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

Properties of Deiters' neurons and inhibitory synaptic transmission in the mouse lateral vestibular nucleus

Deiters' neurons, located exclusively in the lateral vestibular nucleus (LVN), are involved in vestibulospinal reflexes, innervate extensor motoneurons that drive anti-gravity muscles, and receive inhibitory inputs from the cerebellum. We investigated intrinsic membrane properties, short-term plasticity, and inhibitory synaptic inputs of mouse Deiters' and non-Deiters' neurons within the LVN. Deiters' neurons are distinguished from non-Deiters' neurons by their very low input resistance (105.8 vs 521.8 MOhms) respectively, long axons that project as far as the ipsilateral lumbar spinal cord, and expression of the cytostructural protein, non-phosphorylated neurofilament protein (NPNFP). Whole-cell patch clamp recordings in brainstem slices show most Deiters' and non-Deiters' neurons were tonically active (>92%). Short-term plasticity was studied by examining discharge rate modulation following release from hyperpolarization (post-inhibitory rebound firing; PRF) and depolarization (firing rate adaptation; FRA). PRF and FRA gain were similar in Deiters' and non-Deiters' neurons (PRF: 24.9 vs. 20.2 Hz and FRA gain: 231.5 vs. 287.8 spikes/sec/nA respectively). Inhibitory synaptic input to both populations showed GABAergic rather than glycinergic inhibition dominated in Deiters' neurons and GABA_A miniature inhibitory postsynaptic current (mIPSC) frequency was much higher in Deiters' neurons compared to non-Deiters' neurons (~15.9 vs. 1.4 Hz respectively). Our data suggest Deiters' neurons can be reliably identified by their intrinsic membrane and synaptic properties. They are tonically active, glutamatergic, have low sensitivity or 'gain', exhibit little adaptation, and receive strong GABAergic input. Together, these features suggest, since Deiters' neurons have minimal short-term plasticity they are well-suited to a role encoding tonic signals for the vestibulospinal reflex.

Deiters' Nucleus. Its Role in Cerebellar Ideogenesis : The Ferdinando Rossi Memorial Lecture

Otto Deiters (1834-1863) was a promising neuroscientist who, like Ferdinando Rossi, died too young. His notes and drawings were posthumously published by Max Schultze in the book "Untersuchungen über Gehirn und Rückenmark." The book is well-known for his dissections of nerve cells, showing the presence of multiple dendrites and a single axon. Deiters also made beautiful drawings of microscopical sections through the spinal cord and the brain stem, the latter showing the lateral vestibular nucleus which received his name. This nucleus, however, should be considered as a cerebellar nucleus because it receives Purkinje cell axons from the vermal B zone in its dorsal portion. Afferents from the labyrinth occur in its ventral part. The nucleus gives rise to the lateral vestibulospinal tract. The cerebellar B module of which Deiters' nucleus is the target nucleus was used in many innovative studies of the cerebellum on the zonal organization of the olivocerebellar projection, its somatotopical organization, its microzones, and its role in posture and movement that are the subject of this review.

Neural correlates of motor learning in the vestibulo-ocular reflex: dynamic regulation of multimodal integration in the macaque vestibular system

Motor learning is required for the reacquisition of skills that have been compromised as a result of brain lesion or disease, as well as for the acquisition of new skills. Behaviors with well characterized anatomy and physiology are required to yield significant insight into changes that occur in the brain during motor learning. The vestibulo-ocular reflex (VOR) is well suited to establish connections between neurons, neural circuits, and motor performance during learning. Here, we examined the linkage between neuronal and behavioral VOR responses in alert behaving monkeys (Macaca mulatta) during the impressive recovery that occurs after unilateral vestibular loss. We show, for the first time, that motor learning is characterized by the dynamic reweighting of inputs from different modalities (i.e., vestibular vs extravestibular) at the level of the single neurons that constitute the first central stage of vestibular processing. Specifically, two types of information, which did not influence neuronal responses before the lesion, had an important role during compensation. First, unmasked neck proprioceptive inputs played a critical role in the early stages of this process demonstrated by faster and more substantial recovery of vestibular responses in proprioceptive sensitive neurons. Second, neuronal and VOR responses were significantly enhanced during active relative to passive head motion later in the compensation process (>3 weeks). Together, our findings provide evidence linking the dynamic regulation of multimodal integration at the level of single neurons and behavioral recovery, suggesting a role for homeostatic mechanisms in VOR motor learning.

Different neural strategies for multimodal integration: comparison of two macaque monkey species

The integration of neck proprioceptive and vestibular inputs underlies the generation of accurate postural and motor control. Recent studies have shown that central mechanisms underlying the integration of these sensory inputs differ across species. Notably, in rhesus monkey (Macaca mulata), an Old World monkey, neurons in the vestibular nuclei are insensitive to passive stimulation of neck proprioceptors. In contrast, in squirrel monkey, a New World monkey, stimulation produces robust modulation. This has led to the suggestion that there are differences in how sensory information is integrated during self-motion in Old versus New World monkeys. To test this hypothesis, we recorded from neurons in the vestibular nuclei of another species in the Macaca genus [i.e., M. fascicularis (cynomolgus monkey)]. Recordings were made from vestibular-only (VO) and position-vestibular-pause (PVP) neurons. The majority (53%) of neurons in both groups were sensitive to neck proprioceptive and vestibular stimulation during passive body-under-head and whole-body rotation, respectively. Furthermore, responses during passive rotations of the head-on-body were well predicted by the linear summation of vestibular and neck responses (which were typically antagonistic). During active head movement, the responses of VO and PVP neurons were further attenuated (relative to a model based on linear summation) for the duration of the active head movement or gaze shift, respectively. Taken together, our findings show that the brain's strategy for the central processing of sensory information can vary even within a single genus. We suggest that similar divergence may be observed in other areas in which multimodal integration occurs.

Changes of amino acid concentrations in the rat vestibular nuclei after inferior cerebellar peduncle transection

Although there is a close relationship between the vestibular nuclear complex (VNC) and the cerebellum, little is known about the contribution of cerebellar inputs to amino acid neurotransmission in the VNC. Microdissection of freeze-dried brain sections and high-performance liquid chromatography (HPLC) were combined to measure changes of amino acid concentrations within the VNC of rats following transection of the cerebellovestibular connections in the inferior cerebellar peduncle. Distributions of 12 amino acids within the VNC at 2, 4, 7, and 30 days after surgery were compared with those for control and sham-lesioned rats. Concentrations of gamma-aminobutyric acid (GABA) decreased by 2 days after unilateral peduncle transection in nearly all VNC regions on the lesioned side and to lesser extents on the unlesioned side and showed partial recovery up to 30 days postsurgery. Asymmetries between the two sides of the VNC were maintained through 30 days. Glutamate concentrations were reduced bilaterally in virtually all regions of the VNC by 2 days and showed complete recovery in most VNC regions by 30 days. Glutamine concentrations increased, starting 2 days after surgery, especially on the lesioned side, so that there was asymmetry generally opposite that of glutamate. Concentrations of taurine, aspartate, and glycine also underwent partially reversible changes after peduncle transection. The results suggest that GABA and glutamate are prominent neurotransmitters in bilateral projections from the cerebellum to the VNC and that amino acid metabolism in the VNC is strongly influenced by its cerebellar connections.

Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. I. Walking on a level surface

To examine the function of descending brain stem pathways in the control of locomotion, we have characterized the discharge patterns of identified vestibulo- and reticulospinal neurons (VSNs and RSNs, respectively) recorded from the lateral vestibular nucleus (LVN) and the medullary reticular formation (MRF), during treadmill walking. Data during locomotion were obtained for 44 VSNs and for 63 RSNs. The discharge frequency of most VSNs (42/44) was phasically modulated in phase with the locomotor rhythm and the averaged peak discharge frequency ranged from 41 to 165 Hz (mean = 92.8 Hz). We identified three classes of VSNs based on their discharge pattern. Type A, or double peak, VSNs (20/44 neurons, 46%) showed two peaks and two troughs of activity in each step cycle. One of the peaks was time-locked to the activity of extensor muscles in the ipsilateral hindlimb while the other occurred anti-phase to this period of activity. Type B, or single pause, neurons (13/44 neurons, 30%) were characterized by a tonic or irregular discharge that was interrupted by a single pronounced and brief period of decreased activity that occurred just before the onset of swing in the ipsilateral hindlimb; some type B VSNs also exhibited a brief pulse of activity just preceding this decrease. Type C, or single peak, neurons (9/44 neurons, 23%) exhibited a single period of increased activity that, in most cells, was time-locked to the burst of activity of either extensor or flexor muscles of a single limb. The population of RSNs that we recorded included neurons that showed phasic activity related to the activity of flexor or extensor muscles [electromyographically (EMG) related, 26/63, 41%], those that were phasically active but whose activity was not time-locked to the activity of any of the recorded muscles (13/63, 21%) and those that were completely unrelated to locomotion (24/63, 38%). Most of the EMG-related RSNs showed one (15/26) or two (11/26) clear phasic bursts of activity that were temporally related to either flexor or extensor muscles. The discharge pattern of double-burst RSNs covaried with ipsilateral and contralateral flexor muscles. Peak averaged discharge activity in these EMG-related RSNs ranged from 4 to 98 Hz (mean = 35.2 Hz). We discuss the possibility that most VSNs regulate the overall activity of extensor muscles in the four limbs while RSNs provide a more specific signal that has the flexibility to modulate the activity of groups of flexor and extensor muscles, in either a single or in multiple limbs.

Interdependence of spatial properties and projection patterns of medial vestibulospinal tract neurons in the cat

Activity of vestibular nucleus neurons with axons in the ipsi- or contralateral medial vestibulospinal tract was studied in decerebrate cats during sinusoidal, whole-body rotations in many planes in three-dimensional space. Antidromic activation of axon collaterals distinguished between neurons projecting only to neck segments from those with collaterals to C6 and/or oculomotor nucleus. Secondary neurons were identified by monosynaptic activation after labyrinth stimulation. A three-dimensional maximum activation direction vector (MAD) summarized the spatial properties of 151 of 169 neurons. The majority of secondary neurons (71%) terminated above the C6 segment. Of these, 43% had ascending collaterals to the oculomotor nucleus (VOC neurons), and 57% did not (VC neurons). The majority of VOC and VC neurons projected contralaterally and ipsilaterally, respectively. Most C6-projecting neurons could not be activated from oculomotor nucleus (V-C6 neurons) and projected primarily ipsilaterally. All VO-C6 neurons projected contralaterally. The distributions of MADs for secondary neurons with different projection patterns were different. Most VOC (84%) and contralaterally projecting VC (91%) neurons had MADs close to the activation vector of a semicircular canal pair, compared with 54% of ipsilaterally projecting VC (i-VC) and 39% of V-C6 neurons. Many i-VC (44%) and V-C6 (48%) neurons had responses suggesting convergent input from horizontal and vertical canal pairs. Horizontal and vertical gains were comparable for some, making it difficult to assign a primary canal input. MADs consistent with vertical-vertical canal pair convergence were less common. Type II yaw or type II roll responses were seen for 22% of the i-VC neurons, 68% of the V-C6 neurons, and no VOC cells. VO-C6 neurons had spatial properties between those of VOC and V-C6 neurons. These results suggest that secondary VOC neurons convey semicircular canal pair signals to both ocular and neck motor centers, perhaps linking eye and head movements. Secondary VC and V-C6 neurons carry more processed signals, possibly to drive neck and forelimb reflexes more selectively. Two groups of secondary i-VC neurons exhibited vertical-horizontal canal convergence similar to that present on neck muscles. The vertical-vertical canal convergence present on many neck muscles, however, was not present on medial vestibulospinal neurons. Spatial transformations achieved by the vestibulocollic reflex may occur in part on secondary neurons but further combination of canal signals must take place to generate compensatory muscle activity.

Effects of motor cortex and single muscle stimulation on neurons of the lateral vestibular nucleus in the rat

The neuronal responses to stimulation of motor cortical sites and of forelimb single muscles were studied in the lateral vestibular nucleus of anaesthetized rats. Of the 228 neurons tested for response to stimulation of contralateral motor cortex, 63% responded to cortical sites controlling extensor muscles and 30% to those controlling flexors. The corresponding figures for responders to ipsilateral stimulation were 34 and 21%. Vestibulospinal units responded to cortical sites controlling extensor and flexor muscles whereas the remaining lateral vestibular nucleus neurons, very reactive to cortical sites controlling extensor muscles, responded little to contralateral and not at all to ipsilateral cortical sites controlling flexor muscles. The effects evoked by contralateral cortical sites controlling extensors varied, those induced by cortical sites controlling flexors were inhibitory in 77% of cases. The responses to ipsilateral motor cortex stimulation differed not so much by cortical sites controlling extensor or flexor muscles as by whether the neuron was in the dorsal or ventral zone of the lateral vestibular nucleus: mixed in the former, all inhibitory in the latter. Of the lateral vestibular nucleus units tested for response to stimulation of ipsilateral or contralateral forelimb distal muscles, only 11% responded. All the vestibulospinal units responsive to muscle stimulation lay in the dorsal zone of the nucleus. The remainder, dorsal or ventral, were not responsive to contralateral muscles. Single lateral vestibular nucleus cells influenced both by ipsilateral muscle and by contralateral motor cortex made up 24% of the pool, vestibulospinal and non-vestibulospinal. They fell into three groups: responsive to one or both structures but responding more strongly to combined stimulation; responsive to each of the two structures but showing a response to combined stimulation not significantly different from that evoked by the cortex alone; responsive only to combined stimulation. The lateral vestibular nucleus units included in these three groups accounted for 29% of those tested for response to extensor muscles and cortical sites controlling extensors and 15% of those tested for response to flexor muscles and cortical sites controlling flexors. Twenty-five per cent of the vestibulospinal neurons responded both to contralateral muscles and to ipsilateral motor cortex stimulation but none of the non-vestibulospinal neurons responded to both. All the responders to both were in the dorsal zone of the lateral vestibular nucleus and responded to extensor stimuli, always in the same way. These results indicate that motor cortex output exerts a major influence on lateral vestibular nucleus discharges, while the muscle afferents have a modulatory influence on the lateral vestibular nucleus responses to cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of locus coeruleus and subcoeruleus neurons to sinusoidal neck rotation in decerebrate cat

The electrical activity of 99 neurons located in the locus coeruleus-complex, namely in the dorsal (n = 26) and the ventral part of the locus coeruleus (n = 46) as well as the locus subcoeruleus (n = 27), has been recorded in precollicular decerebrate cats during sinusoidal displacement of the neck. This was achieved by rotation of the body about the longitudinal axis of the animal, while maintaining the head stationary. A proportion of these neurons showed some of the main physiological characteristics attributed to the noradrenergic locus coeruleus neurons, i.e. (i) a slow and regular resting discharge, and (ii) a typical biphasic response to fore and hindpaw compression consisting of short bursts of impulses followed by a period of quiescence, due at least in part to recurrent or lateral inhibition of the corresponding neurons. Moreover, 14 out of the 99 neurons were activated antidromically by stimulation of the spinal cord at T12 and L1, thus being considered as coeruleo- or subcoeruleospinal neurons. Among these locus coeruleus-complex neurons tested, 73 out of 99 (i.e. 73.7%) responded to neck rotation at the standard frequency of 0.15 Hz and at the peak amplitude of displacement of 10 degrees. In particular 40 of 73 units (i.e. 54.8%) were excited during side-down neck rotation and depressed during side-up rotation, while 18 of 73 units (i.e. 24.7%) showed the opposite pattern. In both instances the peak of the responses occurred with an average phase lead of +34.2 degrees for the extreme side-down or side-up neck displacement; however, the response gain (impulses/s per deg) was on the average more than two-fold higher in the former than in the latter group of units. The remaining 15 units (i.e. 20.5%) showed phase angle values which were intermediate between those of the two main populations. As to the coeruleo or subcoeruleospinal neurons, 11 of 14 units (78.6%) responded to the neck input, the majority (nine of 11 units, i.e. 81.8%) being excited during side-down neck rotation. Within the explored region, the proportion of responsive units was higher in the locus subcoeruleus (85.2%) than in the locus coeruleus, both dorsal and ventral (69.4%). Moreover, units located in the former structure showed on the average a response gain higher than that found in the latter structures. Similar results were also obtained from the population of locus subcoeruleus-complex neurons which fired at a low rate (less than or equal to 5.0 impulses/s).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of stimulation of vestibular and neck receptors on Deiters neurons projecting to the lumbosacral cord

The activity of lateral vestibular nucleus (LVN) neurons, antidromically identified by stimulation of the spinal cord at T12 and L1, thus projecting to the lumbosacral segments of the spinal cord (IVS neurons), was recorded in precollicular decerebrate cats during rotation about the longitudinal axis either of the whole animal (labyrinth input) or of the body only while the head was kept stationary (neck input). Among the IVS neurons tested for vestibular stimulation, 76 of 129 units (i.e. 58.9%) responded to roll tilt of the animal at the standard parameters of 0.026 Hz, +/- 10 degrees. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.47 +/- 0.44, SD, impulses X s-1 X deg-1 and 3.24 +/- 3.15, SD, %/deg, respectively. As to the response patterns, 51 of 76 units (i.e. 67.1%) were excited during side-down and depressed during side-up tilt, whereas 15 (i.e. 19.7%) showed the opposite behavior. In both instances the peak of the responses occurred with an average phase lead of about +21.0 +/- 27.2, SD, deg with respect to the extreme side-down or side-up position of the animal. Moreover, the former group of units showed almost a twofold larger gain with respect to the latter group (t-test, p less than 0.05). Among the IVS neurons tested for neck stimulation, 75 of 109 units (68.8%) responded to neck rotation at the standard parameters. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.49 +/- 0.40, SD, impulses X s-1 X deg-1 and 3.30 +/- 3.42, SD, %/deg, respectively, thus being similar to the values obtained for the labyrinth responses. However, 59 of 75 units (i.e. 78.6%) were excited during side-up neck rotation and depressed during side-down neck rotation, while 8 of 75 units (i.e. 10.7%) showed the opposite pattern. In both instances the peak of the responses occurred with an average phase lead of +52.0 +/- 18.3, SD, deg for the extreme side-up or side-down neck displacements. Further, the former group of units showed a larger gain than the latter group. Histological controls indicated that 102 of 129 (i.e. 79.0%) IVS neurons tested for labyrinth stimulation and 86 of 109 (i.e. 78.9%) IVS neurons tested for neck stimulation were located in the dorsocaudal part of LVN, the remaining IVS neurons being located in the rostroventral part of LVN.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of roll tilt of the animal and neck rotation on different size vestibulospinal neurons in decerebrate cats with the cerebellum intact

In decerebrate cats with the cerebellum intact we recorded the activity of lateral vestibulospinal neurons projecting to lumbosacral segments of the spinal cord (IVS neurons) and related the resting discharge, as well as the response characteristics of these neurons to roll tilt of the animal and neck rotation, with the cell size inferred from the conduction velocity of the corresponding axons. A slight negative correlation was found between resting discharge rate and conduction velocity of the whole population of IVS neurons responsive and unresponsive to animal tilt and neck rotation, so that the faster the conduction velocity, the lower was the unit discharge at rest. This correlation, however, was found only for the dorsocaudal LVN neurons, which contributed to the majority of IVS units, but not for the rostroventral LVN neurons. Moreover, it affected the units unresponsive but not those responsive to vestibular stimulation; the opposite, however, occurred for the units tested to neck stimulation. These findings indicate that the static properties of the IVS neurons can only in part be related to cell size. If we consider the IVS neurons responsive to roll tilt of the animal (76 neurons) and neck rotation (75 neurons) at the standard parameters of 0.026 Hz, +/- 10 degrees, no significant correlation was found between gain (impulses X s-1 X deg-1) of the labyrinth or neck responses and conduction velocity of the axons. Thus, due to the presence of slight negative relation between resting discharge and conduction velocity of the axons, larger neurons exhibited a greater percentage modulation (sensitivity) to the labyrinth and neck inputs than smaller neurons; this correlation involved particularly the dcLVN neurons. These findings suggest that the afferent pathways driven during dynamic stimulation of labyrinth and neck receptors produce an higher number or density of synaptic contacts on IVS neurons of increasing size. No significant differences in the means of resting discharge, conduction velocity, gain and sensitivity were found between all the IVS units responding to labyrinth and neck inputs. These findings indicate that the effectiveness of the two inputs was almost comparable and did not vary in different units as a function of cell size. The IVS neurons were mainly excited during side-down animal tilt and side-up neck rotation. Although these neurons showed the same spectrum of conduction velocity as those displaying the opposite response patterns, the response gains of the predominant populations of units were on the average higher than those obtained from the remaining populations of units.(ABSTRACT TRUNCATED AT 400 WORDS)

Input-output relations of lobules I and II of the cerebellar anterior lobe vermis in connexion with neck and vestibulospinal reflexes in the cat

In the anaesthetized cat, lobules I and II of the cerebellar anterior lobe vermis were examined to determine their role in the vestibulospinal and neck-vestibulospinal reflexes with respect to: the somatotopic representation of afferent inputs from labyrinth, neck and tail; and the inhibitory influence on vestibulospinal tract (VST) neurones receiving vestibular and neck afferent inputs. After electrical stimulation of the vestibular nerve and of neck afferents, almost identical responses via mossy fibres were evoked in the lobules, with the prominent response in lobules I and IIa of Larsell. Stimulation of the nerve supplying the dorsal region of the tail induced primarily the mossy fibre response, but also the climbing fibre response, in lobule II. The most responsive areas to tail and neck afferent stimulation did not overlap each other. In the lateral vestibular nucleus, 163 antidromically identified VST neurones were recorded extra- or intracellularly. On the basis of the response pattern to contralateral neck afferent stimulation, they were classified into 3 groups: neurones with excitation (n = 45); neurones with inhibition (n = 71); and neurones with no modulation (n = 47). Stimulation of lobules I-IIa inhibited the activities of 44 VST neurones. Out of them, 41 neurones belonged to the first group. They made up 91% of the group. Twenty-nine of these neurones, i.e. neurones receiving excitatory inputs from the neck and inhibitory inputs from the lobules, received additional excitatory input from the labyrinth. Although lobules I-IIa may be regarded as neck area in the anterior lobe vermis from the viewpoint of sensory input, they did not exert inhibitory influence only exceptionally on vestibulocollic neurones, but predominantly on VST neurones sending their axons to lower thoracic or more caudal segments in the spinal cord. It is suggested from these results that lobules I-IIa have a close relationship with the neck reflex and/or interaction of neck and vestibulospinal reflexes being concerned with the postural adjustment of a rather wide area of the body.

Electrophysiological analysis of the tecto-olivocerebellar (lobule VII) projection in the rat

In albino rats the superior colliculus was stimulated and its evoked potentials were explored throughout the posterior vermis of the cerebellum. Climbing fiber responses were identified only in lobule VII, ipsilaterally 1.2-1.6 mm wide. In the medial accessory olive, subnucleus c, in the contralateral side both antidromically evoked potentials from lobule VII and orthodromically evoked potentials from the superior colliculus were recorded. This evidence suggests that they are tecto-olivocerebellar projections.

Lobular distribution of visual climbing fiber responses in the cerebellum

The visual pathway was electrically stimulated at the optic disc, optic nerve and pretectal region in the albino rat. Field potentials of climbing fiber activation of Purkinje cells were explored in a large area of the cerebellum, and obtained in flocculus and posterior vermis. The responses in the posterior vermis were identified in the medial region of lobules VIa-c, VIII, IX, and the dorsal region of lobule X of Larsell. No response was obtained in lobule VII.

Axon collaterals of anterior semicircular canal-activated vestibular neurons and their coactivation of extraocular and neck motoneurons in the cat

We studied the ascending and descending axonal trajectories of excitatory vestibular neurons related to the anterior semicircular canal, by means of local stimulation and spike-triggered signal averaging techniques in anesthetized cats. More than 200 vestibular neurons related to the ampullary nerve of the anterior semicircular canal (ACN) were identified as vestibulo-ocular neurons by antidromic stimulation of the contralateral inferior oblique (IO) muscle motoneuron pool. In the descending, medial and ventral lateral nuclei, about 60% of these vestibulo-ocular neurons were also activated antidromically by upper cervical spinal cord stimulation (vestibulo-ocular-collic (cervical) = VOC). These VOC neurons produced unitary EPSPs in the majority of neck extensor motoneurons located at the C1 segment. None of the VOC neurons had axons descending as far as the thoracic level. Most of these VOC neurons were activated monosynaptically following stimulation of the ACN. The conduction velocity of the descending axons of VOC neurons was approximately 63 m/s, which was significantly faster than that of the ascending axons. The remaining 40% of the vestibulo-ocular neurons were not activated antidromically following spinal cord stimulation at intensities of 1 mA or more (vestibulo-ocular = VO). Most of the VO neurons were activated polysynaptically by ACN stimulation. The superior vestibular nucleus contained VO neurons that were activated mono- and polysynaptically following ACN stimulation.