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.     

The vestibular nerve of the chinchilla. III. Peripheral innervation patterns in the utricular macula

1. Nerve fibers supplying the utricular macula of the chinchilla were labeled by extracellular injection of horseradish peroxidase into the vestibular nerve. The peripheral terminations of individual fibers were reconstructed and related to the regions of the end organ they innervated and to the sizes of their parent axons. 2. The macula is divided into medial and lateral parts by the striola, a narrow zone that runs for almost the entire length of the sensory epithelium. The striola can be distinguished from the extrastriolar regions to either side of it by the wider spacing of its hair cells. Calyx endings in the striola have especially thick walls, and, unlike similar endings in the extrastriola, many of them innervate more than one hair cell. The striola occupies 10% of the sensory epithelium; the lateral extrastriola, 50%; and the medial extrastriola, 40%. 3. The utricular nerve penetrates the bony labyrinth anterior to the end organ. Axons reaching the anterior part of the sensory epithelium run directly through the connective tissue stroma. Those supplying more posterior regions first enter a fiber layer located at the bottom of the stroma. Approximately one-third of the axons bifurcate below the epithelium, usually within 5-20 microns of the basement membrane. Bifurcations are more common in fibers destined for the extrastriola than for the striola. 4. Both calyx and bouton endings were labeled. Calyces can be simple or complex. Simple calyces innervate individual hair cells, whereas complex calyces supply 2-4 adjacent hair cells. Complex endings are more heavily concentrated in the striola than in the extrastriola. Simple calyces and boutons are found in all parts of the epithelium. Calyces emerge from the parent axon or one of its thick branches. Boutons, whether en passant or terminal, are located on thin collaterals. 5. Fibers can be classified into calyx, bouton, or dimorphic categories. The first type only has calyx endings; the second, only bouton endings; and the third, both kinds of endings. Calyx units make up 6% of the labeled fibers, bouton units less than 2%, and dimorphic units greater than 92%. The three fiber types differ in the macular zones they supply and in the diameters of their parent axons. Calyx units were restricted to the striola. The few bouton units were found in the extrastriola.(ABSTRACT TRUNCATED AT 400 WORDS)     

Saccular and utricular inputs to single vestibular neurons in cats

Saccular and utricular organs are essential for postural stability and gaze control. Although saccular and utricular inputs are known to terminate on vestibular neurons, few previous studies have precisely elucidated the origin of these inputs. We investigated the saccular and utricular inputs to single vestibular neurons in whole vestibular nuclei of decerebrated cats. Postsynaptic potentials were recorded from vestibular neurons after electrical stimulation of the saccular and utricular nerves. Ascending and descending axonal projections were examined by stimulating the oculomotor/trochlear nuclei and the cervical segment of the spinal cord, respectively. After each experiment, locations of recorded neurons were identified. The recorded neurons (140) were classified into vestibulo-spinal (79), vestibulo-oculo-spinal (9), and vestibulo-ocular (3) neurons based on antidromic responses; 49 other vestibular neurons were unidentified. The majority of recorded neurons were mainly located in the lateral vestibular nucleus. Most of the otolith-activated vestibular nuclei neurons seemed to participate in vestibulospinal reflexes. Of the total 140 neurons recorded, approximately one third (51) received saccular and utricular inputs (convergent neurons). The properties of these 51 convergent neurons were further investigated. Most (33/51) received excitatory postsynaptic potentials (EPSPs) after saccular and utricular nerve stimulation. These results implied that most of the convergent neurons in this study additively coded mixed information for vertical and horizontal linear acceleration. Based on the latencies of convergent neurons, we found that an early integration process for vertical and horizontal linear acceleration existed at the second-order level.     

The vestibular nerve of the chinchilla. V. Relation between afferent discharge properties and peripheral innervation patterns in the utricular macula

1. The relation between the discharge properties of utricular afferents and their peripheral innervation patterns was studied in the chinchilla by the use of intra-axonal labeling techniques. Fifty-three physiologically characterized units were injected with horseradish peroxidase (HRP) or lucifer yellow CH (LY) and their labeled processes were traced to the utricular macula. For most labeled neurons, the discharge regularity, background discharge, and sensitivity to externally applied galvanic currents were determined, as were the gain (g2 Hz) and phase (phi 2 Hz) of the response to 2-Hz sinusoidal linear forces. Terminal fields were reconstructed and fibers were classified as calyx (n = 13) or dimorphic units (n = 40). No bouton units were recovered. Calyx units were confined to the striola. Dimorphic units were located in the striola (n = 8), the juxtastriola (n = 7), or the peripheral extrastriola (n = 25). 2. To determine whether the intra-axonal sample was representative, the physiological properties of labeled utricular units were compared with those of a larger sample of extracellularly recorded units. A comparison was also made between the morphology of intra-axonally labeled units and those labeled by the extracellular injection of HRP into the vestibular nerve. Most of the discrepancies between the intra-axonal and either extracellular sample can be explained by assuming that small-diameter fibers are underrepresented in the former sample. Dimorphic fibers labeled intra-axonally had more bouton endings and larger terminal trees than did those labeled extracellularly. The latter differences may reflect a sampling bias in the extracellular material. 3. Calyx units were irregularly discharging. The discharge regularity of dimorphic units was related to their macular locations. Only 1/8 dimorphic units in the striola was regularly discharging. The ratio increases to 3/7 in the juxtastriola and to 23/25 in the peripheral extrastriola. Among dimorphic units, there is a tendency for irregularly discharging afferents to have fewer bouton endings. The trend is far from perfect because it is possible to pick a subsample of dimorphic units that have similar numbers of boutons and, yet, have discharge patterns that range from regular to irregular. 4. Published morphological polarization maps can be used to predict the excitatory tilt directions of a unit from its macular location. Predictions were confirmed in 39/41 labeled afferents. 5. The galvanic sensitivity (beta *) of an afferent, irrespective of its peripheral innervation pattern or its epithelial location, was strongly correlated with a normalized coefficient of variation (CV*).(ABSTRACT TRUNCATED AT 400 WORDS)     

Zebrafish pax5 regulates development of the utricular macula and vestibular function.

The zebrafish otic vesicle initially forms with only two sensory epithelia, the utricular and saccular maculae, which primarily mediate vestibular and auditory function, respectively. Here, we test the role of pax5, which is preferentially expressed in the utricular macula. Morpholino knockdown of pax5 disrupts vestibular function but not hearing. Neurons of the statoacoustic ganglion (SAG) develop normally. Utricular hair cells appear to form normally but a variable number subsequently undergo apoptosis and are extruded from the otic vesicle. Dendrites of the SAG persist in the utricle but become disorganized after hair cell loss. Hair cells in the saccule develop and survive normally. Otic expression of pax5 requires pax2a and fgf3, mutations in which cause vestibular defects, albeit by distinct mechanisms. Thus, pax5 works in conjunction with fgf3 and pax2a to establish and/or maintain the utricular macula and is essential for vestibular function.     

Convergence patterns of the posterior semicircular canal and utricular inputs in single vestibular neurons in cats

The convergence of the posterior semicircular canal (PC) and utricular (UT) inputs in single vestibular nuclei neurons was studied intracellularly in decerebrate cats. A total of 160 vestibular neurons were orthodromically activated by selective stimulation of the PC and the UT nerve and classified according to whether or not they were antidromically activated from the spinal cord and oculomotor nuclei into vestibulospinal (VS), vestibulooculospinal (VOS), vestibuloocular (VO), and unidentified vestibular neurons. Fifty-three (33%) of 160 vestibular neurons received convergent inputs from both the PC and UT nerves. Seventy-nine (49%) vestibular neurons responded to PC inputs alone, and 28 (18%) neurons received inputs only from the UT nerve. Of 53 convergent neurons, 8 (15%) were monosynaptically excited from both nerves. Thirty-five (66%) received monosynaptic excitatory inputs from the PC nerve and polysynaptic excitatory or inhibitory inputs from the UT nerve, or vice versa. Approximately one-third of VS and VOS neurons received convergent inputs. A majority of the VS neurons descended to the spinal cord through the lateral vestibulospinal tract, while almost all the VOS neurons descended to the spinal cord through the medial vestibulospinal tract. The convergent neurons were found in all vestibular nuclei but more in the lateral nucleus and descending nucleus. The VS neurons were more numerous than VO neurons or VOS neurons.     

Response of central vestibular neurons to utricular stimulations in cats

Summary In decerebrated, spinal transected cats with neck and forelimbs immobilized by plaster cats, the visual and proprioceptive cues were minimized when the animal was tilted. The contralateral labyrinth was acutely destroyed. The ipsilateral semicircular canals were plugged and the ipsilateral saccule extirpated leaving the ipsilateral utricle intact. Neurons in the vestibular nuclear complex driven by electrical stimulation of the utricle were shown to be highly sensitive to static pitch. Results suggest that the observed response to static pitch was due exclusively to input from the utricle.This research was supported in part by a grant from the Wing Lung Bank Medical Research Fund and Research Grant Committee of the Medical Faculty at the University of Hong Kong     

Neuronal activity in the lateral vestibular nucleus of the cat

The synaptic input to Deiters neurones evoked by stimulation of peripheral somatic nerves was measured by intracellular recordings. EPSPs with broad receptive fields and latencies which indicate polysynaptic connexions were commonly evoked from the FRA. In other cells, low threshold cutaneous afferents were effective at rather short latencies suggesting oligosynaptic connexions from fast ascending fibres. One example was found of EPSPs due to low threshold muscle afferents. IPSPs due to climging fibre activation of Purkinje cells as observed in most of the neurones were evoked by cutaneous volleys above 1.5-2.0T and muscle volleys above 5T (above 3-3.5T in case of Q). Often, IPSPs were evoked by stimulation of nerves, to the segmental level of which the the vestibulospinal neurone under investigation projected. A small proportion of cells received short latency IPSPs involving direct fast mossy fibre tracts, which were evoked from low threshold cutaneous afferents. IPSPs due to polysynaptic mossy fibre activation of Purkinje cells were evoked from the FRA bilaterally and from ipsilateral cutaneous afferents at 1.5-2.0T ("prolonged inhibition"). Prolonged excitatory/inhibitory events mediated by mossy fibre pathways may be involved in quadruped locomotion or other processes making use of a broad motor integration.     

Neuronal activity in the lateral vestibular nucleus of the cat

Summary Experiments were carried out on cats, the chloralose anesthesia having been maintained by small doses of Thiopental in about half of the experiments. Intracellular recordings were evaluated from 256 Deiters neurones, which received EPSPs and/or IPSPs upon stimulation of the spinal cord. In respect to excitation, in 70% of these cells, stimulation of the spinal cord at C₂-level evoked short latency EPSPs, which showed a bimodal latency distribution. Together with EPSP-latencies observed on stimulation of the lumbar cord the results indicate that a small group of EPSPs is evoked monosynaptically, whilst a major group is mediated via disynaptic linkages of fast conducting fibres. The possibility of a trisynaptic linkage is discussed. Deiters neurones can show rhythmic stimuluslocked discharges, probably based on the organization of synaptic excitation.Supported by the Deutsche Forschungsgemeinschaft (grant Br 242/8).     

Neuronal activity in the lateral vestibular nucleus of the cat

The spatial distribution of inhibitory effects mediated by spinoolivocerebellar pathways and evoked by stimulation of the FRA was studied in Deiters' nucleus by intracellular recordings and extracellular measurements of positive field potentials. At a geven recording site individual nerves greatly varied in their effectiveness: Q and Saph, Tib and FDL were usually the most effective, while GS had very little effect. When comparing several recording sites, various patterns of dominating and nondominating nerves were found, resulting in a somewhat patchy distribution of responses. The results were supported by intracellular recordings, which showed that CF-mediated IPSPs can be evoked from a limited number of nerves only. The findings show that a discriminative pattern exists in the FRA-evoked spino-olivocerebellar-mediated inhibition. In addition, Deiters neurones were found in which the inhibition was evoked from a large number of fore- and hindlimb nerves. Such a generalized inhibitory pattern may be involved in interlimb coordination during locomotion.     

Convergence of the anterior semicircular canal and otolith afferents on cat single vestibular neurons

The convergence between the anterior semicircular canal (AC) and utricular (UT) inputs, as well as the convergence between the AC and saccular (SAC) inputs in single vestibular neurons of decerebrated cats were investigated. Postsynaptic potentials were recorded intracellularly after selective stimulation of each pair of vestibular nerves AC/UT or AC/SAC. Neurons were recorded from the central parts of the vestibular nuclei, where the otolith afferents mainly terminate. Of a total of 105 neurons that were activated after stimulation of the AC and UT nerves, 42 received convergent inputs. Thirty-eight of these neurons received excitatory inputs from both afferents. Convergent neurons were further classified into vestibulospinal (n=28) and vestibulooculospinal (n=6) neurons by antidromic activation from the border between the C1 and C2 spinal cord and the oculomotor or trochlear nucleus. Eight neurons that were not antidromically activated from either site were classified as vestibular neurons. Forty three percent of the convergent vestibulospinal neurons and most of the convergent vestibulooculospinal neurons projected to the spinal cord through the medial vestibulospinal tract. The remaining vestibulospinal and vestibulooculospinal neurons descended through the ipsilateral lateral vestibulospinal tract. Of a total of 118 neurons that were activated after stimulation of the AC and/or SAC nerves, 51 received convergent inputs (27 vestibulospinal, 4 vestibulooculospinal, 5 vestibuloocular and 15 vestibular neurons). Forty-two of the convergent neurons received excitatory inputs from both afferents. Thirty seven percent of the convergent vestibulospinal neurons and all of the convergent vestibulooculospinal neurons projected to the spinal cord through the medial vestibulospinal tract. The remaining vestibulospinal and vestibulooculospinal neurons descended through the ipsilateral lateral vestibulospinal tract. Electronic Publication     

Canal-neck interaction in vestibular nuclear neurons of the cat

Summary The convergence and interaction of horizontal semicircular canal and neck proprioceptive inputs were studied in neurons of the caudal two thirds of the vestibular nuclear complex. Extracellular neuron activity was recorded under muscle relaxation and slight anesthesia in chronically prepared cats. The following stimulations were applied: horizontal rotations of (a) the whole body (labyrinth stimulation), (b) the trunk vs. the stationary head (neck stimulation), and (c) the head vs. the stationary trunk (combined labyrinth and neck stimulation).Of 152 neurons investigated, 83 (55%) showed convergence of the two inputs. In about half of these neurons, the neck input was very weak and hardly affected the labyrinthine response during head rotation. Judged from the response pattern, several of these neurons presumably were related to vestibulo-oculomotor function (i.e., vestibular nystagmus). In the other half (i.e., 27% of all neurons), sensitivity of the two inputs was similar. Both labyrinthine and neck responses contained a dynamic (velocity) component; neck responses of more than half of these neurons had, in addition, a static (position) component. The dynamic components were either antagonistic or synergistic as to their convergence during head rotation. When applying this combined stimulation, the dynamic components summed linearly, yielding subtraction in case of antagonistic convergence and addition in case of synergistic convergence. In contrast, the static components of the neck responses remained largely unchanged during head rotation. However, the static head-to-trunk deflection determined the tonic discharge level in such neurons and thus facilitated or disfacilitated the dynamic responses to superimposed labyrinth stimulation.We suggest that the two patterns of labyrinthine-neck interaction observed in vestibular nuclear neurons, i.e., subtraction and addition, may be involved in the postural control of the trunk and head, respectively. In contrast, interference of the neck input with vestibule-oculomotor function appears to be almost negligible in the intact cat.Supported by Deutsche Forschungsgemeinschaft, SFB 70     

Neuronal coding of linear motion in the vestibular nuclei of the alert cat

In the present study we have investigated in the awake cat the response dynamics of vestibular nuclei neurons to visual or/and otolith stimulation elicited by vertical linear motion. Of the 53 units tested during sinusoidal motion at 0.05 Hz (9.1 cm/s), 1 (1.9%) was responsive to the otolith input only, 13 (24.5%) were influenced by the visual input only and 23 (43.4%) responded to both modalities. Neurons were excited either during upward or downward animal or visual surround movement. Most units displayed a firing rate modulation very close to motion velocity. All the neurons receiving convergent visual and otolith inputs (0.05 Hz, 9.1 cm/s) exhibited synergistic patterns of response. Motion velocity coding was improved in terms of input-output phase relationship and response sensitivity when visual and otolith signals were combined. Depending on the units, visual-otolith interactions in single neurons could follow a linear or a nonlinear mode of summation. The dynamic characteristics of visual-otolith interactions were examined in the 0.05 Hz-0.50 Hz frequency bandwidth. Visual signals seemed to predominate over otolith signals at low stimulus frequencies (up to 0.25 Hz), while the contrary was found in the higher frequency range of movement (above 0.25 Hz). The effects of visual stabilization (VS: suppression of visual motion cues) was observed in a small sample of units. As a rule, VS induced a reduction in the amplitude of unit response as compared to visual + otolith stimulation, the lower the motion frequency, the more pronounced the attenuation. VS also decreased the amplitude of the otolith-dependent component of response. The possible modes of visual-vestibular interactions in single cells are discussed. The present study supports the hypothesis that visual and vestibular motion cues are weighted according to their internal relevance.     

Neuronal coding of linear motion in the vestibular nuclei of the alert cat

Summary The aim of the present study was to investigate some aspects of the central processing of otolith information during linear motion. For this purpose, the response characteristics of 69 vestibular nuclei units to sinusoidal otolith stimulation in the vertical Z axis were analysed in the alert cat. Among this population of neurons which responded to a 0.05 Hz, 290 mm translation, 47 units (70%) displayed a firing rate modulation which followed the input frequency (H1 units). The majority of these neurons exhibited an increase in discharge rate during upward displacement, with a response phase close to the motion velocity or slightly leading downward acceleration. The acceleration related units were divided into two groups according to whether they showed clear increases or only a slight change in discharge rate when the stimulus frequency was increased. The former group was characterized by an average –16.3 dB drop in gain (from 43.9±1.8 dB, S.D. to 27.6±7 dB, S.D.) within the 0.05 Hz–0.5 Hz frequency range, while the latter group displayed an average –31.2 dB gain attenuation (from 45.1±1.1 dB, S.D. to 13.9±0 dB) within the same decade. In contrast to differences in response gain, all the units tested exhibited a relatively stable phase lead of about 20° with respect to downward peak acceleration. Conversely, units whose response was close to motion velocity in the lower frequency range (0.05 Hz–0.10 Hz) displayed a strong phase lead of about 100° when the stimulus frequency was increased (up to 0.50 Hz). These neurons were thus characterized by an acceleration related response in the higher frequency range. At the same time, an average –24.8 dB gain attenuation (from 47.7±3.4 dB to 22.9±3.7 dB) was found in the 0.05 Hz–0.5 Hz decade. The remaining 22 neurons (30%) were called H2 units since they displayed a response waveform double that of the input frequency, a response already described during sinusoidal rotation. Unit discharge reached a peak approximately in phase with maximum upward and downward velocity. Asymmetrical change in unit firing rate about the resting discharge level and different dynamic behavior of the upward and downward response components were usually found. These response characteristics suggest that the H2 patterns are centrally constructed and could result from convergence of otolith afferents having opposite polarization vectors. Other evidence suggests that these units which behave like motion-detectors can exert an influence on the neck musculature. Our results corroborate, at least in part, the findings of previous studies on the dynamic responses properties of otolith-dependent central neurons during roll tilt, or pure linear acceleration in the horizontal plane.This research was supported by U.A.-C.N.R.S. 372, INSERM (PRC no 133019) and Ministère de la Recherche et de la Technologie (84.C.1311)     

Properties of utricular-activated vestibular neurons that project to the contralateral vestibular nuclei in the cat

The properties of utricular (UT)-activated vestibular neurons that send axons to the contralateral vestibular nuclei (commissural neurons) were investigated intracellularly or extracellularly in decerebrate cats. A total of 27 vestibular neurons were orthodromically activated by stimulation of UT nerves and antidromically activated by stimulation of the contralateral vestibular nuclei. All neurons tested were classified as vestibulospinal (VS), vestibulooculospinal (VOS), vestibuloocular (VO), and unidentified vestibular neurons (V) after antidromic stimulation of the spinal cord and oculomotor/trochlear nuclei. Most UT-activated commissural neurons (20/27) received monosynaptic inputs. Twelve of 27 commissural neurons were located in the medial vestibular nucleus, 5 were in the lateral vestibular nucleus, 10 were in the descending vestibular nucleus, and no commissural neurons were recorded in the superior vestibular nucleus. Seven of 27 neurons were commissural VS neurons, 9 of 27 were commissural VOS neurons, and 11 of 27 were commissural V neurons. No commissural VO neurons were found. All VOS neurons and 3 VS neurons issued descending axons via the medial vestibulospinal tract. We also studied convergent inputs from the posterior semicircular canal (PC) nerve onto UT-activated commissural neurons. Five of 27 UT-activated commissural neurons received converging inputs from the PC nerves. Electronic Publication