[Projections of the cristae ampullares and utricle into the primary vestibular nuclei. Microphysiological study and anatomo-functional correlations]

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

Differential spatial organization of otolith signals in frog vestibular nuclei

Activation maps of pre- and postsynaptic field potential components evoked by separate electrical stimulation of utricular, lagenar, and saccular nerve branches in the isolated frog hindbrain were recorded within a stereotactic outline of the vestibular nuclei. Utricular and lagenar nerve-evoked activation maps overlapped strongly in the lateral and descending vestibular nuclei, whereas lagenar amplitudes were greater in the superior vestibular nucleus. In contrast, the saccular nerve-evoked activation map coincided largely with the dorsal nucleus and the adjacent dorsal part of the lateral vestibular nucleus, corroborating a major auditory and lesser vestibular function of the frog saccule. The stereotactic position of individual second-order otolith neurons matched the distribution of the corresponding otolith nerve-evoked activation maps. Furthermore, particular types of second-order utricular and lagenar neurons were clustered with particular types of second-order canal neurons in a topology that anatomically mirrored the preferred convergence pattern of afferent otolith and canal signals in second-order vestibular neurons. Similarities in the spatial organization of functionally equivalent types of second-order otolith and canal neurons between frog and other vertebrates indicated conservation of a common topographical organization principle. However, the absence of a precise afferent sensory topography combined with the presence of spatially segregated groups of particular second-order vestibular neurons suggests that the vestibular circuitry is organized as a premotor map rather than an organotypical sensory map. Moreover, the conserved segmental location of individual vestibular neuronal phenotypes shows linkage of individual components of vestibulomotor pathways with the underlying genetically specified rhombomeric framework.

Patterns of canal and otolith afferent input convergence in frog second-order vestibular neurons

Second-order vestibular neurons (2 degrees VN) were identified in the isolated frog brain by the presence of monosynaptic excitatory postsynaptic potentials (EPSPs) after separate electrical stimulation of individual vestibular nerve branches. Combinations of one macular and the three semicircular canal nerve branches or combinations of two macular nerve branches were stimulated separately in different sets of experiments. Monosynaptic EPSPs evoked from the utricle or from the lagena converged with monosynaptic EPSPs from one of the three semicircular canal organs in ~30% of 2 degrees VN. Utricular afferent signals converged predominantly with horizontal canal afferent signals (74%), and lagenar afferent signals converged with anterior vertical (63%) or posterior vertical (37%) but not with horizontal canal afferent signals. This convergence pattern correlates with the coactivation of particular combinations of canal and otolith organs during natural head movements. A convergence of afferent saccular and canal signals was restricted to very few 2 degrees VN (3%). In contrast to the considerable number of 2 degrees VN that received an afferent input from the utricle or the lagena as well as from one of the three canal nerves (~30%), smaller numbers of 2 degrees VN (14% of each type of 2 degrees otolith or 2 degrees canal neuron) received an afferent input from only one particular otolith organ or from only one particular semicircular canal organ. Even fewer 2 degrees VN received an afferent input from more than one semicircular canal or from more than one otolith nerve (~7% each). Among 2 degrees VN with afferent inputs from more than one otolith nerve, an afferent saccular nerve input was particularly rare (4-5%). The restricted convergence of afferent saccular inputs with other afferent otolith or canal inputs as well as the termination pattern of saccular afferent fibers are compatible with a substrate vibration sensitivity of this otolith organ in frog. The ascending and/or descending projections of identified 2 degrees VN were determined by the presence of antidromic spikes. 2 degrees VN mediating afferent utricular and/or semicircular canal nerve signals had ascending and/or descending axons. 2 degrees VN mediating afferent lagenar or saccular nerve signals had descending but no ascending axons. The latter result is consistent with the absence of short-latency macular signals on extraocular motoneurons during vertical linear acceleration. Comparison of data from frog and cat demonstrated the presence of a similar organization pattern of maculo- and canal-ocular reflexes in both species.

Spatial distribution of semicircular canal nerve evoked monosynaptic response components in frog vestibular nuclei

Most second-order vestibular neurons receive a canal-specific monosynaptic excitation, although the central projections of semicircular canal afferents overlap extensively. This remarkable canal specificity prompted us to study the spatial organization of evoked field potentials following selective stimulation of individual canal nerves. Electrically evoked responses in the vestibular nuclei were mapped systematically in vitro. Constructed activation maps were superimposed on a cytoarchitectonically defined anatomical map. The spatial activation maps for pre- and postsynaptic response components evoked by stimulation of a given canal nerve were similar. Activation maps for monosynaptic inputs from different canals tended to show a differential distribution of their peak amplitudes, although the overlap was considerable. Anterior vertical canal signals peaked in the superior vestibular nucleus, posterior vertical canal signals peaked in the descending and in the dorsal part of the lateral vestibular nucleus, whereas horizontal canal signals peaked in the descending and in the ventral part of the lateral vestibular nucleus. A similar, differential but overlapping, spatial organization of the canal inputs was described also for other vertebrates, suggesting a crude but rather conservative topographical organization of semicircular canal nerve projections within the vestibular nuclei. Differences in the precision of topological representations between vestibular and other sensory modalities are discussed.

Cross-striolar and commissural inhibition in the otolith system

Neural connections from the saccular and utricular nerves to the ipsilateral vestibular neurons and the commissural effects were studied by using intracellular recordings of excitatory (E) and inhibitory (I) postsynaptic potentials (PSPs) in vestibular neurons of cats after focal stimulation of the saccular and the utricular maculae. Neural circuits from the maculae to vestibular neurons, termed cross-striolar inhibition, may provide a mechanism for increasing the sensitivity to linear acceleration and tilt of the head. It was examined whether secondary vestibular neurons activated by an ipsilateral otolith organ received a commissural inhibition from a contralateral otolith organ that occupied the same geometric plane. Results suggest that utricular-activated vestibular neurons receiving commissural inhibition may provide a mechanism for increasing the sensitivity to horizontal linear acceleration and tilt of the head. The commissural inhibition of the saccular system was much weaker than that of the utricular system.

Canal-specific excitation and inhibition of frog second-order vestibular neurons

Second-order vestibular neurons (secondary VNs) were identified in the in vitro frog brain by their monosynaptic excitation following electrical stimulation of the ipsilateral VIIIth nerve. Ipsilateral disynaptic inhibitory postsynaptic potentials were revealed by bath application of the glycine antagonist strychnine or of the gamma-aminobutyric acid-A (GABA(A)) antagonist bicuculline. Ipsilateral disynaptic excitatory postsynaptic potentials (EPSPs) were analyzed as well. The functional organization of convergent monosynaptic and disynaptic excitatory and inhibitory inputs onto secondary VNs was studied by separate electrical stimulation of individual semicircular canal nerves on the ipsilateral side. Most secondary VNs (88%) received a monosynaptic EPSP exclusively from one of the three semicircular canal nerves; fewer secondary VNs (10%) were monosynaptically excited from two semicircular canal nerves; and even fewer secondary VNs (2%) were monosynaptically excited from each of the three semicircular canal nerves. Disynaptic EPSPs were present in the majority of secondary VNs (68%) and originated from the same (homonymous) semicircular canal nerve that activated a monosynaptic EPSP in a given neuron (22%), from one or both of the other two (heteronymous) canal nerves (18%), or from all three canal nerves (28%). Homonymous activation of disynaptic EPSPs prevailed (74%) among those secondary VNs that exhibited disynaptic EPSPs. Disynaptic inhibitory postsynaptic potentials (IPSPs) were mediated in 90% of the tested secondary VNs by glycine, in 76% by GABA, and in 62% by GABA as well as by glycine. These IPSPs were activated almost exclusively from the same semicircular canal nerve that evoked the monosynaptic EPSP in a given secondary VN. Our results demonstrate a canal-specific, modular organization of vestibular nerve afferent fiber inputs onto secondary VNs that consists of a monosynaptic excitation from one semicircular canal nerve followed by disynaptic excitatory and inhibitory inputs originating from the homonymous canal nerve. Excitatory and inhibitory second-order (secondary) vestibular interneurons are envisaged to form side loops that mediate spatially similar but dynamically different signals to secondary vestibular projection neurons. These feedforward side loops are suited to adjust the dynamic response properties of secondary vestibular projection neurons by facilitating or disfacilitating phasic and tonic input components.

Development of the human lateral vestibular nucleus: a morphometric evaluation

The development of the human lateral vestibular nucleus was studied on serial sections of the brain of 8 fetuses and neonates at 12-40 weeks of gestation, an infant at 2 months of age and an adult of 63 years using a microscope with a drawing tube and an image-analysing computer system. A morphometric analysis revealed that the lateral vestibular nucleus, whose neurons were distinguished from glia after 16 weeks of gestation, divided cytoarchitectonically into the medial and the lateral subnuclei at 21 weeks of gestation onwards, and showed the moderate development in terms of the columnar length and volume, neuronal size and neuropil.

Vestibular effects in long C3-C5 propriospinal neurones

The effects of stimulation of the vestibular nerve and of regions in and around the vestibular nuclei on long C3-C5 propriospinal neurones (PNs) were investigated with intracellular recording. Disynaptic excitatory postsynaptic potentials were evoked from the contralateral (co) or ipsilateral (i) vestibular nerve in many long PNs but mainly in crossed PNs from the co and in uncrossed from the i nerve. Disynaptic inhibitory postsynaptic potentials were evoked more rarely, mainly from the i vestibular nerve. Threshold mapping revealed an excitatory relay from the co nerve in the medial vestibular nucleus (MVN) and also that the excitatory MVN neurones projecting to the long PNs send collaterals to the abducens and interstitial nucleus of Cajal. Excitation from the i vestibular nerve was relayed in the lateral vestibular nucleus (LVN) and in the MVN. Also, non-second order LVN neurones project to the long PNs. Monosynaptic IPSPs were evoked from the i MVN and i LVN.

Distribution of vestibular afferents that innervate the sacculus and posterior canal in the gerbil

The central distribution of afferents that innervate the macula of the saccule and the crista of the posterior canal was assessed in the gerbil following the direct injection of horseradish peroxidase (HRP) separately into the sensory neuroepithelia of each peripheral receptor organ. Ganglion cells innervating the posterior canal were located in the caudal part of the inferior ganglion, while those cells innervating the saccule were located in the rostral part of the inferior ganglion, scattered in the superior ganglion, and concentrated at the junction (isthmus) between the two. The paths of the central axons of these two groups of ganglion cells through the vestibular root and their division into ascending or descending pathways were similar. However, the distributions of their terminals were different. The posterior canal projected to medial parts of the vestibular nuclear complex. Terminals were found in the medial and superior vestibular nuclei. The posterior canal also projected to the uvula of the cerebellum. The saccule projected to more lateral-lying brainstem areas. Terminal fields were located in the lateral and descending vestibular nuclei and cell group y. Saccule projections outside the vestibular complex were observed to the lateral cuneate nucleus, the N. gigantocellularis, and the cerebellar cortex. Of the eight areas receiving primary afferent projections from these two organs, only within the medial and descending vestibular nuclei and the cerebellar cortex were overlapping projections observed.

Electrophysiological analysis of cerebellar corticovestibular and fastigiovestibular projections to the lateral vestibular nucleus in the cat

In the lateral vestibular nucleus, vestibulospinal tract (VST) neurons were surveyed with microelectrodes in cats anesthetized with sodium pentobarbital. The VST neurons (n = 450) were classified by their properties; axonal courses (LVST and MVST). spinal segmental levels of their axonal termination (C1-3, C4-8, T1-13, L1-4, and L5-neurons), their orthodromic activation by the primary vestibular nerve (second-order and non-second-order vestibular neurons), and their location in the LVN. Inhibitory and excitatory effects of cerebellar stimulation on these classified VST neurons were investigated. 84% (259/308) neurons were observed to receive cerebellar corticovestibular inhibition. The rate was high, and almost the same among classified neurons; C1-3 to L5-neurons, and second-order and non-second-order neurons. However, the rate with MVST neurons (69%) was significantly lower than with LVST cells (87%). These neurons which received cerebellar inhibition were distributed in all areas even deep in the rostroventral region of the LVN, while neurons which did not receive were distributed in the ventral region of the LVN. Electrical stimulation of ipsi- and contralateral fastigial nuclei evoked monosynaptic excitation of the classified VST neurons. Rate of occurrence of crossed fastigiovestibular excitation was higher with cervical neurons (86%) than with lumbar neurons (43%), and higher with second-order neurons (78%) than with non-second-order neurons (41%). Neurons which received monosynaptic excitation from crossed fastigiovestibular fibers were distributed in the ventral region of the LVN. In total, 73% of the neurons were identified to receive either ipsi- or contralateral fastigiovestibular excitation. The results indicated that there was relative scarcity of fastigiovestibular projections in the dorsal region of the LVN. Spinovestibular and other afferents to the LVN were also investigated.

Selective retrograde labelling of vestibular efferent neurons with [3H]choline

Following administration of [3H]choline in the lateral semicircular canal of the cat labyrinth, bidirectional axoplasmic transport [3H]choline and its derivatives was shown by radioautography in the vestibular system. Light-microscopic radioautographs exhibited various patterns of radioautographic labelling. First, a diffuse reaction was observed in vestibular nuclei representing anterograde-labelled, vestibular nerve endings. Second, a heavy labelling limited to perikarya was detected in efferent vestibular neurons and corresponded to retrograde transport. The anterograde migration of [3H]choline is known to be non-selective and is related to synthesis of phospholipids, non-diffusable molecules. In contrast, the retrograde perikaryal labelling seems highly selective and related to the cholinergic specificity of the transmitter. The selectivity of such labelling offers a further possibility of identifying cholinergic neurons and is additional evidence that cholinergic mechanisms are involved in the efferent vestibular control.

Neuronal organization of the vestibulospinal system in the cat

In the lateral and descending vestibular nucleus, vestibulospinal neurons were surveyed extra- and intracellularly in cats anesthetized with sodium pentobarbital. The neurons were investigated both by their antidromic activation from the spinal cord (C1, C4, T1, L1 and L5 spinal levels) and from the oculomotor nucleus region, and by orthodromic activation from the vestibular nerve. Axonal courses of vestibulospinal neurons were determined electrophysiologically at C1 level, as medial (MVST) or lateral (LVST). By single and the same electrodes a number of neurons were recorded in wide regions of the lateral and descending vestibular nucleus from single cats. Thus, it became possible to investigate somatotopical localization systematically for the first time with microelectrode techniques. Neurons of origin of the LVST were localized in the lateral vestibular nucleus. Second-order vestibular neurons were localized in the ventral region of the lateral vestibular nucleus, and in the rostral region of the descending vestibular nucleus. Many second-order, double discharge MVST neurons were identified in the descending and lateral vestibular nucleus. Somatotopical localization were recognized, but non-second-order cervical neurons were identified in the dorsal region, although second-order lumbar neurons were identified in the ventral region of the lateral vestibular nucleus. Specific modes of vestibular activation of these vestibulospinal neurons were discussed, and the vestibulospinal systems in the cat and rabbit were discussed.

Properties of secondary vestibular neurons fired by stimulation of ampullary nerve of the vertical, anterior or posterior, semicircular canals in the cat

Experiments on cats were performed to study the pathway and location of the secondary vestibulo-ocular neurons in response to stimulation of the ampullary nerves of the vertical, anterior or posterior, semicircular canals. Experiments on the medial longitudinal fasciculus transection disclosed that vertical canal-evoked, disynaptic excitation and inhibition were transmitted to the extraocular motoneurons through the contra- and ipsilateral medial longitudinal fasciculus respectively. Secondary vestibular neurons, which receive input from the ampullary nerve of the vertical semicircular canals and send their axons to contralateral medial longitudinal fasciculus, were intermingled in the rostral half of the descending and lateral part of the medial vestibular nuclei. A direct excitatory connection of some of these neurons to the target extraocular motoneurons was confirmed by means of a spike-triggered signal averaging technique. It was also found that neurons activated by antidromic stimulation of ipsilateral medial longitudinal fasciculus were located in the superior vestibular nucleus, some of which made direct inhibitory connections to the target extraocular motoneurons. Both excitatory and inhibitory vestibuloocular neurons made synaptic contact in about half of the impaled target motoneurons.

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 efferents 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.

A new stimulation technique of the crista ampullaris of the lateral canal in the adult cat: study of the action potential of the vestibular nerve

The stimulation of the crista ampullaris of the lateral canal by a short ampullopetal liquid flux produces on the pre-ganglionic vestibular fibres the appearance of a bimodal action potential. The amplitude of this action potential increases with the intensity of the stimulation. This stimulation also provokes the birth of an evoked potential at the level of the vestibular nuclei as well as an ocular jerk.

Labyrinthine input to the vestibular nuclei of the awake cat

The labyrinthine input to the vestibular nuclei was investigated in 24 awake cats. Stimulus consisted of electrical shocks given through bipolar silver wire electrodes, implanted in the utricular and lateral ampullar nerves. Throughout the vestibular nuclei, single units were recorded extracellularly with glass micropipettes filled with Fast Green. The tracts of the penetrating electrodes were identified histologically. In all four nuclei units responding to both labyrinths outnumbered unilaterally responding neurones with certain differences between the individual nuclei. Excitatory as well as inhibitory responses were observed, polysynaptic being more common than mono- or disynaptic ones. No monosynaptic contralateral responses were seen. The latency distribution of contralateral responses closely mirrored that of ipsilateral responses within each nucleus. Both excitatory and inhibitory responses fell into relatively segregated populations, based upon latency distribution. This implies separate pathways for labyrinthine input to the vestibular nuclei.

Functional organization of the superior vestibular nucleus of the squirrel monkey

The response to angular acceleration of units in the superior vestibular nucleus (SVN) of barbiturate-anesthetized, cerebellectomized squirrel monkeys was used to study the distribution of semicircualr-canal inputs to the nucleus. Some so-called intact animals had 6 active semicircular canals. In other animals, the 3 canals on one side were rendered nonresponsive by plugging. In plugged animals, superior, posterior, and horizontal-canal units were encountered on both the plugged and unplugged sides, showing that all 6 canals influence the nucleus. Most units responded bilaterally to labyrinthine polarization; 92.5% of units in intact animals responded to angular acceleration, and this incidence was not decreased in plugged animals. These results suggest that most units in the superior nucleus receive bilateral canal inputs. Convergence of influences arising in orthogonally related canals was detected in less than 10% of units, so the bilateral ampullary influences must arise in parallel canals. Most SVN canal units on the plugged and unplugged sides gave type I responses, indicating that the contralateral canal influence is carried by a crossed inhibitory pathway. Most units influenced by the ipsilateral superior canal were located in the lateral half of the SVN. Posterior-canal units were in the medial half. There was no clear localization of the relatively few horizontal-canal units which were encountered.

Horizontal vestibular nystagmus. I. Identification of medial vestibular neurones

1. The properities of inputs from the horizontal semi-circular canal to neurones of the medial vestibular nucleus have been studied intracellularly in the unanaesthetized encéphale isolé cat. 2. Secondary neurones of the bestibulo-abducens reflex arc were identified by their orthodromic response to labyrinthine stimulation and by antidromic excitation from the contralateral abducens nucleus. 3. The responses of medial vestibular cells receiving only labyrinthine in puts are also described. These were seen to be predominantly excitatory though IPSPs were observed in a few cases. 4. Identified vestibular neurones were intracellularly injected with procion yellow and showed different morphological characteristics correlated with function.

The structural maturation of the stato-acoustic nerve in the cat

The maturation of the stato-acoustic nerve in the cat was studied by light and electron microscopy from the fetal stage to the adult. Measurement of the outer diameter of the fibers and the study of the myelination process revealed that myelination begins earlier for the vestibular nerve than for the cochlear nerve: by the fifty-third day of gestation 64% of the vestibular fibres have already passed the promyelin stage whereas for the cochlear nerve this promyelin stage begins for the majority of fibers on the fifty-seventh gestation day. Afterward, maturation proceeds more rapidly for the cochlear nerve. In the case of both nerves, maturation is still incomplete at two months of age. Concerning the relationship between the thickness of the myelin sheath and the axoplasmic diameter, there is already a good correlation by the fifty-seventh day of gestation in the vestibular nerve, whereas it appears several days after birth in the cochlear nerve.