Pathways for the vestibulo-ocular reflex excitation arising from semicircular canals of rabbits

Vestibular control of the head: possible functions of the vestibulocollic reflex

Here, we review the angular vestibulocollic reflex (VCR) focusing on its function during unexpected and voluntary head movements. Theoretically, the VCR could (1) stabilize the head in space during body movements and/or (2) dampen head oscillations that could occur as a result of the head's underdamped mechanics. The reflex appears unaffected when the simplest, trisynaptic VCR pathways are severed. The VCR's efficacy varies across species; in humans and monkeys, head stabilization is ineffective during low-frequency body movements in the yaw plan. While the appearance of head oscillations after the attenuation of semicircular canal function suggests a role in damping, this interpretation is complicated by defects in the vestibular input to other descending motor pathways such as gaze premotor circuits. Since the VCR should oppose head movements, it has been proposed that the reflex is suppressed during voluntary head motion. Consistent with this idea, vestibular-only (VO) neurons, which are possible vestibulocollic neurons, respond vigorously to passive, but not active, head rotations. Although VO neurons project to the spinal cord, their contribution to the VCR remains to be established. VCR cancelation during active head movements could be accomplished by an efference copy signal negating afferent activity related to active motion. Oscillations occurring during active motion could be eliminated by some combination of reflex actions and voluntary motor commands that take into account the head's biomechanics. A direct demonstration of the status of the VCR during active head movements is required to clarify the function of the reflex.

Peaks and troughs of three-dimensional vestibulo-ocular reflex in humans

The three-dimensional vestibulo-ocular reflex (3D VOR) ideally generates compensatory ocular rotations not only with a magnitude equal and opposite to the head rotation but also about an axis that is collinear with the head rotation axis. Vestibulo-ocular responses only partially fulfill this ideal behavior. Because animal studies have shown that vestibular stimulation about particular axes may lead to suboptimal compensatory responses, we investigated in healthy subjects the peaks and troughs in 3D VOR stabilization in terms of gain and alignment of the 3D vestibulo-ocular response. Six healthy upright sitting subjects underwent whole body small amplitude sinusoidal and constant acceleration transients delivered by a six-degree-of-freedom motion platform. Subjects were oscillated about the vertical axis and about axes in the horizontal plane varying between roll and pitch at increments of 22.5° in azimuth. Transients were delivered in yaw, roll, and pitch and in the vertical canal planes. Eye movements were recorded in with 3D search coils. Eye coil signals were converted to rotation vectors, from which we calculated gain and misalignment. During horizontal axis stimulation, systematic deviations were found. In the light, misalignment of the 3D VOR had a maximum misalignment at about 45°. These deviations in misalignment can be explained by vector summation of the eye rotation components with a low gain for torsion and high gain for vertical. In the dark and in response to transients, gain of all components had lower values. Misalignment in darkness and for transients had different peaks and troughs than in the light: its minimum was during pitch axis stimulation and its maximum during roll axis stimulation. We show that the relatively large misalignment for roll in darkness is due to a horizontal eye movement component that is only present in darkness. In combination with the relatively low torsion gain, this horizontal component has a relative large effect on the alignment of the eye rotation axis with respect to the head rotation axis.

Upbeat-torsional nystagmus and contralateral fourth-nerve palsy due to unilateral dorsal ponto mesencephalic lesion

The central projections of the anterior semicircular canals are thought to be conveyed from the vestibular nuclei to the oculomotor nuclei in the midbrain by three distinct brainstem pathways: the medial longitudinal fasciculus, crossing ventral tegmental tract, and brachium conjunctivum. There is controversy as to whether upbeat nystagmus could result from lesions involving each of these pathways. We report a 52-year-old man who presented with a contralesional fourth-nerve palsy and primary-position upbeat-torsional nystagmus due to a small unilateral dorsal pontomesencephalic lymphomatous deposit. It is postulated that the upbeat-torsional nystagmus was caused by involvement of the brachium conjunctivum, which lies adjacent to the fourth-nerve fascicles at the dorsal pontomesencephalic junction, but involvement of the crossing ventral tegmental tract cannot be excluded. These observations suggest that, in humans, excitatory upward-torsional eye movement signals from the anterior semicircular canals could be partly conveyed to the midbrain by the brachium conjunctivum.

Commissural mirror-symmetric excitation and reciprocal inhibition between the two superior colliculi and their roles in vertical and horizontal eye movements

The functional roles of commissural excitation and inhibition between the two superior colliculi (SCs) are not yet well understood. We previously showed the existence of strong excitatory commissural connections between the rostral SCs, although commissural connections had been considered to be mainly inhibitory. In this study, by recording intracellular potentials, we examined the topographical distribution of commissural monosynaptic excitation and inhibition from the contralateral medial and lateral SC to tectoreticular neurons (TRNs) in the medial or lateral SC of anesthetized cats. About 85% of TRNs examined projected to both the ipsilateral Forel's field H and the contralateral inhibitory burst neuron region where the respective premotor neurons for vertical and horizontal saccades reside. Medial TRNs received strong commissural excitation from the medial part of the opposite SC, whereas lateral TRNs received excitation mainly from its lateral part. Injection of wheat germ agglutinin-horseradish peroxidase into the lateral or medial SC retrogradely labeled many larger neurons in the lateral or medial part of the contralateral SC, respectively. These results indicated that excitatory commissural connections exist between the medial and medial parts and between the lateral and lateral parts of the rostral SCs. These may play an important role in reinforcing the conjugacy of upward and downward saccades, respectively. In contrast, medial SC projections to lateral SC TRNs and lateral SC projections to medial TRNs mainly produce strong inhibition. This shows that regions representing upward saccades inhibit contralateral regions representing downward saccades and vice versa.

Mechanisms of cerebellar gait ataxia

The cerebellum is important for movement control and plays a critical role in balance and locomotion. As such, one of the most characteristic and sensitive signs of cerebellar damage is gait ataxia. How the cerebellum normally contributes to locomotor behavior is unknown, though recent work suggests that it helps generate appropriate patterns of limb movements, dynamically regulate upright posture and balance, and adjust the feedforward control of locomotor output through error-feedback learning. The purpose of this review is to examine mechanisms of cerebellar control of locomotion, emphasizing studies of humans and other animals. Implications for rehabilitation are also considered.

Neural Organization of the Pathways From the Superior Colliculus to Trochlear Motoneurons

The neural organization of the pathways from the superior colliculus (SC) to trochlear motoneurons was analyzed in anesthetized cats using intracellular recording and transneuronal labeling techniques. Stimulation of the ipsilateral or contralateral SC evoked excitation and inhibition in trochlear motoneurons with latencies of 1.1–2.3 and 1.1–3.8 ms, respectively, suggesting that the earliest components of excitation and inhibition were disynaptic. A midline section between the two SCs revealed that ipsi- and contralateral SC stimulation evoked disynaptic excitation and inhibition in trochlear motoneurons, respectively. Premotor neurons labeled transneuronally after application of wheat germ agglutinin-conjugated horseradish peroxidase into the trochlear nerve were mainly distributed ipsilaterally in the Forel's field H (FFH) and bilaterally in the interstitial nucleus of Cajal (INC). Consequently, we investigated these two likely intermediaries between the SC and trochlear nucleus electrophysiologically. Stimulation of the FFH evoked ipsilateral mono- and disynaptic excitation and contralateral disynaptic inhibition in trochlear motoneurons. Preconditioning stimulation of the ipsilateral SC facilitated FFH-evoked monosynaptic excitation. Stimulation of the INC evoked ipsilateral monosynaptic excitation and inhibition, and contralateral monosynaptic inhibition in trochlear motoneurons. Preconditioning stimulation of the contralateral SC facilitated contralateral INC-evoked monosynaptic inhibition. These results revealed a reciprocal input pattern from the SCs to vertical ocular motoneurons in the saccadic system; trochlear motoneurons received disynaptic excitation from the ipsilateral SC via ipsilateral FFH neurons and disynaptic inhibition from the contralateral SC via contralateral INC neurons. These inhibitory INC neurons were considered to be a counterpart of inhibitory burst neurons in the horizontal saccadic system.

Physiological and Anatomical Properties of Mouse Medial Vestibular Nucleus Neurons Projecting to the Oculomotor Nucleus

Neurons in the medial vestibular nucleus (MVN) vary in their projection patterns, responses to head movement, and intrinsic firing properties. To establish whether neurons that participate in the vestibulo-ocular reflex (VOR) have distinct intrinsic physiological properties, oculomotor nucleus (OMN)–projecting neurons were identified in mouse brainstem slices by fluorescent retrograde labeling from the oculomotor complex and targeted for patch-clamp recordings. Such neurons were located in the magnocellular portion of the MVN contralateral to tracer injection, were mostly multipolar, and had soma diameters of around 20 μm. They fired spontaneous action potentials at rates higher than those of other MVN neurons and their spikes were of unusually short duration. OMN-projecting neurons responded to 1-s intracellular current injection with exceptionally high firing rates of >500 spikes/s. Their current–firing relationship was highly linear, with weak firing response adaptation during steady depolarization and little postinhibitory rebound firing after membrane hyperpolarization. Their firing responses were approximately in phase with sinusoidal current injection. The response dynamics of OMN-projecting neurons could be simulated with a simple integrate-and-fire model modified with the addition of small adaptation and rebound conductances. These findings indicate that the membrane properties of OMN-projecting neurons allow them to respond to head movements reliably and with high sensitivity but without substantially altering input dynamics.

Location of efferent terminals of the primate flocculus and ventral paraflocculus revealed by anterograde axonal transport methods

Efferents of the flocculus (FL) and ventral paraflocculus (VP) were examined in seven anesthetized Macaca fuscata by anterograde axonal transport method using wheat germ agglutinin-conjugated horseradish peroxidase or phaseolus vulgaris leucoagglutinin. Several major foci of axon terminals were found in the vestibular nuclear complex and cerebellar nuclei. A difference was seen in the location of efferent terminals between the FL and VP. When the tracer covered the FL, labeled axon terminals were located within the medial and ventrolateral parts of the medial vestibular nucleus, superior vestibular nucleus and y-group. When the tracer covered the VP, labeled axon terminals were located within the caudo-ventral part of posterior interpositus and dentate nuclei, in addition to the medial and ventrolateral parts of the medial vestibular nucleus, superior vestibular nucleus and y-group. Labeled terminals were virtually absent in the basal interstitial nucleus of the cerebellum. On the points of neo- or paleo-cerebellar cortex fiber connections, these results correspond to our previous anatomical observations that the FL received mossy fiber afferents mainly from the vestibular system and nucleus reticularis tegmenti pontis and very little from the pontine nuclei, whereas the VP received mossy afferents mainly from the nucleus reticularis tegmenti pontis and pontine nuclei and very little from the vestibular system. These anatomical observations are consistent with a hypothesis in our previous anatomical and physiological study that the primate FL and VP mediate rather different functional roles in the oculomotor control.

Behavioral analysis of signals that guide learned changes in the amplitude and dynamics of the vestibulo-ocular reflex

We characterized the dependence of motor learning in the monkey vestibulo-ocular reflex (VOR) on the duration, frequency, and relative timing of the visual and vestibular stimuli used to induce learning. The amplitude of the VOR was decreased or increased through training with paired head and visual stimulus motion in the same or opposite directions, respectively. For training stimuli that consisted of simultaneous pulses of head and target velocity 80-1000 msec in duration, brief stimuli caused small changes in the amplitude of the VOR, whereas long stimuli caused larger changes in amplitude as well as changes in the dynamics of the reflex. When the relative timing of the visual and vestibular stimuli was varied, brief image motion paired with the beginning of a longer vestibular stimulus caused changes in the amplitude of the reflex alone, but the same image motion paired with a later time in the vestibular stimulus caused changes in the dynamics as well as the amplitude of the VOR. For training stimuli that consisted of sinusoidal head and visual stimulus motion, low-frequency training stimuli induced frequency-selective changes in the VOR, as reported previously, whereas high-frequency training stimuli induced changes in the amplitude of the VOR that were more similar across test frequency. The results suggest that there are at least two distinguishable components of motor learning in the VOR. One component is induced by short-duration or high-frequency stimuli and involves changes in only the amplitude of the reflex. A second component is induced by long-duration or low-frequency stimuli and involves changes in the amplitude and dynamics of the VOR.

Optokinetic response of simple spikes of Purkinje cells in the cerebellar flocculus and nodulus of the pigmented rabbit

Under anesthesia with N2O (70%) and halothane (2-4%), Purkinje cell activities were extracellularly recorded in the flocculus and nodulus of immobilized pigmented rabbits. Large field (60 degrees x 60 degrees) optokinetic stimulation (OKS) was delivered to the central visual field of one eye with a constant velocity (0.1-4.0 degrees/s) at 0 degrees, 45 degrees, 90 degrees or 135 degrees to the horizontal plane of the eye. Most of the Purkinje cells in the flocculus and the nodulus showed significant simple spike modulations to OKS delivered to either eye. As a whole, the preferred directions of simple spike responses in the flocculus had the same orientation as those of complex spike responses. However, the preferred directions and amplitudes of modulation of simple spike responses did not necessarily correlate with those of complex spike responses in individual flocculus Purkinje cells. On the other hand, the preferred directions of simple and complex spike responses were not necessarily in the same orientation in the nodulus. The optimum velocity for simple spike responses was in the range 0.1-2.0 degrees/s for Purkinje cells in both the flocculus and the nodulus. The amplitude and time to peak of the simple spike responses of nodulus Purkinje cells were significantly smaller and longer, respectively, than those of flocculus Purkinje cells. In both the flocculus and the nodulus, Purkinje cells whose simple spikes preferred the horizontal orientation (H cells) and the vertical orientation (V cells) showed clustering. In particular, zonal organization was noted in the flocculus. H cells were localized in a dorso-ventral zone in the rostral one third of the flocculus, and V cells were in two distinct zones rostral and caudal to the H cell zone. The locations of H and V cells in the flocculus correspond to the H zone and V zones, respectively, determined on the basis of the preferred directions of complex spike responses to OKS. This indicates that the same subdivisions of the flocculus are supplied with optokinetic signals with the same orientation selectively through both mossy and climbing fibers, and suggest that such subdivisions of the flocculus are functional units which control horizontal and vertical components of optokinetic eye movements. The present results indicate that the flocculus and the nodulus are supplied with distinct optokinetic signals through mossy fibers and play different roles in controlling optokinetic eye movements.

Upbeat and downbeat nystagmus occurring successively in a patient with posterior medullary haemorrhage

In a patient with posterior medullary haemorrhage, first upbeat and later downbeat nystagmus occurred in the primary position. The lesion was limited to the posterior and medial part of the medulla. Clinical and electro-oculographic examination first showed upbeat nystagmus in the primary position and upgaze, with downbeat nystagmus in downgaze. Two and a half months later, there was downbeat nystagmus in the primary position and downgaze and upbeat nystagmus in upgaze.

Axonal trajectories of posterior canal-activated secondary vestibular neurons and their coactivation of extraocular and neck flexor motoneurons in the cat

Unit activities of 148 secondary vestibular neurons related to the posterior semicircular canal were recorded extracellularly in anesthetized cats. Axonal projections of these neurons were examined by their antidromic responses to stimulation of the excitatory target motoneurons of the contralateral (c-) inferior rectus muscle (IR) and bilateral (bi-) motoneuron pools of longus capitis muscles, neck flexors, in the C1 segment (C1LC). The neurons were classified into 4 groups according to their axonal projections. The first group of neurons, termed vestibulo-oculo-collic (VOC) neurons, sent axon collaterals both to the c-IR motoneuron pool and to the c-C1LC motoneuron pool. The majority of them (72%) were located in the descending nucleus. The second group of neurons were termed vestibuloocular (VO) neurons and sent their axons to the c-IR motoneuron pool but not to the cervical cord. Most of them (86%) were located in the medial nucleus. The third group of neurons, termed vestibulo-collic (contralateral) (VCc) neurons, sent axons to the c-C1LC motoneuron pool via the contralateral ventral funiculus but not to the oculomotor nuclei. They were mostly (75%) found in the descending nucleus. The last group of neurons were vestibulo-collic (ipsilateral) (VCi) neurons, which gave off axons to the ipsilateral (i-) C1LC motoneuron pool via the ipsilateral ventral funiculus but not to the oculomotor nuclei. One of them also sent an axon collateral to the c-C1LC motoneuron pool. The majority of them (74%) were located in the ventral part of the lateral nucleus. It was also observed in some of the VOC and VCi neurons that they produced unitary EPSPs in the c-C1LC and i-C1LC motoneurons, respectively. Their synaptic sites were estimated to be on the cell somata and/or proximal dendrites of the motoneurons.

The development of the static vestibulo-ocular reflex in the southern clawed toad, Xenopus laevis. II. Animals with acute vestibular lesions

Acute hemilabyrinthectomized tadpoles of the Southern Clawed Toad (Xenopus laevis), younger than stage 47 (about 6 days old), perform no static vestibulo-ocular reflex (Fig. 1). Older acute lesioned animals respond with compensatory movements of both eyes during static roll. Their threshold roll angle, however, depends on the developmental stage. For lesioned stages 60 to 64, it is 75 degrees while stage 52 to 56 tadpoles respond even during a lateral roll of 15 degrees (Figs. 1 and 2). Selective destruction of single macula and crista organs revealed that the static vestibulo-ocular reflex is evoked by excitation of the macula utriculi (Figs. 3 and 4) even in young tadpoles. The results demonstrate that bilateral projections of the vestibular apparatus must have developed at the time of occurrence of the static VOR, that during the first week of life the excitation of a single labyrinth is subthreshold (Fig. 1). We discuss the possibility whether the loss of the static VOR during the prometamorphic period of life (Fig. 2) is caused by increasing formation of multimodal connections in the vestibular pathway.

Topographical representation of vestibulo-ocular reflexes in rabbit cerebellar flocculus

In anaesthetized albino rabbits, the cerebellar flocculus was systematically mapped with a glass microelectrode to identify the location of Purkinje cells that inhibit specific vestibulo-ocular reflex pathways. The effects of microstimulation of the flocculus Purkinje cell layer on vestibular nerve-evoked reflexes to ipsilateral medial rectus, ipsilateral superior rectus and contralateral inferior oblique muscles were explored by recording electromyographically. Visual climbing fibre inputs to the flocculus were also studied by mapping field potentials evoked from both retinae. The results suggest that there are microzones in the flocculus that are related specifically to these three vestibulo-ocular reflex pathways and to different visual climbing fibre pathways.

Inhibitory interaction between the vestibulo-ocular reflexes arising from semicircular canals of rabbits

In anesthetized albino rabbits, electric pulse stimulation was applied to ampullary branches of the vestibular nerve. Reflex discharges evoked from a canal in an extraocular muscle were depressed very effectively by conditioning stimulation at a certain other canal. The present systematic survey revealed that this reflex depression occurred specifically in 3 combinations of conditioning and testing canals; 1. anterior and posterior canals of the same side; 2. anterior and posterior canals of the opposite sides; and 3. horizontal canals of the two sides. Occurrence of postsynaptic inhibition in oculomotor neurons, on the other hand, was indicated by appearance of slow muscle potentials in extraocular muscles. It was confirmed that this motoneuronal inhibition did not contribute to the reflex depression in the above combination (1). Even in combinations (2) and (3), the accompanying motoneuronal inhibition was eliminated by adjusting intensities of canal stimuli or by severing its pathway in the medulla, or it was discriminated from the reflex depression by their different latencies and time courses. Hence, it was concluded that the reflex depression was attributable, at least largely, to non-motoneuronal inhibition, presumably postsynaptic inhibition at relay neurons for vestibulo-ocular reflexes. Slow muscle potentials evoked from a canal were also used as testing responses, but their depression could not be detected after conditioning at other canals.

Postsynaptic inhibition of oculomotor neurons involved in vestibulo-ocular reflexes arising from semicircular canals of rabbits

In anesthetized albino rabbits, electric stimulation of vestibular nerve branches innervating semicircular canals produced not only reflex contraction in certain extraocular muscles, but also a transient relaxation in others. From relaxing muscles was recorded a slow muscle potential that reflected depression of spontaneous spike discharges in muscle fibers. When recorded monophasically, spontaneous spikes of muscle fibers were superposed to form a direct current potential, and depression of the spikes resulted in a transient reduction of this direct current potential, i.e., the slow muscle potential. The slow muscle potential was correlated to the postsynaptic inhibition induced in oculomotor neurons through the vestibulo-ocular reflex are for the following reasons; its latency was compatible with that of the IPSP's recorded from oculomotor neurons; it was removed by severing axons of the inhibitory second-order vestibular neurons; it was blocked by intravenous injection of picrotoxin as were the IPSP's in oculomotor neurons. By recording slow muscle potentials, a specific canal-muscle relationship for the vestibulo-ocular reflex inhibition of oculomotor neurons was shown to be complementary to that obtained for the vestibulo-ocular reflex excitation.