Primary Vestibular Afferents Research Articles

Morphology of single primary vestibular afferents originating from the horizontal semicircular canal in the cat.

The central projections of physiologically characterized vestibular nerve fibers originating from the horizontal semicircular canal were studied in the vestibular nuclei of adult cats after intracellular staining with horseradish peroxidase (HRP). First, primary nerve fibers were physiologically classified as regular or irregular types on the basis of the regularity of the spontaneous discharge pattern. Then, these two types of fibers were morphologically analyzed and compared following HRP intraaxonal injection. The two types of axons showed a basically similar trajectory in the four major vestibular nuclei. They bifurcated into an ascending and a descending branch in the ventrolateral part of the lateral vestibular nucleus (LVN). The ascending branch extended rostrally and gave off one or two collaterals in the superior vestibular nucleus (SVN), although some of the ascending branches further ran rostrally into the cerebellum. The collaterals, while running medially, gave rise to fine terminal branches with en passant boutons in the SVN, and further coursing caudally, they entered the rostral part of the medial vestibular nucleus (MVN). The descending branch, while running caudally in the lateral part of the LVN and the inferior vestibular nucleus (IVN), gave off several thick collaterals to the MVN and extensive terminals were present in the IVN and MVN. In each primary axon, about one-third of the total terminal boutons were distributed in each of the SVN, the MVN, and the IVN. In contrast to this similarity of the overall axonal trajectory within the vestibular nuclei, both types of axons exhibited several marked differences in diameter and in the mode of terminal arborization. In almost every part of the ramification, the irregular-type fibers were thicker than the regular-type fibers. In the regular-type axons, many small terminal boutons (mean size, 2.4 x 1.4 microns, N = 2,739) were located in close proximity (100-150 microns) to the parent collateral. In the irregular-type axons, slightly larger terminal boutons (mean size, 3.0 x 1.7 microns, N = 1,287), were spread more widely (200-300 microns) around their collaterals. These clear morphological differences between the regular-type and the irregular-type terminal axons were consistently observed in any vestibular nucleus.

Current concepts of the vestibular system reviewed: 1. The role of the vestibulospinal system in postural control.

This paper reviews the research findings that support the presence of vestibulospinal reflexes in corrections for head and body instability. Studies of the importance of labyrinthine inputs to the central nervous system organization of eye, head, and body movements demonstrate that the vestibular nuclei are more than a simple relay station for labyrinthine activity. At all levels of the vestibular system beyond the primary vestibular afferents, parallel processing of labyrinthine signals occurs with input from other sensory systems. Thus, output of the vestibular nuclear complex (VNC) is not equivalent to the labyrinthine input. It is the VNC output that influences motor behavior. Various sensory inputs are available to the nervous system to detect and correct postural instability. Most notably, vestibular, visual, and proprioceptive signals contribute significantly to the stabilizing responses in humans. The intent of this paper is to review experimental results rather than to discuss treatment interventions. Wherever possible, conclusions are drawn as to the clinical implications of current research findings.

Electron microscopic comparison of the terminals of two electrophysiologically distinct types of primary vestibular afferent fibers in the cat.

Two types of electrophysiologically distinct vestibular primary afferents (regular-type and irregular-type) were injected intraaxonally with HRP and their terminals were compared using the electron microscope. In general, regular-type terminals were small and two or more terminals contacted a single dendrite side by side, whereas irregular-type terminals were large and covered a wide surface of the dendrites. Both types of terminals contained spherical clear vesicles with almost the same diameter. Prominent postsynaptic membrane specializations were observed in both types. Mitochondria in both types had an elliptical profile with the same elongation index, but mitochondria in the regular-type terminals were thicker than those in the irregular-type ones.

Ultrastructure of contralaterally projecting vestibular efferent neurons in the cat.

Numerous reports have demonstrated that primary vestibular afferent axons terminate in the vicinity of centrally located vestibular efferent (VE) neurons. The goal of this study was to morphologically demonstrate contact of primary vestibular afferents directly on the soma of contralaterally projecting VE neurons, as well as develop a quantitative description of these neurons. A right vestibular neurectomy was performed in three young adult cats followed up three days later by the placement of horseradish peroxidase into the left vestibule. Labeled contralateral (right) VE neurons were examined in a transmission electron microscope for degenerating synaptic profiles contacting the soma. Ultrastructural characteristics of VE neurons, including nuclear size, volume fraction of intracellular organelles, number of synaptic profiles, and cellular shape, were quantitatively analyzed. No degenerating synaptic profiles were observed on labeled VE soma, although degenerating axons were present nearby. Statistical analysis of volume fraction data of the VE neurons suggests a continuum of a single subpopulation of cells rather than multiple subpopulations.

Chapter 2 Vestibular projections to the spinal cord: the morphology of single vestibulospinal axons

The three-dimensional distribution of LVST and MVST axons was examined in the cat cervical spinal cord using an intra-axonal staining method. LVST and MVST axons were electrophysiologically identified by their responses to stimulation of the vestibular nucleus, bilateral vestibular primary afferents, the LVST and the MVST were stained with injection of HRP. The axonal trajectory was reconstructed from serial histological sections. LVST axons were found to have multiple axon collaterals in the cervical cord. The maximum number of the identified collaterals for one neuron was 7. These collaterals were observed in either LVST axons terminating at the cervical cord or those projecting below Th2. The rostro-caudal extension of terminals for each collateral was very restricted (mean = 760 μm) and much narrower than intercollateral intervals (mean = 1470 μm). In the gray matter, collaterals ramified successively, pursued a delta-like path, and terminated mainly in lamina VIII and in the medial part of lamina VII and many boutons made apparent contact with the cell bodies and the proximal dendrites of motoneurons in the ventromedial nucleus. Some terminals were also distributed to the ventrolateral part of lamina VII adjacent to lamina IX. One group of LVST axons projected to lamina IX in the lateral ventral horn and terminated on large neurons there, probably motoneurons of forelimb muscles. MVST axons had one to seven axon collaterals at C1–C3 within the range of the stained axon. Stem axons ran in the ventromedial funiculus and primary collaterals arose from them at right angles. Each collateral had a very nar-row rostrocaudal spread as in LVST axons. Terminals were distributed in laminae VIII and IX, including the ventromedial nucleus, the spinal accessory nucleus, and the commissural nucleus. Many terminals seemed to make contact with retrogradely labelled motoneurons of neck muscles. Both crossed and uncrossed MVST axons had these characteristics.

Sensory-to-motor transformations in the vestibular system.

The vestibulo-ocular reflex is a compensatory reflex that results in eye movements that are 180 degrees out of phase with movements of the head but that match head velocity. Because of these reflex eye movements that are equal, but opposite to head movement, the viewed object remains on the fovea of the retina during head movement, thus resulting in visual acuity that is not degraded by visual image slip on the retina. This reflex is compensatory over a large spectrum of head movements in any plane of space. This is accomplished by a spatial and temporal transformation of the input from the vestibular semicircular canals to the motoneurons that innervate the extraocular muscles. The reflex is a three-neuron arc. The middle leg of the reflex is accomplished by secondary vestibular neurons whose axons branch to innervate more than one extraocular muscle. These secondary neurons thus program an eye movement rather than the contraction of a single extraocular muscle. These programmed eye movements that match the plane of the particular semicircular canal that is the input to the reflex constitute the spatial transformation. Primary vestibular afferents innervating the semicircular canals have a broad range of response dynamics that either lead, lag or are in phase with head velocity. The predominant vestibular primary afferent input to the middle leg of the reflex, the same secondary neurons as mentioned above, is parcellated so that afferents more in phase with head velocity predominate.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparisons of the immunocytochemical localization of choline acetyltransferase in the vestibular nuclei of the monkey and rat

Immunocytochemical studies of the brainstem were done in the squirrel monkey and rat using the same polyclonal antisera for choline acetyltransferase (ChAT). Cells immunoreactive for ChAT (ChATir) were evident in large numbers in visceral and motor cranial nerve nuclei in both species, but virtually no ChATir cells were seen in the vestibular nuclear complex of the rat. In the monkey ChATir cells were distributed in caudal parts of the medial (MVN) and in dorsal parts of the inferior (IVN) vestibular nuclei. Only a few immunoreactive cells were seen in the rostral MVN and none were found in cell group f of the IVN. Nearly all cells of group z and x, which do not receive primary vestibular afferents, were immunoreactive to ChAT. None of the cells in the superior and lateral vestibular nuclei, cell group y, the infracerebellar nucleus or the interstitial nucleus of the vestibular nerve were immunoreactive for ChAT. Cells immunoreactive to ChAT were present in large numbers in the rostral part of the nucleus prepositus in the monkey, but not in the rat. The relatively small number and distribution of ChATir cells in the MVN suggested they could constitute only a small fraction of the MVN neurons that contribute to a massive commissural system. Significant differences in cholinergic vestibular neurons appear to exist between the rat and the monkey.

Vestibular signals carried by pathways subserving plasticity of the vestibulo-ocular reflex in monkeys

The vestibulo-ocular reflex (VOR) is subject to long-term adaptive changes that minimize retinal image slip and keep eye movement equal to and opposite head movement. As a step toward identifying the site of neural changes, we have used a transient vestibular stimulus to study the dynamic response properties of the vestibular signals carried by the modifiable pathways. In normal monkeys, "rapid changes in head velocity" (30 degrees/sec in 50 msec) evoke a VOR that has a slight overshoot and reaches a steady-state gain (eye velocity divided by head velocity) of 1.0. Adaptation to magnifying spectacles causes changes in both the steady-state gain and the degree of overshoot in the eye velocity of the VOR. When the steady-state gain is decreased, the transient overshoot increases, so that peak eye velocity is twice steady-state. When the steady-state gain is increased, the overshoot decreases, so that peak eye velocity is nearly equal to steady-state. The discharge of vestibular primary afferents suggests an explanation for the inverse relationship between the transient overshoot and the steady-state gain of the VOR. In normal monkeys, 73 afferents showed a range of transient responses during rapid changes in head velocity. The afferents with the most regular spontaneous discharge had little overshoot in firing rate. Afferents with less regular discharge had large overshoots in firing; the peak change in firing was 2-6 X the steady-state change. We suggest that the large overshoot in eye velocity when VOR gain is low represents the contribution of vestibular signals from afferents with large transient responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology of amino acid receptors on vertebrate primary afferent nerve fibres

1. 1. Structure-activity of primary afferent depolarising action (PAD) mediated by γ-aminobutyrate (GABA) analogues suggests a difference between subsynaptic receptors located at fibre terminations within the dorsal horn and axonal receptors which are distributed throughout non-synaptic regions. 2. 2. The interaction of the bicuculline-sensitive GABA receptor (GABA A) ionophore complex with barbiturates and benzodiazepines suggests that at least three binding sites are required to explain the independent GABA-mimetic, GABA-potentiating and picrotoxin-reversing effects of such agents. 3. 3. Difficulties with explanation of the depressant effects of baclofen on spinal transmission, in terms of the bicuculline-resistant GABA (GABA B) receptor hypothesis, are mentioned. 4. 4. Glutamate-induced PAD of low threshold afferents is mediated indirectly through release of potassium. However, such terminals possess receptors (possibly autoreceptors for l-glutamate), activated by (+)2-amino-4-phosphonobutyrate, which cause depression of transmitter release. 5. 5. Primary afferent C-fibres possess receptors which are selectively activated by kainate and which mediate picrotoxin-resistant PAD. Such receptors may be involved in the presynaptic conditioning of C-fibre transmitter release. 6. 6. The peripheral terminals of vestibular primary afferents, in amphibia, possess excitatory amino acid receptors which are probably activated by the transmitter released from hair cells.

Action of the efferent vestibular system on primary afferents in the toadfish, Opsanus tau

Spinalized toadfish were held in a lucite chamber and perfused through the mouth with running seawater. Primary vestibular afferents and vestibular efferent axons and somas were studied with glass microelectrodes. Vestibular semicircular canal afferent and efferent axons were visually identified and penetrated with glass microelectrodes. Afferents responded to pulses of injected current with trains of action potentials, whereas efferents responded with only a single spike. This differential response to injected current served to further distinguish these two classes of nerve fibers that share the same canal nerve for part of their course. When current pulses were injected into efferent somadendritic recording sites, cells responded with trains of action potentials similar to those seen in other central nervous system neurons. Semicircular canal afferents were spontaneously active and occupied the same spectrum of regularity as vestibular afferents recorded in other species. Behavioral arousal evoked by lightly touching the fish on the snout or over the eye resembled spontaneous arousal observed in the field and consisted of eye withdrawal, fin erection, and attempted swimming. Efferent vestibular neurons were spontaneously active and increased their frequency of discharge when the fish was behaviorally aroused. Most efferents were briskly activated by behavioral arousal, but the time constant of the decay of their responses was variable ranging from 100 to 600 ms. Not only touch, but multimodal stimuli were capable of increasing the level of spontaneous activity of efferent vestibular neurons. The shortest latency to behavioral activation was 160 ms. Vestibular primary afferents also manifested increase in neuronal activity with behavioral activation. Irregularly discharging afferents were much more responsive than regularly discharging afferents. One rare case of transient inhibition in a regularly discharging afferent is illustrated. Severing the efferent vestibular nerve blocked behavioral activation in vestibular primary afferents. Electrical stimulation of the efferent vestibular nerve produced excitatory postsynaptic potentials (EPSPs) at latencies within the monosynaptic range in vestibular primary afferents. These monosynaptic EPSPs could produce action potentials in primary afferents or could sum with subthreshold depolarizations produced by current passed through the microelectrode to initiate impulses.(ABSTRACT TRUNCATED AT 400 WORDS)

Central projections of primary vestibular fibers in the bullfrog: I. The vestibular nuclei.

The central projections of the vestibular end organs in the bullfrog Rana calesbeiana were analyzed by using horseradish peroxidase labeling of the primary vestibular afferents. Separate extracellular injections were made of the anterior branch, the posterior branch, the ampullary nerve of each of the three semicircular canals, and the branch to the saccule. The anterior and posterior branches of the bullfrog eighth nerve, each containing both vestibular and auditory fibers, merge and enter the brain stem as a single nerve root. The thin caliber fibers of the anterior branch enter the brain stem on the ventral-posterior aspect of the nerve and immediately divide dichotomously into ascending (rostral) and descending (caudal) branches. The thick caliber fibers of the anterior branch enter the brain stem on the ventral-anterior aspect of the eighth nerve and traverse medially into the alar plate before dividing into ascending and descending branches. The primary afferent fibers of the vestibular nerve innervate an ipsilateral area of the brain stem which extends caudally from the rhombencephalon at the level of the twelfth nerve nucleus and rostrally up to and including the cerebellar nucleus and the cerebellum. The following vestibular nuclei can be identified by the fact that they receive primary vestibular afferents: the ventral vestibular nucleus, medial vestibular nucleus, descending vestibular nucleus, superior vestibular nucleus, and cerebellar nucleus. The dorsal (acoustic) nucleus, which receives primary auditory input, also receives afferents from the saccule. In addition to these nuclei, the cerebellum and the reticular formation receive significant primary input from the various vestibular receptors. The primary vestibulo-cerebellar fibers terminate mainly among the granular cells of the lobus auricularis and of the corpus cerebelli on the ipsilateral side. Each of the three semicircular canals projects into the cerebellum, while no such projection was observed in the saccular nerve preparations. Fibers from each of the three semicircular canals project to all of the vestibular nuclei. Heavily labeled large neurons, presumably vestibular efferents, are seen in the ipsilateral reticular formation, adjacent to the seventh motor nucleus.

Distribution of primary vestibular fibers in the brainstem and cerebellum of the monkey

Attempts were made to determine the central projections of ganglion cells innervating individual semicircular ducts in the monkey by implanting or injecting tritiated amino acids (leucine and/or proline), or horseradish peroxidase (HRP), selectively into a single ampulla. Central transport via the vestibular ganglion in animals receiving isotope implants or injections fell into three categories: (1) transport from ganglion cells innervating all receptive elements of the labyrinth, (2) transport from ganglion cells innervating the three semicircular ducts, and (3) transport from cells of the inferior vestibular ganglion innervating the posterior semicircular duct. Transneuronal transport of isotope was observed in secondary vestibular fibers in animals where proline was used and survival exceeded 12 days. Transneuronal labeling of secondary auditory fibers was independent of the [ 3H]amino acid used, and occurred with survivals of 10 or more days. HRP implanted into the ampulla of the lateral semicircular duct in several animals produced retrograde transport to efferent vestibular and cochlear neurons, but did not result in transganglionic labeling of primary vestibular or auditory fibers. Primary vestibular fibers terminate throughout the superior (SVN) and medial vestibular nuclei (MVN). Within SVN, terminals are most pronounced in its central large-celled portion, but extend into peripheral parts of the nucleus, except for a small medial area near its junction with the oral pole of MVN. Primary projections to MVN are homogenously distributed throughout the nucleus excepting a small circular area of sparse terminals along its ventral margin. Primary vestibular afferents terminate mainly in rostral and caudal portions of the inferior vestibular nucleus (IVN), but do not reach cell group ‘f’. Projections to the lateral vestibular nucleus (LVN) are restricted to its ventral part. Primary projections to the accessory vestibular nuclei reach the interstitial nucleus of the vestibular nerve (NIVN) and cell group ‘y’. Fibers project beyond the vestibular nuclei (VN) to terminate ipsilaterally in the accessory cuneate nucleus (ACN), the subtrigeminal lateral reticular nucleus (SLRN), and well-defined portions of the reticular formation (RF). Projections to SVN and MVN are derived primarily from ganglion cells innervating the semicircular ducts, while projections to caudal IVN, cell group ‘y’ and ACN are related mainly to macular portions of the vestibular ganglion. NIVN receives both macular and duct afferents. Posterior duct afferents terminate in medial portions of SVN, in rostrolateral portions of MVN, and in rostral IVN. Transneuronal transport of isotope increases the volume of terminal label in the ipsilateral VN, but not in dorsal LVN, or cell groups ‘f’ or ‘x’. The quality of transneuronal transport in secondary vestibular fibers is dependent upon: (1) survival time, (2) proximity to the VN, and (3) the excitatory or inhibitory nature of the projection. Primary vestibulocerebellar fibers terminate heavily in the ipsilateral nodulus and ventral uvula. Lesser projections reach the flocculus, deep folia of vermal lobules V and VI, and the lingula. Primary vestibulocerebellar projections terminate as mossy fiber rosettes in the granular layer of these cortical areas. No primary vestibular fibers terminate in the primate fastigial nuclei.

Frequency-selective adaptation: evidence for channels in the vestibulo- ocular reflex?

The vestibulo-ocular reflex (VOR) is under long-term adaptive regulation to minimize retinal image slip during head movement; normally this process keeps VOR gain (eye velocity divided by head velocity) near 1.0. It has been common to think of the adaptive mechanism as a single pure gain element, although some properties of the system (e.g., frequency-selective changes in the gain of the VOR) argue that it must be more complex. We now report new observations on the frequency selectivity of the adaptive mechanism. Our data suggest a new model in which the VOR operates as a series of parallel, temporal frequency channels, each of which has an independently adjustable gain element. Adaptive changes were produced by oscillating monkeys sinusoidally at a single temporal frequency (0.2 or 2.0 Hz) in visual conditions that cause either increases (toward two) or decreases (toward zero) in VOR gain. When tested in darkness at the adapting frequency, the VOR showed large changes in gain and little or no change in phase. When tested at frequencies other than the adapting frequency, the VOR showed less pronounced changes in gain and unexpected changes in phase. The phase changes were orderly but depended in a complex way on adapting frequency, testing frequency, and VOR gain. We have tested the channels concept by calculating the response properties of a mathematical model that processed its inputs in parallel pathways. The model reproduced our data when we assumed that the vestibular primary afferents were distributed in an orderly way to parallel brain channels that had differing dynamics: vestibular inputs with more phase lead projected to higher frequency channels, which themselves had faster dynamics than their low frequency counterparts. Such an organization, when regulated by an adaptive controller that can selectively alter the gain of one channel, could play a key role in establishing and maintaining the frequency-independent performance seen in the adult VOR.

Central course and terminal arborizations of single primary vestibular afferent fibers from the horizontal canal in the cat

Physiologically identified single primary vestibular afferents originating from the horizontal canal in the cat were intraaxonally stained with horseradish peroxidase. All afferents examined gave off one collateral to the interstitial nucleus of Cajal located in the vestibular root bundle in the brainstem. Then these fibers bifurcated into ascending and descending branches in the lateral vestibular nucleus. The ascending branch distributed terminals mainly in the middle part of the superior vestibular nucleus. The descending branch issued several collaterals in the ventrolateral regions of the lateral and inferior vestibular nuclei. These collaterals ran horizontally and reached the medial vestibular nucleus. Terminals were observed mainly in the boundaries between the medial nucleus and the lateral or inferior nucleus. Two types of primary vestibular afferents may be distinguished. Type A fibers had terminal arborizations with plenty of boutons en passage and boutons terminaux, while type B fibers those with a few boutons. The axonal diameter of the main trunk was smaller in type A fibers than in type B fibers.

Physiological and anatomical characteristics of primary vestibular afferent neurons in the bullfrog.

Intracellular recordings were made in the VIIIth nerve of the bullfrog (Rana catesbiana) to measure the membrane characteristics and obtain records of spontaneous and evoked spike activity of primary semicircular canal afferents. Physiological stimulation of the canals was achieved by rotating the preparation on a servomotor driven turntable with the animals' head centered in the rotational axis. The responses of each neuron to sinusoidal rotations at frequencies of 0.05Hz, 0.5Hz and for impulsive accelerations of 400 deg/sec2 were obtained. Membrane characteristics measured included the cell resting and action potential amplitude, and spike-activation threshold for applied currents. Physiologically characterized neurons were injected with horseradish peroxidase by applying pneumatic pressure and/or iontophoretic currents to the micropipettes containing 5% HRP in 1 M KCI. Following survival times of 12--48 h, the VIIIth nerve and attached vestibular end organ was removed for histochemical processing using a diaminobenzidine procedure to visualize the HRP reaction product. Light microscopy was used to discern the anatomical features of the neurons and to trace their peripheral dendritic trajectories from the ganglion to their termination(s) in the crista. Our studies have revealed that the bullfrog's primary vestibular afferents are characterized by a broad range of soma and axon diameters which correspond to an equally broad range of spontaneous and evoked activity characteristics. The largest neurons had more irregular spontaneous firing rates and consistently exhibited the greatest gain and smallest phase shifts with respect to head acceleration. These neurons consistently terminated at or near the central region of the crista. On the other hand, the smallest neurons were characterized by having the most regular spontaneous discharge patterns, the lowest gains, and greatest phase shifts with respect to head acceleration. Our findings are thus consistent with the view that the anatomical features of the primary vestibular neurons are important in determining the neuron's physiological characteristics. In terms of response dynamics our observations indicate that the receptors in the frog's crista ampullaris are heterogeneous and differentially sensitive to a wide range of stimulus frequencies.

Dogfish horizontal canal system: Responses of primary afferent, vestibular and cerebellar neurons to rotational stimulation

The activities of horizontal canal primary afferents, vestibular neurons and neurons in the auricular lobe of the cerebellum were sampled during horizontal head rotation in the dogfish Scyliorhinus canicula. Sinusoidal head rotation produces a smooth sinusoidal modulation of discharge frequency in primary vestibular afferents. The information content of this afferent signal is rather faithfully represented by the output of both the vestibular nuclei and the vestibulocerebellum. A degree of information processing does take place in that: 1. The spontaneous activity of central neurons is generally lower than that of primary afferents, with the result that many are silenced during the inhibitory portion of their response. 2. Some vestibular and cerebellar neurons responded maximally 180° out-of-phase with the ipsilateral primary afferents (type II response). 3. Information from opposite canals is integrated to produce an increased rotation sensitivity in some vestibular neurons and a bidirectional increase in firing (type III response) in other neurons. 4. At stimulus frequencies below 0.16 Hz Purkinje cells showed a steeper reduction of phase lag than granular layer units or primary afferents whereas vestibular neurons did not show a clear tendency towards reduced phase lag at lower frequencies.

The fine structure of the feline superior vestibular nucleus: Identification and synaptology of the primary vestibular afferents

The superior vestibular nucleus of the cat and its primary vestibular afferents were examined by light and electron microscopy. The primary vestibular afferents branch within the nucleus in a sheet-like pattern, in the transverse plane. The dendritic fields of many secondary neurons are shaped like discs and are also oriented in the transverse plane. This relation between the primary afferents and dendritic fields may be relevant to the convergence of primary afferents innervating particular endorgans onto secondary neurons. Synaptic boutons in the SV were divided into 3 putative types on the basis of the size and shape of their synaptic vesicles. The primary afferent bouton was identified by comparing the SV of the two sides after unilateral lesions of the vestibular ganglion. Its boutons contain round vesicles of 40 nm average diameter and are associated with prominent postsynaptic densities; the two other putative bouton types contain smaller, round vesicles, and pleomorphic vesicles. The primary afferent boutons largely contact proximal dendrites, their appendages, and cell somata of the secondary neurons. In animals receiving unilateral lesions of the vestibular ganglion and allowed to survive long enough for the primary afferent boutons to disappear (5–6 days), there occurs in the denervated as compared to normal SV: (1) a decrease in the fraction of the somal surface of the secondary neurons covered by boutons with small round vesicles; and (2) a decrease in the ratio: number of boutons with small round vesicles to number of boutons with pleomorphic vesicles. In addition, there appears on the lesioned side a new group of boutons with pleomorphic vesicles smaller than those in boutons from the control side. These observations suggest plastic changes in response to deafferentation, and may be related to the marked behavioral recovery which occurs within a few days after lesion of the vestibular ganglion.

The cerebellar projection of the vestibular nerve in the cat.

The projection of the vestibular nerve to the cerebellum of the cat was examined with silver degeneration methods after complete lesions of the vestibular ganglion. The majority of the primary vestibular afferents were traced to the cortex of the ipsilateral nodulus and uvula, relatively fewer entering the ipsilateral flocculus. Fibers were not traced to the paraflocculus, lingula or lateral cerebellar nucleus. A sparse projection to the ipsilateral fastigial nucleus may exist, but it remains equivocal until confirmed with additional methods. Light microscopic examination of plastic sections confirmed these observations and showed further details of the organization of the primary vestibular projection to the nodulus and uvula. These results show that the region of the cerebellum densely innervated by primary vestibular afferents is smaller than previously believed.

Efferent Controlled Integrating Functions of Primary Vestibular Afferents

The vestibular type II receptor cells of mammalians are multiply innervated. Their afferents are integrating neurons consisting of many inputs but only one output. They transform the irregular spontaneous input activity into a regular output. Under the influence of efferent activity following the stimulation of the contralateral labyrinth, this regular output activity becomes irregular. This efferent influence upon afferent spontaneous activity is analysed by means of an existing computer model of an integrating cell. The analysis confirms a high functional interdependence of both labyrinths.

Response properties of vestibular afferents in alert cats during optokinetic and vestibular stimulation

Unlike secondary vestibular neurons primary vestibular afferents of the alert cat do not respond to rotation of large-field visual patterns. The phases and gains of the responses of horizontal canal afferents to head rotation as well as the resting discharges found in the alert animal do not significantly differ from those measured in animals anesthetized with barbiturates.