Atypical "reversed" paroxysmal positioning nystagmus in benign paroxysmal positional vertigo

An atypical paroxysmal positioning nystagmus (PPNy), "reversed" in its directional components, is a rare finding in patients suffering from benign paroxysmal positioning vertigo (BPPV). It poses problems of pathophysiogenetic interpretation, differential diagnosis with a CNS lesion and therapeutic strategy. Such a finding was observed in 7 patients (out of a total of 450 BPPV) and took on the typical PPNy pattern after repeated diagnostic manoeuvres. These aspects are incompatible with the "heavy cupula" mechanism (cupulolithiasis) and are better explained by the presence of "free endolymph aggregates" (canalolithiasis) within the posterior semicircular canal (p.s.c). Moreover, canalolithiasis would also permit a more convincing interpretation of all the clinical findings observed in typical p.s.c.-BPPV.

Input patterns and pathways from the six semicircular canals to motoneurons of neck muscles. I. The multifidus muscle group

1. The pattern of connections between the six semicircular canals and neck motoneurons of the multifidus muscle group was investigated by recording intracellular potentials from motoneurons in the upper cervical cord of anesthetized cats. 2. Synaptic potentials were recorded in motoneurons of the rectus capitis posterior (RCP) muscle at C1, the obliquus capitis inferior (OCI) muscle at C1 and C2, and the cervical multifidus muscle (Multi) at C4 in response to electrical stimulation of individual ampullary nerves of the six semicircular canals. Excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs, respectively) were evoked by separate stimulation of individual ampullary nerves in all of the neck motoneurons. Virtually all of the neck motoneurons received convergent inputs from the six ampullary nerves. 3. Motoneurons that supplied a single muscle had a homogeneous pattern of input from the six semicircular canals. There were two patterns of input from the six semicircular canals to motoneurons of the multifidus muscle group. RCP and Multi motoneurons were excited by stimulation of the bilateral anterior canal nerves (ACNs) and the contralateral lateral canal nerve (LCN) and inhibited by stimulation of the bilateral posterior canal nerves (PCNs) and the ipsilateral LCN. This input pattern is similar to that previously observed in other dorsal extensor muscles, whereas the other input pattern observed in OCI motoneurons is entirely new. OCI motoneurons at C1 and C2 were excited by stimulation of the ipsilateral ACN, PCN, and the contralateral LCN and inhibited by stimulation of the contralateral ACN, PCN, and the ipsilateral LCN. 4. Most postsynaptic potentials (PSPs) were disynaptic, but there were trisynaptic inhibitory connections between the contralateral ACN and PCN and OCI motoneurons, and between the contralateral PCN and RCP motoneurons. 5. The pathways for mediating these inputs from different semicircular canals to neck motoneurons were determined by making lesions in the lower medulla. Transection of the ipsilateral medial longitudinal fascicle (MLF) abolished the following potentials: all disynaptic PSPs in RCP motoneurons except the disynaptic EPSPs from the ipsilateral ACN, and in OCI motoneurons, disynaptic PSPs from the bilateral LCNs, and disynaptic IPSPs from the contralateral PCN. Complete bilateral section of the MLF did not affect the disynaptic EPSPs from the ipsilateral ACN in RCP motoneurons, the disynaptic EPSPs from the ipsilateral ACN and PCN in OCI motoneurons, nor the trisynaptic IPSPs from the contralateral ACN and PCN in COI motoneurons and from the contralateral PCN in RCP motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of hyaluronan in the middle and inner ear. A light microscopical study in the rat using a hyaluronan-binding protein as a specific probe

The histochemical distribution of hyaluronan (hyaluronic acid, HYA) was analyzed in middle and inner ear tissues from rats by use of microwave-aided fixation, a hyaluronan-binding protein and the avidin-biotin/peroxidase staining procedure. In the middle ear HYA was mainly localized in the pars flaccida, the ossicles, the round window membrane and in the endomysium of the middle ear muscles. The subepithelial stroma of the Eustachian tube was strongly HYA-positive. In the inner ear, the spiral ligament of the cochlea and the connective tissue surrounding the nerve fibers emerging from the crista ampullaris stained intensely for HYA. Except for a weak staining of the basilar membrane, the Corti organ was devoid of HYA. The heterogenous distribution of HYA may indicate its specific involvement in middle and inner ear physiology and pathology.

Sensory transduction of head velocity and acceleration in the toadfish horizontal semicircular canal

1. Sinusoidal mechanical indentation of the long-and-slender limb of the horizontal semicircular canal and/or utricle was used to produce adequate stimulation of the labyrinth. Indentation of the canal increased, while indentation of the utricle decreased the afferent discharge rate. This follows because indentation of the canal and utricle produce oppositely directed mechanical stimuli as defined by endolymph flow, transcupular pressure, and cupular deflection. Simultaneous in-phase indentations of both the canal and utricle, with amplitudes adjusted to produce equal (but opposite) magnitudes of afferent response modulation, generate destructive interaction that minimizes the afferent modulation, whereas sinusoidal indentation 180 degrees out-of-phase generates constructive interaction that maximizes the afferent modulation. This observation correlates directly with analysis of the labyrinthine elasto-hydrodynamics which predicts that balanced in-phase indentations minimize macromechanical endolymph flow through the ampullary cross section and maximize the dilatational pressure within the ampulla acting equally on both sides of the cupula and across the labyrinthine wall. 2. Two groups of afferents are identified according to their response to balanced sinusoidal indentation of the canal limb and the utricle. In one group there is complete destructive interaction and the afferent response can be effectively nulled by adjusting the relative amplitude and phase of the two stimuli. In the second group a residual afferent response remains that cannot be nulled. The residual is described in the model as unit-specific sensitivity to dilatational pressure acting equally on both sides of the cupula.

Vestibular morphology in relation to age and circling behavior

The DBA/2 strain of mice have genetically induced vestibular dysfunction that presents grossly at an early age as circling behavior and abnormal righting reflexes. The vestibular morphology of this strain has not previously been examined. DBA/2 mice of different ages that showed circling behavior were sacrificed and then had their inner ears immediately removed and fixed in glutaraldehyde. The specimens were prepared for light, transmission and scanning electron microscopy. Additional specimens at 10 months of age were fixed with paraformaldehyde for immunohistochemical investigation and labelling of alpha-actinin. Non-circling litter mates served as controls. The morphology and immunohistochemistry of the vestibular end-organs were evaluated as a function of age and circling behavior. The sensory epithelium of the ampulla and utricle in the circling mice showed softening of the cuticle, hair cell cytoplasmic herniation, expelled cellular debris, fused stereocilia and giant hair cells that progressively increased in severity with age. The non-circling litter mates showed similar but less severe pathology of the vestibular sensory epithelium. The immunohistochemical analysis showed no differences at a magnification of 400 x.

Ascending projections of posterior canal-activated excitatory and inhibitory secondary vestibular neurons to the mesodiencephalon in cats

The axonal projections of 62 posterior canal (PC)-activated excitatory and inhibitory secondary vestibular neurons were studied electrophysiologically in cats. PC-related neurons were identified by monosynaptic activation elicited by electrical stimulation of the vestibular nerve and activation following nose-up rotation of the animal's head. Single excitatory and inhibitory neurons were identified by antidromic activation following electrical stimulation of the contralateral and ipsilateral medial longitudinal fasciculus, respectively. The oculomotor projections of identified neurons were confirmed with a spike-triggered averaging technique. The axonal projections of the identified neurons were then studied by systematic, antidromic stimulation of the mesodiencephalon. Excitatory neurons showed two main types of axonal projections. In one type, axonal branches were issued to the interstitial nucleus of Cajal, central gray, and thalamus including the ventral posterolateral, ventral posteromedial, ventral lateral, ventral medial, centromedian, central lateral, lateral posterior, and ventral lateral geniculate nuclei. The other type was more frequently observed, giving off axon collaterals to the above-mentioned regions and to Forel's field H as well. Inhibitory neurons issued axonal branches to limited areas which included the central gray, interstitial nucleus of Cajal, its adjacent reticular formation and caudalmost part of Forel's field H, but not the rostral part of the Forel's field H and the thalamus. These results suggest that PC-related excitatory neurons participate in the genesis of vertical eye movements and in the perception of the vestibular sensation, and that PC-related inhibitory neurons seem to take part only in the genesis of vertical eye movements.

Functional architecture of vestibular primary afferents from the posterior semicircular canal of a turtle, Pseudemys (Trachemys) scripta elegans

Physiological studies in many vertebrates indicate that vestibular primary afferents are not a homogeneous population. Such data raise the question of what structural mechanisms underlie these physiological differences and what functional role is played by afferents of each type. We have begun to answer these questions by characterizing the architecture of 110 afferents innervating the posterior canal of Pseudemys scripta. We emphasize their spatial organization because experimental evidence suggests that afferent physiological properties exhibit significant spatial heterogeneity. The sensory surface of the posterior canal comprises paired, triangular hemicristae, which are innervated by two afferent types. Bouton afferents (66% of total afferents) are found over the entire sensory surface. They differ significantly in the shape and size of their collecting areas, number of boutons, soma size, and axon diameter; this morphological variation is systematically related to the afferent's spatial position. In addition, multivariate analyses suggest that bouton afferents may comprise two subtypes: alpha afferents have delicate processes and are found throughout the crista; beta afferents are more robust and are concentrated preferentially toward the canal center. Calyx-bearing afferents comprise two morphological subtypes: dimorphs (13% of total afferents) bear calyceal and bouton endings; calyceal afferents (21%) bear calyceal endings only. Both types occur exclusively in an elliptical region near the center of each hemicrista; their morphology varies with radial distance from the center of this elliptical region. Our data provide evidence that in Pseudemys: (1) the classical vestibular afferent types (bouton, calyx, dimorph) are structurally heterogeneous, and (2) their spatial sampling characteristics are highly structured and distinctive for each type. These spatial patterns may shed light on regional differences in physiological profiles of vestibular afferents, and they raise questions about the role of this spatial heterogeneity in signaling head movement.

Organization of inner ear endorgan projections in the goldfish, Carassius auratus

Cytoarchitectural analysis of the octavolateralis area of the goldfish, Carassius auratus, reveals that as in other teleosts, five first-order octaval nuclei are present: the anterior octaval, magnocellular, descending, tangential, and posterior octaval nuclei. The descending nucleus appears to be anatomically specialized relative to that of the halecomorph Amia calva and many teleosts in that a large dorsomedial subpopulation of the nucleus lies medial to nucleus medialis, a first-order lateral line nucleus. In addition to this dorsomedial zone, the descending nucleus is made up of an intermediate and a ventral zone. Application of horseradish peroxidase (HRP) to individual inner ear endorgans reveals that the distribution of these afferents to the octaval nuclei is generally similar to that in another otophysan, Ictalurus punctatus [McCormick and Braford, 1993]. Nucleus magnocellularis receives a diffuse projection from all of the endorgans. The semicircular canals project heavily to the nucleus tangentialis, the entire ventral zone and portions of the intermediate zone of the descending nucleus, the ventral portion of the caudal anterior nucleus, and the bulk of the rostral anterior nucleus. The macula neglecta projects to the intermediate zone of the descending nucleus and to ventral locations within the dorsal half of the caudal anterior nucleus. The otolithic endorgans--the saccule, lagena, and utricle--project, in an overlapping manner, to the dorsal half of the caudal anterior nucleus and minimally to the rostral anterior nucleus. The inputs of the otolithic endorgans to the intermediate zone of the descending nucleus are more segregated, though a given region is sometimes supplied by more than one endorgan. The projections of the saccule tend to be concentrated more medially than those of the other two endorgans. The dorsomedial zone of the descending nucleus receives the majority of its primary input from the saccule, and a much smaller input from the lagena, over most of its rostrocaudal extent. At caudal-most levels of the dorsomedial zone, afferents from the three otolithic endorgans overlap.

Second-order vestibular neuron morphology of the extra-MLF anterior canal pathway in the cat

Second-order vestibular neurons form the central links of the vestibulo-oculomotor three-neuron arcs that mediate compensatory eye movements. Most of the axons that provide for vertical vestibulo-ocular reflexes ascend in the medial longitudinal fasciculus (MLF) toward target neurons in the oculomotor and trochlear nuclei. We have now determined the morphology of individual excitatory second-order neurons of the anterior semicircular canal system that course outside the MLF to the oculomotor nucleus. The data were obtained by the intracellular horseradish peroxidase method. Cell somata of the extra-MLF anterior canal neurons were located in the superior vestibular nucleus. The main axon ascended through the deep reticular formation beneath the brachium conjunctivum to the rostral extent of the nucleus reticularis tegmenti pontis, where it crossed the midline. The main axon continued its trajectory to the caudal edge of the red nucleus from where it coursed back toward the oculomotor nucleus. Within the oculomotor nucleus, collaterals reached superior rectus and inferior oblique motoneurons. Some axon branches recrossed the midline within the oculomotor nucleus and reached the superior rectus motoneuron subdivision on that side. Since these neurons did not give off a collateral toward the spinal cord, they were classified as being of the vestibulo-oculomotor type and are thought to be involved exclusively in eye movement control. The signal content and spatial tuning characteristics of this anterior canal vestibulo-oculomotor neuron class remain to be determined.

"Velocity leakage" in the pigeon vestibulo-ocular reflex

The transfer characteristics of the vestibulo-ocular reflex (VOR), and of the semicircular canal primary afferents (SCPAs) that drive it, have been studied in several species. In monkeys and cats, the dominant time constant describing horizontal VOR dynamics (tau hv) is longer than that (tau c) of horizontal SCPAs. This lengthening of the time constant has been attributed to a "velocity storage" mechanism that has been modeled as a positive feedback loop in the VOR pathways. We have studied the transfer characteristics of horizontal and vertical VOR and SCPAs in unanesthetized pigeons. In this species the dominant time constants of both the horizontal and vertical VOR (tau hv and tau vv) are shorter that tau c. This finding indicates that time constants characterizing the lower frequency response of the VOR can be lengthened or shortened depending on the species. We propose that in the pigeon the "velocity leakage" mechanism can be modeled by substituting negative feedback for positive feedback in the model of the VOR pathways. Negative feedback can also account for the further shortening of tau hv and tau vv as VOR gain increases with arousal. Additionally, making the negative feedback loop nonlinear can model the dependency of lower frequency VOR phase on amplitude, and skew in VOR waveforms. Pigeon VOR and SCPA dynamics also differ in their adaptive properties and higher frequency behavior. A predominance of input from highly adaptive SCPAs is proposed to account for the increased adaptation of the vertical VOR as compared with SCPAs overall. A pure time-delay associated with VOR operation can explain the phase lag of the VOR relative to SCPAs at higher frequencies.

The influence of middle ear pressure changes on the primary vestibular neurons in guinea pigs

The responses of primary vestibular neurons and perilymphatic pressure changes to middle ear pressure stimuli in guinea pigs were investigated in order to clarify the direct effects of pressure stimulus on the vestibular apparatus. The vestibular response was related to the amount of middle ear pressure change applied at a rate of +/- 100 mmH2O/s. The neural response rates of vestibular units to positive pressure in the middle ear were significantly larger than those to negative pressure. The time course pattern of the perilymphatic pressure change resembled that of the response of the vestibular units, indicating that the vestibular response is elicited by middle ear pressure via the pressure transmitted in the inner ear.

Bilateral communication between vestibular labyrinths in pigeons

Extracellular action potentials from single horizontal semicircular canal primary afferent fibers were recorded in paralysed decerebrate pigeons during pulse mechanical stimulation of the contralateral horizontal semicircular canal. Clear responses to the contralateral membranous duct displacement stimuli were observed in 51% of the tested 158 horizontal semicircular canal afferents. Generally, three different types of responses were obtained in the primary afferent fibers including excitation, inhibition, and a few complex type neural activity profiles. Inhibitory responses were of larger amplitude and had longer time constants than did excitatory responses. The few complex type responses observed were characterized by an initial excitatory discharge followed by a longer duration decrease in the fiber's firing rate. The sensitivity to stimulation and type of response obtained for each afferent was significantly correlated with the fiber's coefficient of variation value. Regular firing afferents were less sensitive and exhibited primarily excitatory responses (71%) to contralateral canal stimulation. Irregular firing afferents were more sensitive and exhibited mostly inhibitory responses (84%). The present results demonstrate that a communication network for information exchange between the bilateral labyrinths exists in pigeons. The observed responses in primary afferent fibers to contralateral horizontal semicircular canal stimulation are proposed to be mediated by the vestibular efferent system, which could provide an anatomical pathway for information exchange from vestibular receptors on opposite sides of the head.

Responses of pigeon vestibulocerebellar neurons to optokinetic stimulation. II. The 3-dimensional reference frame of rotation neurons in the flocculus

1. The complex spike activity of Purkinje cells in the flocculus in response to rotational flowfields was recorded extracellularly in anesthetized pigeons. 2. The optokinetic stimulus was produced by a rotating "planetarium projector." A light source was placed in the center of a tin cylinder, which was pierced with numerous small holes. A pen motor oscillated the cylinder about its long axis. This apparatus was placed above the bird's head and the resultant rotational flow-field was projected onto screens that surrounded the bird on all four sides. The axis of rotation of the planetarium could be oriented to any position in three-dimensional space. 3. Two types of responses were found: vertical axis (VA; n = 43) neurons responded best to visual rotation about the vertical axis, and H-135i neurons (n = 34) responded best to rotation about a horizontal axis. The preferred orientation of the horizontal axis was at approximately 135 degrees ipsilateral azimuth. VA neurons were excited by rotation about the vertical axis producing forward (temporal to nasal) and backward motion in the ipsilateral and contralateral eyes, respectively, and were inhibited by rotation in the opposite direction. H-135i neurons in the left flocculus were excited by counterclockwise rotation about the 135 degrees ipsilateral horizontal axis and were inhibited by clockwise motion. Thus, the VA and H-135i neurons, respectively, encode visual flowfields resulting from head rotations stimulating the ipsilateral horizontal and ipsilateral anterior semicircular canals. 4. Sixty-seven percent of VA and 80% of H-135i neurons had binocular receptive fields, although for most binocular cells the ipsilateral eye was dominant. Binocular stimulation resulted in a greater depth of modulation than did monocular stimulation of the dominant eye for 69% of the cells. 5. Monocular stimulation of the VA neurons revealed that the best axis for the contralateral eye was tilted back 11 degrees, on average, to the best axis for ipsilateral stimulation. For the H-135i neurons, the best axes for monocular stimulation of the two eyes were approximately the same. 6. By stimulating circumscribed portions of the monocular receptive fields of the H-135i neurons with alternating upward and downward largefield motion, it was revealed that the contralateral receptive fields were bipartite. Upward motion was preferred in the anterior 45 degrees of the contralateral field, and downward motion, was preferred in the central 90 degrees of the contralateral visual field.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of caudal medullary raphe neurons to natural vestibular stimulation

1. Over two thirds of caudal medullary raphespinal neurons respond to electrical stimulation of the vestibular nerve, and it has been suggested that these neurons may participate in the generation of vestibulospinal and vestibulosympathetic reflexes. The objective of the present study was to determine which vestibular endorgans (semicircular canals or otolith organs) provide inputs to these cells. 2. Experiments were conducted on decerebrate cats that were baroreceptor denervated and vagotomized, and that had a cervical spinal cord transection so that inputs from tilt-sensitive receptors outside of the labyrinth did not influence the units we recorded. 3. In most experiments, vertical vestibular stimulation was used to stimulate the anterior and posterior semicircular canals and the otolith organs. The plane of whole body rotation that produced maximal modulation of a neuron's firing rate (response vector orientation) was measured at one or more frequencies between 0.1 and 0.5 Hz. Neuron dynamics were then studied with sinusoidal (0.02-1 Hz) stimuli aligned with this orientation. Alternatively, in two animals horizontal rotations at 0.5 and 1.0 Hz were employed to stimulate the horizontal semicircular canals. 4. The properties of raphespinal neurons were similar to those of a larger sample of raphe neurons studied that either could not be antidromically activated from the cervical spinal cord or were not tested for a spinal projection. In response to vertical vestibular stimulation, > 85% of caudal medullary raphe neurons had response gains that remained relatively constant across stimulus frequencies, like regularly firing otolith afferents.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphological correlations between spontaneously discharging primary vestibular afferents and vestibular nucleus neurons in the cat

Synaptic connections between physiologically classified primary vestibular afferents (PVAs) and their target vestibular nucleus (VN) neurons were examined by a combination of intra-axonal staining and electron microscopic techniques. PVAs originating from the horizontal semicircular canal were electrophysiologically classified as either regular- or irregular-type based on the regularity of their spontaneous discharge patterns, and were intra-axonally labeled with horseradish peroxidase (HRP). HRP-labeled PVAs of both types had many swellings along their course that contacted VN neurons. These swellings contained spherical synaptic vesicles and showed asymmetric postsynaptic specialization. Target VN neurons of both types of PVAs were distributed primarily in the superior, medial, and inferior VN. Irregular-type PVAs made more axosomatic contacts than did regular-type PVAs. The soma size of target VN neurons and the number of terminal boutons per target VN neuron were larger for irregular-type PVAs than for regular-type PVAs. Large VN neurons (presumably kinetic neurons) were innervated exclusively by irregular-type PVAs. Small VN neurons were innervated by PVAs of the regular-type and the irregular-type. These results demonstrate that there is a correlation between the physiological properties and morphological characteristics of PVAs and their target VN neurons.

Acoustic response properties of single units in the torus semicircularis of the goldfish, Carassius auratus

Single units of the goldfish torus semicircularis (TS) were recorded in response to pure tones. Response areas (RA) were obtained by recording the number of spikes evoked by tones in a range of frequencies and levels within the units' dynamic range. RAs gave estimates of best sensitivity (BS), characteristic frequency (CF), most excitatory frequency at each level (BF), and Q10dB. Peri-stimulus-time histograms (PSTH), interspike interval histograms (ISIH), and period histograms were obtained at various frequencies and levels to describe the units' temporal response patterns. The distribution of CF is nonuniform with modes at 155, 455, and 855 Hz. The distribution of the coefficient of synchronization to standard tones is also nonuniform, revealing a dichotomy between units with little or no phase-locking and those that phase-lock strongly. PSTHs for units without significant phase-locking vary widely and include patterns resembling those of the mammalian auditory brainstem. Compared with saccular afferents, torus units tend to have lower spontaneous rates, greater sensitivity, and sharper tuning. Unlike saccular afferents, BF is independent of level for most torus units. Some torus units are similar to saccular afferents while others reveal significant transformations of information between the periphery and the midbrain.

The octavolateral systems in the stingray, Dasyatis sabina. I. Primary projections of the octaval and lateral line nerves

The central projections of the electrosensory, mechanosensory, and octaval nerves of the Atlantic stingray were examined by transganglionic transport of horseradish peroxidase. Particular attention was paid to the relation of the projections to cell plates C1 and C2, and to a newly described cell plate, C3. The electroreceptors in the stingray are found in three groups on the dorsal and ventral sides of the body. The electroreceptors are represented topographically on the ipsilateral dorsal nucleus. Those of the rostral part of the head and pectoral fins are represented on the rostroventral part of the nucleus, and those on the caudal part of the head and the pectoral fin on the dorsocaudal part of the nucleus. Mechanosensory lateral line afferents terminate within the ipsilateral intermediate and caudal nuclei, and the lateral granular mass of the vestibulocerebellum. Anterior lateral line afferents also project to the magnocellular octaval nucleus. A topographic representation of the mechanosensory lateral line periphery is present on the intermediate and caudal nuclei. Mechanoreceptors on the trunk are represented laterally and those on the head medially. The terminal field of the anterior lateral line afferents on the intermediate nucleus surrounds cell plates C1 and C2. The anterior lateral line afferents also project to the medial part of the lateral granular mass, whereas the posterior lateral line afferents project to the lateral portion. Sparse projections of anterior lateral line afferents to the periventricular octaval nucleus were also observed. The octaval nerve afferents terminate largely within the octaval column. Octaval nerve projections were also observed to the reticular formation, the periventricular octaval nucleus, the deep cerebellar nucleus, the vestibulocerebellum, particularly the lower lip and medial granular mass, and the intermediate nucleus. A sparse projection to cell plate C3 was found. The relation between cell plates C1 and C2 and the anterior lateral line afferents suggests that these cell plates are related to processing lateral line information. While the relationship between cell plate C3 and the octaval afferents is not strong, the sparse octaval projection C3 receives suggests that it is relaying octaval information.

Corticofugal projections to the vestibular nuclei in squirrel monkeys: further evidence of multiple cortical vestibular fields

Single- and multiple-unit recordings were made from nerve cells located in the different nuclei of the brainstem vestibular nuclear complex (VNC) of anaesthetized squirrel monkeys (Saimiri sciureus) by conventional stereotaxic techniques. After neurons responding to semicircular canal stimulation in a yaw, roll, or pitch direction or to otholith stimulation were identified, small amounts of retrograde tracer substances were deposited at the recording sites. Up to three different tracers were administered to different parts of the VNC in the same animal (Fast Blue, HRP-WGA, and Rhodamine-dextranes). After adequate survival times, the animals were sacrificed. Following histological processing, the cortical grey matter was screened systematically for cells labelled with the retrograde tracers (fluorescence microscopy or light microscopy for HRP processing). Labelled nerve cells which clearly project to the VNC directly were found predominantly in the cytoarchitectonic layer 5 of seven different cortical areas: 1) The parieto-insular vestibular cortex PIVC, which in squirrel monkeys consists mainly of the medial area Ri and parts of the anterior area Ig; 2) area 7ant, which presumably corresponds to the macaque area 2v; 3) area 3aV, a vestibular field of area 3a; 4) the temporal area T3 bordering on area Ri; 5) the premotor area 6a; and 6, 7) the areas 6c and 23c of the anterior cingulate cortex. The PIVC, area 7ant, and area 3aV form the "inner cortical vestibular circuit" (Guldin et al.: J. Comp. Neurol. 326:375-401, '92), while the other cortical areas mentioned also have direct projections to the structures of the inner cortical vestibular circuit. It is speculated that the direct projections of the cortical vestibular structures to the brainstem vestibular nuclei regulate the vestibulo-ocular, the vestibulo-spinal, and the optokinetic reflexes mediated through the VNC, thus preventing counteractions of these reflexes during voluntary, goal-directed head movements or locomotion.

The correlated blanching of synaptic bodies and reduction in afferent firing rates caused by transmitter-depleting agents in the frog semicircular canal

Synaptic bodies (SBs) associated with rings of synaptic vesicles and well-defined, pre- and post-synaptic membrane structures are indicators of maturity in most hair cell-afferent nerve junctions. The role of the SBs remains elusive despite several experiments showing that they may be involved in storage of neurotransmitter. Our results demonstrate that SBs of the adult posterior semicircular canal (SCC) cristae hair cells become less electron dense following incubation of the SCC with the transmitter-depleting drug tetrabenazine (TBZ). Objective quantification and comparison of the densities of the SBs in untreated and TBZ-treated frog SCC demonstrated that TBZ significantly decreased the electron density of SBs. This reduction in electron density was accompanied by a reduction in firing rates of afferent fibers innervating the posterior SCC. A second transmitter-depleting drug, guanethidine, previously shown to reduce the electron density of hair cell SBs, also reduced the firing rates of afferent fibers innervating the posterior SCC. In contrast, the electron density of dense granules (DG), similar in size and shape to synaptic bodies (SB) in hair cells, did not change after incubation in TBZ, thus indicating that granules and SBs are not similar in regard to their electron density. The role of SBs in synaptic transmission and the transmitter, if any, stored in the SBs remain unknown. Nonetheless, the association of the lessening of electron density with a reduction in afferent firing rate provides impetus for the further investigation of the SB's role in neurotransmission.

Contribution of irregular semicircular canal afferents to the horizontal vestibuloocular response during constant velocity rotation

1. The effects of constant anodal currents (100 microA) delivered bilaterally to both labyrinths on the horizontal vestibuloocular response (VOR) were studied in squirrel monkeys during steps of angular velocity in the dark. We report that bilateral anodal currents decreased eye velocity approximately 30-50% during the period of galvanic stimulation without a change in the time constant of VOR. The decrease in eye velocity, present during steps of angular velocity, was not observed during sinusoidal head rotation at 0.2, 0.5, and 1 Hz. The results suggest that responses from irregular vestibular afferents influence VOR amplitude during constant velocity rotation.

Experimental study of a frog semicircular canal nerve: influence of the initial cupular position

The posterior semicircular canal of the frog was stimulated by mechanical endolymphatic flow toward either an ampullofugal (excitatory) or ampullopetal (inhibitory) direction. The magnitudes of the responses as the cupula was moved to various positions were compared. When the same amount of stimulus was given in the same direction, the responses were equal, regardless of the initial position of the cupula.

Glial fibrillary acidic protein (GFAP)-like immunoreactivity in the vestibular endorgan of the rat

The distribution of glial fibrillary acidic protein (GFAP)-like immunoreactivity in the peripheral vestibular nervous system of normal adult rats was studied using immunohistochemical methods. The immunoreactivity was demonstrated at the light-microscopic level. GFAP-like immunoreactivity could be seen at the vestibular fibers terminating in the hair cells.

Calcitonin gene-related peptide in the efferent system of the inner ear. A review

Many calcitonin gene-related peptide-like immunoreactive (CGRP-IR) fibers were found in the vestibular end-organs and the cochlea of rats. CGRP-IR fibers in the vestibular end-organs originated in the bilateral cell groups dorsolateral to the genu of the facial nerve. These fibers formed a fiber plexus at the base of the sensory epithelia in the maculae of the otolith organs and the ampullae of the semicircular canals, and formed synaptic contacts with the nerve chalice on type I vestibular sensory cells. CGRP-IR fibers in the cochlea originated in the ipsilateral lateral superior olivary nucleus. Most of them existed in the inner spiral bundle under the inner hair cells, and formed synaptic contacts with afferent terminals on the inner hair cells. These findings suggest a postsynaptic modulation of CGRP on the vestibular and cochlear information at the level of type I vestibular sensory cells and the inner hair cells.

Vestibular control of skeletal geometry in the guinea pig: a problem of good trim?

Motor control of different segments of the body with multiple degrees of freedom appears to be coordinated by utilizing preferred axes of motor activity. This hypothesis may also be applied to vestibular control of posture. To explore this question we studied the anatomical relationship between the head and the cervical vertebral column by taking radiographs of the head-neck region in unrestrained alert guinea pigs. We determined that biomechanical constraints contribute to the stereotypical skeletal geometry observed in the resting animal and to a functional segmentation of the head-neck movement apparatus. Subsequent lesion studies of vestibular end organs with quantification of the resulting postural syndromes suggest that the functional segmentation of the cervical vertebral column corresponds to a functional partitioning of vestibular afferents. Our findings also indicate that the sensorimotor transformation mechanisms necessary to convert a given head velocity signal into the appropriate neck motor frame are already embedded in the networks provided by second-order vestibular neurons. Good trim of postural control will be the end result of an appropriate internal representation of the objective vertical.