Responses of Purkinje cells of the cerebellar vermis to neck and macular vestibular inputs

Distinct representations of body and head motion are dynamically encoded by Purkinje cell populations in the macaque cerebellum

The ability to accurately control our posture and perceive our spatial orientation during self-motion requires knowledge of the motion of both the head and body. However, while the vestibular sensors and nuclei directly encode head motion, no sensors directly encode body motion. Instead, the integration of vestibular and neck proprioceptive inputs is necessary to transform vestibular information into the body-centric reference frame required for postural control. The anterior vermis of the cerebellum is thought to play a key role in this transformation, yet how its Purkinje cells transform multiple streams of sensory information into an estimate of body motion remains unknown. Here, we recorded the activity of individual anterior vermis Purkinje cells in alert monkeys during passively applied whole-body, body-under-head, and head-on-body rotations. Most Purkinje cells dynamically encoded an intermediate representation of self-motion between head and body motion. Notably, Purkinje cells responded to both vestibular and neck proprioceptive stimulation with considerable heterogeneity in their response dynamics. Furthermore, their vestibular responses were tuned to head-on-body position. In contrast, targeted neurons in the deep cerebellar nuclei are known to unambiguously encode either head or body motion across conditions. Using a simple population model, we established that combining responses of~40-50 Purkinje cells could explain the responses of these deep cerebellar nuclei neurons across all self-motion conditions. We propose that the observed heterogeneity in Purkinje cell response dynamics underlies the cerebellum’s capacity to compute the dynamic representation of body motion required to ensure accurate postural control and perceptual stability in our daily lives.

Neck muscle fatigue impacts plasticity and sensorimotor integration in cerebellum and motor cortex in response to novel motor skill acquisition

The cerebellum undergoes neuroplastic changes in response to motor learning. Healthy human individuals demonstrate reduced cerebellar inhibition (CBI) following motor learning. Alterations in neck sensory input due to muscular fatigue are known to impact upper limb sensorimotor processing, suggesting that neck fatigue may also impact cerebellum to motor cortex (M1) pathways in response to motor learning. Therefore, this study aimed to determine whether cervical extensor muscle (CEM) fatigue alters CBI in response to motor learning. We examined 16 participants (8 CEM fatigue and 8 CEM control). A double cone transcranial magnetic stimulation (TMS) coil stimulated the ipsilateral cerebellar cortex 5 ms before application of contralateral test stimuli of the M1 to the right first dorsal interosseous muscle. Cerebellar-M1 activity curves were established pre- and post-motor skill acquisition (consisting of tracing sinusoidal-pattern waves with the index finger) and following either the CEM fatigue or control intervention. The control group showed greater cerebellar disinhibition than the fatigue group following motor skill acquisition (P < 0.006), while the fatigue group showed similar levels of CBI pre- and post-motor skill acquisition. Both groups improved in accuracy following acquisition (P = 0.012) and retention (P = 0.007), but the control group improved significantly more (17% at acquisition and 22% at retention) versus lower (6% and 9%) improvements for the fatigue group. Lessened cerebellar disinhibition in the CEM fatigue versus control group, coupled with diminished motor learning, suggests that CEM fatigue affects the cerebellar-M1 interaction, influencing the cerebellum's ability to adjust motor output to acquire and learn a novel motor task.NEW & NOTEWORTHY Normally motor learning decreases cerebellar inhibition (CBI) to facilitate learning of a novel skill. In this study, neck fatigue before motor skill acquisition led to less of a decrease in CBI and significantly less improvement in performance accuracy relative to a control group. This study demonstrated that neck fatigue impacts the cerebellar-motor cortex interaction to distal hand muscles, a highly relevant finding due to the altered neck postures and fatigue accompanying increased technology use.

Structured Variability in Purkinje Cell Activity during Locomotion

The cerebellum is a prominent vertebrate brain structure that is critically involved in sensorimotor function. During locomotion, cerebellar Purkinje cells are rhythmically active, shaping descending signals and coordinating commands from higher brain areas with the step cycle. However, the variation in this activity across steps has not been studied, and its statistical structure, afferent mechanisms, and relationship to behavior remain unknown. Here, using multi-electrode recordings in freely moving rats, we show that behavioral variables systematically influence the shape of the step-locked firing rate. This effect depends strongly on the phase of the step cycle and reveals a functional clustering of Purkinje cells. Furthermore, we find a pronounced disassociation between patterns of variability driven by the parallel and climbing fibers. These results suggest that Purkinje cell activity not only represents step phase within each cycle but also is shaped by behavior across steps, facilitating control of movement under dynamic conditions.

Effects of body to head rotation on the labyrinthine responses of rat vestibular neurons

Vestibulospinal reflexes elicited by head displacement in space depend on the direction of body displacement, because the neuronal responses to labyrinthine stimulation are tuned by neck displacement: a directional tuning takes place in the medial cerebellum and in spinal motoneurons, while a gain and a basal activity tuning can be observed in the reticular formation, a target structure of the medial cerebellum. In the present study, we investigated whether also the response of vestibular nuclear neurons (another target of the medial cerebellum) to labyrinthine stimulation is tuned by neck displacement and which parameters of the response are modulated by it. In urethane-anaesthetized Wistar rats, single-unit activity was recorded from the vestibular nuclei at rest and during wobble of the whole animal at 0.156 Hz. This stimulus tilted the animal's head by a constant amplitude (5°), in a direction rotating at a constant velocity over the horizontal plane, either in clockwise or counter clockwise direction. The gain and the direction of neuronal responses to wobble were evaluated through Fourier analysis, in the control position (with coincident head and body axes) and following a body-to-head rotation of 5-30° over the horizontal plane, in both directions. Most of the vestibular neurons modified their response gain and/or their basal activity following body-to-head rotation, as it occurs in the reticular formation. Only few neurons modified their response direction, as occurs in the cerebellum and in spinal motoneurons. The different behaviour of cerebellar neurons and of their vestibular and reticular target cells, suggests that the role played by the cerebellum in the neck tuning of vestibulospinal reflexes has to be reconsidered.

Effects of leg-to-body position on the responses of rat cerebellar and vestibular nuclear neurons to labyrinthine stimulation

The spatial organization of vestibulospinal (VS) reflexes, elicited by labyrinthine signals and related to head motion, depends on the direction of body tilt, due to proprioceptive neck afferents acting through the cerebellar anterior vermis. The responses of Purkinje cells located within this region to labyrinthine stimulation are modulated by the head-to-body position. We investigated, in urethane-anesthetized rats, whether a 90° leg-to-trunk displacement modifies the responses of corticocerebellar and vestibular nuclear neurons to the labyrinthine input, which would indicate that VS reflexes are tuned by the leg-to-trunk position. With this aim, unit activity was recorded during "wobble" stimuli that allow evaluating the gain and spatiotemporal properties of neuronal responses. The response gain of corticocerebellar units showed a significant drop in the leg-rotated position with respect to the control one. Following a change in leg position, a proportion of the recorded neurons showed significant changes in the direction and phase of the response vector. In contrast, vestibular nuclear neurons did not show significant modifications in their response gain and direction. Thus, proprioceptive afferents signaling leg-to-trunk position seem to affect the processing of directional labyrinthine signals within the cerebellar cortex.

The cerebellum and sensorimotor coupling: Looking at the problem from the perspective of vestibular reflexes

Cerebellar modules process afferent information and deliver outputs relevant for both reflex and voluntary movements. The response of cerebellar modules to a given input depends on the whole array of signals impinging on them. Studies on vestibular reflexes indicate that the response of the cerebellar circuits to the vestibular input is modified by the integration of multiple visual, vestibular and somatosensory afferent signals. In this way the cerebellum slowly adapts these reflexes when they are not adequate to the behavioural condition and allows their fast modifications when the relative position of the body segments and that of the body in space are changed. Studies on voluntary movements indicate that the cerebellum is responsible for motor learning that consists of the development of new input-output associations. Several theoretical, anatomical and clinical studies are consistent with the hypothesis that the cerebellum allows the delivery of motor commands which vary according to the condition of the motor apparatus. Finally, the cerebellum could change the relation between visual information and aimed reaching movements according to the position of the eyes in the orbit and of the neck over the body. We propose that, due to the large expansion of its cortex, an important function of the cerebellum could be that of expanding the range of sensorimotor associations according to all the factors characterizing the behavioural condition. Indeed, following cerebellar lesion, learning is often lost, the movement results impaired and requires an increased attention. In the light of the recently discovered connections of the cerebellum with the rostral regions of the frontal lobe, it can be suggested that the ability of cerebellar circuits to modify the rules of input-output coupling according to a general context is a fundamental property allowing the cerebellum to control not only motor but also cognitive functions.

Responses of Purkinje-cells of the cerebellar anterior vermis to stimulation of vestibular and somatosensory receptors

In decerebrate cats, sinusoidal rotation of the forepaw around the wrist modifies the activity of the ipsilateral forelimb extensor triceps brachii (TB) and leads to plastic changes of adaptive nature in the gain of vestibulospinal (VS) reflexes (VSRs). Both effects are depressed by functional inactivation of the cerebellar anterior vermis, which also decreases the gain of VSRs. In order to better understand the mechanisms of these phenomena, the simple spike activity of Purkinje (P-) cells was recorded from the vermal cortex of the cerebellar anterior lobe during individual and/or combined stimulation of somatosensory wrist, neck and vestibular receptors. About one third of the recorded units were affected by sinusoidal rotation of the ipsilateral forepaw around the wrist axis (0.16 Hz, +/-10 degrees ). Most of these neurons ( approximately 60%) increased their activity during ventral flexion of the wrist and decreased it during the oppositely directed movement, with an average phase lag of -141 degrees with respect to the position of maximal dorsiflexion. The remaining cells ( approximately 40%) were excited during dorsiflexion of the wrist, with an average phase lead of 59 degrees with respect to the extreme dorsal flexion. Both populations showed comparable response gains, with an average value of 0.42+/-0.52, S.D., imp/s/deg. About half of the recorded units were also tested during sinusoidal roll tilt of the animal around the longitudinal axis (0.16 Hz, +/-10 degrees ), leading to stimulation of labyrinthine receptors. When both stimuli were applied simultaneously, the responses to combined stimulation usually corresponded to the sum of individual responses. While the phase distribution of somatosensory responses was clearly bimodal, vestibular responses showed phase angle values uniformly scattered between +/-180 degrees and 0 degrees , so that, during combined stimulation, each neuron could be maximally activated by coupling the two stimuli with a particular phase relation. Finally, a proportion of the recorded neurons was also tested during sinusoidal rotation of the body around its longitudinal axis, with the head fixed in space, leading to stimulation of neck receptors. The proportion of neurons affected by individual stimulation of vestibular or neck receptors (81% and 72%, respectively) was larger than that of wrist-driven neurons. Convergence of signals from vestibular, somatosensory wrist and neck receptors was found in 18% of the neurons analyzed. In conclusion, the results of this study show that somatosensory signals from the forelimb: i) modulate the activity of a sizeable proportion of neurons located within the cerebellar anterior vermis and ii) interact widely with labyrinthine and neck signals at this level. Moreover, iii) this corticocerebellar region is largely dominated by vestibular and neck signals that may be utilized to build up a neuronal representation of the position of body in space. These findings suggest that: 1) the modulation of TB activity induced by rotation of the ipsilateral wrist may at least partially depend upon the simultaneous changes in P-cell activity and 2) the interaction of vestibular and somatosensory wrist signals at P-cell level may represent the substrate of the plastic changes that affect the VSR when animal tilt and wrist rotation are driven together. A preliminary report of these data has been presented [ Responses of cerebellar Purkinje cells to forepaw rotation in decerebrate cat. Pflügers Arch 440:R31].

The cerebellum may implement the appropriate coupling of sensory inputs and motor responses: evidence from vestibular physiology

Starting from a survey of current ideas on the role of the cerebellum in sensorimotor transformations, the present review summarizes the results of recent experiments showing that (a) somatosensory signals modify the spatial organization of the postural reflexes, thus leading to body stability, and (b) otolith input changes the plane of reflex eye movements, by keeping it perpendicular to the gravito-inertial vector. Evidence will be given that both transformations require the integrity of specific cerebellar regions. These data indicates that the cerebellum allows an optimal input-output coupling in relation to the ultimate behavioural goal of the motor activity.

Adaptive modification of the cats vestibulospinal reflex during sustained and combined roll tilt of the whole animal and forepaw rotation: cerebellar mechanisms

In decerebrate cats, the electromyogram (EMG) activity of the forelimb extensor triceps brachii (TB) increases during side-down roll tilt of the whole animal (vestibulospinal reflex, VSR) at about 0.15 Hz. (+/-10 degrees ), while decreases during side up tilt. On the other hand, the TB activity increases during dorsal flexion of the ipsilateral forepaw (0.15 Hz, +/-5 degrees-10 degrees ), but decreases during ventral flexion. In six experiments, these stimuli were synergistically associated (side-down tilt coincided with dorsal flexion of the forepaw), so that the EMG modulation of the TB activity was greater than that induced by the individual stimuli. During a 3-h period of this sustained stimulation, the amplitude of the pure VSR progressively increased to reach the maximum value at the end of the third hour and persisted unmodified during the post-adaptation period (1 h). In three experiments, animal tilt and forepaw rotation were antagonistically associated (side-down tilt coincided with ventral flexion of the forepaw). In these instances the VSR gain remained on the average stable, but, at the end of the 3-h period of combined stimulation, a proportion of TB responses to animal tilt showed a phase reversal. In a digitigrade animal like the cat, a dorsal flexion of the wrist is associated with a decrease in limb length and would occur when the extensor tone is not appropriate to support body weight; we propose, therefore, that somatosensory volleys elicited by wrist rotation modify the gain of VSR so as to maintain postural stability. Inactivation, on the side of muscle recording, of the corticocerebellar region which projects to the lateral vestibular nucleus of Deiters, by local microinjection of the GABA-A agonist muscimol (0.5 microl at 16 microg/microl), decreased the already adapted gain of VSR. In conclusion, the results of this study suggest that somatosensory reafferent inputs to the cerebellar vermis are used to plastically modify the gain of VSR, when external forces produce changes in the final posture of the foot during animal tilt.

Neck input modifies the reference frame for coding labyrinthine signals in the cerebellar vermis: a cellular analysis

The activity of 68 neurons, mainly Purkinje cells, was recorded from the cerebellar anterior vermis of decerebrate cats during wobble of the whole animal (at 0.156 Hz, 5 degrees), a mixture of tilt and rotation, leading to stimulation of labyrinth receptors. Most of the neurons (65/68) were affected by both clockwise and counterclockwise rotations. Twenty-four units showing responses of comparable amplitude to these stimuli (narrowly tuned cells) were represented by a single vector (Smax), whose preferred direction corresponded to the direction of stimulation giving rise to the maximal response. The remaining 41 units, however, showed different amplitude responses to these rotations (broadly tuned cells) and were characterized by two spatially and temporally orthogonal vectors (Smax and Smin), suggesting that labyrinthine signals with different spatial and temporal properties converged on these cells. All these units were tested while the body was aligned with the head (control position), as well as after static displacement of the body under a fixed head by 15 degrees and/or 30 degrees around a vertical axis passing through C1-C2, thus leading to stimulation of neck receptors. The orientation component of the response vector of the Purkinje cells to vestibular stimulation changed following body-to-head displacement. Moreover, the amplitude of vector rotation corresponded, on the average, to that of body rotation. Changes in temporal phase, gain and tuning ratio of the responses were also observed. We propose that information from neck receptors regulates the convergence of labyrinthine signals with different spatial and temporal properties on corticocerebellar units. Due to their strict relationship with the motor system, these units may give rise to appropriate responses in the limb musculature, by modifying the spatial organization of the vestibulospinal reflexes according to the requirements of body stability. The cerebellar vermis may thus represent an important structure, where frames of reference can be altered to account for changes in position of trunk, head and neck.

Noradrenergic influences on the cerebellar cortex: effects on vestibular reflexes under basic and adaptive conditions

Experiments performed either in decerebrate cats or in intact rabbits have shown that functional inactivation of the cerebellar anterior vermis or the flocculus decreased the basic gain of the vestibulospinal or the vestibulo-ocular reflex, respectively. These findings were attributed to the fact that a proportion of the vermal or floccular Purkinje cells, which are inhibitory in function, discharge out of phase with respect to the vestibulospinal or the vestibulo-ocular neurons during sinusoidal animal rotation, thus exerting a facilitatory influence on the gain of the vestibular reflexes. Intravermal injection of a beta-noradrenergic agonist slightly increased the gain of the vestibulospinal reflex, whereas the opposite result was obtained after injection of beta-antagonists. Similarly, intrafloccular injection of a beta-noradrenergic agonist slightly facilitated the gain of the vestibulo-ocular reflex in darkness (but not in light), whereas a small decrease of the reflex occurred after injection of a beta-antagonist. It was postulated that the noradrenergic system acts on Purkinje cells by enhancing their amplitude of modulation to a given labyrinth signal, thus increasing the basic gain of the vestibular reflexes. The Purkinje cells of the cerebellar anterior vermis and the flocculus also exert a prominent role on the adaptation of vestibulospinal and vestibulo-ocular reflexes, respectively. In particular, intravermal or intrafloccular injection of beta-noradrenergic antagonists decreased or suppressed the adaptive capacity of the vestibulospinal and vestibulo-ocular reflexes that always occurred during sustained out-of-phase neck-vestibular or visual-vestibular stimulation, whereas the opposite result was obtained after local injection of a beta-noradrenergic agonist. The noradrenergic innervation of the cerebellar cortex originates from the locus coeruleus complex, whose neurons respond to vestibular, neck, and visual signals. It was postulated that this structure acts through beta-adrenoceptors to increase the expression of immediate-early genes, such as c-fos and Jun-B, in the Purkinje cells during vestibular adaptation. Induction of immediate-early genes could then represent a mechanism by which impulses elicited by sustained neck-vestibular or visuovestibular stimulation are transduced into long-term biochemical changes that are required for cerebellar long-term plasticity.

Spatiotemporal response properties of cerebellar Purkinje cells to neck displacement

The activity of 184 Purkinje cells and 58 unidentified neurons located within the cerebellar anterior vermis was recorded in decerebrate cats during wobble of the body under a fixed head. This stimulus induced a neck displacement of constant amplitude (2.5 degrees) whose direction rotated at the constant velocity of 56.2 degrees/s on the horizontal plane, both in the clockwise and counterclockwise directions. It was then possible to evaluate the spatiotemporal characteristics of unit responses to neck displacement in the vertical planes; 131 of 184 Purkinje cells (71%) and 35 of 58 unidentified cells (60%) responded to clockwise and/or counterclockwise rotations. In particular, among the responsive units, 44% of the Purkinje cells and 37% of the unidentified cells showed an equal amplitude modulation during clockwise and counterclockwise rotations. These units are expected to show a maximal response sensitivity for neck displacement in a preferred direction, a null response for perpendicularly oriented stimuli and a constant temporal phase (narrowly tuned neurons). In 28% of the Purkinje cells and 40% of the unidentified cells, responses of different amplitudes were observed during clockwise and counterclockwise rotations. These neurons should display a preferred direction of response to neck displacement, lack of null response directions and a temporal phase changing with the stimulus direction (broadly tuned neurons). Finally, 27% of the Purkinje cells and 23% of the unidentified cells responded only to wobble in the clockwise or counterclockwise direction (unidirectional units). This behavior predicts equal sensitivities for all the directions of neck displacement and a response phase changing linearly with the direction of neck displacement. A maximal sensitivity vector (Smax), aligned with the preferred direction of the neuron, was evaluated for the bidirectional narrowly tuned and broadly tuned units. Its amplitude and temporal phase corresponded to the response characteristics expected for stimuli in the preferred direction of the cell. Smax directions were distributed over the horizontal plane. Most of them, however, were closer to the pitch than to the roll axis and pointed towards the animal's tail. Among pitch-related Purkinje cells, the temporal phase of Smax was small with a predominance of phase lags; phase leads of rather large amplitude were usually observed for roll-related Purkinje cells. The possibility that the recorded population of units coded the direction of neck displacement was tested by assuming that each cell gave a vectorial contribution related to its response properties and that the vectorial sum of such contributions represented the outcome of the population code. Dynamic body-to-head displacements in four different directions were simulated and for each direction 12 population vectors were evaluated at regular intervals of the stimulus cycle. The direction of the population vector was related to that of the stimulus, but the correspondence was close only for the pitch direction. Moreover, the amplitude of the population vector depended upon the direction of the stimulus, being larger for pitch than for roll displacements. Due to the efferent connections of the explored cerebellar region, the neuronal signals generated by the Purkinje cell population are probably transferred to the spinal cord, where they may differentially affect the amplitude and the spatial properties of the neck reflexes according to the direction of neck displacement.

Spatiotemporal response properties of cerebellar Purkinje cells to animal displacement: a population analysis

The hypothesis that corticocerebellar units projecting to vestibulospinal neurons contribute to the spatiotemporal response characteristics of forelimb extensors to animal displacement was tested in decerebrate cats in which the activity of Purkinje cells and unidentified cells located in the cerebellar anterior vermis was recorded during wobble of the whole animal. This stimulus imposed to the animal a tilt of fixed amplitude (5 degrees) with a direction moving at a constant angular velocity (56.2 degrees/s), both in the clockwise and counterclockwise directions over the horizontal plane. Eighty-three percent (143/173) of Purkinje cells and 81% (42/52) of unidentified cells responded to clockwise and/or counterclockwise rotations. In particular, 116/143 Purkinje cells (81%) and 32/42 unidentified cells (76%) responded to both clockwise and counterclockwise rotations (bidirectional units), while 27/143 Purkinje cells (19%) and 10/42 unidentified cells (24%) responded to wobble in one direction only (unidirectional units). For the bidirectional units, the direction of maximum sensitivity to tilt (Smax) was identified. Among these units, 24% of the Purkinje cells and 26% of the unidentified cells displayed an equal amplitude of modulation during clockwise and counterclockwise rotations, indicating a cosine-tuned behavior. For this unit type, the temporal phase of the response to a given direction of tilt should remain constant, while the sensitivity would be maximal along the Smax direction, declining with the cosine of the angle between Smax and the tilt direction. The remaining bidirectional units, i.e. 57% of the Purkinje cells and 50% of the unidentified cells displayed unequal amplitudes of modulation during clockwise and counterclockwise rotations. For these neurons, a non-zero sensitivity along the null direction is expected, with a response phase varying as a function of stimulus direction. As to the unidirectional units, their responses to wobble in one direction predict equal sensitivities along any tilt direction in the horizontal plane and a response phase that changes linearly with the stimulus direction. By comparing these data with those obtained previously during selective stimulation of macular receptors by a 5 degrees off-vertical axis rotation, it appeared that the directions of maximum sensitivity to tilt were distributed over the whole horizontal plane of stimulation, in both conditions. However, co-stimulation of macular and canal receptors during wobble decreased the proportion of unidirectional units, while that of the bidirectional, namely broadly tuned units, increased. In addition, while the average gain of the Smax vector of the bidirectional units was comparable, the temporal phase of this vector tended to show a more prominent phase leading behavior during wobble with respect to off-vertical axis rotation. The possibility that the tested neurons formed a population which coded the direction of head tilt in space was also investigated using a modified version of the classical population vector analysis. It was shown that for each selected time in the tilt cycle the direction of the population vector closely corresponded to that of the head tilt, while its amplitude was related to that of the stimulus. We conclude that the broad distribution of the response vector orientation of units located in the cerebellar anterior vermis represents an appropriate substrate for the cerebellar control of vestibulospinal reflexes involving extensor muscles during a variety of head tilts. In view of their efferent projections to the vestibular and fastigial nuclei, the cerebellar anterior vermis may provide a framework for the spatial coding of vestibular inputs, giving equal emphasis to both side-to-side and fore-aft stability.

Responses of Purkinje cells in the cerebellar anterior vermis to off-vertical axis rotation

Responses of 67 Purkinje cells (P-cells) and 44 unidentified neurons (U-cells) located in the cerebellar anterior vermis were recorded in decerebrate cats during off-vertical axis rotation (OVAR). This stimulus consisted of a slow constant velocity (9.4%/s) rotation in the clockwise (CW) and counterclockwise (CCW) directions around an axis inclined by 5 degrees with respect to the vertical. OVAR imposes on the animal head a 5 degrees tilt, whose direction changes continuously over the horizontal plane, thus eliciting a selective stimulation of macular receptors. A total of 27/67 P-cells (40%) and 24/44 U-cells (55%) responded to both CW and CCW rotations. For these bidirectional units, the direction of maximum sensitivity to tilt (Smax) could be identified. Smax directions were distributed over the whole horizontal plane of stimulation. Among bidirectional neurons, 48% of the P-cells and 33% of the U-cells displayed an equal amplitude of modulation during CW and CCW rotations, indicating a cosine-tuned behaviour. In these instances, the temporal phase of the unit response to a given direction of tilt remained constant, while the sensitivity was maximal along the Smax direction and declined with the cosine of the angle between Smax and the tilt direction. The remaining bidirectional units displayed unequal amplitudes of modulation during CW and CCW rotations. For these neurons, a nonzero sensitivity along the null direction was expected and the response phase varied as a function of stimulus direction. Finally, 31% and 23% of P-cells and U-cells, respectively, responded during OVAR in one direction only (unidirectional units). This behaviour predicts equal sensitivities along any tilt direction in the horizontal plane and a response phase that changes linearly with the stimulus direction. The posibility that the tested neurons formed a population which coded the direction of head tilt in space was also investigated. The data from the whole population of cells were analysed using a modified version of vectorial analysis. This model assumes that for a particular tilt each cell makes vectorial contributions; the vectorial sum of these contributions represent the outcome of the population code and points in the direction of head tilt in space. Thus, a dynamic head tilt along four representative directions was simulated. For each of the four directions, 12 population vectors were calculated at regular time intervals so as to cover an entire cycle of head tilt. The results indicate that for each selected time in the cycle the direction of the population vector closely corresponded to that of the head tilt, while its amplitude was related to the amount of head tilt. These data were particularly obtained for the P-cells. In view of their efferent connections, the cerebellar anterior vermis may provide a framework for the spatial organization of vestibulospinal reflexes induced by stimulation of otolith receptors.

Role of muscarinic receptors in the cerebellar control of the vestibulospinal reflex gain: cellular mechanisms

Most of the inhibitory Purkinje (P-) cells of the cerebellar anterior vermis fire out-of-phase with respect to the excitatory vestibulospinal neurons during roll tilt of the animal, thus exerting a positive influence on the gain of the vestibulospinal reflex (VSR). The responses of these P-cells depend on activation of glutamatergic excitatory mossy fibers-granule cells, but they are likely to be shaped by GABAergic inhibitory interneurons. The cerebellar cortex contains cholinergic fibers and both muscarinic and nicotinic receptors. In decerebrate cats intravermal injection of the muscarinic agonist bethanechol increased the VSR gain. The cellular mechanisms underlying these gain changes were studied in anesthetized Sprague-Dawley rats by microiontophoresis. Application of bethanechol (10-60 nA, 300 s) increased the response of vermal P-cells to pulses of glutamate (22/33 cells) or GABA (23/25 cells). These effects, which were blocked by the muscarinic antagonist scopolamine, lasted up to 15-40 min and occurred regardless of whether bethanecol altered the basal firing rate of the cells. We propose that the increase of P-cell responses to both excitatory and inhibitory neurotransmitters following activation of muscarinic receptors enhances the amplitude of modulation of these neurons to animal tilt, thus increasing the gain of the VSR.

Role of the spinocerebellum in adaptive gain control of cat's vestibulospinal reflex

In decerebrate cats, a 3-h period of sustained roll tilt of the head (at 0.15 Hz. +/- 10) leading to selective stimulation of labyrinth receptors, associated with a synchronous roll tilt of the body (at 0.15 Hz., +/- 12.5) leading to 2.5 degrees out-of-phase neck rotation produced an adaptive increase in gain of the vestibulospinal reflex (VSR) elicited by roll tilt of the animal at 0.15 Hz, +/- 10 degrees. This increase reached the maximum at the end of the third h of stimulation and persisted unmodified during the first h after stimulation. Microinjection into zone B of the cerebellar anterior vermis of the GABA-A agonist muscimol (0.25 microliter at 8 micrograms/microliters saline), producing only a slight or negligible depression of the VRS gain in non-adaptive conditions, prevented the occurrence of the adapted increase in gain of the VSR following a 3-h period of sustained head-body rotation. Moreover, intravermal injection of the GABA-A agonist muscimol or the GABA-B agonist baclofen (0.25 microliter at 8 or 2 micrograms/microliters saline, respectively) suppressed the already adapted VSR gain. It is postulated that the adaptive increase in gain of the VSR following a sustained neck-vestibular stimulation depends on plastic changes which affect the Purkinje cells of the cerebellar anterior vermis.

Noradrenergic agents in the cerebellar vermis affect adaptation of the vestibulospinal reflex gain

In precollicular decerebrate cats, the vestibulospinal reflex (VSR) was intermittently recorded from the triceps brachii during sinusoidal roll tilt of the whole animal (at 0.15 Hz, +/- 10 degrees), leading to selective stimulation of labyrinth receptors. This reflex, tested during and after a 3-h period of sustained animal tilt at the same parameters indicated above, showed an adaptive increase in gain in some experiments but not in others. In a second group of experiments, however, rotation of the head (at 0.15 Hz, +/- 10 degrees) was associated with a synchronous body rotation (at 0.15 Hz, +/- 12.5 degrees) which led to an additional neck input, due to 2.5 degrees of out-phase body-to-head displacement. In these experiments, the VSR, tested every 10-15 min, consistently showed an adaptive increase in gain during and after a 3-h period of sustained vestibular and neck stimulation. Microinjection into the cerebellar anterior vermis of beta-adrenergic agents (0.25 microliters at 8 micrograms/microliters saline) produced slight and short-lasting changes in the basic amplitude of the VSR, due to the neuromodulatory influence of these agents on the Purkinje cells activity. In addition, the beta-adrenergic agonist isoproterenol brought to the light an adaptive process in those experiments in which no adaptation occurred during a sustained roll tilt of the whole animal. On the other hand, the beta-adrenergic antagonists propranolol or sotalol either suppressed the increase in gain of the VSR which occurred in other experiments during sustained animal rotation, or prevented the occurrence of an adaptive increase in gain during a continuous out-phase head and body rotation. We conclude that the adaptive changes in gain of the VSR are facilitated by the noradrenergic system acting within the cerebellar cortex through beta-adrenoceptors.

Adaptive modification of the cat's vestibulospinal reflex during sustained vestibular and neck stimulation

In decerebrate cats, rotation about the longitudinal axis of the whole animal at 0.15 Hz, +/- 10 degrees produced an increased electromyogram (EMG) activity of the triceps brachii during side-down tilt and a decreased activity during side-up tilt. This vestibulospinal reflex (VSR) was tested before, during and after a sustained (3-h) period of roll tilt of the head at the parameters indicated above, associated with a synchronous roll tilt of the body at 0.15 Hz, but at the peak amplitude of either 12.5 degrees or 7.5 degrees. This additional stimulus led to 2.5 degrees of neck rotation, which was respectively out of phase (condition A) or in-phase (condition B) with head rotation. In a few instances the peak amplitude of neck rotation was increased to 5 degrees. In the first experimental condition A, the gain of the VSR (tested every 10-15 min) progressively increased, starting from the first hour of out of phase neck-vestibular stimulation to reach, on average, 241% of the control value at the end of the third hour of stimulation. On the other hand, in the second experimental condition B, the mean gain of the VSR first decreased to 82% during the first hour of in-phase neck-vestibular stimulation, but then increased to 165% of the corresponding control during the last hour of recording. In other experiments an adaptive increase in gain of the pure VSR occurred during a sustained (3-h) period of selective roll tilt of the whole animal, but it was less consistent and, on average, smaller in amplitude than that obtained during out of phase neck-vestibular stimulation. The adaptive changes in gain of the VSR described above were not associated with changes in the phase angle of the responses, and were also observed during the post-adaptation period. Further experiments indicated that the gain of the N-VSR, i.e. of the EMG responses to combined neck-vestibular stimulation, displayed a prominent adaptive increase during the sustained out of phase stimulation, but not during the in phase stimulation.

Temporal response properties of lumbar-projecting vestibulospinal neurons to roll tilt in decerebrate cats

In decerebrate cats, rotation about the longitudinal axis of the animal, leading to sinusoidal stimulation of labyrinth receptors, produces a tonic contraction of limb extensors during side-down tilt (alpha responses) and of dorsal neck extensors during side-up tilt (beta responses). These changes in posture are mediated, at least in part, by lateral vestibular nucleus (LVN) neurons, with response characteristics to stimulation of macular and/or canal receptors that have so far been evaluated at the level of either unidentified vestibulospinal (VS) neurons or vestibulo-collic neurons projecting to the upper cervical cord. In the present study we investigated the dynamics of the responses of VS neurons projecting to the lumbosacral segments of the spinal cord to increasing frequencies of tilt (from 0.026 to 0.32 Hz, +/- 10 degrees). All the recorded units showed an average phase lead with respect to position of +25.6 +/- 5.5 degrees (SE) at the tilt frequency of 0.026 Hz. Most of these neurons (n = 32) were particularly activated during side-down tilt (alpha responses) and showed either a stable phase or an increase in phase lead of the responses with increasing frequency of tilt. At the tilt frequency of 0.026 Hz, the smaller the phase lead of the responses, the larger was the response gain. Moreover, the smaller the phase lead of the responses at that frequency of tilt, the smaller the increase in gain but the larger was the increase in lead of the responses obtained by increasing the stimulation frequency up to 0.32 Hz. Through this set of finely organized changes in unit response characteristics, the overall output of this population of neurons increased, while the phase angle of the responses reached the mean value of +64.9 +/- 2.6 degrees (SE), thus becoming more related to the velocity than to the positional signal. The remaining units (n = 7), which were mainly activated during side-up tilt (beta responses), displayed an increase in phase lag of the responses to increasing frequency of stimulation, which reached the mean value of -118.9 +/- 14.5 degrees (SE) at 0.32 Hz. The differences in the dynamic properties of these VS neurons projecting to the lumbosacral cord, with respect to those of previously recorded populations of VS neurons, are discussed.

Noradrenergic and cholinergic modulations of corticocerebellar activity modify the gain of vestibulospinal reflexes

In addition to mossy fibers and climbing fibers, the cerebellar cortex receives noradrenergic and cholinergic afferents. Since the Purkinje (P) cells of the cerebellar vermis (culmen) respond to roll tilt of the animal with a discharge pattern that is out of phase with respect to that of the related lateral vestibular neurons, thus exerting a facilitatory influence on the gain of the vestibulospinal (VS) reflex, we tested the effects of local microinjection into the anterior vermis of noradrenergic and cholinergic agents on these reflexes. In decerebrate cats, unilateral microinjection in the paramedial zone B of the culmen of 0.25 microliters of small doses of alpha 1-, alpha 2-, and beta-noradrenergic agonists (i.e., metoxamine, clonidine, and isoproterenol, respectively) increased the response gain (in impulses/second per deg) of the EMG response of the ipsilateral and to some extent also of the contralateral triceps brachii to animal tilt (at 0.15 Hz, +/- 10 degrees). On the other hand local injection of the corresponding antagonists (i.e., prazosin, yohimbine, and propranolol) either decreased the gain of the ipsilateral triceps brachii to labyrinth stimulation or else prevented the occurrence of the effects induced by the corresponding agonists. An increase in gain of the VS reflexes was also elicited in other experiments by unilateral microinjection either of the nonselective cholinergic agonist carbachol or of the anticholinesterase eserine sulfate. Thus, the effects could be produced by increasing the naturally present amount of acetylcholine. Further experiments indicated that a bilateral increase in the response gain of the triceps brachii to labyrinth stimulation occurred after microinjection of a selective muscarinic (bethanechol) or nicotinic agonist (nicotine), while just the opposite result was obtained after microinjection of the corresponding muscarinic (scopolamine) and nicotinic (hexamethonium, D-tubocurarine) blockers. The effects of the noradrenergic and cholinergic agonists, which persisted for about two hours after the injection, were site specific and dose dependent. It appears, therefore, that the noradrenergic and cholinergic afferents to the cerebellar vermis intervene in the gain regulation of the VS reflexes, possibly by increasing the amplitude of modulation of the P cells to labyrinth stimulation.

Changes in gain of vestibulospinal reflexes after local injection of beta-adrenergic substances in the cerebellar vermis of decerebrate cats

Experiments were performed in decerebrate cats to find out whether the noradrenergic afferent system to the cerebellar cortex modified the gain of the vestibulospinal (VS) reflexes and, if so, whether this effect was mediated through beta-adrenoceptors. Unilateral injection into the vermal cortex of the culmen of the beta-adrenergic agonist isoproterenol (0.25-0.50 microliter at 8-16 micrograms/microliters of saline) increased the gain of the ipsilateral and to a lesser extent also of the contralateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. The opposite result was obtained after local injection of an equal dose of the beta-adrenergic antagonist, propranolol. Only slight changes in the phase angle of the responses were obtained. The effective sites were located within the parasagittal zone B which upon stimulation inhibited the activity of the ipsilateral triceps brachii. Since most of the Purkinje (P)-cells of this corticocerebellar area, which project to the lateral vestibular nucleus, fire out-of-phase with respect to the related VS neurons, we postulate that the beta-adrenergic system enhances the amplitude of the P-cell responses to labyrinth stimulation, thus increasing the gain of the VS reflex.

Noradrenergic agents into the cerebellar anterior vermis modify the gain of vestibulospinal reflexes in the cat

The noradrenergic (NA) afferent projection to the cerebellar cortex, which originates mainly from the locus coeruleus (LC), may act on the target neurons by utilizing both alpha- and beta-adrenoceptors. Experiments performed in decerebrate cats have shown that unilateral injection into the vermal cortex of the cerebellar anterior lobe of 0.25 microliter of the alpha 1-adrenergic agonist metoxamine or the alpha 2-agonist clonidine (at 2-8 micrograms/microliters of saline) as well as of the non-selective beta-agonist isoproterenol (at 8-16 micrograms/microliters) decreased the postural activity in the ipsilateral forelimb, while the extensor tonus either remained unmodified or slightly increased on the contralateral side. The same agents also increased the gain of the vestibulospinal (VS) reflexes elicited by recording the multiunit EMG responses of the ipsilateral and the contralateral triceps brachii to roll tilt of the animal (at 0.15 Hz, +/- 10 degrees), leading to sinusoidal stimulation of labyrinth receptors. The crossed effects were more prominent for the alpha 2- than for the alpha 1- and beta-agonists. Only slight changes in the phase angle of the responses were observed. The effects described above appeared 5-10 min after the injection, reached the peak values after 15-30 min and disappeared within 2 h. The effective area was located within the third and/or the fourth folium of the culmen rostral to the fissura prima, 1.4-1.8 mm lateral to the midline. This area corresponded to zone B of the cerebellar cortex, which projects to the ipsilateral lateral vestibular nucleus (LVN), on which it exerts a prominent inhibitory influence. In fact, monopolar stimulation of this area with three negative pulses (at 300/sec) performed prior to the local injection inhibited the spontaneous EMG activity of the ipsilateral triceps brachii. The effects described above were dose-dependent; injection of an equal volume of saline was ineffective. All changes in posture and reflexes elicited by metoxamine or clonidine were impaired by previous injection into the same corticocerebellar area of the corresponding alpha 1- or alpha 2-adrenergic antagonist prazosin or yohimbine, respectively (0.25 microliters at 8-16 micrograms/microliters). However, cross-interactions between alpha 1- and alpha 2-adrenergic agonists and antagonists were also observed. In fact, injection of the alpha 2-adrenergic antagonist yohimbine prevented the occurrence of all the metoxamine effects, while administration of the alpha 1-adrenergic antagonist prazosin prevented the occurrence of the ipsilateral, but not of the contralateral effects induced by clonidine injection.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of locus coeruleus and subcoeruleus neurons to sinusoidal stimulation of labyrinth receptors

In precollicular decerebrate cats the electrical activity of 141 individual neurons located in the locus coeruleus-complex, i.e. in the dorsal (n = 41) and ventral parts (n = 67) as well as in the locus subcoeruleus (n = 33), was recorded during sinusoidal tilt about the longitudinal axis of the whole animal, leading to stimulation of labyrinth receptors. Some of these neurons showed physiological characteristics attributed to the norepinephrine-containing locus coeruleus neurons, namely, (i) a slow and regular resting discharge, and (ii) a typical biphasic response to fore- and hindpaw compression consisting of short impulse bursts followed by a silent period, which has been attributed to recurrent and/or lateral inhibition of the norepinephrine-containing neurons. Furthermore, 16 out of the 141 neurons were activated antidromically by stimulation of the spinal cord at T12 and L1, thus being considered coeruleospinal or subcoeruleospinal neurons. A large number of tested neurons (80 out of 141, i.e. 56.7%) responded to animal rotation at the standard frequency of 0.15 Hz and at the peak amplitude of 10 degrees. However, the proportion of responsive neurons was higher in the locus subcoeruleus (72.7%) and the dorsal locus coeruleus (61.0%) than in the ventral locus coeruleus (46.3%). A periodic modulation of firing rate of the units was observed during the sinusoidal stimulus. In particular, 45 out of the 80 units (i.e. 56.2%) were excited during side-up and depressed during side-down tilt (beta-responses), whereas 20 of 80 units (i.e. 25.0%) showed the opposite behavior (alpha-responses). In both instances, the response peak occurred with an average phase lead of about + 18 degrees, with respect to the extreme side-up or side-down position of the animal; however, the response gain (imp./s per deg) was, on average, more than two-fold higher in the former than in the latter group. The remaining 15 units (i.e. 18.7%) showed a prominent phase shift of this response peak with respect to animal position. Similar results were obtained from the subpopulation of locus coeruleus-complex neurons which fired at a low rate (less than 5.0 imp./s), as well as for the antidromically identified coeruleospinal neurons. The response gain of locus coeruleus-complex neurons, including the coeruleospinal neurons, did not change when the peak amplitude of tilt was increased from 5 degrees to 20 degrees at the fixed frequency of 0.15 Hz. This indicates that the system was relatively linear with respect to the amplitude of displacement.(ABSTRACT TRUNCATED AT 400 WORDS)

Convergence and interaction of neck and macular vestibular inputs on locus coeruleus and subcoeruleus neurons

Extracellular recordings were obtained in precollicular decerebrate cats from 90 neurons located in the noradrenergic area of the dorsal pontine tegmentum, namely in the dorsal (LCd, n = 24) and the ventral part (LC alpha, n = 40) of the locus coeruleus (LC) as well as in the locus subcoeruleus (SC, n = 26). Among these units of the LC complex, 13 were coerulospinal (CS) neurons antidromically identified following stimulation of the spinal cord at T12-L1. Some of these neurons showed the main physiological characteristics of the norepinephrine (NE)-containing LC neurons, i.e., a slow and regular resting discharge and a typical biphasic response to fore- and hindpaw compression consisting of a short burst of excitation followed by a period of quiescence, due, in part at least, to recurrent and/or lateral inhibition. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotation about the longitudinal axis at 0.15 Hz, +/- 10 degrees peak amplitude. Among the 90 LC-complex neurons, 60 (66.7%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 67 (74.4%) responded to neck rotation. Convergence of macular and neck inputs was found in 52/90 (57.8%) LC-complex neurons; in these units, the gain and the sensitivity of the first harmonic of the response corresponded on the average to 0.34 +/- 0.45, SD, impulses.s-1.deg-1 and 3.55 +/- 2.82, SD, %/deg for the neck responses and to 0.23 +/- 0.29, SD, impulses.s-1.deg-1 and 3.13 +/- 3.04, SD, %/deg for the macular responses. In addition to these convergent units, 8/90 (8.9%) and 15/90 (16.7%) LC-complex units responded to selective stimulation either of macular or of neck receptors only. These units displayed a significantly lower response gain and sensitivity to animal tilt and neck rotation with respect to those obtained from convergent units. Most of the convergent LC-complex units were maximally excited by the direction of stimulus orientation, the first harmonic of responses showing an average phase lead of about +31.0 degrees with respect to neck position and +17.6 degrees with respect to animal position. Two populations of convergent neurons were observed. The first group of units (43/52, i.e., 82.7%) showed reciprocal ("out of phase") responses to the two inputs; moreover, most of these units were excited during side-down neck rotation, but inhibited during side-down animal tilt.(ABSTRACT TRUNCATED AT 400 WORDS)

Dynamic adaptation of the blind pointing characteristic to stepwise lateral tilts of body, head, and trunk

Blind pointing (i.e. pointing to visual targets without seeing the pointing arm) was investigated in normal subjects in response to stepwise lateral tilts (20 degrees) of the body, head, and trunk. Blind pointing positions of the right index finger on the outer surface of a hemispherical screen were measured relative to the positions of visual targets that were presented along a horizontal line (+/- 30 degrees in head coordinates) on the inner screen surface, thus yielding a blind pointing characteristic (BPC). (1) BPC is highly reproducible and can be subdivided into separate branches for the ipsi- and contralateral hemifields. These branches are rotated relative to the target line by individually different BPC angles pi i and pi c. (2) pi i exhibits characteristic time courses (measured within 10 min following a stepwise tilt) for each paradigm. (3) Body tilt (left ear down) causes a step-like increase in pi i of up to 14 degrees; body tilt (right ear down) causes a step-like decrease in pi i to about zero. (4) Trunk tilt (right shoulder down) produces a gradual decrease in pi i of up to 6 degrees (average time constant tau T = 5 min); trunk tilt (left shoulder down) produces a gradual increase in pi i of up to 4 degrees. (5) Head tilt (left ear down) causes an increase in pi i of up to 9 degrees followed by a gradual decrease (average time constant tau H = 6 min); head tilt (right ear down) causes a step-like decrease in pi i with unsignificant further changes. These findings are discussed in terms of a neural sensorimotor coordinate transformation process receiving separate, dynamic otolith and neck afferent influences.

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)

Influence of Renshaw cells on the response gain of hindlimb extensor muscles to sinusoidal labyrinth stimulation

The contraction of limb extensor muscles during side-down roll tilt of the animal depends upon an increased discharge of excitatory vestibulospinal (VS) neurons (alpha-response) and a reduced discharge of inhibitory reticulospinal (RS) neurons of the medulla (beta-response), both acting on ipsilateral limb extensor motoneurons. In the decerebrate cat, a modulation of the multiunit EMG activity was clearly present in forelimb extensors, but was extremely weak or absent in hindlimb extensors. Experiments performed in decerebrate cats with the deefferented GS muscle fixed at a constant length have shown that Renshaw (R)-cells, monosynaptically coupled with gastrocnemius-soleus (GS) motoneurons, were either unresponsive or displayed only very weak, small amplitude alpha-responses to sinusoidal stimulation of labyrinth receptors elicited during slow head rotation after bilateral neck deafferentation. This effect was attributed to excitatory VS volleys acting on GS motoneurons and, through their recurrent collaterals, on the related R-cells. In these instances the recurrent inhibition of the GS motoneurons contributed to the very low gain of the EMG response of the corresponding muscles to labyrinth stimulation. Intravenous injection of an anticholinesterase (eserine sulphate, 0.05-0.1 mg/kg) at a dose that in previous experiments increased the firing rate of medullary RS neurons, while decreasing the decerebrate rigidity, slightly increased the discharge rate of R-cells linked with the GS motoneurons in the animal at rest; these findings suggest that the RS system inhibits the extensor motoneurons by exciting the related R-cells. All the R-cells, which prior to the injection were either unresponsive or showed an alpha-response to head rotation (at 0.026-0.15 Hz, +/- 10 degrees), after eserine sulphate showed a beta-response for the same parameters of labyrinth stimulation. In particular, a reduced discharge of the R-cells linked with the GS motoneurons occurred during side-down head rotation as shown for the majority of the RS neurons. It appears therefore that the same R-cells, which in the normal decerebrate cat responded to the excitatory VS volleys acting through the GS motoneurons, were now decoupled from their input motoneurons during head rotation, thus behaving as if they underwent the most efficient direct excitatory control of the RS system. The reduced discharge of the R-cells linked with the GS motoneurons during side-down head rotation would lead to disinhibition of these motoneurons, thus enhancing the response gain of the corresponding muscle to labyrinth stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of Renshaw cells coupled with hindlimb extensor motoneurons to sinusoidal stimulation of labyrinth receptors in the decerebrate cat

Contraction of ipsilateral limb extensors during side-down roll tilt of the head, leading to selective stimulation of labyrinth receptors, is attributed to an increased discharge of excitatory vestibulospinal (VS) neurons (alpha-responses) and a decreased discharge of medullary inhibitory reticulospinal (RS) neurons (beta-responses), both of which act on ipsilateral extensor motoneurons. Experiments were performed in decerebrate cats, with the de-efferented gastrocnemius-soleus (GS) muscle fixed at a constant length, to find out whether Renshaw (R) cells linked with GS motoneurons responded to labyrinth stimulation elicited by head rotation, while the neck had been bilaterally deafferented. We hoped in this way to clarify the role and the mechanism by which these inhibitory interneurons act on limb extensor motoneurons during the vestibular reflexes. 72.7% of the R-cells, disynaptically excited by group I volleys elicited by single shock stimulation of the GS nerve, weakly responded to head rotation at frequencies of 0.026-0.15 Hz and at a peak amplitude of 10 degrees. For the frequency of head rotation of 0.026 Hz, +/- 10 degrees C, most of the GS R-cells increased their firing rate during side-down head displacement (alpha-responses); some responses were related to head position, but others showed some phase lead or lag with respect to head position. The gain of the first harmonic of these unit responses was very low and corresponded on the average to 0.084 +/- 0.062, S.D. imp./s/deg, while the sensitivity corresponded to 2.14 +/- 2.35, S.D.%/deg (base frequency, 6.85 +/- 5.97, S.D. imp./s). These responses were attributed to the activity of VS neurons, the increased discharge of which during side-down head rotation exerts a weak excitatory influence on a limited number of GS motoneurons and, through their recurrent collaterals, on the related R-cells. The modulation of the firing rate of R-cells coupled with the GS motoneurons increased linearly by increasing the peak amplitude of displacement from 5 degrees to 20 degrees at the frequency of 0.026 Hz, so that the response gain remained almost unchanged. An increase in frequency of head rotation from 0.026 to 0.32 Hz at a fixed amplitude of 10 degrees, thus changing the maximal angular acceleration from 0.26 degrees/s2 to 41.7 degrees/s2, reversed the response pattern of R-cells reported above. The resulting beta-responses, which also showed some phase lead or lag with respect to head position, were attributed to vestibular activation of RS neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Central projections from cat suboccipital muscles: a study using transganglionic transport of horseradish peroxidase

Central projections of suboccipital muscle nerves were examined following exposure of cut peripheral nerves to the tracer horseradish peroxidase. Labelled fibers entered the C1 and C2 dorsal roots and accumulated in the dorsolateral part of the dorsal funiculus. Many labelled fibers entered the grey matter of C1 to C3 in ventrally directed bundles which passed medially to the base of the dorsal horn. No terminal labelling was apparent in superficial layers of the dorsal horn. However, labelled fibers ramified extensively throughout medial parts of the intermediate laminae, in and around the central cervical nucleus. Labelled fibers also projected toward the ventral horn. In cats subjected to ventral root section at the time of peripheral nerve exposure, a modest distribution of reaction product was observed deep in the ventral horn. In cats which did not undergo ventral root section, anterograde projections in the ventral horn were obscured by the simultaneous retrograde filling of motoneurons both in the ventromedial nucleus and on the medial and lateral borders of the gray matter. Labelled axons also coursed rostrally into the medulla where they formed a circumscribed bundle between the main cuneate nucleus and the spinal nucleus of V. Three consistent regions of HRP deposition could be identified at medullary levels. Dense accumulations of reaction product were present in circumscribed regions of the external cuneate nucleus (ECN) throughout its rostrocaudal extent. A second zone of dense labelling occurred in the intermediate nucleus of Cajal, where it appeared to form a continuing column rostral to the central cervical nucleus in C1-C3. Sparse labelling was restricted to a third zone in the ventrolateral part of the main cuneate nucleus.

Convergence and interaction of neck and macular vestibular inputs on reticulospinal neurons

Extracellular recordings were obtained in decerebrate cats from neurons located in the inhibitory area of the medullary reticular formation, namely in the medial aspects of the nucleus reticularis gigantocellularis, magnocellularis and ventralis. Of 127 medullary reticular units examined, 77 were reticulospinal neurons antidromically identified following stimulation of the spinal cord at T12-L1; the remaining 50 neurons were not activated antidromically. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotations about the longitudinal axis at 0.026 Hz, 10 peak amplitude. Among the 127 reticular units, 84 (66.1%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 93 (73.2%) responded to neck rotation. Convergence of macular and neck inputs was found in 71/127 (55.9%) reticular neurons; in these units, the gain as well as the sensitivity of the first harmonic of response corresponded on the average to 0.49 +/- 0.41, SD imp/s/deg and 5.10 +/- 4.27, SD %/deg for the neck responses and to 0.40 +/- 0.39, SD imp/s/deg and 3.90 +/- 3.80, SD %/deg for the macular responses, respectively. Most of the convergent reticular units were maximally excited by the direction of stimulus orientation, the first hormonic or responses showing an average phase lead of about +42.7 with respect to neck position and +24.9 with respect to animal position. Two populations of convergent neurons were observed. The first group of units (58/71, i.e. 81.7%) showed reciprocal ("out of phase") responses to the two inputs in that they were mainly excited during side-down neck rotation, but inhibited during side-down animal tilt. The remaining group of units (13/71, i.e. 18.3%) showed parallel ("in phase") responses to the two inputs and they were mainly excited by side-down neck rotation and animal tilt. The response characteristics of medullary reticular neurons to the combined neck and macular inputs, elicited during head rotation, closely corresponded to those predicted by a vectorial summation of the individual neck and macular responses. In particular, "out of phase" units displayed small amplitudes and large phase leads of the responses with respect to head position, when both types of receptors were costimulated. In contrast, "in phase" units displayed large amplitude and small phase leads during head rotation.(ABSTRACT TRUNCATED AT 400 WORDS)

Visual (optokinetic), somesthetic and vestibular inputs to the frog cerebellum

Unitary response to visual (optokinetic), somesthetic (neck and limb) and vestibular stimulation were recorded from the Purkinje cell layer throughout most of the dorsal surface of the frog cerebellum. Simple spike activity in Purkinje cells and activity from cells without complex spikes were considered. Optokinetic responses (types I-III) were restricted to the dorsal rim and auricular lobes. Units were sensitive to very small velocity of optokinetic cylinder rotation (0.02-0.03 degrees/s) with peak sensitivity at about 1 degree/s. On the average an approximately linear relation of response amplitude to stimulus velocity was observed from 0.02 to 1 degree/s. The response progressively diminished above 1 degree/s to become very small at 30 degrees/s. Asymmetric response and silencing of firing during part of the cycle were nonlinearities observed with sinusoidal optokinetic stimulation in the range of 0.02-1 Hz, +/- 5-10 degrees (peak velocities 0.8-30 degrees/s). Somesthetic responses were recorded throughout most of the corpus cerebelli proper but the strongest input was to rostral regions. No somatotopic arrangement was found. Rather, convergence from more than one limb and neck was relatively common. Adaptation to successive cycles of stimulation was characteristic of somesthetic responses. Vestibular responses (type I-IV) were recorded throughout most of the explored area but the strongest input was to the dorsal rim and auricular lobes. From the analysis of unitary activity, the dorsal rim and auricular lobes are shown to be functionally linked to the vestibular and optokinetic systems whereas the explored part of the corpus is linked to the somesthetic and vestibular systems.

Responses of medullary reticulospinal neurons to sinusoidal rotation of neck in the decerebrate cat

The electrical activity of 132 neurons located in the inhibitory area of the medullary reticular formation, namely, in the medial aspects of the nucleus reticularis gigantocellularis, magnocellularis and ventralis 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. In particular, 85 neurons were activated antidromically by stimulation of the spinal cord at T12 and L1, the remaining 47 units were not activated antidromically. Among these reticular neurons tested, 66 out of 85 (i.e. 77.6%) of the neurons that were, and 31 out of 47 (i.e. 66.0%) of the neurons that were not antidromically activated responded to slow neck rotation at the frequency of 0.026 Hz and at the peak amplitude of displacement of 10 degrees. The units influenced by neck rotation showed a periodic modulation of the firing rate in response to sinusoidal stimulation of neck receptors. In particular, 70 of 97 units (i.e. 72.2%) were excited during side-down neck rotation and depressed during side-up rotation, while 19 of 97 units (i.e. 19.6%) showed the opposite pattern. In both instances, the peak of the responses occurred with an average phase lead of +41 degrees for the extreme side-up or side-down neck displacement. The remaining 8 units (i.e. 8.2%) showed a prominent phase shift of the peak of their response relative to neck position. The proportion of units excited during side-down neck rotation were almost equally distributed throughout the whole rostro-caudal extent of the reticular structures explored. Responses to neck rotation were detectable at 0.25 degrees of peak displacement. The gain (imp./s/deg.) and the sensitivity (%/deg., i.e. percentage change of the mean firing rate per degree of displacement) in responses of reticulospinal neurons decreased by increasing the peak amplitude of neck rotation from 1 to 10 degrees at a frequency of 0.026 Hz. Therefore, the system did not behave linearly with respect to amplitude of stimulation. By increasing the frequency of stimulation from 0.008 to 0.32 Hz at the fixed amplitude of 10 degrees, the gain, sensitivity and phase lead of responses increased for frequencies of neck rotation above 0.051 Hz. Reticulospinal neurons may thus monitor changes in neck position as well as in velocity of neck rotation. Responses of reticulospinal neurons to neck rotation are discussed in relation to the responses to the same stimulus recently described of vestibulospinal neurons originating from the lateral vestibular nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Relation between cell size and response characteristics of medullary reticulospinal neurons to labyrinth and neck inputs

The activity of presumably inhibitory reticulospinal neurons with cell bodies located in the medial aspects of the medullary reticular formation and axons projecting to lumbosacral cord has been recorded in decerebrate cats and their response characteristics to sinusoidal stimulation of labyrinth receptors (134 neurons) and neck receptors (110 neurons) have been related to cell size inferred from the conduction velocity of the corresponding axons. No significant correlation was found between resting discharge and conduction velocity of the axons. Among the recorded reticulospinal neurons, 64/134 (i.e. 47.8%) units responded to roll tilt, while 66/110 (i.e. 60.0%) units responded to neck rotation (0.026 Hz, +/- 10 degrees). A positive correlation was found between gain (imp./s/deg) of the labyrinth and neck responses and conduction velocity of the axons. Thus, due to absence of correlation between resting discharge and conduction velocity of the axons, larger neurons exhibited a greater percentage modulation (sensitivity) to the labyrinth and the neck input than smaller neurons. These findings are attributed to an overall increase in density or efficacy of the synaptic contacts made by the vestibular and neck afferent pathways on reticulospinal neurons of increasing size. Units receiving neck-macular vestibular convergence showed on the average an higher gain of the neck (GN) response with respect to the labyrinth (GL) response (GN/GL: 1.95 +/- 1.49, S.D.; n = 43); however, due to a parallel increase in gain of the reticulospinal neurons to both neck and labyrinth inputs, the relative effectiveness of the two inputs did not vary in different units as a function of cell size. The reticulospinal neurons were mainly excited by the direction of animal orientation and/or neck displacement. In particular, most of these positional sensitive units were excited by side-up animal tilt (37/58, i.e. 63.8%) and by side-down neck rotation (47/60, i.e. 78.3%). These predominant response patterns were particularly found between large size neurons, whereas small size neurons tended to show also other response patterns. The evidence indicates that in addition to intrinsic neuronal properties related to cell size, the quantitative and qualitative organization of synaptic inputs represents the critical factor controlling the responsiveness of reticulospinal neurons to vestibular and neck stimulation.

Cervico-vestibular and visuo-vestibular interaction. Self-motion perception, nystagmus, and gaze shift

In 8 healthy subjects we studied self-motion perception and nystagmus due to sinusoidal stimulation (amplitude 90 degrees peak to peak, frequency 0.05 Hz) of the horizontal semicircular canals, the cervical proprioceptors, and the retina. We used an electrically driven rotatory chair and optokinetic drum combination. For cervical stimulation the subject's head was placed in a clamp, attached to the drum. Eye movements were recorded by means of electrooculography, d.c. amplification. Subjects signalled the estimated head position by means of a 'joystick'. In the present series of experiments the vestibular and cervical informations were played off against each other in combined stimulation conditions with an interstimulus phase lag of 0 to 315 degrees, in steps of 45 degrees. Similarly, the vestibular and visual informations were played off against each other. Concerning estimated head position, our main finding is that both the visually and the cervically induced illusion of head rotation overrule the vestibular sensation of head motion. The ocular response to combined vestibular plus cervical stimulation shows that both nystagmus slow phases and saccades of the cervical and the vestibular responses add up by vectorial summation.

Responses of vestibulospinal neurons to neck and macular vestibular inputs in the presence or absence of the paleocerebellum

1. The role of the paleocerebellum in determining the responses of lateral vestibular nucleus (LVN) neurons either to independent or combined stimulation of macular vestibular and neck receptors has been investigated in decerebrate cats. Sinusoidal rotation around the longitudinal axis at 0.026 Hz, 5-10 degrees, represented the constant input parameters. Among the tested neurons, 100 and 131 units were recorded in animals with intact cerebellum and following partial cerebellectomy, respectively. The units were classified according to their anatomical location in either the rostroventral (cLVN) or dorsocaudal (ILVN) part of the LVN; units also were activated antidromically from the spinal cord. 2. The majority of units responded to stimulation of macular receptors both in preparations with intact cerebellum (75.0%) or with partial cerebellectomy (71.8%), the response being primarily in phase with the direction of animal orientation. The proportion of responding units and their response sensitivity were greater in the cLVN than ILVN in each preparation; no significant differences in mean firing rate and response sensitivity were observed between the two preparations for each subdivision of the LVN. In animals with cerebellum intact, the majority of cLVN and ILVN units were excited during side-down tilt; following partial cerebellectomy, this predominant response pattern still was present in cLVN but was reversed in ILVN. 3. About one-half of the units responded to sinusoidal stimulation of neck receptors in both preparations, the response being mainly in phase with the direction of neck orientation. In the intact cerebellum preparations, the proportion of cLVN units responding to neck rotation was greater than that of ILVN units, but no difference in response sensitivity was observed between these units. Following partial cerebellectomy, the proportion of cLVN units responsive to the neck input was reduced but that of ILVN units was not; however, the average response sensitivity was halved for both cLVN and ILVN units. In preparations with cerebellum intact, most of the cLVN units were excited during side-down neck rotation, whereas ILVN units were excited mainly by rotation in the opposite sense; following partial cerebellectomy, the majority of units were excited during side-up neck rotation, not only in ILVN but also in cLVN. 4. Units receiving a convergent input from both receptors were more numerous in cLVN (72.7%) than ILVN (41.8%) in preparations with intact cerebellum; following partial cerebellectomy, this disproportion of responsive units in the two divisions (45.3% and 44.9%, respectively) disappeared, due to a reduced number of cLVN units responding to the neck input. In both preparations, the macular input had a relatively greater influence on the cLVN whereas the neck input was more effective on the ILVN. 5...

Vestibular inputs to the fastigial nucleus; evidence of convergence of macular and ampullar inputs

1. Experiments have been undertaken on 11 decerebrate cats to investigate the effects of natural vestibular stimulation on the activity of cerebellar fastigial neurons. 2. From recordings in the rostral portion of the nucleus during sinusoidal lateral (roll) and horizontal (yaw) rotation, distinctive patterns of response were observed. 3. The majority of neurons sensitive to vestibular stimulation showed responses to a single modality of vestibular activation. During lateral tilt some neurones showed positional sensititivy, others gave responses related tothe velocity of movement. Other neurones responded in phase with the velocity of movement in the horizontal plane. 4. Aside from these neuronal responses, others provided indications of a convergence of inputs from different sets of vestibular receptors. In particular, several neurons showed a pattern of response that indicated tht they received inputs from otolith receptors and ampullar receptors of the vertical canal. At low velocities of movement their response was positional but with inreasing velocity the magnitude of the response increased and there was a marked phase shift of the discharge towards head velocity. 5. Neurons responding to horizontal rotation often showed positional responses during lateral tilt. There were also indications of a convergence of ampullar inputs from both vertical and horizontal canals. 6. The neural pathways mediating these resonses are discussed in consideration of previous neuroanatomical and neurophysiological data. We consider it likely that several pathways may act to evoke the patterns of response observed, and a role of the cerebellar cortex is indicated.