Intracellular study of frog's vestibular neurons in relation to the labyrinth and spinal cord

Summary Field and intracellular potentials were recorded in the vestibular nuclei of the frog following stimulation of the anterior branch of the ipsilateral vestibular nerve and the spinal cord. The field potential induced by stimulation of the vestibular nerve consisted of an early positive-negative wave followed by a slow negativity and that recorded during spinal cord stimulation was composed of an antidromic potential followed by a slow negative wave. These potentials were most prominent in the ventral region of the stato-acoustic complex. Mono- and polysynaptic EPSPs were recorded from vestibular neurons following vestibular nerve stimulation. Short latency depolarizations of small amplitude preceded the monosynaptic EPSPs in some neurons. Spike-like partial responses were commonly superimposed on the EPSPs. These all-or-none depolarizations probably originated in the dendrites. In a group of vestibular neurons stimulation of the vestibular nerve evoked full action potentials with latencies ranging from 0.2 to 1.1 msec. They are presumably caused by antidromic activation of neurons which send their axons to the labyrinth. The presence of efferent neurons in the vestibular nuclei was confirmed by their successful staining with Procion Yellow following axonal electrophoresis.After stimulation of the spinal cord, antidromic spike potentials and EPSPs were recorded in vestibular neurons. In addition, short-latency depolarizing potentials (EDPs) were evoked by spinal stimulation, with latencies similar to those of antidromic potentials. The EDPs are suggested to be induced by electrotonic transmission from the neighboring cell and likely to be active spike potentials produced at some distance away from the soma.     

Functional organization of vestibulofastigial projection in the horizontal semicircular canal system in the cat

Spike potentials of fastigial nucleus neurons were recorded extracellularly in decerebrate, unanesthetized cats. The neurons responding to head rotation in the horizontal plane with a type I fashion were located mainly in the middle and caudal regions of the fastigial nucleus. Three fourth of these fastigial type I neurons were antidromically activated by stimulation of the contralateral vestibular nuclei. These neurons were excited transsynaptically from the ipsilateral vestibular nerve or nuclei. Intra cellular recordings were made from those neurons which were located in the caudal half of the fastigial nucleus and were activated antidromically from the contralateral vestibular nuclei. Stimulation of the ipsilateral vestibular nerve produced EPSPs in these neurons with latencies of 1.0-6.6 msec. The shortest conduction time along primary vestibular aggerents from the labyrinth to the ipsilateral fastigial nucleus was 0,7 msec. The EPSPs with the shortest latency of 1.0 msec were therefore postulated to be due to monosynaptic connections of primary vestibular afferents with fastigial neurons. Stimulation of ipsilateral vestibular nuclei also produced monosynaptic EPSPs in fastigial neurons. These EPSPs were facilitated by conditioning stimulation of the ipsilateral vestibular nerve, indicating the existence of polysynaptic activation of fastigial neurons from the ipsilateral vestibular nerve through the vestibular nuclei.     

The vestibulo-ocular reflex arc in the newborn kitten

Summary Field potentials and postsynaptic potentials were recorded in the vestibular and abducens nuclei and neurons following vestibular nerve stimulation in anesthetized newborn kittens (within 72 h after birth). Stimulation of the ipsilateral vestibular nerve evoked an initial P wave and an N₁ field potential in the vestibular nuclei. No N₂ potential was evoked. Latencies of the peak of the P wave, the onset and the peak of the N₁ potential were 0.99±0.16 ms, 1.66±0.18 ms, and 2.51±0.23 ms, respectively. Ipsilateral vestibular nerve stimulation evoked monosynaptic excitatory postsynaptic potentials (EPSPs) and polysynaptic inhibitory postsynaptic potentials (IPSPs) in vestibular nuclear neurons. Stimulation of the contralateral vestibular nerve evoked polysynaptic IPSPs in vestibular nuclear neurons. In abducens motoneurons, ipsilateral vestibular nerve stimulation evoked monosynaptic EPSPs and disynaptic IPSPs; contralateral vestibular nerve stimulation produced disynaptic EPSPs. We conclude that short circuit pathways of the vestibul-ovestibular and vestibulo-ocular reflex arc are present in the kitten already at birth.Supported by the Japanese Ministry of Education, Science, and Culture Grants-in-Aid for Scientific Research nos. 572 140 30 and 575 700 53     

Lesion-induced vestibular plasticity in the frog: are N-methyl-D-aspartate receptors involved?

Summary The synaptic excitation of central vestibular neurons in the isolated superfused brainstem of chronic hemilabyrinthectomized (HL) frogs and of controls was studied electrophysiologically and pharmacologically. Central vestibular neurons were excited either through vestibular afferent fibers or through the vestibular commissural pathway by means of electrical stimulation of the ipsilateral or the contralateral VIIIth nerve. In chronic HL frogs, commissural field potential amplitudes were on the average larger than those of intact frogs and the shape parameters of intracellularly recorded commissural EPSPs of chronic animals were on the average shifted towards those of vestibular afferent EPSPs. In control frogs, vestibular afferent EPSPs were generated independently from N-methyl-D-aspartate (NMDA) receptors, whereas commissural EPSPs exhibited a delayed NMDA receptor mediated component. Commissural EPSPs of HL frogs exhibited a NMDA receptor mediated component as well. The size of this EPSP component was larger when the time to peak of the EPSP was longer. EPSPs with similar rise times exhibited NMDA mediated components of similar size, irrespective of whether they originated from chronic animals or controls. The tendency of these EPSPs towards shorter rise times in chronic animals was paralled by a similar decrease of the relative size of their NMDA receptor mediated component. It is concluded that the increased synaptic efficacy of commissural fibers observed in chronic HL frogs does not result from an increased NMDA receptor component.     

Synaptic mechanisms of interaction between Deiters' nucleus and the nuclei of some cranial nerves

The effects of stimulation of the vestibular nerve, spinal trigeminal nucleus, facial and hypoglossal nuclei of the cranial nerves on the neuronal activity in the lateral vestibular nucleus of Deiters were studied in cats anaesthetized with pentobarbitone. Stimulation of these nuclei was found to produce antidromic and synaptic activation of Deiters' neurons. Descending axon collaterals of the vestibular neurons to these brainstem structures were revealed. Stimulation of the VIIIth nerve, spinal trigeminal and facial nuclei evoked mono- and polysynaptic excitatory postsynaptic potentials in Deiters' neurons. Stimulation of the spinal trigeminal nucleus evoked mono- and polysynaptic inhibitory postsynaptic potentials and disfacilitation in Deiters' neurons. In some vestibular neurons inhibitory postsynaptic potentials were also evoked by stimulation of the nucleus hypoglossus. Convergence of influences from these structures on Deiters' neurons was shown to exist. The peculiarities and functional significance of the effects mentioned are discussed.     

Vestibulospinal effects on hindlimb motoneurons of the frog

Summary Field and intracellular potentials were recorded in the lumbar spinal cord of the frog following stimulation of the anterior branch of the vestibular nerve and vestibular nucleus. The field potential recorded in the motoneuron pool after VIIIth nerve stimulation consisted of two presynaptic positive-negative potentials (latencies 1.7 and 2.6 msec) followed by a slow negative wave. The latency of the first presynaptic field potential was only 0.6 msec longer than the presynaptic field potential evoked by stimulation of the vestibular nucleus; it is suggested that electrotonic coupling in the vestibular nuclei is responsible for the fast vestibulospinal transmission.Whereas VIIIth nerve stimulation produced EPSPs in both flexor (peroneal) and extensor (tibial) motoneurons, IPSPs were found only in extensor motoneurons. The functional implication of these findings was discussed. Comparison of PSP latencies with the extracellular presynaptic field potentials generated by VIIIth nerve or nucleus stimulation indicated that EPSPs were produced by the excitatory action of vestibulospinal axons on motoneurons. The longer latencies of the vestibular induced IPSPs suggested that they were generated indirectly by inhibitory spinal interneurons. Preliminary experiments on the interaction of segmental and vestibular induced PSPs suggest that the latter are generated close to the soma of motoneurons.     

The organization of the vestibulo-oculomotor and trochlear reflex pathways in the rabbit

Summary Intracellular and extracellular responses were recorded with glass micro-electrodes from motoneurons in the IIIrd and IVth cranial nuclei of anesthesized rabbits. Five subgroups of neurons innervating the superior rectus (SR), inferior oblique (IO), inferior rectus (IR), medial rectus (MR), and superior oblique (IVth) extraocular muscles were identified by their antidromic activation from the branches of the IIIrd and IVth cranial nerves. The relative positions of the subgroups thus determined were consistent with the histological data on the rabbit. In the SR, IO, IR, and IVth subgroups the effects of ipsilateral VIIIth nerve stimulation were inhibitory, producing disynaptic IPSPs, while the effects of contralateral VIIIth nerve stimulation were excitatory, producing disynaptic EPSPs. In the MR subgroup, however, a mixture of EPSPs and IPSPs was produced by VIIIth nerve stimulation: this was particularly clear on the ipsilateral side. Sites relaying these VIIIth nerve effects to each of the five subgroups were explored by direct stimulation of various brain stem sites. Stimulation of the superior vestibular nucleus (SV) produced IPSPs monosynaptically in all five subgroups on the ipsilateral side as well as in the contralateral MR subgroup. Stimulation of the medial vestibular nucleus (MV) produced EPSPs monosynaptically in all of the five subgroups on the contralateral side as well as in the ipsilateral MR subgroup. Stimulation of the brachium conjunctivum (BC) also produced EPSPs monosynaptically in the contralateral SR, IO, and IR subgroups. Further, while the recording electrode was placed within each of the five subgroups to observe the extracellular potentials corresponding to the intracellularly recorded IPSPs and EPSPs, the medulla and cerebellum were systematically tracked with a monopolar stimulating electrode. It was thus confirmed that the SV is the sole inhibitory relay site, while excitation is relayed by both the MV and the BC. The origin of the BC pathway was traced to the Y-Group for the IO, to the lateral nucleus of the cerebellum (LN) for the IR, and to both the Y-Group and the LN for the SR subgroup.     

Postsynaptic influences on the vestibular non-deiters nuclei from primary vestibular nerve

Summary Neurones in the descending, medial and superior vestibular nuclei of the cats were explored with intracellular microelectrodes. Cerebellar- and spinal-projecting neurones were identified by their antidromic invasion from the region of fastigial nuclei and from the second cervical segment, respectively, and the others by their location. The central actions of the primary vestibular impulses upon these non-Deiters vestibular nuclei neurones were investigated by using electric stimulation of the ipsilateral vestibular nerve. Many of these cells received excitatory postsynaptic potentials (EPSPs) monosynaptically, similar to those evoked in the ventral Deiters neurones, as described elsewhere, except that the unitary EPSPs are often larger. Some cells received only polysynaptic EPSPs or IPSPs and a few cells were not influenced at all.     

Inhibitory connections of nystagmus-related reticular burst neurons with neurons in the abducens,prepositus hypoglossi and vestibular nuclei in the cat

Summary Action potentials of inhibitory burst neurons (IBNs) were extracellularly recorded in the pontomedullary reticular formation in the cat. These neurons were identified by their burst activity coincident with the quick inhibitory phase of the contralateral abducens nerve during vestibular nystagmus and by their antidromic activation from the contralateral abducens nucleus.During extracellular recording from the soma of single IBNs, another electrode for microstimulation was systematically tracked throughout the brain stem. For each IBN investigated, the effective sites for antidromic activation were invariably found in the contralateral abducens, prepositus hypoglossi, medial vestibular nuclei and the area ventral to the prepositus hypoglossi nucleus. Stimulation of neither the ipsilateral brain stem nor the oculomotor nuclei evoked antidromic responses in IBNs.Extracellular spikes of single IBNs and neurons in the overlying projection area were recorded simultaneously. Their correlation was examined by using peri-spike time histograms. Shortly after the spikes of single IBNs, the activity of motoneurons and internuclear interneurons in the abducens nucleus, and of type II neurons in the prepositus hypoglossi and vestibular nuclei, was depressed.Connections of IBNs with ipsilateral medial rectus motoneurons were studied by spike-triggered averaging of membrane potentials of the motoneurons and action potentials of the medial rectus nerve. Single IBN spikes induced a di- or polysynaptic disfacilitation in the motoneurons. This disfacilitation was concluded to be mediated by some of the above-described interneurons which were directly inhibited by IBNs. Their depressant effect on medial rectus motoneuronal spike activity was comparable to that on the spike activity of contralateral abducens motoneurons.     

Synaptic linkage in the vestibulo-ocular and cerebello-vestibular pathways to the VIth nucleus in the rabbit

Summary Intra- and extra-cellular responses were recorded with glass microelectrodes from motoneurons in the VIth cranial nuclei of anesthesized rabbits. VIth nucleus motoneurons were identified by their antidromic activation from the VIth nerve. In these motoneurons stimulation of the ipsilateral VIIIth nerve produced IPSPs with disynaptic latencies (mean and S.D., 1.08 ± 0.1 msec) while stimulation of the contralateral VIIIth nerve produced EPSPs with disynaptic latencies (mean and S.D., 1.20 ± 0.18 msec). Correspondingly, direct stimulation of the ipsilateral medial vestibular nucleus (MV), produced IPSPs with monosynaptic latencies (mean and S.D., 0.61±0.15 msec) while direct stimulation of the contralateral MV produced EPSPs with monosynaptic latencies (mean and S.D., 0.61±0.09 msec). Further, with the recording electrode placed within the VIth nucleus to observe the extracellular potentials corresponding to the intracellularly recorded IPSPs and EPSPs, the medulla was systematically tracked with a monopolar stimulating electrode. It was demonstrated that the inhibitory relay cells could be effectively stimulated in the rostral half of the ipsilateral MV and the excitatory relay cells in the rostral half of the contralateral MV.Pharmacological investigation suggested that the inhibitory transmitter involved in the vestibular inhibition is gamma amino-butyric acid or a related substance.Electric stimulation of the flocculus produced a prominant depression in the inhibitory vestibulo-ocular reflex pathway to the VIth nucleus, while the excitatory pathway was free of any similar flocculus inhibition.     

Direct projections from vestibular nuclei to facial nucleus in cats

Postsynaptic potentials were recorded from motoneurons in the facial nucleus in response to stimulation of the vestibular and trigeminal nerves. The motoneurons were identified by antidromic activation from their peripheral axons. Disynaptic excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) and mixed EPSP/IPSPs were recorded in response to vestibular nerve stimulation, ranging in latency from 0.9 to 2.1 ms, with most at 1.5 ms. Activity in secondary vestibular axons recorded within the facial nucleus occurred at a latency of 0.7-1.1 ms. The amplitudes of the vestibular postsynaptic potentials were small, generally less than a millivolt, but double shocks produced marked summation. The average time to peak of ipsilateral vestibular EPSPs, 1.1 ms, was faster than that of either ipsilateral IPSPs, 1.6 ms, or contralateral EPSPs, 1.4 ms. The double-spiked vestibular activity was detectable in double-peaked PSPs. Disynaptic EPSPs, ranging in latency from 2.0 to 3.0 ms, were recorded in response to trigeminal nerve stimulation. The average time to peak was 1.3 ms. The multiple-spiked activity of the trigeminal neurons was detectable in multipeaked EPSPs. Inhibitory ipsilateral effects (Vi IPSPs) were recorded twice as often as excitatory ipsilateral effects (Vi EPSPs), being found in 29% versus 15% of the motoneurons. Contralateral effects were found in 13% of the motoneurons studied, and almost all were excitatory. Analysis of synaptic potential shapes suggested that the excitatory and inhibitory vestibular synapses probably contact distal dendrites preferentially, with the excitatory connections being somewhat closer to the soma. The trigeminal inputs probably contact the facial motoneurons more extensively near the soma. Horseradish peroxidase was injected into the facial nucleus, and retrograde uptake by vestibular neurons was studied. The majority of filled vestibular neurons was ipsilateral to the injection site, especially in the medial vestibular nucleus, ventral y group, and supravestibular nucleus. On the contralateral side, filled vestibular cells were found almost exclusively in the medial nucleus. Filled cells were also noted in the trigeminal nucleus, predominantly ipsilaterally at all rostrocaudal levels. We have demonstrated monosynaptic projections to facial motoneurons from both vestibular and trigeminal nuclei. The trigeminal input is likely to be involved in facial reflexes, especially blinking and grimacing. The afferent vestibular population overlaps that going to the oculomotor and cervical motoneurons; these projections may be collaterals of single vestibular neurons.4+.     

Electrophysiological properties of the somatotopic organization of the vestibulospinal system in the frog

In experiments on the preparation of a frog perfused brain (Rana ridibunda), field and intracellular potentials were recorded from neurons of the vestibular nuclear complex following stimulation of the ipsilateral vestibular nerve and different levels of the spinal cord. Stimulation of the vestibular nerve evoked mono- and polysynaptic excitatory postsynaptic potentials and orthodromic action potentials. In parallel, an antidromic activation of vestibular neurons sending their axons to the labyrinth was recorded. Vestibulospinal neurons sending their axons to the cervical (C neurons) and lumbar (L neurons) enlargements of the spinal cord were identified by their antidromic activation. A rather high conduction velocity along vestibulospinal fibres (mean 15.47 m/s) was observed. A somatotopic arrangement of the vestibulospinal system was established in spite of extremely large overlapping zones for the fore- and hindlimb representations in the vestibular nuclear complex. The hindlimbs were represented more poorly than the forelimbs. Antidromic potentials of C and L neurons were recorded in the medial, descending and with the highest density in the lateral vestibular nuclei (Deiters' nucleus). C neurons were evenly distributed in the other vestibular nuclei studied, while L neurons were located predominantly in the caudal parts of the vestibular nuclear complex. The multiplicity of the origin of the vestibulospinal axons was established. Peculiarities of the functional correlation between the vestibular input and vestibulospinal system are discussed.     

Synaptic linkage in the vestibulo-ocular reflex pathway of rabbit

Summary Microelectrodes were inserted into IIIrd cranial nucleus of anaesthetized rabbit. IIIrd nucleus was identified by observing the field potentials evoked antidromically by stimulation of IIIrd cranial nerve. After stimulation of VIIIth nerve extracellular field potentials, spike potentials in secondary vestibular fibers, and postsynaptic potentials in IIIrd nucleus neurones were recorded. VIIIth nerve impulses either excite or inhibit IIIrd nucleus neurones postsynaptically with disynaptic latencies around 1.7 msec. By local stimulation of the medulla, it was found that the secondary vestibular impulses inhibiting IIIrd nucleus neurones are mediated by the superior nucleus. The excitatory impulses are relayed by the rostral half of the medial nucleus as well as a certain structure(s) relevant to the brachium conjunctivum. Preliminary pharmacological investigations on the inhibition of IIIrd nucleus neurones are reported.     

Cerebellar inhibitory control of the vestibulo-ocular reflex investigated in rabbit IIIrd nucleus

Summary In anaesthetized rabbits, the vestibulo-ocular reflex was evoked by electric stimulation of VIIIth nerve and was observed by recording postsynaptic potentials and relevant field potentials in Illrd nucleus. The electric stimulation of flocculus produced a prominent inhibition of the vestibulo-ocular reflex in both the inhibitory component relayed by the superior vestibular nucleus and the excitatory component mediated by the brachium conjunctivum. The excitatory component mediated by the medial vestibular nucleus appeared to be free of the flocculus inhibition. The flocculus inhibition was blocked very effectively by systemic injection of picrotoxin. That the flocculus inhibitory action is due to monosynaptic postsynaptic inhibition of secondary vestibular neurones was demonstrated by direct stimulation of, and also by recording from, the superior nucleus. Recording from the superior nucleus was also performed in anaesthetized cats. All of these above results indicate that Purkinje cells in flocculus projecting to vestibular and cerebellar nuclei cells have inhibitory synaptic action. Flocculus stimulation produced also an excitatory effect upon vestibular nuclei neurones. However, this effect could be attributed to intracerebellar activation of the primary vestibular fibers which pass into the flocculus.     

The effects of stimulating the cerebellar nodulus in the cat on the responses of vestibular neurons

In a first series of experiments, recordings were obtained from cat abducens and trochlear motorneurons and from axons of secondary vestibular neurons terminating in these motor nuclei, and the effects of cerebellar nodulus stimulation on utricular- and canal-evoked responses in these neurons were studied. Ultricular activation of vestibular axons recorded in the ipsilateral VIth and contralateral IVth nuclei was probably monosynaptically inhibited by nodular stimulation provided conditioning-test intervals were in the range between 0-10 ms and the test stimuli were close to threshold intensities. Of the vestibular axons activated by stimulation of the semicircular canal nerves only those evoked by the horizontal canal stimulation and recorded in the ipsilateral VIth nucleus were weakly inhibited. When the vestibular stimuli were strong enough to produce clear field potentials in the motor nuclei and/or postsynaptic potentials in motorneurons, nodular stimulation had practically no effect on their amplitudes. It is concluded that inhibition of vestibuloocular transmission is weak as compared to floccular inhibition studied previously. In a second series of experiments, recordings were obtained from vestibular neurons which were activated antidromically and/or transsynaptically by stimulation of the contralateral fastigial nucleus, and the effects of ipsilateral nodular stimulation on these responses were studied. It was found that nodular stimulation inhibited both antidromic as well as transsynaptic fastigial activations of vestibular neurons. Most of these vestibular neurons were located in the descending vestibular nucleus and received polysynaptic vestibular and spinal inputs. It is concluded that in addition to its weak inhibitory effect on vestibuloocular transmission the nodulus exerts a powerful inhibition on vestibular neurons transmitting vestibular and spinal inputs to cerebellar nuclei and/or cortex. It is suggested that the nodulus controls cerebellar projecting vestibular neurons which carry vestibular and spinal information to the cerebellum. The vestibular, proprioceptive and visual information which is present in the nodulus may aid the role of the nodulus in controlling body posture.     

Vestibulo-reticular projections in adult lamprey: their role in locomotion

This study describes the anatomical projections from vestibular secondary neurons to reticulospinal neurons in the adult lamprey and the modulation of vestibular inputs during fictive locomotion. Anatomical tracers were applied in the posterior (PRRN) and middle rhombencephalic reticular nuclei as well as to the proximal stumps of cut vestibular nerve branches to identify the neurons projecting to the reticular nuclei that were in close proximity with vestibular primary afferents. Labeled neurons were found in the intermediate (ION) and posterior (PON) octavomotor nuclei, and were more numerous on the side of the injection (around 56-87 and 101-107 for the ION and the PON, respectively). Morphologies varied but cells were mostly round or oval. Axonal projections from the PON formed a dense bundle, whereas those from the ION were less densely packed. Based on their morphology and the distribution of their projections, most vestibulo-reticular neurons were presumed to be vestibulospinal cells. Reticulospinal cells from the PRRN were recorded intracellularly in the in vitro brainstem-spinal cord preparation and large excitatory post-synaptic potentials (EPSPs) were evoked following stimulation of the ipsilateral anterior and the contralateral posterior branches of the vestibular nerves, whereas inhibitory post-synaptic potentials (IPSPs) or smaller EPSPs were elicited by stimulation of the ipsilateral posterior or of the contralateral anterior branches. During fictive locomotion, both the excitatory and the inhibitory responses displayed phasic changes in amplitude such that the amplitude of the EPSPs was minimal when the spinal cord activity switched from the ipsilateral to the contralateral side of the recorded reticulospinal cell. The IPSPs were then of maximal amplitude. We propose that this modulation could serve to reduce the influence of vestibular inputs in response to head movements during locomotion.     

Excitatory connections between neurons of the central cervical nucleus and vestibular neurons in the cat

 The central cervical nucleus (CCN) of the cat receives input from upper cervical muscle afferents, particularly primary spindle afferents. Its axons cross in the spinal cord, and while in the contralateral restiform body give off collaterals to the vestibular nuclei. In order to study the connections between CCN axons and vestibular neurons, we stimulated the area of the CCN in decerebrate cats while recording intra- or extracellularly from neurons in the contralateral vestibular nuclei. CCN stimulation evoked excitatory postsynaptic potentials (EPSPs) or extracellularly recorded firing in the lateral, medial and descending vestibular nuclei. The latency of EPSPs (mean 1.6 ms) was on average 0.4 ms longer than the latency of antidromic spikes evoked in the CCN by stimulation of the contralateral vestibular nuclei (mean 1.2 ms), demonstrating that the excitation was typically monosynaptic. The results provide further evidence that the CCN is an important excitatory relay between upper cervical muscle afferents and neurons in the contralateral vestibular nuclei. Received: 1 August 1996 / Accepted: 16 December 1996     

Compensation of labyrinthine lesions: effects of trigeminal neurotomy on vestibular field potentials

The aim of the research was to analyse the vestibular nuclear activity before and after the section of the 5th cranial nerve in chronically hemilabyrinthectomized guinea pigs during the stage of compensation. The animals were hemilabyrinthectomized (chloroform and vaselin oil into the right middle ear) and upon compensation (24–45 days), field potentials were recorded from the vestibular nuclei of the intact side following electrical stimulation of the ipsilateral labyrinthine receptors. Then the left trigeminal trunk was sectioned ventrally through the foramen lacerum and ovale which are fused in the guinea pig and the vestibular field potentials were again recorded for a period of 120 min. Trigeminal neurotomy modified the vestibular field potentials determining an increment in amplitude of 30–50% of N₁ and N₂ waves, configuration and latency remained unaltered. The dependence of vestibular compensation on trigeminal afferents is discussed on the light of reported results.     

Functional characteristics of the input-output correlation in the vestibular nuclear complex of the frog

Experiments on perfused frog brains were used to record focal and intracellular potentials of neurons in the vestibular nuclear complex produced in response to stimulation of the anterior branch of the ispilateral vestibular nerve and the spinal cord. Stimulation of the vestibular nerve evoked mono- and polysynaptic EPSP with orthodromic action potentials. These were accompanied by recordings of antidromic activation (with a mean latent period of 0.75 sec) of neurons which send their axons into the labyrinth. Antidromic action potentials from vestibular neurons arose with latent periods of the order of 1.43 msec in response to stimulation of the cervical thicknening and 2.19 msec in response to stimulation of the lumbar thickening of the spinal cord. Bursts from the spinal cord often evoked EPSP with orthodromic action potentials in vestibular neurons. The characteristics of the functional correlation between the vestibular input and the vestibulospinal system are discussed. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 84, No. 10, pp. 1085–1092, October, 1998.     

Nucleus ventroposterior inferior (VPI) as the vestibular thalamic relay in the rhesus monkey I. Field potential investigation

Summary In order to investigate the thalamic relay of the vestibulo-cortical pathway, field potentials were recorded in the rhesus thalamus under pentobarbital anesthesia. Short latency responses (2.5 msec on the average) upon stimulation in isolation of the vestibular nerve were recorded in the inferior ventroposterior nucleus (VPI). These potentials were abolished after transection of the vestibular nerve but were not affected by total cerebellectomy. Projection of VPI neurons to the primary vestibular cortex was demonstrated by antidromic stimulation. Field potentials with latencies of those observed in the vestibular cortex (about 5 msec) in response to vestibular nerve stimulation were recorded in other areas of the thalamus (ventrobasal, ventrolateral, posterior group, including magnocellular medial geniculate nuclei). Thus, the VPI rather than the other nuclei with long latency responses is likely to be the thalamic relay in the vestibulo-cortical path. The close topographical relationship between vestibular and somatic areas in the cortex is parallelled in the thalamus, the VPI being closely related to VPL and VPM nuclei.1 Abbreviations used in this Communication AI primary auditory projection area - BCI brachium colliculi inferioris - CM ncl. centrum medianum - IPS intraparietal sulcus, the lower bank of the anterior end of which contains the primary vestibular field - LM medial lemniscus - mc magnocellular - MG medial geniculate body - PO posterior group - SI primary somatosensory cortex - V.c.pc. ncl. ventrocaudalis parvocellularis. - VL ncl. ventralis lateralis - VPI ncl. ventroposterior inferior - VPL ncl. ventroposterior lateralis - VPM ncl. ventroposterior medialis Guest researcher from Abteilung Neurologie, Universität Ulm, W.-Germany.