greatest development of cochlear nuclei; Eryx cochlear nuclei were not quite as large but were better differentiated than in Xenopeltis; and Cylindrophis cochlear nuclei were fairly large but not as well developed nor as well differentiated as in either Xenopeltis or Eryx. The cochlear nuclei of the three non-burrowing snakes, Epicrates, Natrix, and Pituophis, were not as large nor as well developed as those of the burrowing snakes. There is some, but not complete, correlation between cochlear development and papilla basilaris length and number of hair cells. Thus, Xenopeltis and Eryx, with well-developed cochlear nuclei, have relatively long papillae basilares; but the boid, Epicrates, with less well-developed cochlear nuclei, has a fairly well-developed papilla basilaris. Cylindrophis, a burrowing species, shows only a moderate degree of cochear nuclei and papilla basilaris development. The non-burrowers, Natrix and Pituophis, have both small cochlear nuclei and relatively short papillae basilares.

Neuromorphometric features and dimensional analysis of the vestibular end organ in the little brown bat (Myotis lucifugus)

Neuromorphometric parameters of the vestibular system were determined from serial sections of temporal bones from four little brown bats. Well-developed eminentiae cruciatae project from the cristae ampullares of the anterior and posterior membranous ampullae. A total of 4,500 bipolar ganglion cells were enumerated within the vestibular ganglion. The widths of the cell somas varied from 2.5 to 20 micrometers, with 70% of them having widths between 5.0 and 12.5 micrometers. Two maxima were observed in a curve of ganglion cell density as a function of the length of the ganglion. The first maximum indicated a density of 4,800 cells per mm2 at a length 0.20 from the apex of the ganglion (in the pars dorsalis); the second, a density of 4,750 cells per mm2 at 0.38 mm (in the pars ventralis). The morphometric parameters studied were the radii of curvature of the semicircular ducts, the cross-sectional diameters of the semicircular canals and ducts, the dimensions of the cristae ampullares and their membranous ampullae, and dimensions pertaining to the statoconial organs. Surface areas (measured from graphic projections) were determined as 0.098 mm2 and 0.016 mm2 and hair cell count 500 and 1,300 cells for the saccular and utricular maculae, respectively. The radii of curvature of the three semicircular ducts, R, were dissimilar, with the anterior duct having the largest radius (R = 0.91 mm) and the posterior duct the smallest one (R = 0.69 mm). The average cross-sectional diameters of the anterior, lateral, and posterior ducts were measured as 0.11 mm, 0.14 mm, and 0.13 mm, respectively. Some of the morphological parameters were used to ascertain information regarding the dynamics of semicircular--canal function. In particular, the coefficients theta and II in the torsion pendulum model (Steinhausen, '31; Egmond et al., '49), and the time constants xi L congruent to II/delta and xi S congruent to theta/II of the torsion pendulum model were estimated for the little brown bat from these parameters. Where appropriate, comparisons were made to time constants obtained for other species.

Axonal branching in the trochlear and oculomotor nuclei of single vestibular neurons activated from the posterior semicircular canal nerve in the cat

Axonal branches of single vestibular neurons activated by stimulation of the ampullary nerve of the posterior semicircular canal in the cat were studied by means of local antidromic stimulation in the trochlear and the oculomotor nucleus. These vestibulo-ocular neurons were located in the rostral half of the descending vestibular nucleus and the lateral part of the medial vestibular nucleus. The majority of vestibulo-ocular neurons projecting to the inferior rectus motoneuron pool in the contralateral oculomotor nucleus was activated antidromically from the contralateral trochlear nucleus as well. This suggests that axonal branches of a single vestibular neuron project to both nuclei.

Neurodynamic response analysis of anterior semicircular canal afferents in the pigeon

1. The neurodynamic responses to sinusoidal and pulse angular accelerations were studied in anterior semicircular canal afferents in the barbiturate-anesthetized pigeon. 2. The resting discharge frequency, aS, varied from 7.4 to 149.0 impulses/s. For most units, aS remained fairly constant for long periods of time during the experiment. 3. The neural-response harmonic distortion, resulting from stimulation by sinusoidal angular accelerations, varied in different units. Percent distortions from as low as 3% to those as high as 57% were determined. 4. Intensity-function plots of peak first harmonic neural response as a function of the peak sinusoidal angular acceleration (with frequency, f, as a parameter) are of two types: one has a linear relationship between the variables; the other demonstrates pronounced nonlinearities ("saturation," particularly for low values of f). In saturation-type units, the data of which fit a power law function, the exponent of the function is frequency dependent, becoming closer and closer to unity with increasing f. 5. Data for all units fit the transfer function, G'(s) = Csk/(tauLS + 1), where G'(s) relates the unit response to angular acceleration, C is a gain constant, 0 < k < 1, and tauL is the so-called long time constant of the classical torsion pendulum model. tauL varied from 4.45 to 22.17 S (mean +/- SE = 10.24 +/- 1.20 S). This may be interpreted as an indication of a regional distribution of tauL'S within the neuroepithelium. Arguments are advanced to show that this is consistent with our present understanding of the ampullary end organ. 6. The degree of regularity of the spontaneous discharge (as determined by the coefficient of variation, CV) was significantly correlated with the parameter k in G'(S). The larger the CV, the larger is the corresponding k. Further work indicated that the larger the value of k, the more adaptation a unit exhibited (k varied from 0.017 to 0.66). 7. The time-domain response of G(S) = G'(S)/(tauSS + 1) to different durations of pulse angular acceleration stimuli agreed well with the neural response to these stimuli (tauS = 2.27 ms is the short time constant of the torsion pendulum model). 8. The term Sk was decomposed into an expression containing a series of polynomials in S in the numerator and denominator. The first term in this expansion K0tau1S/(tau1S + 1), has previously been shown to describe so-called adaptation properties in the dynamics of the semicircular canals. A mean (+/-SE) tau1 = 71.56 (+/-10.01) S was determined. Evidence is presented that Sk probably represents a relaxation phenomenon comprised of a time-varying intracellular Na+/K+-transport process, components of which are summed with the generator potential in the afferent terminal(S) of the receptor hair cell.

Singular neurectomy: hypotympanotomy approach

A modified surgical technique is described for singular neurectomy. The hypotympanotomy approach provides greater ease in exposing the singular canal. Delayed round window overhang removal provides decreased risk of round window trauma. Eight cases done by this technique indicated a lower complication rate than any reported transmeatal series. Selection criteria, results, and complications are discussed. The risk of cochlear damage limits singular neurectomy to patients who are severely incapacitated by benign paroxysmal positional vertigo. Compared with vestibular neurectomy, singular neurectomy avoid risks inherent in intracranial procedures, and the vertigo caused by surgical intervention is less pronounced.

Vestibular inputs to the fastigial nucleus; evidence of convergence of macular and ampullar inputs

1. Experiments have been undertaken on 11 decerebrate cats to investigate the effects of natural vestibular stimulation on the activity of cerebellar fastigial neurons. 2. From recordings in the rostral portion of the nucleus during sinusoidal lateral (roll) and horizontal (yaw) rotation, distinctive patterns of response were observed. 3. The majority of neurons sensitive to vestibular stimulation showed responses to a single modality of vestibular activation. During lateral tilt some neurones showed positional sensititivy, others gave responses related tothe velocity of movement. Other neurones responded in phase with the velocity of movement in the horizontal plane. 4. Aside from these neuronal responses, others provided indications of a convergence of inputs from different sets of vestibular receptors. In particular, several neurons showed a pattern of response that indicated tht they received inputs from otolith receptors and ampullar receptors of the vertical canal. At low velocities of movement their response was positional but with inreasing velocity the magnitude of the response increased and there was a marked phase shift of the discharge towards head velocity. 5. Neurons responding to horizontal rotation often showed positional responses during lateral tilt. There were also indications of a convergence of ampullar inputs from both vertical and horizontal canals. 6. The neural pathways mediating these resonses are discussed in consideration of previous neuroanatomical and neurophysiological data. We consider it likely that several pathways may act to evoke the patterns of response observed, and a role of the cerebellar cortex is indicated.

Pathways mediating optokinetic responses of vestibular nucleus neurons in the rat

1. The effects of various brain lesions on the responses of vestibular nuclear neurons (Vn) of the horizontal semicircular canal system to optokinetic stimulations were studied to elucidate the optokinetic path from the retina to the vestibular nuclei. A previous study performed in intact rats served as a control [2]. 2. It was shown that the pretectal region including the n. of the optic tract is the first central relay in the optokinetic path; it receives its functionally effective input from the contralateral eye. Unilateral lesions of this area rendered all Vn responses unidirectional when tested with binocular stimulation. Lesions of other visual centers such as the superior colliculi or visual cortices had no influence on the optokinetic response properties of Vn. 3. The area of the n. reticularis tegmenti pontis (NRTP) proved to be an important link between pretectum and vestibular nuclei: Unilateral lesions produced effects similar to those described for pretectal lesions. Pretectal axons to NRTP descend lateral to the MLF and tectospinal tract. 4. It was demonstrated that the vestibular commissure plays the crucial role in mediating the mirror image optokinetic effects to Vn on the opposite side and assures the bidirectionality of the responses to binocular stimulation. 5. Cerebellectomy did not significantly affect the Vn responses to the optokinetic stimuli presented in this study. 6. Electrical stimulation of the pretectum excited type II and inhibited type I Vn ipsilaterally and had the opposite effect on Vn located on the opposite side. NRTP stimulation excited type II and inhibited type I ipsilaterally; latency analysis of these effects suggested that the pretectal stimuli excited opsilateral NRTP neurons which, in turn, excited ipsilateral type II Vn. Ipsilateral type I inhibition as well as the concurrent contralateral type II inhibition and type I excitation are produced by the inhibitory action of type II on type I and the commisural system. 7. Systemic application of picrotoxin abolished all optokinetic responses of Vn except the type II activation. This finding further supports the hypothesis described above. 8. Unilateral pretectal or NRTP lesions abolished OKN to surround motion in the direction of the lesion.

Clinical inferences from recent observations on vestibular neuro-anatomy

Recent knowledge of the neuro-anatomy of the vestibular system gained as the result of retrograde axonal tracer methods is reviewed. The new findings help explain some clinical features. The findings include separate termination in the vestibular nuclei of the first order neurons from the semicircular canals of the macular; the detailed connections between the eye cerebellar and spinal paths are commented upon.

Axonal branches and terminations in the cat abducens nucleus of secondary vestibular neurons in the horizontal canal system

Single vestibular neurons functionally identified as secondary neurons receiving primary afferents from the horizontal canal in the cat were intracellularly stained with horseradish peroxidase (HRP). The vestibular neurons projecting to the ipsilateral abducens nucleus distributed terminal branches in a relatively narrow band in the nucleus. The stem axon of contralaterally projecting vestibular neurons bifurcated into ascending and descending branches in the contralateral medial longitudinal fasciculus. The collaterals emerging from these branches distributed terminals in a relatively wide area in the abducens nucleus. Collateral branches extended into the medial vestibular nucleus, prepositus hypoglossi nucleus and reticular formation.

[Responses of frog primary vestibular afferents to direct vibration of a semicircular canal]

Responses of primary afferents (PA) of lateral semicircular canal to sinusoidal vibration of the canal wall were studied within the range 0.05--200 Hz (mean amplitudes 5--15 microns) in immobilized frogs. Dynamic characteristics (gain, phase) relative linear velocity of the vibrator (micron.s-1-1) were examined. At 0.2 Hz, the gain was 5.35 +/- 3.19 imp-s-1/micro.s-1 (mean; S. D.; n=14) and linearly decreased if the frequency rose. Phase lag of responses relative velocity at 0.05 Hz was 49.8 degrees +/- 16.5 degrees (n=13) and at 1 Hz 97 degrees +/- 9.4 degrees (n=22). At 100 Hz phase lag was about 240 degrees. Three groups of PA were described: wide range PA reacting in the range from 0.05 up to 60--180 Hz; high frequency PA responding in the range from 20--40 up to 100--150 Hz; low frequency PA responding in the range from 0.05 up to 2--20 Hz.

Branch of the singular nerve (posterior ampullary nerve) in the otic capsule

The frequency of occurrence, location and course of the branch of the singular nerve in the otic capsule in man was investigated by examining the histological sections of 223 human temporal bones. In the proximal portion near the internal auditory meatus the branch of the singular nerve was observed in 194 bones (87%), and was seen to join the main trunk of the singular nerve in 181 of those bones. In the distal portion near the posterior semicircular canal ampulla in 58 bones (26%) and in 45 of those bones it was seen to arise directly from the main trunk of the singular nerve. In 13 bones the branch was seen taking its course in the otic capsule independently from the main trunk all the way to the posterior canal crista. The accessory portion of the branch of the singular nerve reached the dura mater of the posterior cranial fossa near the internal auditory meatus in 11 of 223 bones.

Horizontal canal input to cat extraocular motoneurons

Synaptic potentials were recorded in identified extraocular motoneurons in the anesthetized cat, following stimulation of the horizontal canal nerve (HCN). Weak stimulation of the HCN evoked disynaptic EPSPs in ipsilateral medial rectus (i-MR), contralateral lateral rectus (c-LR) and disynaptic IPSPs in i-LR motoneurons. Weak stimulation of the HCN produced longer latency IPSPs (probably trisynaptic) in c-MR motoneurons. It is suggested that the HCN projects to an excitatory interneuron in the vestibular nucleus whose axon in turn projects to a third order inhibitory interneuron in the IIIrd nucleus which finally projects to c-MR motoneurons. Essentially there is no influence of the HCN stimulation on bilateral superior rectus (SR), inferior rectus (IR), superior oblique (SO) and inferior oblique (IO) motoneurons.

Vestibular nuclei activity in the alert monkey during suppression of vestibular and optokinetic nystagmus

Single neurons were recorded in the vestibular nuclei of monkeys trained to suppress nystagmus by visual fixation during vestibular or optokinetic stimulation. During optokinetic nystagmus vestibular nuclei neurons exhibit frequency changes. With the suppression of optokinetic nystagmus this neuronal activity on average is attenuated by 40% at stimulus velocities of 40 degrees/s. At a stimulus velocity of 5 degrees/s responses are, under both conditions, close to threshold. For steps in velocity, suppression of vestibular nystagmus shortens the time constants of the decay of neuronal activity from 15--35 s to 5--9 s, while the amplitude of the response remains unchanged. The results are discussed in relation to current models of visual-vestibular interaction. These models use a feedback mechanism which normally operates during vestibular and optokinetic nystagmus. Nystagmus suppression interrupts this feedback loop.

The velocity response of vestibular nucleus neurons during vestibular, visual, and combined angular acceleration

In alert Rhesus monkeys neuronal activity in the vestibular nuclei was measured during horizontal angular acceleration in darkness, acceleration of an optokinetic stimulus, and combined visual-vestibular stimulation. The working ranges for visual input velocity and acceleration extend up to 60 degrees/s and 5 degrees/s2. The corresponding working range for vestibular input acceleration is wider and time-dependent. During combined stimulation, that is acceleration of the monkey in the light, a linear relation between neuronal activity and velocity could be established for all neurons. Type I vestibular plus eye movement neurons displayed the greatest sensitivity and had a small linear range of operation. Other vestibular neurons were less sensitive but had a larger range of linear response to different values of acceleration. Accelerating the animal and visual surround, simultaneously but in opposite directions, results in neuronal activity proportional to relative velocity over a limited range.

Comparison of dynamic properties of canal-evoked vestibulospinal reflexes of the neck and forelimb in the decerebrate cat

The responses of neck and forelimb muscles to sinusoidal polarization of the horizontal canal nerve were compared by recording from these muscles simultaneously. Contrary to results on the vestibulocollic reflex, the central phase lag in the vestibulo-forelimb reflex increases with increasing frequencies up to 3 Hz. This demonstrates a difference in the organization of vestibular-driven pathways to neck and forelimb muscles.

The development of function of horizontal semicircular canal primary neurons in the rat

The activity of primary horizontal semicircular canal neurons in newborn rats was compared to similar cells in adults. All animals were anesthetized with ether. Neurons were categorized as regular or irregular on the basis of their spontaneous activity. Horizontal semicircular canal neurons can respond to angular acceleration stimulation at birth. In newborn rats no regularly firing cells could be found, but the percentage of these cells and their average resting rate increased during growth. Neurons in newborn rats differ from those in the adult by having a lower average resting rate, a lower sensitivity to long-duration angular acceleration and taking longer to reach peak increase in firing during the stimulus. Sensitivity reaches adult values by about 4 days although the canal dimensions continue to increase until about 20 days.

Vestibular mechanisms

It is apparent from this and other reviews of the subject that our knowledge of vestibular function is most complete for the primary canal and otolithic afferents. Relatively little progress has been made in the understanding of receptor mechanisms and the functional importance of the efferent vestibular system. Since most of it has been summarized previously the latter were not considered here. Considerably more knowledge has accumulated in the field of central vestibular mechanisms, particularly those related to eye movements. Recent advances in functional synaptology of direct and indirect vestibuloocular pathways are described. It appears that the indirect pathways are essential for the central integration of the peripheral head velocity into a central eye position signal. Candidates for the neural integrator are presented and discussed and their connectivity described both for the horizontal and the relatively poorly studied vertical eye movement system. This field will certainly be studied extensively during the next years. Another interesting field is the role of the cerebellum in the control the vestibuloocular reflex. Recent data and hypotheses, including the problem of cerebellar plasticity, are summarized and evaluated. That the vestibular nuclei are by no means a simple relay system for specific vestibular signals destined for other sensory or motor centers is evidenced in this review by the description of multiple canal-canal, canalotolith, and visual-vestibular convergence at the nuclear level. Canal-otolith and polysensory convergence in vestibular neurons enables them to correct for the inherent inadequacies of the peripheral canal system in the low frequency range. The mechanisms of polysensory interaction in the central vestibular system will undoubtedly be an important and interesting field for future research.

Horizontal ampullary nerve fiber projections to the crista angustarum in the crista ampullaris

Individual nerve fibers were traced on consecutive serial 1-micron-thick cross-sections to determine their position of origin within the horizontal ampullary nerve trunk (HANT) and the site at which they penetrated the basilar lamina of the horizontal crista in the guitarfish, Rhinobatos productus. Of the 132 fibers successfully traced, 87.1% followed an anatomical projection pattern consistent with the dynamic response characteristics of the nerve fibers. That projection pattern was: from the peripheral HANT region to the ipsilateral slope; from the central HANT region to the crest; and from the intermediate HANT region both to the ipsilateral slope and the crest of the horizontal crista. The 12.9% of "error" fibers not following this projection pattern were also consistent with the functional neurophysiological data. The terms crista bulbus and crista angustarum were introduced to designate specific anatomical regions of the dumb-bell shaped crista ampullaris.

Cupular movement and nerve impulse response in the isolated semicircular canal

The aim of the study was to record nerve impulse responses to controlled displacements of the cupula and relate these to observations of cupular motion. The preparation was the skate semicircular canal, isolated and maintained in a chamber. The canal was cut and tied tightly to an oil-filled tube driven by a 1 microliter syringe. In some experiments dye was used to make the face of the cupula visible. Small volume displacements (0.02 microliter) caused a billowing of the central portion of the cupula, whereas for repeated large displacements (0.1 microliter) the top of the cupula moved along the roof of the ampulla. Displacements of the latter amount were close to saturation in the characteristic functions (spike rate versus volume displacement) of nerve firing. It seems that the discharge rate of tonically responding units in the skate crista ampullaris rather faithfully record the force acting on the cupula. The motion of the base of the cupula rather than its tip appears to be the relevant parameter in setting the level of excitation in the sensory cells within the physiological range of stimulation. Such a mode of operation would ensure the highest sensitivity of the sense organ.