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

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

Role of the primate flocculus in adaptation of the vestibulo-ocular reflex

Neuronal events associated with adaptation of the horizontal vestibulo-ocular reflex (HVOR) induced by sustained vestibular-visual mismatching were investigated in the primate flocculus. The floccular area related to the HVOR (H-zone) was identified by electrical micro-stimulation which induced ipsilaterally directed horizontal eye movement. It was thus found that Purkinje cells in the H-zone consistently changed their simple spike responses to head rotation in parallel with the adaptive HVOR gain change. This was demonstrated by observing the change of simple spike firing of Purkinje cells during adaptation of HVOR either in a population study or an individual study. Since similar changes occurred even after bilateral lesioning of vestibular nuclei had extinguished the HVOR, these changes appear to represent vestibular, but not eye velocity, mossy fiber responsiveness. The complex spike discharge, on the other hand, modulated during vestibular-visual stimulation with a reciprocal pattern to the adaptive changes in the simple spike discharge. These results are consistent with the hypothesis that the flocculus Purkinje cells adaptively control the HVOR through their simple spike activity under influences of retinal error signals conveyed by visual climbing fiber pathways.     

Behavior of floccular Purkinje cells correlated with adaptation of horizontal optokinetic eye movement response in pigmented rabbits

Summary Single unit spike activities of Purkinje cells in the cerebellar flocculus were examined during sustained horizontal sinusoidal oscillation (0.33 Hz, 2.5° peak-to-peak) of a striped screen around an alert pigmented rabbit. The floccular area specifically related to horizontal reflex eye movement (H-zone) was identified by means of local stimulation that induced abduction of the ipsilateral eye. In control states, simple spike discharge of most of the H-zone Purkinje cells was enhanced by backward screen movement and depressed by forward screen movement, while complex spike discharge was modulated reciprocally. After one-hour sustained oscillation of the screen, the gain of horizontal optokinetic eye movement response (HOKR) increased by 0.16 on average. Correspondingly, simple spike modulation in most of H-zone Purkinje cells tested significantly increased in amplitude, while complex spike modulation tended to decrease. No such systematic changes were observed in other Purkinje cells. These results are consistent with the hypothesis that the floccular H-zone Purkinje cells adaptively control the optokinetic eye movement through modification of the visual mossy fiber responsiveness under the influence of the retinal error signals conveyed by the visual climbing pathway.     

Transient direct connection of vestibular mossy fibers to the vestibulocerebellar Purkinje cells in early postnatal development of kittens

Postnatal development of mossy fiber afferents from the vestibular and the visual system to the vestibulocerebellum was studied electrophysiologically and morphologically. In kittens anesthetized with pentobarbital sodium and N2O plus halothane, extracellular simple and complex spikes of Purkinje cells were recorded in the flocculus, nodulus and uvula. In the flocculus, stimulation of the VIIIth, but not the optic nerve, evoked simple spike responses with a latency of 16 ms at the day of birth which decreased to 5 ms by day 15 (short latency group). On the other hand, another group of simple spike responses with much longer latencies (50-80 ms) began to be elicited on day 7 via both the optic and VIIIth nerves. The latency decreased to 24 ms by day 15 and 10 ms on day 30. These latencies further shortened with development to the adult latency value (3-5 ms). Simple spike responses of the short latency group were also evoked in the nodulus and uvula from the VIIIth nerve with a slightly longer latency than that in the flocculus (23 ms on day 3 and 12 ms on day 17). Because of the immaturity of granule cells in early postnatal days, short latency simple spike responses from the VIIIth nerve suggested the direct synaptic connection of vestibular mossy fibers with Purkinje cells. Horseradish peroxidase was injected into the white matter of the flocculus, nodulus and uvula in slice preparations. Mossy fibers labeled with horseradish peroxidase showed fine branches extending to reach Purkinje cell somata from mossy swellings in the internal granular layer during days 2-20. Electron microscopy showed that the labeled mossy fibers made intimate contacts with Purkinje cell somata and the terminals contained many round synaptic vesicles. Pre and postsynaptic densities were occasionally found. After day 20, direct mossy fiber connections with Purkinje cells could not be observed. During days 7-20, these direct connections, as well as mossy fiber-granule cell connections could be observed. It was demonstrated that during early postnatal development, vestibular mossy fibers temporarily make direct contact with Purkinje cells, through which impulses could be transmitted to elicit simple spikes in Purkinje cells.     

Zonal organization of flocculo-vestibular connections in rats

Anatomical and electrophysiological evidence has contributed to the hypothesis that microzones in the mammalian flocculus are organized to reflect control of eye movements in the planes of semicircular canals. Adult male Long-Evans rats received iontophoretic injections of FluoroGold and/or tetramethylrhodamine dextran amine (10,000 molecular weight, "FluoroRuby") into the vestibular nuclei. The distribution of retrogradely labeled Purkinje cells revealed that efferent projections from the dorsal surface of the flocculus and the ventral paraflocculus to the superior vestibular nucleus, rostral medial vestibular nucleus, ventral lateral vestibular nucleus, and caudal aspect of the vestibular nuclear complex (caudal medial vestibular nucleus, inferior vestibular nucleus and nucleus prepositus hypoglossi) tended to correspond to previously identified climbing fiber zones [Ruigrok et al. (1992) J. comp. Neurol. 316, 129-150] in a manner consistent with other mammals. However, vestibular nucleus projections from the ventral surface of the flocculus did not appear to respect climbing fiber zonal boundaries. Rather, climbing fiber zones each contained interdigitated groups of Purkinje cells that project to different vestibular nuclear regions. It is suggested that this pattern of flocculus efferent organization is a specialization for controlling the activity of primary and accessory extraocular muscle pairs to confine vestibulo-ocular reflexes within semicircular canal planes when the "center of regard" is located at different eccentricities.     

Motor dynamics encoding in the rostral zone of the cat cerebellar flocculus during vertical optokinetic eye movements

The complex spike (CS) and simple spike (SS) activities of Purkinje cells in the rostral zone of the cerebellar flocculus were recorded in alert cats during optokinetic responses (OKR) elicited by a stimulus sequence consisting of a constant-speed visual pattern movement in one direction for 1 s and then in the opposite direction for 1 s. The quick-phase-free trials were selected. Ninety-eight cells were identified as rostral zone cells by the direction-selective CS activity that was modulated during vertical but not horizontal stimuli. In most of the majority population (88 cells), with an increasing CS firing rate during upward OKR and an increasing SS rate during downward OKR, the inverse dynamics approach was successful and the time course of the SS rate was reconstructed (mean coefficient of determination, 0.70 and 0.72 during upward and downward stimuli, respectively) by a linear weighted superposition of the eye acceleration, velocity, position, and constant terms, at a given time delay (mean 10 ms) from the unit response to the eye-movement response. Standard regression coefficient (SRC) analysis revealed that the contribution of the velocity term (mean SRC 0.98 for upward and 0.80 for downward) to regression was dominant over acceleration (mean SRC 0.018 and 0.058) and position (-0.14 and -0.12) terms. The velocity coefficient during upward stimuli (6.6 spikes/s per degree/s) was significantly (P<0.01) larger than that during downward stimuli (4.9 spikes/s per degree/s). In most of the minority population (10 cells), with both CS and SS firing rates increasing during upward OKR, the inverse dynamics approach was not successful. It is concluded that 1) in the cat rostral zone Purkinje cells, in which the preferred direction is upward for CS and downward for SS, eye velocity and acceleration information is encoded in SS firing to counteract the viscosity and inertia forces, respectively, on the eye during vertical OKR; 2) the eye position information encoded in SS firing is inappropriate for counteracting the elastic force; 3) encoding of eye velocity information during upward OKR is quantitatively different from that during downward OKR: SS firing modulation is larger for upward than for downward OKR of the same amplitude; and 4) encoding of motor dynamics is obscure in cells in which the preferred direction is upward for both CS and SS.     

Distribution of granule cells projecting to focal Purkinje cells in mouse uvula-nodulus

Mossy and climbing fibers convey a broad array of signals from vestibular end organs to Purkinje cells in the vestibulo-cerebellum. We have shown previously that Purkinje cell simple spikes (SSs) and climbing fiber-evoked complex spikes (CSs) in the mouse uvula-nodulus are arrayed in 400 mum wide sagittal climbing fiber zones corresponding to the rotational axes of the vertical semicircular canals. It is often assumed that mossy fibers modulate a higher frequency of SSs through the intermediary action of granule cells whose parallel fibers course through the Purkinje cell dendritic tree. This assumption is complicated by the diffuse topography of vestibular primary afferent mossy fiber projections to the uvula-nodulus and the dispersion of mossy fiber signals along folial axes by parallel fibers. Here we measure this parallel fiber dispersion. We made microinjections of neurobiotin into the molecular layers of different folia within the mouse vestibulo-cerebellum and measured the distribution of granule cells retrogradely labeled by the injected neurobiotin. Sixty-two percent of labeled granule cells were located outside a 400 mum sagittal zone flanking the injection site. The dispersion of labeled granule cells was approximately 2.5 mm along folial axes that were 2.7-2.9 mm wide. Our data suggest that topographic specificity of SSs could not be attributed to the topography of vestibular primary afferent mossy fiber-granule cell projections. Rather the response specificity of SSs must be attributed to other mechanisms related to climbing fiber-evoked Purkinje cell and interneuronal activity.     

Acute adaptation of the vestibuloocular reflex: signal processing by floccular and ventral parafloccular Purkinje cells

The gain of the vertical vestibuloocular reflex (VVOR), defined as eye velocity/head velocity was adapted in squirrel monkeys by employing visual-vestibular mismatch stimuli. VVOR gain, measured in the dark, could be trained to values between 0.4 and 1.5. Single-unit activity of vertical zone Purkinje cells was recorded from the flocculus and ventral paraflocculus in alert squirrel monkeys before and during the gain change training. Our goal was to evaluate the site(s) of learning of the gain change. To aid in the evaluation, a model of the vertical optokinetic reflex (VOKR) and VVOR was constructed consisting of floccular and nonfloccular systems divided into subsystems based on the known anatomy and input and output parameters. Three kinds of input to floccular Purkinje cells via mossy fibers were explicitly described, namely vestibular, visual (retinal slip), and efference copy of eye movement. The characteristics of each subsystem (gain and phase) were identified at different VOR gains by reconstructing single-unit activity of Purkinje cells during VOKR and VVOR with multiple linear regression models consisting of sensory input and motor output signals. Model adequacy was checked by evaluating the residual following the regressions and by predicting Purkinje cells' activity during visual-vestibular mismatch paradigms. As a result, parallel changes in identified characteristics with VVOR adaptation were found in the prefloccular/floccular subsystem that conveys vestibular signals and in the nonfloccular subsystem that conveys vestibular signals, while no change was found in other subsystems, namely prefloccular/floccular subsystems conveying efference copy or visual signals, nonfloccular subsystem conveying visual signals, and postfloccular subsystem transforming Purkinje cell activity to eye movements. The result suggests multiple sites for VVOR motor learning including both flocculus and nonflocculus pathways. The gain change in the nonfloccular vestibular subsystem was in the correct direction to cause VOR gain adaptation while the change in the prefloccular/floccular vestibular subsystem was incorrect (anti-compensatory). This apparent incorrect directional change might serve to prevent instability of the VOR caused by positive feedback via the efference copy pathway.     

Increase in Purkinje cell gain associated with naturally activated climbing fiber input

These experiments were designed to test the hypothesis that climbing fiber inputs evoked by a peripheral stimulus increase the responsiveness of Purkinje cells to mossy fiber inputs. This hypothesis was based on a previous series of observations demonstrating that spontaneous climbing fiber inputs are associated with an accentuation of the Purkinje cell responses to subsequent mossy fiber inputs (10, 12). Furthermore, short-term nonpersistent interactions between climbing and mossy fiber inputs have been an important aspect of many theories of cerebellar function (5, 7, 8, 12, 36). Extracellular unitary recordings were made from Purkinje cells in lobule V of decerebrate, unanesthetized cats. To activate mossy and climbing fiber inputs, the forepaw was passively flexed by a Ling vibrator system. A data analysis was developed to sort the simple spike trials into two groups, based on the presence or absence of complex spikes activated by the stimulus. In addition, during those trials in which complex spikes were activated, the simple spike train was aligned on the occurrence of the complex spike. For each simple spike response to the forepaw input, the average firing rate during the response was compared to background both in those trials in which complex spikes were activated and in those in which they were not. The ratio of the response amplitudes in the histograms constructed from these two groups of trials permitted a quantification of the change in responsiveness when climbing fiber inputs were activated. The results show that both excitatory and inhibitory simple spike responses are accentuated when associated with the activation of a complex spike. Using an arbitrary level of a gain change ratio of 120% as indicating a significant modification, 64% of the response components analyzed increased their amplitude when climbing fiber input was present. Simple spike response components occurring prior to complex spike activation were usually not accentuated, although in a few cells the amplitude of this component of the response increased. In addition, in a small number of cells the occurrence of complex spikes was associated with a new simple spike component. For excitatory responses, the magnitude of the gain change ratio was shown to be inversely related to the amplitude of the simple spike response evoked by the mossy fiber inputs. The data obtained is consistent with the hypothesis that the climbing fiber input is associated with an increase in the responsiveness of Purkinje cells to mossy fiber inputs. The increased responsiveness occurs whether the simple spike modulation evoked by the peripheral stimulus is excitatory or inhibitory. The change in responsiveness is short term and nonpersistent. It is argued that the activation of climbing fiber inputs to the cerebellar cortex is associated with an increase in the gain of Purkinje cells to mossy fiber inputs activated by natural peripheral stimuli.     

Afferents to the cerebellar flocculus in cat with special reference to pathways conveying vestibular, visual (optokinetic) and oculomotor signals

Summary Horseradish peroxidase (HRP) was injected into the cerebellar flocculus of 20 cats to determine: (a) the proportions of afferents from the various brain stem nuclei; (b) possible projections from the basilar pontine nuclei; and (c) sources of saccadic eye movement signals recorded from flocculus Purkinje cells. Results confirm earlier findings that the flocculus receives large numbers of mossy fibre afferents from the vestibular and perihypoglossal nuclei, bilaterally, and climbing fibres from the contralateral inferior olive (dorsal cap, ventrolateral outgrowth, medial accessory olive, ventral bend of principal olive). In addition, large numbers of HRP-labeled neurons have been identified within: (i) the basilar pontine nuclei, bilaterally, where they are distributed in columns in the dorsolateral, lateral, ventral medial and dorsomedial nuclei; (ii) the nucleus reticularis tegmenti pontis; (iii) several of the cranial motor nuclei, VI, VII, X (retrofacial n.), XI (n. ambiguus), and XII; (iv) the raphe magnus, pontis and obscurus; (v) the lateral reticular nucleus, pars subtrigeminalis.Finally, new information is presented which shows that large numbers of flocculus projecting neurons are located within the medial longitudinal fasciculus at two locations; one just rostral to the hypoglossal nucleus and another group extends 2–3 mm rostral to the abducens nucleus. These groups are bilateral, and have been termed, respectively, the caudal and intermediate interstitial nucleus of the medial longitudinal fasciculus. Both groups correspond in location to physiologically identified neurons in cat which fire in relation to saccadic eye movements. Their projection to the flocculus, in part, explains the saccadic discharge of Purkinje cells in the flocculus.     

Effects of nucleus prepositus hypoglossi lesions on visual climbing fiber activity in the rabbit flocculus

The caudal dorsal cap (dc) of the inferior olive is involved in the control of horizontal compensatory eye movements. It provides those climbing fibers to the vestibulocerebellum that modulate optimally to optokinetic stimulation about the vertical axis. This modulation is mediated at least in part via an excitatory input to the caudal dc from the pretectal nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system. In addition, the caudal dc receives a substantial GABAergic input from the nucleus prepositus hypoglossi (NPH). To investigate the possible contribution of this bilateral inhibitory projection to the visual responsiveness of caudal dc neurons, we recorded the climbing fiber activity (i.e., complex spikes) of vertical axis Purkinje cells in the flocculus of anesthetized rabbits before and after ablative lesions of the NPH. When the NPH ipsilateral to the recorded flocculus was lesioned, the spontaneous complex spike firing frequency did not change significantly; but when both NPHs were lesioned, the spontaneous complex spike firing frequency increased significantly. When only the contralateral NPH was lesioned, the spontaneous complex spike firing frequency decreased significantly. Neither unilateral nor bilateral lesions had a significant influence on the depth of complex spike modulation during constant velocity optokinetic stimulation or on the transient continuation of complex spike modulation that occurred when the constant velocity optokinetic stimulation stopped. The effects of the lesions on the spontaneous complex spike firing frequency could not be explained when only the projections from the NPH to the inferior olive were considered. Therefore we investigated at the electron microscopic level the nature of the commissural connection between the two NPHs. The terminals of this projection were found to be predominantly GABAergic and to terminate in part on GABAergic neurons. When this inhibitory commissural connection is taken into consideration, then the effects of NPH lesions on the spontaneous firing frequency of floccular complex spikes are qualitatively explicable in terms of relative weighting of the commissural and caudal dc projections of the NPH. In summary, we conclude that in the anesthetized rabbit the inhibitory projection of the NPH to the caudal dc influences the spontaneous firing frequency of floccular complex spikes but not their modulation by optokinetic stimulation.     

Purkinje cell activity in the middle zone of the cerebellar flocculus during optokinetic and vestibular eye movement in cats

Based on the inverse dynamics theory, a previous paper reconstructed simple-spike (SS) firing rates of Purkinje cells in the cat's flocculus middle-zone by a linear-weighted summation of eye acceleration, velocity, and position during optokinetic response (OKR). The present study investigated the SS rates during combined optokinetic and vestibular stimuli of the cells recorded in the previous paper. During the sinusoidal vestibuloocular reflex (VOR) in the light (VORL) and in the dark (VORD) the firing modulation was small. During VOR suppression (VORS) by head and visual-pattern rotation in the same direction, the modulation was deep, with the peak coinciding roughly with peak ipsiversive head velocity. During VOR enhancement (VORE), the modulation was deep, with the peak coinciding roughly with peak contraversive head velocity. If we interpret these data in relation to eye and head movements, the cells in the cat were comparable to the horizontal-gaze-velocity Purkinje cells in the monkey that encode a linear summation of eye and head velocity signals. Alternatively, if we interpret the data on the basis of the inverse dynamics theory, the SS rates during VORL, VORS, and VORE were well-fitted by the OKR components of the movements (subtraction of VORD from VORL, VORS, and VORE eye movements, respectively), but not by the whole movements, using the coefficients calculated during OKR. It is concluded that the data are interpretable by both theories when the VOR gain (eye movement/head movement) is close to 1 and the firing is dominated by eye velocity information.     

Behavior of floccular Purkinje cells correlated with adaptation of vestibulo-ocular reflex in pigmented rabbits

Summary The responsiveness of floccular Purkinje cells to head oscillations was examined in alert pigmented rabbits subjected to adaptation of horizontal vestibulo-ocular reflex (HVOR) under three different combinations of turntable and screen oscillations. Purkinje cells involved in the HVOR control (H-zone cells) were identified by local stimulation effects that induced horizontal eye movements. In control states, simple spike discharages of H-zone cells were modulated predominantly out of phase with the velocity of sinusoidal turntable oscillation (0.1 Hz, 5° peak-to-peak). A sustained 180° outphase combination (5° turntable and 5° screen oscillation) was found to increase the average HVOR gain by 0.16, at which point the majority of H-zone cells increased the outphase simple spike modulation. A sustained inphase combination (5° turntable and 5° screen oscillation) decreased the average HVOR gain by 0.09, with the majority of H-zone cells decreasing the outphase simple spike modulation or becoming converted to the inphase modulation. With a vision-reversal combination (5° turntable and 10° screen oscillation), there was no change in the gain of the HVOR, but a moderate advancement in the phase. In this case, H-zone cells showed no appreciable changes in their simple spike modulation. Complex spike discharges of all H-zone cells tested were modulated in response to optokinetic stimuli involved in the combinations of turntable and screen oscillations. These results support the hypothesis that H-zone cells adaptively control HVOR dynamic characteristics through modification of mossy fiber responsiveness to head oscillation under influences of retinal error signals conveyed by climbing fiber afferents.     

The mossy fiber projection of the nucleus reticularis tegmenti pontis to the flocculus and adjacent ventral paraflocculus in the cat

The pontine projection of the flocculus and adjacent ventral paraflocculus was investigated with antegrade and retrograde axonal tracer techniques. Injections of horseradish peroxidase into the floccular complex revealed subsets of labeled neurons in the nucleus reticularis tegmenti pontis, the nucleus raphe pontis and the medial lemniscus. Following injections of tritiated leucine in these subsets, the topographical distribution of labeled mossy fibers in the floccular complex was studied. Cells clustered in the central part of the nucleus reticularis tegmenti pontis project to the rostral flocculus and the rostral part of the caudal flocculus. The terminal field of cells in the nucleus raphe pontis and of cells associated with the lateral aspect of the medial lemniscus covered the same area. The number of mossy fiber terminals arising from these cells is small and concentrated in a medial position. The medial extension of the ventral paraflocculus and its most caudal sublobule do receive a very dense mossy fiber projection from cells associated with the medial edge of the medial lemniscus next to the rostral nucleus reticularis tegmenti pontis and beyond. Concomitantly, a collateral projection terminates in a restricted part of the uvula. Labeled mossy fiber terminals were never observed in the nodulus. The nucleus reticularis tegmenti pontis does not project to any part of the lower brain stem. The connections described in this paper are discussed in relation to the possible role of the nucleus reticularis tegmenti pontis as a relay nucleus in brain stem pathways transmitting visual information. It is concluded that in the cat this nucleus is an exclusively pre-cerebellar relay, not involved as a final link in the non-cerebellar pathway transmitting visual information to the vestibular nuclei.     

Visual-vestibular interaction in the flocculus of the alert monkey

Summary The activity of Purkinje cells (P-cells) was recorded in the flocculus of alert Rhesus monkeys under different conditions of visual-vestibular stimulation. Stimulus conditions were vestibular, optokinetic, combined and conflicting. About 10–20% of all P-cells were activated in their simple spike activity during conflicting stimulation to the recording side (type I) and gave no response or much less during vestibular stimulation. About half of these P-cells were also activated during optokinetic stimulation to the recording side at velocities above 40–60 deg/s. Simple and complex spike activity behaved in a reciprocal way with overlapping but not identical working ranges. Simple spike modulation was unidirectional, complex spike activity always bidirectional. Modulation of simple spike activity cannot be related to one single parameter of the sensory input or the oculomotor output. The hypothesis is put forward that the vestibular nuclei and the flocculus behave in a complementary fashion in processing visual-vestibular information, the flocculus being specialized for high velocity optokinetic nystagmus and suppression of vestibular nystagmus.Supported by a grant from the Swiss National Foundation for Scientific Research 3.343-2.78     

Nature of optokinetic response and zonal organization of climbing fiber afferents in the vestibulocerebellum of the pigmented rabbit

In pigmented rabbits anesthetized with N2O (70%) and halothane (2-4%), Purkinje cells were extracellularly recorded in the flocculus. A large central visual field (60 degrees x 60 degrees) was used to optokinetically stimulate either the ipsi- or contralateral eye, and the direction and velocity selectivities of complex spike responses were examined. For optokinetic stimulation (OKS) delivered to the ipsilateral eye (n = 129), the preferred direction was forward (F, n = 57) or upward (U, n = 37), while the remaining cells (n = 35) showed no response (N). For OKS delivered to the contralateral eye (n = 107), the preferred direction was backward (B, n = 11), downward (D, n = 42) or upward (U, n = 2), and the rest (n = 52) showed N. Cells tested with both eyes (n = 89) fell into five categories based on the preferred direction to ipsi- and contralateral OKS: (1) ipsi-F and contra-B (F/B type, n = 9), (2) ipsi-F but contra-N (F/N type, n = 28), (3) ipsi-U and contra-D (U/D type, n = 13), (4) ipsi-U but contra-N (U/N type, n = 17), and (5) ipsi-N but contra-D (N/D type, n = 22). The optimum velocity was within 0.1-2.0 degrees/s for all cells. On the average, the best response was obtained at 0.2-0.5 degrees/s stimulation. All ipsi-F cells responded to electrical stimulation of the optic tract (OT), while most cells preferring ipsi-U, contra-B and contra-D directions did not respond. No characteristic feature was found in cells innervated with collateralized climbing fiber branches to the nodulus. In the flocculus, cells preferring horizontal orientation (H cells, preferring ipsi-F and/or contra-B directions) were localized in a narrow dorsoventral zone (less than 1.0 mm) along the caudal border of the rostral one third, while those preferring the vertical orientation (V cells, preferring ipsi-U and/or contra-D directions) were in two distinct narrow zones located rostral and caudal to the H cell zone. H and V cells were intermingled in the central portion of the ventral flocculus. These four zones are in good agreement with previously defined H, anterior V, posterior V and R zones, respectively. The results indicate that the subdivision of the flocculus which controls horizontal (vertical) eye movements receives information regarding movements of the visual surround in the horizontal (vertical) orientation through visual climbing fiber afferents, thus being organized in olivo-cortico-nuclear functional units for control of eye movements.     

Sensory separation in climbing and mossy fiber inputs to cat vestibulocerebellum

Summary Electrical or flash stimulation of the visual pathway evoked in the vestibulocerebellum of barbiturate anesthetized cats, field and unitary potentials characteristic of climbing fiber (CF) activation of Purkinje cells. The latency of the CF field potentials was 11–15 msec in the flocculus and 14–19 msec in the nodulus/ventral uvula. Mossy fiber (MF) related field and unitary responses were not observed following visual stimulation. Conversely, electrical stimulation of the VIIIth nerve evoked in the vestibulocerebellum MF-related field and unitary potentials, exclusively. Despite this dichotomy, the field potentials evoked by visual and vestibular stimulation frequently overlapped within the cerebellar cortex. This overlap was shown at the level of individual Purkinje cells by means of extra- and intracellular recordings which demonstrated vestibulo-visual convergence. These observations indicate that a given sensory modality may reach specific cerebellar areas utilizing only one of the two cerebellar afferent systems. It is concluded that the MF and CF afferent systems, when considered as sensory inputs, can operate as independent information channels.     

Responses of sagittally aligned Purkinje cells during perturbed locomotion: relation of climbing fiber activation to simple spike modulation.

1. The purpose of these experiments is to test the hypothesis that the synchronous activation of sagittally aligned Purkinje cells by a physiologically relevant stimulus is associated with an increase in the simple spike responses of the same neurons. 2. This hypothesis was tested using a perturbed locomotion paradigm in decerebrate locomoting ferrets. The responses of 3-5 sagittally aligned Purkinje cells were recorded simultaneously in response to the intermittent perturbation of the forelimb during swing phase. A data analysis is introduced, the real time postsynaptic response (RTPR), that permits the quantification of the simple spike responses of Purkinje cells in a manner that can be related to their complex spike responses on a trial-by-trial basis. 3. The data support the above hypothesis by illustrating that the amplitude of the combined simple spike responses across the population of Purkinje cells is correlated with the extent to which their climbing fiber inputs are synchronously activated. These findings together with an analysis of the gain-change ratio support the view that the synchronous climbing fiber input may be responsible for mediating this increased responsiveness. 4. More generally, the data suggest that the task- and/or behaviorally dependent activation of sagittal strips of climbing fiber inputs may provide a mechanism whereby the responsiveness of discrete populations of Purkinje cells can be selectively regulated, specifying the groups of neurons that will be most dramatically modulated by mossy fiber inputs activated by the same conditions.     

Differences in mossy and climbing afferent sources between flocculus and ventral and dorsal paraflocculus in the rat

 Sources of mossy and climbing fiber inputs to the flocculus (FL), ventral paraflocculus (VP) and/or dorsal paraflocculus (DP) were identified in the vestibular ganglion, medulla oblongata and pons of 19 Wistar rats after 26 local injections of horseradish peroxidase, wheat-germ agglutinin-conjugated horseradish peroxidase, fast blue or diamidino yellow into the FL, VP and/or DP. There were large differences in the sources of mossy fibers to the FL and VP/DP. Labeled neurons after injections into the FL were observed mainly in the ipsilateral vestibular ganglion, bilaterally in the vestibular and prepositus hypoglossal nuclei, and in the caudal part of the nucleus reticularis tegmenti pontis. Labeled neurons were rarely observed in the pontine nuclei after localized injections into the FL. By contrast, after injections into the VP and/ or DP, numerous labeled neurons were observed in the pontine nuclei with a contralatetral predominance and in the rostral part of the nucleus reticularis tegmenti pontis bilaterally, but not in the vestibular nuclei in either side. Sources of climbing fibers to the FL and paraflocculus were completely contralateral to the injection side. After injection into the FL, labeled neurons were observed in the caudal dorsal cap and ventrolateral outgrowth of the inferior olivary nucleus. After injections into the VP, labeled neurons were observed mainly in the rostral dorsal cap, ventral medial accessory olivary nucleus (MAO) and caudal half of the ventral leaf of the principal olivary nucleus. After injections into the DP, labeled neurons were observed in the ventral MAO and caudal half of the ventral leaf of the principal olivary nucleus. These differences in the sources of mossy and climbing fiber inputs may suggest functional differences between the FL and VP/DP. The present results are consistent with our previous observations in monkey that the FL and VP/DP exhibit quite different mossy fiber input organizations. Received: 5 June 1998 / Accepted: 7 September 1998