The inhibitory vestibular efferent system and its relation to the cerebellum in the frog

Context-independent encoding of passive and active self-motion in vestibular afferent fibers during locomotion in primates

The vestibular system detects head motion to coordinate vital reflexes and provide our sense of balance and spatial orientation. A long-standing hypothesis has been that projections from the central vestibular system back to the vestibular sensory organs (i.e., the efferent vestibular system) mediate adaptive sensory coding during voluntary locomotion. However, direct proof for this idea has been lacking. Here we recorded from individual semicircular canal and otolith afferents during walking and running in monkeys. Using a combination of mathematical modeling and nonlinear analysis, we show that afferent encoding is actually identical across passive and active conditions, irrespective of context. Thus, taken together our results are instead consistent with the view that the vestibular periphery relays robust information to the brain during primate locomotion, suggesting that context-dependent modulation instead occurs centrally to ensure that coding is consistent with behavioral goals during locomotion.

Using experimental and computational approaches the authors show that the vestibular efferent system does not modulate peripheral coding during locomotion. Instead, vestibular afferents unambiguously convey information in a context independent manner.

Effects of Efferent Activity on Hair Bundle Mechanics

Hair cells in both the auditory and vestibular systems receive efferent innervation. A number of prior studies have indicated that efferent regulation serves to diminish the overall sensitivity of the auditory system. The efferent pathway is believed to affect the sensitivity and frequency selectivity of the hair cell by modulating its membrane potential. However, its effect on the mechanical response of the hair cell has not been established. We explored how stimulation of the efferent neurons affects the mechanical responsiveness of an individual hair bundle. We tested this effect on in vitro preparations of hair cells in the sacculi of American bullfrogs of both genders. Efferent stimulation routinely resulted in an immediate increase of the frequency of hair bundle spontaneous oscillations for the duration of the stimulus. Enlarging the stimulus amplitude and pulse length, or conversely, decreasing the interpulse interval led to oscillation suppression. Additionally, we tested the effects of efference on the hair bundle response to mechanical stimulation. The receptive field maps of hair cells undergoing efferent actuation demonstrated an overall desensitization with respect to those of unstimulated cells.SIGNIFICANCE STATEMENT The efferent system is an important aide for the performance of the auditory system. It has been seen to contribute to sound detection and localization, ototoxicity prevention, and speech comprehension. Although measurements have demonstrated that efference suppresses basilar membrane movement, there is still much unknown about how efferent activity affects hearing mechanics. Here, we explore the mechanical basis for the efferent system's capabilities at the level of the hair bundle. We present optical recordings, receptive field maps, and sensitivity curves that show a hair bundle is desensitized by efferent stimulation. This supports the hypothesis that efferent regulation may be a biological control parameter for tuning the hair bundle's mechanical sensitivity.

Efferent control of the electrical and mechanical properties of hair cells in the bullfrog's sacculus

Background

Hair cells in the auditory, vestibular, and lateral-line systems respond to mechanical stimulation and transmit information to afferent nerve fibers. The sensitivity of mechanoelectrical transduction is modulated by the efferent pathway, whose activity usually reduces the responsiveness of hair cells. The basis of this effect remains unknown.

Methodology and Principal Findings

We employed immunocytological, electrophysiological, and micromechanical approaches to characterize the anatomy of efferent innervation and the effect of efferent activity on the electrical and mechanical properties of hair cells in the bullfrog's sacculus. We found that efferent fibers form extensive synaptic terminals on all macular and extramacular hair cells. Macular hair cells expressing the Ca²⁺-buffering protein calretinin contain half as many synaptic ribbons and are innervated by twice as many efferent terminals as calretinin-negative hair cells. Efferent activity elicits inhibitory postsynaptic potentials in hair cells and thus inhibits their electrical resonance. In hair cells that exhibit spiking activity, efferent stimulation suppresses the generation of action potentials. Finally, efferent activity triggers a displacement of the hair bundle's resting position.

Conclusions and Significance

The hair cells of the bullfrog's sacculus receive a rich efferent innervation with the heaviest projection to calretinin-containing cells. Stimulation of efferent axons desensitizes the hair cells and suppresses their spiking activity. Although efferent activation influences mechanoelectrical transduction, the mechanical effects on hair bundles are inconsistent.

Response of vestibular nerve afferents innervating utricle and saccule during passive and active translations

The distinction between sensory inputs that are a consequence of our own actions from those that result from changes in the external world is essential for perceptual stability and accurate motor control. In this study, we investigated whether linear translations are encoded similarly during active and passive translations by the otolith system. Vestibular nerve afferents innervating the saccule or utricle were recorded in alert macaques. Single unit responses were compared during passive whole body, passive head-on-body, and active head-on-body translations (vertical, fore-aft, or lateral) to assess the relative influence of neck proprioceptive and efference copy-related signals on translational coding. The response dynamics of utricular and saccular afferents were comparable and similarly encoded head translation during passive whole body versus head-on-body translations. Furthermore, when monkeys produced active head-on-body translations with comparable dynamics, the responses of both regular and irregular afferents remained comparable to those recorded during passive movements. Our findings refute the proposal that neck proprioceptive and/or efference copy inputs coded by the efferent system function to modulate the responses of the otolith afferents during active movements. We conclude that the vestibular periphery provides faithful information about linear movements of the head in the space coordinates, regardless of whether they are self- or externally generated.

Ultrastructural localization of ChAT-like immunoreactivity in the human vestibular periphery

Acetylcholine (ACh) has long been considered a neurotransmitter candidate in the efferent vestibular system of mammals. Recently, choline acetyltransferase (ChAT), the synthesizing enzyme for ACh, was immunocytochemically localized in all five end-organs of the rat vestibule (Kong et al. (1994) Hear. Res. 75, 192-200). However, there is little information in the literature concerning the cholinergic innervation in the vestibular periphery of man. In the present study the ultrastructural localization of the ChAT-like immunoreactivity in the human vestibular periphery was investigated in order to reveal the cholinergic innervation in the human vestibular end-organs. A modified method of pre-embedding immunoelectron microscopy was applied. It was found that the ChAT-like immunoreactivity was located in the bouton-type vesiculated nerve terminals in the vestibular neurosensory epithelia of man. These ChAT-like immunostained nerve terminals make synaptic contacts either with afferent chalices surrounding type I vestibular sensory hair cells, or with type II vestibular sensory hair cells. These results show that the ChAT-like immunoreactivity in the human vestibular periphery is confined to the efferent vestibular system. The ChAT-containing efferents innervate both type I hair cells and type II hair cells, making postsynaptic and presynaptic contacts, respectively. This study presents evidence that ACh is a neurotransmitter candidate in the efferent vestibular system of man.

Efferent neurons and vestibular cross talk in the frog

A galvanic stimulus (30- to 120-s, 0.3-mA constant current pulse) was used to depolarize the spike-generating region of horizontal and anterior canal afferent neurons. The galvanically induced spike activity from these neurons served as a driving input to the efferent vestibular system in the bullfrog. Efferent-mediated effects were assessed by intracellular recordings of posterior canal afferent spike activity, either ipsilateral or contralateral to the driving stimulus. Ipsilateral to the driving stimulus, efferent-mediated spike rate changes occurred in 62 (39%) of 158 posterior canal afferent neurons. Ipsilateral efferent-mediated effects were overwhelmingly excitatory (92%). Of responding units, 3% were inhibited during stimulus application and 5% showed mixed responses involving 3-20 s of inhibition followed by facilitation. Contralateral to the driving stimulus, efferent-mediated spike rate changes occurred in 18 (23%) of 77 posterior canal afferent neurons. Contralateral efferent-mediated effects were overwhelmingly inhibitory (95%). Only one unit was facilitated during stimulation and no mixed responses to contralateral stimulation were observed. Analysis of the coefficient of variation in interspike intervals (CV) before and during stimulation showed no significant efferent-mediated effects on spike train noise. Comparisons of resting spike rates between units showing efferent-mediated effects and those that did not were in general agreement with previous studies. Responding units had a lower mean spike rate (6.8 +/- 0.70 spikes/s, mean +/- SE) than did nonresponding units (10.7 +/- 0.42 spikes/s, mean +/- SE; P < 0.001; 2-tailed t-test of log-normalized data). Comparison between groups in the regularity of their resting spike rates, as quantified by CV, showed considerable overlap. When responding and nonresponding units with similar resting spike rates were compared, responding units had more irregular resting spike rates than did nonresponding units (P < 0.004; 2-tailed, paired t-test). In most cases (77%) the temporal pattern and general shapes of efferent-mediated responses mirrored the driving input of the galvanically activated afferent neurons. The other 23% of efferent-mediated responses exhibited a marked adaptation of the response. Adapting and nonadapting units were not significantly different in their mean resting spike rates or in the regularity of their resting spike rates.

The cholinergic pharmacology of the frog saccule

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

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.

Variations in receptor cell innervation in the saccule of a teleost fish ear

Using transmission electron microscopy, we quantitatively analyzed the afferent and efferent synapses on 67 sensory hair cells along the saccular epithelium of the oscar (Astronotus ocellatus), a cichild fish with a non-specialized ear. The synaptic profile (number of afferent and efferent synapses per cell) varied considerably among cells. The number of synapses per hair cell ranged from three to 24, and all but six of the 67 hair cells had both afferent and efferent synapses. Statistical analysis showed that the synaptic profiles did not significantly vary anywhere on the saccular epithelium except at the edges. There, hair cells had significantly fewer efferent synapses than hair cells in other epithelial regions. This statistical variation in efferent synapse distribution in different epithelial regions corresponds with the lengths of ciliary bundles in these regions. The synapses on hair cells showed a regional specificity in position. In all cells synapses were never located more apically than the top of the nucleus. On hair cells towards the periphery, the most apical synapse on the hair cells tended to be afferent. On more centrally located cells, the most apical synapse was efferent in 92% of the cells.

Behavior of primary horizontal canal neurons in alert and anesthetized guinea pigs

The activities of the primary horizontal canal neurons were investigated in alert and anesthetized guinea pigs. The average resting rate in the alert guinea pigs was 47.3 +/- 21.2 spikes/s. This value was significantly higher than that in anesthetized animals which had an average rate of 39.1 +/- 20.5 spikes/s. The average gain from the total of 66 units tested at 0.3-Hz sinusoidal rotation was 0.42 +/- 0.37 spikes/s/degree/s, and this value was the same as that of anesthetized guinea pigs. The average phase lag to the angular acceleration was 57.3 +/- 20.7 degrees in the alert guinea pigs, which was significantly smaller than that in the anesthetized animals. The number of cutoff neurons at 0.3 Hz was 1 out of 67 neurons in the alert guinea pigs and 7 out of 64 neurons in the anesthetized guinea pigs. The functional meaning of these differences in primary semicircular canal neurons is discussed.

Stimulus-specific long-term habituation of visually guided orienting behavior toward prey in toads: a 14C-2DG study

The regional distribution of cerebral glucose utilization, revealed by the 14C-2DG technique, was compared between (i) toads after stimulus-specific long-term habituation of the orienting response toward a repeatedly presented prey dummy ('habituation group') and (ii) non-habituated toads, readily orienting toward the repetitively presented prey stimulus ('naive group'). In the 'habituation group', the ventral medial pallium (vMP), a certain portion of the preoptic area (PO), and the dorsal hypothalamus (dHYP) showed a statistically significant increase in 14C-2DG-uptake; decrease was observed in the ventral layers of the optic tectum (vOT), a portion of the tegmental reticular formation (RET), the ventral cerebellum (vCB), and the striatum (STR). The results suggest that stimulus-specific long-term habituation of prey-catching affects both, components of the stimulus-response mediating circuit (e.g., involving OT), and structures extrinsic to it, (e.g., vMP, PO, dHYP), which may belong to a modulatory circuitry. Bilateral lesions to vMP strongly delay habituation. Our results are suggesting that damping of the adequate behavioral motor response during habituation involves active inhibitory processes of a modulatory system that develops in strength during stimulus repetition so as to suppress response output, which basically supports Sokolov's hypothesis (1975).

Presynaptic actions of cholinergic agents upon the hair cell-afferent fiber synapse in the vestibular labyrinth of the frog

Spontaneous activity of semicircular canal afferents in the isolated labyrinth of the frog is altered by bath application of cholinergic agonists. Muscarinic agonists can produce an increase in action potential frequency of individual afferents. This increase develops slowly and is prolonged in the time course of its action. Nicotinic agonists can either increase (most cases) or decrease afferent activity. These effects occur rapidly and decay during the period of activation, suggesting desensitization. Muscarinic effects are blocked by prior administration of atropine and nicotinic effects (both increases and decreases in action potential frequency) by curare. Intracellular recordings reveal that the nicotinic effects on afferent action potential frequency are the result of alterations in the frequency of spontaneous synaptic potentials, indicating a presynaptic site of action on the hair cells for these compounds. This conclusion is supported by the fact that in the presence of high Mg2+/low Ca2+, which blocks hair cell release of transmitter, cholinergic agonists do not affect the resting membrane potential of the vestibular afferent. Electrical stimulation of the VIIIth cranial nerve can result in either an increase or a decrease in spontaneous synaptic potential and action potential frequency of an afferent. These effects are blocked by prior administration of curare or of nicotinic agonists. Repetitive or continuous stimulation of the VIIIth nerve results in a reversible reduction of the evoked response, suggesting desensitization. Transection of the VIIIth cranial nerve two weeks prior to recording eliminates these actions of electrical stimulation, but not the responses to cholinergic agonists, indicating that the effects of electrical stimulation are mediated by centrally arising efferents. These findings confirm that acetylcholine is probably the transmitter released from centrally arising vestibular efferents, and, in addition, demonstrate that efferent-mediated effects are predominantly expressed through nicotinic receptors. Studies comparing the effects of isolation of the semicircular canal alone versus the intact labyrinth suggest that the method of isolation may be an important factor in determining whether efferent activity results in a predominant increase or decrease in afferent activity.

Curare and the efferent vestibular system. An electrophysiological study in the frog

In the frog we have studied the action of d-tubocurarine on the spontaneous discharge recorded from the whole horizontal semicircular canal (HC) nerve and from single HC afferent fibres. Gross activity was recorded in isolated head preparations. In each case, the spontaneous frequency was measured 6 min before and 16 min after a drop of D-tubocurarine (0.1 microliter, 5.10(-6)M) dissolved in Ringer or after a similar drop of Ringer was injected into the perilymph near the HC ampulla. Curare elicited a significant increase of the discharge frequency in about 60% of the preparations and a decrease in 6-8% of the cases, whether the contralateral eighth nerve was cut or not, while Ringer had no effect. After severing of the ipsilateral eighth nerve anterior branch, curare had no significant effect on the mean resting rate (calculated from 25 preparations), while in a few cases it evoked an increase or a decrease of the discharge frequency. Single afferent discharges were recorded in intact frogs. The mean frequency calculated from about 200 afferent fibres was 21.4 +/- 1.7 spikes/s after curare treatment, while it was only 15.6 +/- 1.3 spikes/s after Ringer injection; these two values are highly significantly different. From these results we conclude that the ampullary receptors are tonically subject to an inhibitory influence probably due to the tonic activity of the cholinergic efferent vestibular system. Moreover, the activity of the hair cells is probably also controlled by another, as yet unknown cholinergic mechanism.

Peripheral organization of the vestibular efferent system in the frog: an electrophysiological study

The distribution and the properties of efferent fibers in vestibular nerve were studied in the isolated frog labyrinth. Electrical stimulation of the central stump of any vestibular nerve branchlet elicited compound action potentials in all the other eighth nerve branchlets, indicating the existence of neural links between the various vestibular organs. The same experimental paradigm, when repeated in frogs with chronic section of the eighth nerve roots, demonstrated that these pathways are efferent collaterals extending to all vestibular organs. There are more collaterals linking the 3 semicircular canals than the otolith organs and the otoliths with the canal organs. Efferent connections in the eighth nerve were preserved in full after ablation of the ipsilateral hemi-cerebellum, suggesting that the efferent pathways probably originate in the brainstem. Intracellular recordings from single afferent fibers of both canal and otolith organs revealed that efferent fiber activation could elicit either inhibition or facilitation of the receptor discharge. It was concluded that the frog efferent vestibular system is endowed with non-selective control channels which allow single neurons to influence the receptor activity of different labyrinthine organs.

Clinical investigation of the efferent inhibition of the vestibular function

We have investigated the modulation of nystagmus in homo by rotating dizzy patients with constantly increasing acceleration according to a certain computerized scheme or by oscillation with a triangular waveform. We found that the modulation varied greatly in different patients. We saw all variations between hyperreflexive patients, completely lacking inhibition and are flexive patients with a hyperactive inhibition which, however, could be broken by asking the patient to count backwards simultaneously. Habituation of hyperreflexive patients completely lacking their inhibition allowed a normal capacity to habituate. This indicates that there are two different inhibition systems, one fast, appearing during intensive nystagmus and showing a short duration, short time constant (STC), high gain and high reaction readiness, and another type, which is slow and habituating and coming first after iterated stimuli, having a long duration, long time constant (LTC), low initial gain and low reaction readiness. The first type seems to be initiated in higher centres, the second has its effect on the peripheral labyrinth.

Responses of afferent and efferent neurons to visual inputs in the vestibular nerve of the frog

In the frog immobilized by intralymphatic injection of D-tubocurarine, stimulation of the visual apparatus with either electrical shocks applied to the optic chiasma or light pulses elicited, in many cases, an increase and a decrease of firing of efferent and afferent vestibular neurons, respectively, recorded from the horizontal semicircular canal nerve. Optokinetic stimulation was completely inefficient in modulating the efferent and afferent discharge. These results show that stimulation of the visual system can modify vestibular apparatus fuctioning at the most peripheral level. However, it is likely that the effects observed were due to an arousal phenomenon or/and to a motor corollary discharge.

Tonic influence from one labyrinth onto the contralateral one. An electrophysiological study in the frog

In the frog's isolated head preparations, spontaneous activity was recorded from the whole nerve of the left horizontal semicircular canal (HC) for 6 min before and 16 min after destruction of the right labyrinth by heating or administration of d-tubocurarine (0.5 microliter, 5.10(-6)M) into the perilymph of the right labyrinth. Just after destruction on the right labyrinth, spontaneous activity of the left HC nerve abruptly increased by 20-400% in 24 preparations out of the 40 studied; activity then increased slowly and regularly and in most cases reached a steady level. In the 16 other preparations such destruction had no effect (15 preparations) or elicited a slight decrease of the HC nerve activity (one preparation). After curare administration, the spontaneous activity of the left HC nerve decreased by 20-100% in 27 preparations out of the 40 studied; in most cases such a decrease was reversed 25-80 min after administration of the drug. The spontaneous discharges were unaffected in 11 preparations and slightly increased in the two others. Destruction of the right labyrinth or administration of curare never modified spontaneous activity recorded from the left HC nerve when the connections between the two labyrinths had been interrupted either by sagittal section of the medulla oblongata or section of the right vestibular nerve close to the brain stem. These results demonstrate that one labyrinth has a tonic inhibitory influence on the contralateral one.

Origin of centrifugal fibers to the labyrinth in the frog (Rana esculenta). A study with the fluorescent retrograde neuronal tracer 'Fast blue'

After injecting a solution of a fluorescent retrograde neuronal tracer (Fast blue, Diamidino compound 253/50) into the perilymphatic space of the frog labyrinth (Rana esculenta), labeled cells were found in the ventral and dorsal nuclei of the VIIIth nerve and in the nucleus reticularis medius. We consider these labeled cells to be the origin of the efferent innervation of the frog labyrinth. No evidence was found for the existence of a direct cerebello-labyrinthine connection.

Effects of efferent stimulation on the saccule of goldfish

1. The effect of single or repetitive stimulation applied to efferent nerve fibres on afferent nerve activity and microphonic potentials was studied in the saccule of goldfish. 2. The sound-evoked excitatory post-synaptic potentials (e.p.s.p.s) recorded intracellularly from afferent eighth nerve fibres were reduced in size or completely abolished by efferent stimulation. The maximum inhibitory effect produced by repetitive efferent stimulation was equivalent to reducing the sound intensity by 10-25 db. Spontaneous miniature e.p.s.p.s were also suppressed by efferent stimulation. 3. The effect of single efferent stimulation appeared with a delay of 607 msec and lasted for about 40 msec, reaching its peak at about 12 msec. The slow and prolonged time course makes a sharp contrast with the very fast time course of afferent synaptic action. 4. The application of hyperpolarizing current through the recording micro-electrode revealed no sign of a post-synaptic increase in membrane conductance during inhibition. Hence, the inhibition was mostly attributable to a presynaptic action, i.e. to a suppression of transmitter release from hair cells. 5. Individual e.p.s.p.s were evoked in response to each wave of sound without any change in latency, but reached their peak much earlier during inhibition than in the control period. A likely explanation for this finding is that transmitter is released from hair cells during inhibition only in the early part of the stimulatory phase of the sound wave. 6. The extracellularly recorded microphonic potentials showed a slight increase in amplitude during efferent stimulation. 7. The nature and site of action of efferent nerve action are discussed along with some drug effects.

Efferent activity in the goldfish vestibular nerve and its influence on afferent activity

Out of 326 fibres in the horizontal semicircular canal branch of the goldfish vestibular nerve, 7 fibres could be identified as efferents. They showed irregular spontaneous activity and responded to rotatory stimuli with double frequency. Additionally in the central stump of the dissected nerve, efferent fibres were found, the spontaneous and stimulus modulated activity of which could not be differentiated from afferents. Efferents could be driven by a number of stimuli (vestibular, visual, somatosensory). Disruption of the efferent influence upon the receptors by dissection of the nerve or by pharmacological means (Gallamine) led to an increase of spontaneous afferent activity by 50%, showing that there is tonic efferent inhibition. Transfer functions of afferents were not changed after release from efferent influence. Electrical stimulation of efferents in 41% of the fibres led to an increase of afferent activity instead of the expected inhibition, which was seen in another 32%.

Activation of the efferent system in the isolated frog labyrinth: effects on the afferent EPSPs and spike discharge recorded from single fibres of the posterior nerve

Intra-axonal recordings were obtained from single afferent fibres of the posterior nerve in the isolated labyrinth of the frog (Rana esculenta). EPSPs and spike discharge were recorded both at rest and during rotatory stimulation of the canal. Electrical stimulation of either the distal end of the cut posterior nerve or of the central stumps of the anterior-horizontal nerves elicited a frequency-dependent inhibitory effect on the afferent discharge arising from the posterior canal. Denervation experiments revealed that inhibition is mediated by efferent fibres exhibiting a high degree of branching in the proximal part of the eighth nerve. The inhibitory effect was selectively cancelled by (1)D-tubocurarine 10(-6) M; (2) atropine 5 x 10(-5) M; (3) acetylcholine or carbachol 10(-4) M; (4) eserine 10(-5) M. Inhibition is thus most likely to be sustained by the release of acetylcholine from the efferent nerve terminals. Experiments in which the ionic composition of the external medium was modified suggest that the transmitter acts mainly by opening the chloride ion channels of the hair cell membrane. In some units the same stimulation pattern evoked a consistent increase in both EPSP and spike discharge, instead of inhibition. Such facilitation was unaffected by drugs for ionic modifications which block the efferent synapse, but disappeared after denervation. Inhibition and facilitation, therefore, act as two control mechanisms which are able to modify substantially, at the first stage of processing, the sensory information which is sent to the vestibular second order neurons.

Mechanisms of compensation for vestibular deficits in the frog. II. Modification of the inhibitory Pathways

In hemilabyrinthectomized frogs inhibitory responses of central vestibular neurons to electrical stimulation of the remaining vestibular nerve were recorded extra- and intracellularly at different stages (0, 3, and 60 days) after the operation. In acute animals inhibition of vestibular neurons following stimulation of the VIIIth nerve is rarely observed. In chronic animals about 30% of the vestibular neurons on the partially deafferented side and about 15% of the vestibular neurons on the intact side are inhibited. The distribution of the latencies of these inhibitory responses is bimodal with ranges from 4 to 14 ms and 18 to 24 ms. Removal of the cerebellum reduced the number of inhibited vistibular neurons and picrotoxin abolished all inhibitory responses. The vestibular input to the cerebellar dorsal rim is bilateral. In chornic animals excitation of Purkinje cells was similar as in acute preparations but many more cells were disfacilitated. Inhibition of partially deafferented vestibular neurons by cerebellar and brain stem neurons increases in parallel with their excitatory commissural input. The balance between these plastic changes may be crucial for the functional recovery of appropriate compensatory reflexes.

A mechanism for type III vestibular responses of frog cerebellar Purkinje cells

Type III Purkinje cells (P-cells), which are excited with both directions of horizontal rotation, are found in high numbers in the frog auricular lobe and adjacent cerebellar areas. To examine the mechanisms underlying these responses, recordings were made from P-cells in curarized animals during rotational stimulation of the horizontal canals. The horizontal canal input to these cells was then modified unilaterally by VIIth nerve section, intraperilymphatic injection of local anesthetic, or by caloric stimulation. Control recordings were also obtained from peripheral canal neurons. Type III responses were abolished by unilateral lesions or reversible blockage of the VIIIth nerve with local anesthetic. The remaining responses were attributable only to the unaffected horizontal canal, ie. only type II or type I responses were observed upon interruption of the ipsi-or contralateral nerve, respectively. The level of spontaneous activity of cerebellar input fibers was low and during rotation produced 'cell silencing' response waveform asymmetries (facilitation greater than disfacilitation). When the level of peripheral resting activity was increased (warm water irrigation), thereby increasing horizontal canal response symmetry, type III responses were reduced in magnitude or abolished. Conversely, cold water irrigation, which decreases the resting rate and response symmetry of input fibers, enhanced type III response magnitudes. On the basis of these results, it is suggested that type III responses result from the fact that single P-cells receive a facilitatory input from both horizontal canals. Since these inputs are 180 degrees phase-reversed and their response waveforms asymmetrical, their resulting postsynaptic effect is a net excitation during both portions of the stimulus cycle.

Neurophysiology of Spastic, a behavior mutant of the mexican axoloti: altered vestibular projection to cerebellar auricle and area acoustico-lateralis

The spastic mutant of Ambystoma mexicanum shows deficiencies in swimming coordination and equilibrium. Behavior "phenocopy" experiments done previously indicated that vestibular projections to cerebellum and hindbrain interneurons might be responsible for mutant behavior patterns. To test function in mutant vestibular projections, single unit recordings were carried out in the vestibulo-cerebellum (auricle) and hindbrain area acoustico-lateralis (AAY) of wild-type and mutant animals in response to natural vestibular stimulation. Vestibular unit types responding during longitudinal tilting or to sustained tilt were encountered in equal proportions in both animal types. However, mutants showed a significant increase in spontaneously active units in these areas indicating possible deficiencies in inhibitory circuitry. In addition, the topographic location of vestibular units changed under the influence of the spastic gene. In mutants, significant numbers of units were found "translocated" into a ventro-caudal auricular zone abutting the AAL. Anatomical studies detailed in the following paper have shown this same area to contain grossly "translocated" cerebellar cells and afferent fiber tracts in mutants. These data are drawn together in a model in which deficiencies in the (form and) inhibitory function of the vestibulo-cerebellum is postulated to underly the behavioral abnormalities of the spastic phenotype.

Functional characterization of primary vestibular afferents in the frog

1. In order to more accurately identify the nature of the vestibular input to central neurons, the response properties of single semicircular canal and otolith units in the frog VIIth nerve were studied in curarized preparations. 2. An equation describing the response plane was calculated for each canal on the basis of null point measurements. These results show that the ipsilateral canal planes are orthogonal within 2-5 degrees, and the pairs of right-left synergists are essentially coplanar. A head position of 10-20 degrees maxilla nose up produces optimal horizontal canal and minimal vertical canal activation with horizontal rotation. 3. The frequency response of the horizontal canal was examined in the range 0.025-0.5 Hz. Comparatively shorter phase-lags and a 10 fold greater acceleration gain in this frequency range distinguish the frog from the mammalian species studied. 4. Otolithic responses were tonic, phasic-tonic, and phasic in nature. The preponderance of the latter two groups is stressed (94%). Tonic responses were proportional to the gravitational vector change. Phasic responses were proportional to velocity during transitions in head position and phase-led displacement (30-80%) with sinusoidal acceleration in roll and pitch. 5. Efferent vestibular neurons respond to rotation in the horizontal (usually Type III) as well as vertical planes. Responses in the vertical planes result from canal and/or otolithic input to these neurons indicating that the vestibular efferent system receives extensive multi-labyrinthine convergence.

Inhibition by efferent nerve fibres: action on hair cells and afferent synaptic transmission in the lateral line canal organ of the burbot Lota lota

1. Intracellular recordings were made from morphologically identified hair cells in the lateral line canal organs of the burbot Lota lota. 2. I.p.s.p.s were recorded from hair cells when the efferent fibres were excited by electrical stimulation of the lateral line nerve. The i.p.s.p.s were abolished when the fish was injected with immobilizing concentration of Flaxedil which is known to block the efferent synapses. 3. The i.p.s.p.s are accompanied by a decrease in the resistance of the hair cell membrane and an increase in the intracellular receptor potential. 4. Spontaneous and mechanically evoked e.p.s.p.s which were recorded intracellularly from the post-synaptic afferent nerve terminals were reduced in amplitude for the duration of the i.p.s.p.

Cerebellar control of vestibular neurons of the frog

Intra- and extracellular potentials were recorded from vestibular neurons of the frog following stimulation of the ipsilateral cerebellar cortex (auricular lobe). Single neurons were identified as vestibular neurons by their location within the borders of the vestibular nuclei and by their activation via stimulation of the anterior branch of the VIIIth nerve. 2. Stimulation of the cerebellar auricular lobe provoked in some vestibular neurons apparently pure IPSPs with latencies as short as 1.5 msec suggesting a monosynaptic linkage between Purkinje cells and vestibular neuron. Vestibular-evoked responses were inhibited by cerebellar conditioning stimuli for as long as 50 msec. 3. Cerebellar stimulation also elicited an EPSP-IPSP sequence or occasionally pure EPSPs in vestibular neurons. These EPSPs were probably generated by axon reflex activation via the excitation of axons of primary and secondary vestibular neurons terminating in the cerebellum since they persisted after chronic VIIIth nerve section. Some vestibular evoked resonses were facilitated by cerebellar stimuli for 40-50 msec. 4. Occasionally an inhibitory effect was also induced in vestibular neurons by paired ipsilateral VIIIth nerve stimulation; it's latency of onset and time course suggested that a vestibulo-cerebello-vestibular loop was mainly responsible for the inhibition.

Experimental studies on the nerve--sensory cell relationship during degeneration and regeneration in ampullar nerves of the frog labyrinth

The degeneration and regeneration of afferent and efferent nerves to the crista ampullaris in the frog were studied after transsection of the ampullar nerves. In some experiments one or both rostral ampullar nerves were simply divided and left in place. In other experiments the ampullar nerves to the horizontal and to the anterior vertical crista were divided and the branch to the former was brought into contact with the distal stump of the anterior vertical ampullar nerve. Sensory cells in the labyrinth survived total denervation for 1-2 weeks without regular signs of ultrastructural damage. The afferent synaptic structures in these cells were often preserved in the denervated stage. The afferent nerve fibres degenerated earlier than the efferent ones. Both type of fibres reinnervated the vestibular epithelium after division of the ampullar nerve. The fibres from a horizontal ampullar nerve could reinnervate a denervated anterior vertical crista, resulting in misdirected reflexes. About 40 days after division both function and morphological organization had been restored to normal.

The post-synaptic action of efferent fibres in the lateral line organ of the burbot Lota lota

1. The post-synaptic action of efferent fibres on lateral line organs in the burbot was investigated with extracellular electrodes.2. Selective excitation of efferent fibres or stimulation of the lateral line nerve causes a negative efferent potential in the epithelium, an increase in the microphonic potential, and inhibition of afferent nerve impulses.3. Stimulation of the lateral line nerve causes antidromic impulses in the afferent fibres in addition to exciting the efferent fibres. However, stimulation of the lateral line nerve has no post-synaptic influence on lateral line organs when the efferent synapses are selectively blocked by gallamine.4. The influence of efferent stimulation on the microphonic potential is greater at large amplitudes of mechanical stimulation than at low.5. The similarities between the effects of efferent stimulation in the lateral line organ and in the cochlea suggest similar synaptic mechanisms.