Tonic conjugated action of receptor-receptor fibers and of the efferent vestibular system on the spontaneous afferene activity of a semicircular canal in the frog]

In the frog, the joint action of non-afferent vestibular systems [i.e. the efferent vestibular system and the receptor-receptor fibre system] on the afferent vestibular activity is null or very small. The receptor-receptor fibre system being inhibitory, it seems that the efferent vestibular system as a whole is facilitatory, which agrees with previous results.     

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

Summary In the frog, the spontaneous discharges of afferent fibres from the horizontal semicircular canal (HC) and of efferent vestibular units were recorded by means of glass micropipettes filled with 2 mol/l NaCl as well as during acoustic stimulation; pure tones 300–2,000 Hz and clicks 150/s, 80–100 dB re 10^–5 N/m² were used. The activity of 56% of the efferent fibres recorded was increased by such stimulations while the discharge of the others was not modified. In intact preparations the activity of 34.4% of the afferent fibres recorded was either increased or decreased by sound stimulation depending on the unit; the discharge of the others (65.6%) was not modified (Fig. 3). Section of both saccular nerves did not change the percentage of the units modulated by sound showing that the saccules have probably no effect on this modulation (Fig. 4). In preparations where the contralateral auditory papillae were eliminated, 21.1% of the afferent units were facilitated and no unit was inhibited (Fig. 5), while in preparations where the ipsilateral auditory organs were eliminated 21.1% of the afferent units were inhibited and no unit was facilitated (Fig. 6). Therefore, in intact preparations one can assume that decrease and increase of the HC afferent fibre discharges were due to stimulation of the contralateral and the ipsilateral auditory organs, respectively. Such a modulation of canal afferent discharges being mediated by efferent vestibular fibres, it can be postulated that the efferent vestibular system has a double influence upon the hair cells of the vestibular epithelium: one inhibitory and the other facilitatory. Such a double effect is discussed.Abbreviations EVS efferent vestibular system - HC horizontal semicircular canal     

Interrelations between auditory and vestibular receptors in the frog's labyrinth

Summary The influence of the efferent vestibular system being eliminated, the spontaneous activity of afferent fibres of the ampullary nerves of the horizontal and vertical anterior semicircular canals was recorded in the frog. By functionally eliminating either both papillae or all the vestibular receptors except for the papillae, and then using statistical methods, as well as by stimulating the papillae by sounds or the papillary nerve fibres by electrical stimulus, it has been shown that the auditory papillae have a facilitatory influence on the spontaneous afferent activity from the horizontal and vertical anterior canals. This influence is most likely mediated by receptor-receptor fibres arising from the auditory organs and innervating the semicircular canals.Abbreviations HC horizontal canal - VAC vertical anterior canal This research was supported by a grant from D.G.R.S.T. (Aide à la Recherche n 77.7.1127)     

Efferent neurons of the lateral-line system and the VIII cranial nerve in the brainstem of anurans

Summary The octavo-lateral efferent system of several anuran species was studied by means of retrograde transport of horseradish peroxidase. This system is organized similarly in all larval anurans and in all adult aglossids. All have two groups of efferent neurons in the nucleus reticularis medialis between the VIIIth and the IXth motor nucleus. The caudal group consists of efferent neurons that supply the posterior lateral-line nerve (NLLp) and a considerably smaller group of neurons supplying both the NLLp and the anterior lateral-line nerve (NLLa). The rostral group is composed of efferent neurons supplying the NLLa, neurons projecting to the inner ear and neurons supplying both the inner ear and the NLLa. Efferent neurons of the VIIIth cranial nerve exhibit a rostrocaudal cytoarchitectonic differentiation. Caudal perikarya, which are rounder in shape than those of the rostral part, have a dendritic projection to the superior olive. It is suggested that this differentiation reflects a functional differentiation of acoustic and vestibular efferent neurons.Labeled neurons were ipsilateral to the site of application of HRP. None were found in the vestibular nuclei or in the cerebellum.Efferent axons projecting to neuromasts of the NLLa leave the medulla with the VIIth nerve, axons projecting to neuromasts of the NLLp exit via the IXth nerve. Cell counts and the observation of axonal branching revealed that efferent units of both the lateral-line and the VIIIth-nerve system supply more than one receptor organ. In contrast to the lateral-line system, dendrites of efferent neurons of the VIIIth nerve project dorsally onto its nuclei, and afferents of the VIIIth nerve project onto efferent neurons. These structures most probably represent a feedback loop between the afferent and efferent systems of the VIIIth cranial nerve.     

Bilateral otolith contribution to spatial coding in the vestibular system

Recent work on the coding of spatial information in central otolith neurons has significantly advanced our knowledge of signal transformation from head-fixed otolith coordinates to space-centered coordinates during motion. In this review, emphasis is placed on the neural mechanisms by which signals generated at the bilateral labyrinths are recognized as gravity-dependent spatial information and in turn as substrate for otolithic reflexes. We first focus on the spatiotemporal neuronal response patterns (i.e. one- and two-dimensional neurons) to pure otolith stimulation, as assessed by single unit recording from the vestibular nucleus in labyrinth-intact animals. These spatiotemporal features are also analyzed in association with other electrophysiological properties to evaluate their role in the central construction of a spatial frame of reference in the otolith system. Data derived from animals with elimination of inputs from one labyrinth then provide evidence that during vestibular stimulation signals arising from a single utricle are operative at the level of both the ipsilateral and contralateral vestibular nuclei. Hemilabyrinthectomy also revealed neural asymmetries in spontaneous activity, response dynamics and spatial coding behavior between neuronal subpopulations on the two sides and as a result suggested a segregation of otolith signals reaching the ipsilateral and contralateral vestibular nuclei. Recent studies have confirmed and extended previous observations that the recovery of resting activity within the vestibular nuclear complex during vestibular compensation is related to changes in both intrinsic membrane properties and capacities to respond to extracellular factors. The bilateral imbalance provides the basis for deranged spatial coding and motor deficits accompanying hemilabyrinthectomy. Taken together, these experimental findings indicate that in the normal state converging inputs from bilateral vestibular labyrinths are essential to spatiotemporal signal transformation at the central otolith neurons during low-frequency head movements.     

Influence of the efferent vestibular system on vestibulo-spinal reflexes in the guinea pig

The influence of the efferent vestibular system on vestibulo-spinal activity was investigated during experiments on guinea pigs decerebrated and following cerebellar extirpation at precollincular level. Efferent vestibular neurons forming compact groups ventromedially to the vestibular nuclei were excited by means of electrical stimulation. Electromyographic activity in the triceps brachii extensor muscles of the right and left forelimbs was adopted as a test reaction (crossed extensor reflex and locomotor activity produced by stimulating the mesencephalic locomotor region). Adequate stimulation of the vestibular apparatus was accomplished by static tilting and cyclic shifting of the animal around its longitudinal axis at angles of ±20°. The efferent vestibular system was found to exert a bilateral inhibitory action on vestibulo-spinal activity. Vestibular efferent stimulation produced a reduction in the intensity of vestibulo-fugal influences: it does not change the dynamics of vestibulo-spinal reflex effects, however. Mechanisms of vestibular efferent action on vestibular control of spinal motor activity are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 1, pp. 78–86, January–February, 1989.     

Choline acetyltransferase immunoreactivity in the human vestibular end-organs

Acetylcholine (ACh) is believed to play a major role in the efferent vestibular system in several animal models, however no information regarding the role of ACh in the human efferent vestibular system has been published. Post-embedding immunohistochemistry in a hydrophilic resin was used to investigate the choline acetyltransferase immunoreactivity (ChATi) and acetylcholinesterase (ACHE) histochemistry in human vestibular end-organs. ChATi and AChE activity was found in numerous bouton-type terminals at the basal area of the vestibular hair cells. These terminals were found to contact type II vestibular hair cells and the afferent chalices surrounding type I hair cells. This study provides the first evidence that the human efferent vestibular axons and terminals are cholinergic.     

Chemosensory integration in the spiny lobster: Ascending activity in the olfactory-globular tract

Summary In the frog,Rana esculenta, when the influence of the efferent vestibular system was eliminated, the spontaneous activity of single afferent fibres recorded from one branch of the nerve of the horizontal semicircular canal (HC) or of the nerve of the vertical anterior canal (VAC) was inhibited in 16–17% of the cases when stimulating electrically the other branch of the same ampullary nerve. Moreover, the spontaneous activity of about 200 afferent fibres was recorded from the nerves of the HC and VAC in three experimental situations. In the first one, the brain was destroyed, or the left vestibular nerve cut as it enters the brain stem, and all the branches of the left vestibular nerve were cut except for the one recorded (VAC or HC nerve); in the second one, recordings were made on the peripheral end of the ampullary nerve previously cut near the ampulla; in the third situation they were made on the ampullary nerve after having cut the vestibular nerve between the periphery and Scarpa's ganglion close to Scarpa's ganglion. Statistical comparisons of the distribution of the spontaneous frequencies and of the mean activities between the experimental situations show that the activities were greater in the second or third experimental situations than in the first one. These results could be explained by the existence of an inhibitory feedback loop outside the brain including Scarpa's ganglion and mediated by receptor-receptor fibres.Abbreviations HC horizontal semicircular canal - PE peripheral end of the ampullary nerve - VAC vertical anterior semicircular canal This research was supported by a grant from D.G.R.S.T. (Aide à la Recherche n 77.7.1127)     

Renorenal reflexes: interaction between efferent and afferent renal nerve activity.

In rats, stimulation of renal mechanoreceptors by increasing ureteral pressure results in a contralateral inhibitory renorenal reflex response consisting of increases in ipsilateral afferent renal nerve activity, decreases in contralateral efferent renal nerve activity, and increases in contralateral urine flow rate and urinary sodium excretion. Mean arterial pressure is unchanged. To study possible functional central interaction among the afferent renal nerves and the aortic and carotid sinus nerves, the responses to renal mechanoreceptor stimulation were compared in sinoaortic denervated rats and sham-denervated rats before and after vagotomy. In contrast to sham-denervated rats, there was an increase in mean arterial pressure in response to renal mechanoreceptor stimulation in sinoaortic-denervated rats. However, there were no differences in the renorenal reflex responses among the groups. Thus, our data failed to support a functional central interaction among the renal, carotid sinus, and aortic afferent nerves in the renorenal reflex response to renal mechanoreceptor stimulation. Studies to examine peripheral interaction between efferent and afferent renal nerves showed that marked reduction in efferent renal nerve activity produced by spinal cord section at T6, ganglionic blockade, volume expansion, or stretch of the junction of superior vena cava and right atrium abolished the responses in afferent renal nerve activity and contralateral renal function to renal mechanoreceptor stimulation. Conversely, increases in efferent renal nerve activity caused by thermal cutaneous stimulation increased basal afferent renal nerve activity and its responses to renal mechanoreceptor stimulation. These data suggest a facilitatory role of efferent renal nerves on renal sensory receptors.     

The corticofugal pathways of the visual system

Cortical neurons belonging to the same topological ensemble send axons to thalamic and mesencephalic structures and also to contra and ipsilateral cortical areas. The projections are called the corticofugal system. This review addresses the organization and the functions of the efferent cortical fibers within the visual network. For example, the cortico-geniculate fibers participate in shaping the structure of the concentric receptive fields of geniculate cells. Namely, the size of the surround area depends on descending impulses from the cortex. By contrast, cortico-mesencephalic fibers have a more global influence on visual responses. Following the interruption of cortical activity all responses to visual stimuli decline; although in rodents and lagomorphs cortical inactivation does not eliminate those visual responses that are sent to the superior colliculus or pretectum directly from the retina. In each hemisphere it has been demonstrated that contra-lateral cortico-cortical fibers participate in the continuity of the two visual hemi-fields, as the interruption of the callosal impulses results in a truncated field in which the contralateral part of the receptive field is missing., overlaps the vertical meridian is missing. Finally, ipsilateral cortico-cortical fibers allow a consolidation of visual properties of cortical cells. It must be added that there are considerable differences among species in the organization of cortico-cortical relationships. However, this survey seems to indicate that all corticofugal axons are excitatory.     

Eye movements and brainstem neuronal responses evoked by cerebellar and vestibular stimulation in chicks

Summary The vestibulo-ocular reflex undergoes adaptive changes that require inputs from the cerebellar flocculus onto brainstem vestibular neurons. As a step toward developing an in vitro preparation in chicks for studying the synaptic basis of those changes, we have elucidated the organization of the pathways through which the flocculus influences vestibulo-ocular movements. Electrical stimulation of the vestibular ampulla evoked brief, contralaterally directed movements in both eyes. Although single current pulses to the flocculus elicited no response, conjunctive stimulation of the flocculus and the vestibular apparatus significantly reduced the vestibularly-evoked movement. Trains of current pulses applied to the flocculus and ampulla evoked eye movements directed toward and away from the side of stimulation, respectively. Recordings from the brainstem revealed neurons that were activated by ipsilateral vestibular stimulation and inhibited by ipsilateral floccular stimulation. Our sample included neurons in the lateral vestibular nucleus, the ventrolateral portion of the medial vestibular nucleus, and the superior vestibular nucleus. Similarities between these findings and those of similar studies in mammals indicate that the chick will provide a good model system for cellular studies of adaptive changes in the vestibulo-ocular reflex.Abbreviations FTN flocculus target neuron - VOR vestibuloocular reflex     

Expression of Calcitonin Gene-Related Peptide in Efferent Vestibular System and Vestibular Nucleus in Rats with Motion Sickness

Motion sickness presents a challenge due to its high incidence and unknown pathogenesis although it is a known fact that a functioning vestibular system is essential for the perception of motion sickness. Recent studies show that the efferent vestibular neurons contain calcitonin gene-related peptide (CGRP). It is a possibility that the CGRP immunoreactivity (CGRPi) fibers of the efferent vestibular system modulate primary afferent input into the central nervous system; thus, making it likely that CGRP plays a key role in motion sickness. To elucidate the relationship between motion sickness and CGRP, the effects of CGRP on the vestibular efferent nucleus and the vestibular nucleus were investigated in rats with motion sickness.

Methods

An animal model of motion sickness was created by subjecting rats to rotary stimulation for 30 minutes via a trapezoidal stimulation pattern. The number of CGRPi neurons in the vestibular efferent nucleus at the level of the facial nerve genu and the expression level of CGRPi in the vestibular nucleus of rats were measured. Using the ABC method of immunohistochemistry technique, measurements were taken before and after rotary stimulation. The effects of anisodamine on the expression of CGRP in the vestibular efferent nucleus and the vestibular nucleus of rats with motion sickness were also investigated.

Results and Discussion

Both the number of CGRPi neurons in the vestibular efferent nucleus and expression level in the vestibular nucleus increased significantly in rats with motion sickness compared to that of controls. The increase of CGRP expression in rats subjected to rotary stimulation 3 times was greater than those having only one-time stimulation. Administration of anisodamine decreased the expression of CGRP within the vestibular efferent nucleus and the vestibular nucleus in rats subjected to rotary stimulation. In conclusion, CGRP possibly plays a role in motion sickness and its mechanism merits further investigation.     

Ultrastructural features of the synaptic complex of the vestibular nuclei of Lampetra planeri (Bloch)

Summary The vestibular fibres of ammocoetes of Lampetra planeri form a peculiar type of synaptic contact with the large nerve cells of the vestibular nuclei. In fact, a large expansion of the fibre is enveloped by the nerve cell cytoplasm, which makes a sort of spoon or actual channel by sealing its own plasma membrane with adhesion plaques around the fibre.In the whole area of juxtapposition between the fibre and the cell membranes, three different types of specialized contacts occur: desmosomoid junctions, which are the most numerous; zones having the characteristics of chemically active synapses on account of the presence of vesicles clustered on the presynaptic side; gap-junctions, where electrical transmission might occur. A puzzling feature is the presence of an enormous number of presynaptic vesicles located in the central part of the axoplasm of the fibre expansion.Supported in part by a grant to the Centre of Neuroembryology from the National Research Council.We wish to thank Prof. Aldo Rossi for supplying the perfusion technique and Mr. Dino Scorsini for his skilful technical assistance.     

Development of the inner ear efferent system across vertebrate species

Inner ear efferent neurons are part of a descending centrifugal pathway from the hindbrain known across vertebrates as the octavolateralis efferent system. This centrifugal pathway terminates on either sensory hair cells or eighth nerve ganglion cells. Most studies of efferent development have used either avian or mammalian models. Recent studies suggest that prevailing notions of the development of efferent innervation need to be revised. In birds, efferents reside in a single, diffuse nucleus, but segregate according to vestibular or cochlear projections. In mammals, the auditory and vestibular efferents are completely separate. Cochlear efferents can be divided into at least two distinct, descending medial and lateral pathways. During development, inner ear efferents appear to be a specific motor neuron phenotype, but unlike motor neurons have contralateral projections, innervate sensory targets, and, at least in mammals, also express noncholinergic neurotransmitters. Contrary to prevailing views, newer data suggest that medial efferent neurons mature early, are mostly, if not exclusively, cholinergic, and project transiently to the inner hair cell region of the cochlea before making final synapses on outer hair cells. On the other hand, lateral efferent neurons mature later, are neurochemically heterogeneous, and project mostly, but not exclusively to the inner hair cell region. The early efferent innervation to the ear may serve an important role in the maturation of afferent responses. This review summarizes recent data on the neurogenesis, pathfinding, target selection, innervation, and onset of neurotransmitter expression in cholinergic efferent neurons.     

Comparative view of the central organization of afferent and efferent circuitry for the inner ear

In all vertebrates, eighth nerve fibres from the inner ear distribute to target nuclei situated in the dorsolateral wall of the rhombencephalon. In amniotes, primary auditory and vestibular nuclei are readily delineated in that acoustic nuclei lie dorsal and sometimes rostral to vestibular nuclei. Fishes and aquatic amphibians have, in addition to labyrinthine organs, hair cell receptors in the lateral line system. Eighth nerve and lateral line fibres from these sense organs project to the octavolateralis region of the rhombencephalon. In this region, the primary nuclei cannot be easily divided into functionally distinct units. However, modality-specific zones seem to be present for auditory as well as lateral line projections lie dorsal and sometimes rostral to those from vestibular organs. Projections from the primary auditory and vestibular nuclei to higher order centres follow pathways which are conservative in their architecture among vertebrates. Ascending auditory fibres project either directly or via relay nuclei to a large midbrain center, the torus semicircularis (inferior colliculus) and hence to the forebrain. In fishes and aquatic amphibians, the lateral line system also sends a projection to the midbrain and information from this system may be integrated with auditory input at that level. The organization of vestibulospinal and vestibulo-ocular pathways shows little variation throughout vertebrate phylogeny. The sense organs of the inner ear of all vertebrates and of the lateral line system of anamniotes receive an efferent innervation. In anamniotes and some reptiles, the efferent supply originates from a single nucleus (Octavolateralis Efferent Nucleus) while that of "higher" vertebrates arises from separate auditory and vestibular efferent nuclei. The biological significance of this innervation for all vertebrates is not yet understood. However, an important feature common to all is the association of the efferent system with the motor centres of the hindbrain.     

Input and output organization of cerebral neurones responding to tactile and taste stimuli applied to the lip of Helix pomatia L

Mechano- and chemoafferent responsiveness as well as outputs of identified cerebral neurones were investigated by electrophysiological methods in Helix pomatia L. the axonal projections of the identified cells were studied by intracellular staining. The studied neurones proved to be unipolar, their main axon branches were found in ipsilateral lip nerves. They could be divided into several groups according to their spontaneous activity, input and output organization and the selectivity of their responses to different tactile and taste stimuli applied to the lip. The activity of most of the neurones could be influenced by both ipsi- and contralateral inputs. They receive afferent input mostly through the medial lip nerves and their efferent information is transferred to the periphery mainly through the pair of inner lip nerves. There were seven neurones among the identified cells which responded selectively to taste stimuli identified in previous behavioural tests as phagostimulants. They can be considered as elements of the cerebral system regulating taste discrimination and feeding.     

A neuro-synaptic model of the auditory masking and unmasking process

A model of bilateral information processing in the auditory system was presented on the basis of the interaction of postsynaptic potentials intra- and internuclei in order to analyze the mechanism of binaural unmasking as well as monaural masking. The system was composed of a bilateral pair of auditory nuclei, which were organized in two parallel afferent systems as well as an efferent system. In the model, bilateral inputs were processed in three stages, i.e., the detection of interaural differences by the first afferent system, the equalization of relative neural timing by the efferent system, and the cancellation of specific spectral components by the second afferent system. Assuming the masking process to be forward and backward inhibitions on the auditory memory of signal by the ones of noise, the unmasking process could be explained as a result of disinhibition by the cancellation of bilateral masker inputs.This study was supported in part by a Grant-in-Aid for Scientific Research (No. 59870082) from the Japanese Ministry of Education, Science, and Culture     

Activity of premotor vestibular neurons in the alert cat

The activity of medial vestibular nucleus neurons projecting to the contralateral abducens nucleus (premotor vestibular neurons) has been recorded during spontaneous and vestibular induced eye movements in the alert cat. Recorded neurons were identified by their antidromic activation from the abducens nucleus and by the post-synaptic field potential induced in this nucleus. The activity of identified medial vestibular neurons increased significantly with horizontal eye position and velocity toward the contralateral side, and decreased abruptly during ipsilateral saccades. The activity of these neurons was also related to head velocity toward the ipsilateral side. The functional role and origin of eye position and velocity signals present in these vestibular neurons are discussed.     

Normal gait is differentially influenced by the otoliths

Postural control depends on the integration of vestibular, somatosensory and visual orientation signals. The otolith contribution to postural control is achieved by the integration of otolith inputs and peripheral afferent inputs involved in crossed reflex pathways. This study shows that a functional linkage between otolith signals and activity in lower limb muscles is detectable in normal human gait. The otolith input appears to dominate particularly the neck proprioceptive and gaze motor influences during normal gait. This is demonstrated by an increase of tibialis anterior muscle activity during retroflexion of the head/neck, leading to an increased stability and counteracting possible perturbations. It is also shown by decrease of coordination during the movement caused by larger displacement of the centre of gravity demonstrated in vector diagrams.