Responses of a spino-olivo-cerebellar pathway in the cat

1. Surface potentials, similar to those found by earlier workers, have been recorded from the vermis of the anterior lobe of the cerebellum following stimulation of muscular, cutaneous and articular nerves of the ipsilateral hind limb. The most conspicuous component of the response consisted of a positive potential succeeded by a smaller negative potential.

2. Micro-electrode recordings showed that this component coincided both with climbing fibre responses in individual Purkinje cells, and with extracellular field potentials within the cerebellar cortex which closely resembled those found by Eccles, Llinás & Sasaki (1966) following electrical stimulation of the inferior olive.

3. Stimulation of the cerebellar surface, in the region where the responses to limb nerve stimulation were largest, led to antidromic invasion of neurones of the contralateral inferior olive. The antidromic action potentials were sometimes followed by up to three orthodromic spikes. Histological techniques were used to show that these neurones were located in the caudal parts of the dorsal and medial accessory olives.

4. Stimulation of nerves of the hind limb evoked discharges of the same neurones of the dorsal accessory olive which were antidromically invaded from the vermis of the anterior lobe. The nerves used (quadriceps, gastrocnemius-soleus, sural and the posterior nerve to the knee joint) were shown to excite heavily overlapping populations of neurones.

5. Those neurones of the medial accessory olive, which were identified antidromically from the anterior lobe vermis, were not discharged by stimulation of hind limb nerves.

6. Simultaneous recording from the surface of the anterior lobe and from the dorsal accessory olive showed that the onset of olive cell discharges occurred about 5 msec before the onset of the positive potential at the cerebellar surface.

     

A Rubro-Olivary pathway II. Simultaneous action on dynamic Fusimotor neurones and the activity of the posterior lobe of the cerebellar cortex

Summary In the preceding paper (Appelberg and Molander 1967) the caudal part of the red nucleus and parts of the inferior olivary nucleus were shown to cause increased dynamic sensitivity of muscle spindles when stimulated repetitively. The results to be presented will show that single shock electrical stimulation in the caudal part of the red nucleus evoked a field potential in the inferior olivary nucleus. This response seemed to be monosynaptically evoked and was observed only in parts of the olive where repetitive stimulation caused increased dynamic sensitivity of muscle spindles. Stimulation in the red nucleus as well as single shock stimulation in the actual part of the inferior olive also caused a potential in the vermis of the posterior cerebellar lobe. In conditioning — test experiments with the two stimuli the conditioning shock was seen to cause alternating periods of decreased and increased responsiveness in the pathway concerned. The same type of interaction was seen between two responses caused by double shock stimulation in the red nucleus.It is concluded that information from the caudal part of the red nucleus reaches dynamic fusimotor neurones in the spinal cord via a relay in the inferior olivary nucleus; an additional relay in the pathway is also predicted. The cerebellum seems to receive information about ongoing activity in the pathway but mesencephalic stimulation was seen to cause spindle effects also in decerebellated animals.     

Fetal cortical transplants in the cerebral hemisphere of newborn rats: a retrograde fluorescent analysis of connections

Summary The activity of cerebellar Purkinje cells and interpositus neurones was recorded during and after periods of high frequency (2.5–7.5 Hz) climbing fibre activation in barbiturate-anaesthetized cats. 1. During the high frequency conditioning stimulation, the Purkinje cell simple spike (SS) firing was initially silenced in all zones studied. After a few seconds, the SS reappeared and the frequency increased to well above that of the control level after approximately 10 s. Thereafter, the SS rate started to decline so that, after 15–20 s, the Purkinje cells fired no more SS. This SS silence lasted up to 60 s, whether or not the stimulation was continued. 2. The Purkinje cells responded with a complex spike (CS) to every stimulus. If the high-frequency stimulation lasted for at least 15 s, the spontaneous CS discharge of the Purkinje cells in the c1, c2, and c3 zones was suppressed after the conditioning stimulation had ended. This suppression lasted for approximately the same length of time as the SS silence. In the b zone, however, no CS suppression was observed. 3. Interpositus neurones displayed an increased discharge rate after periods of conditioning stimulation, thus displaying a mirror image of the Purkinje cell SS firing. 4. The behaviour of the neurones agrees well with the behaviour predicted by an hypothesis of the olivo-cerebello-olivary loop (Andersson and Hesslow 1987). 5. The results suggest that the cerebello-olivary projection is topographically organized and matches the microzonal organization in the olivo-cerebellar projection.     

Long term modification of cerebellar inhibition after inferior olive degeneration

Summary The long term effects of inferior olive destruction on the activities of the Purkinje cells and their target neurones in the cerebellar nuclei were studied in the rat. Careful observations were also made of motor behaviour throughout the study. Albino rats were injected with 3-acetylpyridine to produce a neurotoxic destruction of the inferior olive and then were used for acute recording experiments at 1–2 days, 5–7 days, 12–18 days, 35–38 days, 75–97 days and 230–252 days. After degeneration of the inferior olive, there was an initial period lasting for a few days, characterized by a high firing frequency of Purkinje cells associated with a very low level of activity of the neurones in the cerebellar nuclei. During this period, there was a deep depression of motor activity. A period of adaptation follows during the first month, characterized by a slow recovery of the initial firing frequency of the cerebellar units and a gradual recovery of spontaneous locomotion; nevertheless the firing pattern and motor behaviour remain abnormal. From one month on the unit activities disturbances and the motor deficiencies stabilize. The hypothesis is advanced that Purkinje cell inhibition on their target neurones, which increases during the initial period, gradually diminishes during the adaptation time, and then stabilizes to a subnormal state.     

Evidence for a GABA-mediated cerebellar inhibition of the inferior olive in the cat

Summary 1. Climbing fibres were activated by peripheral nerve stimulation at high frequencies (>3 Hz) for 15–25 s and then at 0.9 Hz for about 1 min. The high frequency activation induced a post-conditioning inhibition, lasting up to about 1 min, of climbing fibre responses recorded from the cerebellar surface. 2. Electrolytic lesions were made in the superior cerebellar peduncle (brachium conjunctivum). After the lesion, the post-conditioning inhibition was completely eliminated. 3. Injections of the GABA-receptor blocker bicuculline methiodide into the inferior olive reversibly blocked the post-conditioning inhibition. 4. The results support the hypothesis proposed by Andersson and Hesslow (1987a), that post-conditioning inhibition is mediated by a GABA-ergic interposito-olivary pathway.     

Branching of olivary axons to innervate pairs of sagittal zones in the cerebellar anterior lobe of the cat

Summary Four of the eight sagittal zones in the cortex of the cerebellar anterior lobe, the x, c1, c3 and d2 zones, receive similar climbing fibre input from the ipsilateral forelimb through pathways ascending in the dorsal funiculus (DF-SOCPs) and have disynaptic relays in the main cuneate nucleus (Ekerot and Larson 1979a).The present investigation demonstrates that the forelimb areas of these four zones are innervated by climbing fibres from three groups of olivary neurones with branching axons (Fig. 6). The termination sites of climbing fibre branches were determined by identifying the low-threshold spots on the cerebellar surface from which short-latency (3.1–7.5 ms), direct climbing fibre responses could be evoked in Purkinje cells. In some Purkinje cells these responses were followed by late (8.2–13.5 ms) climbing fibre responses (olivary reflex responses).Each group projects to a pair of these zones which is separated by an intervening zone innervated from a private group of olivary neurones. The three groups of olivary neurones have been denoted the x-c1 group, the c1–c3 group, and the c3–d2 group in accordance with the zones they innervate. They project to the following areas: (a) The x-c1 group to the x zone in the vermal cortex and the lateral part of the c1 zone in the intermediate cortex; (b) the c1–c3 group to the medial parts of the c1 and c3 zones in the intermediate cortex; and (c) the c3–d2 group to the lateral part of the c3 zone in the intermediate cortex and the d2 zone in the extreme lateral part of the anterior lobe.Olivary axons belonging to the x-c1 and c1–c3 groups often send several climbing fibres to each projection area, whereas only single termination sites were found in the c3 zone for olivary axons belonging to the c3-d2 group.The two projection areas of each group of olivary neurones have a similar topographical organization: the areas innervated by the x-c1 group lack distinct somatotopical organization, whereas the areas innervated by the c1-c3 and c3-d2 groups have a detailed somatotopical representation of the ipsilateral forelimb.The low threshold spots for the late climbing fibre responses (olivary reflex responses) were restricted to the same sagittal strips as the low-threshold spots for the direct responses.Supported by grants from the Medical Faculty, University of Lund, and to Dr. O. Oscarsson from the Swedish Medical Research Council (Project 01013)     

Origin in the medial accessory olive of climbing fibres to the x and lateral c1 zones of the cat cerebellum: a combined electrophysiological/WGA-HRP investigation

Summary The climbing fibres to the x and lateral c1 zones of the anterior lobe of the cerebellum arise as branches of common stem olivary axons. Anatomical studies have shown that the c1 zone receives its climbing fibres from the dorsal accessory olive (DAO). It has, therefore, been assumed that the x-zone also receives its climbing fibres from this olivary subnucleus. The present study demonstrates that both the x-zone and the lateral part of the c1 zone in fact receive their climbing fibre input from the middle portion of the medial accessory olive (MAO) (approximate antero-postero-levels P10-13). Electrophysiological techniques were used to define the extent of these cerebellar zones and small volumes (15–50 nl) of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were pressure injected into the defined zone. These small pressure injections resulted in injection sites with minimal spread to adjacent zones. The sensitive tetramethylbenzidine (TMB) reaction was used to visualize both the injection site and retrogradely labelled cells in the inferior olive. This combination of electrophysiological and neuroanatomical techniques gave extremely reproducible results. The results suggest that the zone previously named lateral c1 would be better designated cx.     

Input from trigeminal cutaneous afferents to neurones of the inferior olive in rats

Summary Extracellular recordings were obtained from inferior olivary neurones of the rat. The responses of fifty neurones evoked by electrical stimulation of a branch of the trigeminal nerve were recorded. Maxillary nerve stimulation was most effective. The response was characterized by an early discharge (single spike and wave, typically with latencies between 16 and 30 msec) and a weak late discharge which followed a period of inhibition of about 100 msec. Half of the neurones responded to one branch of the trigeminal nerve only whereas the other neurones displayed a varying degree of convergence, including sometimes a convergence from limb nerves. Forty-nine olivary neurones were tested for cutaneous receptive fields. Ten out of these had small receptive fields (<20% of the contralateral face) and a low threshold to mechanical stimuli. Twenty neurones which had larger receptive fields responded also to low-threshold or to medium-threshold (i.e. non-nociceptive) mechanical stimuli. None of the neurones displayed receptive fields more extensive than half of the contralateral face and some of the larger fields had a small, low-threshold focus. Olivary neurones responding to electrical stimulation of trigeminal nerves or mechanical stimulation of the face were located in the medial segment of the olivary complex (dorsal accessory and principal olive). A few cells only were located in the lateral segment.It is concluded that neurones of the inferior olive receive a substantial input from trigeminal afferents and are capable of transmitting precise somatotopical information to the cerebellum.     

Responses in the inferior olive to stimulation of the cerebellar and cerebral cortices in the cat

1. Extracellular field potentials and single unit responses have been recorded from the inferior olive of the cat following stimulation of the surface of the contralateral paramedian lobule of the cerebellum, and of the ipsilateral cerebral cortex. 2. Cerebellar stimulation results in antidromic invasion of inferior olivary neurones via the climbing fibres. These responses are followed by synaptic discharges which may be generated through climbing fibre recurrent collaterals. 3. Precise histological controls have shown that these responses to stimulation of the paramedian lobule are located in the ventral lamella of the principal olive. 4. Unifocal stimulation of the sensori-motor cortex with surface-anodal pulses evokes synaptically generated discharges of neurones in the central lamella, with a latency of 8-9 msec. The area of cortex yielding responses has been mapped at chosen stimulus intensities and the limitations of the maps have been discussed. 5. It has been shown that the initial excitatory responses obtained from either cortex are followed by an inhibition which lasts about 100 msec, and gives way to a period of recovery or facilitation. This, in turn, is succeeded by a further period of inhibition. Possible neural substrates for these changes have been discussed.     

Evoked responses of the superior olive to amplitude-modulated signals

Evoked potentials of some auditory centers of Rhinolophidae bats to amplitude-modulated signals were studied. A synchronization response was found in the cochlear nuclei (with respect to the fast component of the response) and in the superior olivary complex (with respect to both fast and slow components of the response) within the range of frequency modulation from 50 to 2000 Hz. In the inferior colliculus a synchronized response was recorded at modulation frequencies below 150 Hz, but in the medial geniculate bodies no such response was found. Evoked responses of the superior olivary complex were investigated in detail. The lowest frequencies of synchronization were recorded within the carrier frequency range of 15–30 and 80–86 kHz. The amplitude of the synchronized response is a function of the frequency and coefficient of modulation and also of the angle of stimulus presentation.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 63, No. 5, pp. 626–631, May, 1977.     

Mode of induction of long-term depression at parallel fibre--Purkinje cell synapses in rabbit cerebellar cortex

In a superficial folium of the dorsal paraflocculus of high decerebrate rabbits, extracellular unitary spikes were recorded from a Purkinje cell, while two parallel fibre beams impinging onto that Purkinje cell were separately stimulated in the molecular layer. Climbing fibre afferents were stimulated at the contralateral inferior olive. Quisqualate was ionophoretically applied to the dendrite of the Purkinje cell intersecting one of the stimulated parallel fibre beams (test beam). Long-term depression (longer than 45 min) occurred in Purkinje cell responsiveness to the test beam, but not to the other beam (control beam), when quisqualate was applied for 4 min in conjunction with 2 Hz stimulation of climbing fibres. This effect was completely abolished by simultaneous application of a glutamate blocker, kynurenate, during conjunctive quisqualate-climbing fibre stimulation. Application of quisqualate alone caused a small degree of depression in parallel fibre-Purkinje cell transmission. This effect was abolished when spontaneous activity of climbing fibres was blocked by injection of tetrodotoxin or lidocaine to the contralateral inferior olive, and therefore was due to conjunction of quisqualate with spontaneous climbing fibre inputs that normally occurred at 0.5-1.2 Hz. These findings suggest that the occurrence of long-term depression is strictly dependent on conjunction of climbing fibre activity with quisqualate receptor activation.     

Termination and functional organization of the dorsal spino-olivocerebellar path

1. The spino-olivocerebellar path ascending through the dorsal funiculus (DF-SOCP) was investigated in decerebrate cats with the cord transected in the third cervical segment except for the dorsal funiculi. The climbing fibre responses evoked in Purkinje cells were studied by recording the mass activity at the cerebellar surface and by recording from single cells.2. The DF-SOCP forms a disynaptic path from the spinal cord to the cerebellar cortex as shown by latency measurements. Anatomical studies have recently demonstrated that the relays are in the rostral part of the dorsal funiculus nuclei and in the dorsal accessory olive.3. The DF-SOCP projects to sagittal zones in the pars intermedia and vermis of the anterior lobe. The somatotopical organization is predominantly transverse in the pars intermedia and predominantly longitudinal in the vermis.4. The olivary neurones in the DF-SOCP are activated by the flexor reflex afferents from wide receptive fields. The fields are restricted to one ipsilateral limb and the majority of the olivary neurones could be activated from all the nerves tested in this limb.5. Natural stimulation of receptors evoked excitation in about half of the olivary neurones investigated. This excitation was elicited by pressure against deep structures. Inhibitory effects were rarely observed.6. The dorsal and ventral spino-olivocerebellar paths are compared.     

Role of climbing fibers in determining the spatial patterns of activation in the cerebellar cortex to peripheral stimulation: an optical imaging study

The spatial patterns of activation in the rat cerebellar cortex evoked by ipsilateral face stimulation were mapped using optical imaging based on the pH sensitive dye, Neutral Red. The aims of the study were to characterize the optical responses evoked by peripheral stimulation and test the hypothesis that the resultant parasagittal banding is due to climbing fiber activation. In the anesthetized rat Crus I and II of the cerebellar cortex were stained with Neutral Red. Epi-fluorescent changes produced by a train of stimuli (5-10s and 4-20 Hz) to the ipsilateral face were monitored in time using a fast, high resolution charge-coupled device camera. The patterns of activation were quantified using a two-dimensional fast Fourier transform analysis that removed signals with high spatial frequencies and minimized the contribution of horizontal structural elements (i.e. blood vessels). The dominant spatial pattern of activation evoked by face stimulation was that of parasagittal bands. The bands were highly frequency-dependent and were elicited most strongly by stimulus frequencies in the range of 6-8 Hz. There was a large fall-off in the response for frequencies above and below. The optical signal evoked by face stimulation built up over a period of 10s and then gradually decayed. Within a folium the individual parasagittal bands exhibited some frequency and temporal specificity. Stimulation of the contralateral inferior olive also resulted in the activation of parasagittal bands with characteristics similar to the bands evoked by face stimulation, including a preferred stimulus frequency which peaked at 10 Hz. Injection of lidocaine into the contralateral inferior olive blocked the parasagittal bands evoked by ipsilateral face stimulation, while control injections of saline had no effect.The results confirm that a parasagittal banding pattern is a dominant feature of the functional architecture of the cerebellar cortex. The parasagittal banding pattern observed with Neutral Red is due primarily to the activation of climbing fiber afferents. The frequency tuning of the responses, with the preference for peripheral stimuli of 6-8 Hz, is in agreement with previous findings that the inferior olive is inherently rhythmic. These observations support the hypothesis that inferior olivary neurons are dynamically coupled into groups that activate parasagittal bands of Purkinje cells in the cerebellar cortex. The frequency tuning also supports the hypothesis that the climbing fiber system is involved with timing. Activation of this afferent system may require stimuli with appropriate frequency content and stimuli synchronized to the rhythmicity of the inferior olive.     

Synaptic actions of peripheral nerve impulses upon Deiters neurones via the climbing fibre afferents

1. The cerebellar integration of sensory inputs to Deiters neurones was studied in cats under Nembutal anaesthesia.2. Stimulation of peripheral nerves produced in the Deiters neurones a sequence of an initial excitatory post-synaptic potential (e.p.s.p.) and a later inhibitory post-synaptic potential (i.p.s.p.), or a relatively small e.p.s.p.3. The Deiters neurones were classified as forelimb (FL)- or hind limb (HL)-type cells according to the location of the most effective peripheral nerve. In the FL cells stimulation of the forelimb nerves produced the e.p.s.p.-i.p.s.p. sequence (dominant response), while stimulation of the hind limb nerves was ineffective or produced the small e.p.s.p. (non-dominant response). In contrast, in the HL cells the non-dominant response was evoked from the forelimb nerves, and the dominant response from the hind limb nerves.4. The stimulus intensity-response relation indicates that Group I and II muscle afferents and low and high threshold cutaneous afferents contribute to the dominant and non-dominant responses.5. Antidromic identification of these Deiters neurones revealed that 90% of the HL cells and 85% of the FL cells project to the lumbo-sacral and cervico-thoracic segments of the spinal cord, respectively, while 10% of the HL cells and 15% of the FL cells innervate the cervico-thoracic and lumbo-sacral segments, respectively.6. The mean latency of the e.p.s.p. evoked from the forelimb nerves was 14 msec in the FL cells and 13 msec in the HL cells, and the latency of the e.p.s.p. evoked from the hind limb nerves was 17 msec in the FL cells and 18 msec in the HL cells. The later i.p.s.p. regularly followed the onset of the e.p.s.p. with a delay of 3-5 msec.7. The dominant and non-dominant responses in both types of cells exhibited the following three characteristic features: (i) a strong depression after conditioning stimulation of the inferior olive, (ii) an increase of the inferior olivary excitability during the responses, and (iii) a striking frequency depression with stimulation at relatively low frequency (5-10/sec).8. Consequently it was concluded that all of the responses were produced through the climbing fibres originating from the inferior olive, the i.p.s.p.s due to inhibition from Purkyne cells activated by the climbing fibres and the e.p.s.p.s due to excitation from the collaterals of the climbing fibres.     

Studies of the Synaptic Plasticity of Field CA3 of the Hippocampus During Tetanization of the Perforant Path

Studies on living slices of hippocampus-entorhinal cortex formation from adult rats were performed to investigate changes in responses in field CA3 to stimulation of mossy fibers in conditions of perforant path tetanization with different parameters. Tetanization of the perforant path at frequencies of 10 and 100 Hz induced depression of responses in CA3 on testing of this same path. Tetanization of the perforant path at a frequency of 10 Hz and an amplitude subthreshold for potentiating mossy fiber synapses in CA3 became threshold if preceded by tetanization of the perforant path at a frequency of 100 Hz. Tetanization of mossy fibers at 10 Hz resulted in potentiation of the input to CA3, while tetanization at 100 Hz induced depression. High-frequency tetanization of the perforant path (100 Hz) delivered in trains following at the frequency of the theta rhythm, led mainly to depression of field CA3 responses to stimulation of mossy fibers.__________Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti, Vol. 54, No. 4, pp. 542–547, July–August, 2004.     

Anatomical demonstration of branching olivocerebellar fibres by means of a double retrograde labelling technique

It has been shown physiologically that axons of neurones in the inferior olive may branch and supply different areas within a particular longitudinal cerebellar zone. In the present study such branching neurones have been demonstrated anatomically.Small amounts of horseradish peroxidase and tritiated inactivated horseradish peroxidase were injected in somatotopically corresponding regions of the paramedian lobule and the intermediate part of the anterior lobe in three adult cats. The occurrence in the inferior olives of labelled cells was carefully mapped. Cells showing labelling with both markers (double labelled cells) were found in all major divisions of the olive: medial and dorsal accessory olive and dorsal and ventral lamella of the principal olive. The olivary regions containing double labelled cells project to cerebellar zones C₂, C₁–C₃, D₁ and D₂, respectively. The distribution of double labelled olivary cells in the particular cases is in agreement with the extent of spreading of the injected marker solutions to various of the longitudinal cerebellar zones in each case.These findings appear to establish that the ventral lamella projects to the lateralmost cerebellar zone D₂ of Voogd not only in the paramedian lobule but also in the anterior lobe. It is emphasized that, although neurones with branching axons occur in all major olivary subdivisions, no conclusions can be made as to whether all the minor parts of the olive harbour such neurones or whether branching is equally prolific for all neurones.     

Inhibition of the inferior olive during conditioned responses in the decerebrate ferret

Output from the interpositus nucleus can inhibit the inferior olive, probably via the GABA-ergic nucleo-olivary pathway. It has been suggested that the function of this inhibition might be to regulate synaptic plasticity resulting from parallel fibre/climbing fibre interaction in cerebellar Purkinje cells, by providing negative feedback information to the olive. Thus, when a learned response, generated by the interpositus nucleus, reaches a sufficient amplitude, the olive would be inhibited and further learning blocked. This suggestion was tested in a classical conditioning paradigm. Decerebrate ferrets were trained using electrical skin stimulation of the forelimb as the conditioned stimulus (CS) and periorbital stimulation as the unconditioned stimulus (US). Climbing fibre responses evoked in Purkinje cells by the US were recorded as surface field potentials in the part of the c3 zone controlling eyeblink. It was found that the CS did not inhibit the olive at the beginning of training, but when conditioned responses were large, the olive was inhibited by the CS in some animals. After a number of unpaired CS presentations, which caused extinction of the conditioned response, the inhibition disappeared. The size of individual conditioned responses correlated negatively with the size of the climbing fibre responses evoked by the US. Climbing fibre responses evoked by direct stimulation of the olive were also inhibited. It was concluded that cerebellar output during performance of a conditioned response inhibits the inferior olive. The results thus support the hypothesis of a cerebellar locus of conditioning and are consistent with the proposed role of cerebello-olivary inhibition.     

The olivo-cerebellar system: Functional properties as revealed by harmaline-induced tremor

Summary Intracellular recording from Purkinje cells in cat cerebellar cortex demonstrated an 8–10/sec burst activity following intravenous administration of harmaline (10 mg/kg), a drug known to produce tremor at the same frequency. The burst activation of Purkinje cells was generated by large all-or-none depolarizations similar to climbing fiber (CF) excitatory postsynaptic potentials (EPSPs). Polarization of the cell membrane through the recording electrode (via a Wheatstone bridge) revealed that the all-or-none depolarization had an equilibrium potential and time course identical to the electrically evoked CF-EPSP, demonstrating directly that tremor is associated with specific activation of the CF afferent system.Interspike frequency histograms of the burst responses of Purkinje cells show that the rhythmic CF activity may continue for several hours with approximately 10% frequency scatter, the actual frequency depending on the level of anesthesia. Simultaneous extracellular recordings from Purkinje cells near the midline vermis indicated that CFs projecting to this area fire in a synchronous manner, while simultaneous recording from three Purkinje cells at different lateralities from the midline showed that the rhythmic activity is reduced in the lateral vermis and may be absent in the cerebellar hemispheres.Intra- and extracellular recordings from cerebellar nuclear cells (fastigial) disclosed a bursting type of activation following harmaline; a similar type of activity could be recorded in the reticular formation neurons and at inferior olive level. At spinal cord level, harmaline induced a repetitive and rhythmic activation of motoneurons which was not modified by dorsal root section. Cooling of the cerebellar cortex produced a definite desynchronization of the rhythmic motoneuronal firing. However, the basic 10/sec firing of the spinal cord motoneurons could still be observed. Following lesion of the inferior peduncles which interrupted the olivo-cerebellar pathway, the rhythmic activation of Purkinje cells, nuclear cells, vestibular and reticular cells and motoneurons disappeared. However, the rhythmic activity was maintained at inferior olivary level. It is suggested that harmaline acts directly on the inferior olive since in animals with low decerebration, cerebellectomy and spinal transection, rhythmic activity of the inferior olive could still be observed.The results of these experiments strongly suggest that the inferior olive is able to generate the activation of motoneurons and that such influence can only take place through the activation of the cerebellar nuclei. Possible functions of the inferior olive as a generator of fast muscular transients are discussed.     

Harmaline induced tremor III. A combined simple units,horseradish peroxidase,and 2-deoxyglucose study of the olivocerebellar system in the rat

Summary Purkinje cells were recorded extracellularly and mapped in the cerebellar cortex of the rat under tremogenic doses of harmaline. Four différent types of responses were encountered, of which two were considered as being responsible for the harmaline tremor. The latter had a regular firing pattern of complex spikes at 5 to 10 Hz and were mostly found in the vermis. Their number decreased in the more lateral region of the cerebellar cortex until they eventually disappeared. Horseradish peroxidase was injected into all the areas of the cerebellar cortex containing Purkinje cells with harmaline-induced activity. Labeled neurons were in all cases traced to the medial accessory olive. The metabolic activity of the inferior olive under harmaline was measured with 2-deoxyglucose. Increased labeling was only found in the medial accessory olive. Such an increase was demonstrated as being due to a direct effect of the drug on the inferior olivary neurons, indicating that the medial accessory olive is responsible for the harmaline tremor in the rat. Our results point out that, in the rat, there is an inverse relationship between serotoninergic innervation of a region in the inferior olivary nucleus and that with harmaline sensitivity, therefore a serotoninergic mechanism hypothesis for the harmaline tremor needs further investigation.Assistant from University Paris VII     

Cerebellar inhibition of inferior olivary transmission in the decerebrate ferret

Stimulation around the superior cerebellar peduncle or within the deep cerebellar nuclei is known to inhibit the inferior olive with a very long latency. It has been suggested that this inhibition is mediated by the GABA-ergic nucleo-olivary pathway, but alternative explanations such as activation of an indirect excitatory pathway or a pathway via the red nucleus are possible. A long-latency inhibition via the nucleo-olivary pathway would have profound implications for cerebellar function and the present study was performed to test alternative explanations and to characterize the nucleo-olivary inhibition. Climbing fibre responses (CFRs), evoked by periorbital stimulation and recorded from the cerebellar cortex, could be inhibited by stimulation of two distinct mesencephalic areas. One was located within the superior cerebellar peduncle and the other about 1 mm further ventrally. Inhibition evoked from either area occurred in the inferior olive and was independent of a red nucleus relay. Single Purkinje cell recordings revealed that inhibition from the ventral area was not secondary to olivary activation. It is concluded that stimulation of the ventral area activated nucleo-olivary fibres. The inhibition elicited by stimulation within the peduncle probably resulted from indirect activation on the nucleo-olivary fibres via antidromic activation of the interpositus nucleus. The time courses of the inhibition from the two areas were indistinguishable. The duration of the strongest inhibition was short and had a sharp peak at about 30 ms. It is suggested that the time course of the inhibition is important for temporal regulation of learned responses.