D.M. Armstrong

An investigation of the cerebellar corticonuclear projections in the rat using an autoradiographic tracing method. II. Projections from the hemisphere

Abstract Autoradiographic tracing was used to study the distribution within the intracerebellar nuclei of axon terminals from Purkinje cells in the cerebellar hemisphere of the rat. Only selected areas of hemisphere were studied but terminal labelling was found in most of nucleus lateralis and nucleus interpositus indicating extensive convergence with projections from uninvestigated cortex. The projections were entirely ipsilateral. Injections into rostral crus II and nearby caudal curs I showed that these regions project largely to different portions of the nuclear complex (terminations from crus II predominantly dorsal to those from crus I). Each region projected to all three deep nuclei and there was an obvious mediolateral localisation in the connexions. There was a substantial projection from the medial part of curs II to the dorsolateral protuberance of nucleus fastigius (Fdlp) and, although the labelled projection to fastigius from medial crus I was small, a larger projection from crus I may in fact exist because medial portions of lobulus simplex and paramedian lobule also projected substantially to Fdlp. These observations suggest that, in the rat, the lateral part of fastigius derives its main input from a rostrocaudal strip of cortex in the medial part of the hemisphere (cf. Goodman et al.). The ventral paraflocculus projected to the caudoventral portion of lateralis and the adjoining small-celled portion of interpositus posterior. Finally, major projections from two somatosensory areas of cerebellar cortex (the lateral part of the anterior lobe and the copula pyramidis plus paramedian lobule) converged onto the medial part of interpositus anterior. This convergence presumably has significance for cerebellar control of the rubrospinal tract.

Topographical localization in the olivocerebellar projection in the rat: An autoradiographic study

The topographical organization of the olivocerebellar projection in the rat was examined by making small localized injections of tritiated leucine into the inferior olive (12 cases). The overall projection pattern is displayed in the form of a topographical map, revealing that the olivocerebellar projection is organized in sagittally orientated strips of cerebellar cortex, each of which receives its terminations from a discrete portion of the olive. The projection pattern is compared to that in the cat. In addition, the laterality of the projection is discussed.

The spatial organisation of climbing fibre branching in the cat cerebellum

SummaryThe spatial distribution of branches from climbing fibre afferents to the cerebellar cortex has been studied using electrophysiological techniques. Recordings were made from a large number of individual Purkinje cells in which climbing fibre responses were evoked by stimulation of the cerebellar surface. For each cell one or more distinct low threshold points were found from which such responses were evoked via an axon reflex pathway.For each climbing fibre studied in this way, from one to three terminations were located. These were frequently very widely distributed in the rostro-caudal direction, but were generally found to lie within a sector of the cerebellar cortical sheet which was very narrow in the medio-lateral direction.A method for estimating the conduction times in the parent axon and in each of its branches is described and applied to the experimental data relating to one axon.Quantitative measurements were made to define the parameters necessary to activate climbing fibre terminals by stimulation of the cortical surface. Using a cerebellar stimulus whose parameters were chosen on the basis of these measurements, the pattern of climbing fibre axon reflex connexions was determined over most of the accessible cerebellar surface by mapping the climbing fibre evoked potentials.Connexions formed by branching climbing fibres were found throughout the accessible cortex. They appeared always to be orientated at light angles to the long axes of the cerebellar folia, whatever the orientation of the folia with respect to the sagittal plane of the animal.

Spino-olivocerebellar pathways to the posterior lobe of the cat cerebellum

SummarySurface- and micro-electrode recordings were made from the left paramedian lobule of the cat cerebellum in order to investigate responses generated by stimulation of the superficial radial nerves. The existence was demonstrated of two climbing fibre (CF) paths from the forelimb, closely resembling two well-known paths terminating in the anterior lobe of the cerebellum — the dorsal funiculus spino-olivocerebellar path (DF-SOCP) and the lateral funiculus climbing fibre spinocerebellar path (LF-CF-SCP).Simultaneous recordings from the paramedian lobule and the anterior lobe showed that “corresponding” CF responses in the two areas had identical latencies, thresholds, stimulus/response relationships, recovery cycles and similar reactions to certain spinal cord and brain stem lesions. The characteristic fluctuations which occur in the amplitude and latency of CF responses in the anterior lobe were synchronous with similar fluctuations in the paramedian lobule. It was concluded that the DF-SOCP and the LF-CF-SCP reach both areas.Further recordings showed that the LF-CF-SCP terminates in a narrow zone of cortex which extends continuously from the intermediate anterior lobe across lobulus simplex and crus I onto the paramedian lobule. The DF-SOCP has a dual termination in the intermediate anterior lobe; one terminal zone was shown to extend from anterior lobe to paramedian lobule but the second (lateral) zone could not be traced behind lobulus simplex.Micro-electrode recordings from the right inferior olive showed cells projecting to the terminal zones of each of the two pathways. Their responses to nerve stimulation were compatible with their being relay cells on the DF-SOCP and the LF-CF-SCP.

The olivocerebellar projection in the rat: An autoradiographic study

The extent of the olivocerebellar projection was examined in the rat using autoradiographic techniques. In animals in which injections of [3H]leucine encompassed the whole olive unilaterally (4 cases), the vast majority of olive cells was densely labelled and climbing fibres were heavily labelled throughout the contralateral hemicerebellum, except for some small gaps which were not consistently located between cases. The multiple injections required to cover the oliver inevitably labelled cells in the reticular formation surrounding the olive, and it is possible that these neurones might also provide climbing fibres to the cerebellum. To control for this possibility, the inferior olive was pharmacologically destroyed (4 cases) prior to [3H]leucine injections similar in size and placement to those given to normal animals. Examination of the cerebellar cortex of these pretreated animals revealed no molecular layer labelling despite identification of labelled reticular neurones. It was thus demonstrated that all regions of the cerebellar cortex receive afferents from the inferior olive which terminate as climbing fibres. The distribution of these terminations over the entire cortex permits the conclusion that the inferior olive is the major source of climbing fibres in the rat. The same conclusions are reached using [3H]methionine as the tracer (4 cases).

Cerebello-cerebellar responses mediated via climbing fibres

SummaryA sequence of three evoked potentials was recorded on the paramedian lobule of the cat cerebellum following single shock stimulation of the intermediate portion of the ipsilateral anterior lobe. The second and third responses have been analysed using micro-electrode recording techniques and each was shown to reflect near-synchronous climbing fibre activation of many Purkinje cells in the paramedian lobule. The first response has not been fully studied but did not involve the climbing fibres.The first climbing fibre evoked potential was constant in amplitude and latency and able to follow at stimulation frequencies as high as 350/sec. It was scarcely affected by section of the ipsilateral cerebellar peduncles. The second was very variable in amplitude, never followed at frequencies greater than c. 10/sec and was completely abolished by pedunculotomy. Intracellular recordings from Purkinje cells showed that the first climbing fibre response was associated with a single excitatory post-synaptic potential but the second with excitatory post-synaptic potentials which were usually multiple.The above characteristics, together with the results of impulse collision experiments, have been satisfactorily accounted for by postulating that the climbing fibre afferents branch widely after entering the cerebellum. The earlier response arose as a result of axon reflexes and the later response as a result of orthodromic impulses generated by the cells of origin of the fibres. Evidence is presented which suggests that some at least of these cells were situated in the inferior olive.

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.

SummaryThe 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.

Zonal organization of cortico-nuclear and nucleo-cortical projections of the paramedian lobule of the cat cerebellum. 1. The C1 zone

Abstract The cortico-nuclear (C–N) and nucleo-cortical (N–C) projections of the C1 cortical zone in pars anterior (pa) and pars copularis (pc) of the paramedian lobule (PML) in the posterior lobe of the cat cerebellum were investigated with a combined electrophysiological and neuroanatomical technique. In each experiment the medio-lateral boundaries of the zone were located on the cortical surface by recording field potentials mediated via climbing fibres and evoked in the zone by activity elicited in spino-olivocerebellar paths through percutaneous stimulation of fore- and hindlimbs; a small (15–30 nl) injection of WGA-HRP was then made into the zone. The distributions in the deep cerebellar nuclei were determined (with light microscopy) both for terminal labelling due to anterograde axonal transport by Purkinje cells and for cell bodies labelled due to retrograde transport in N–C axons. The extent to which injection sites were confined to the C1 zone was assessed both by comparing injection site and zone widths and by determining the distributions of retrogradely labelled neurones within the contralateral inferior olive. The C–N projection from the part of the zone in PML pa (a forelimb part) terminates almost exclusively (perhaps exclusively) in nucleus interpositus anterior (NIA), primarily in caudal and dorsal parts, where it overlaps heavily with the C–N projections from the lobule V parts (also forelimb parts) of the C1 and C3 zones as previously defined. The C–N projection from the part of the zone in PML pc (a hindlimb part) also terminates virtually exclusively in NIA but primarily in almost all parts of the medial third of the nucleus. There is, nevertheless, sufficient overlap between the PML pa and PML pc projections that approximately one third of the termination territory of each projection overlaps that of the other. The PML pc part of the zone is almost entirely lacking in a N–C projection, as previously found for the lobule V part of the C1 zone (and C3 zone). However, the PML pa part of the zone receives N–C projections that arise, in descending order of size, from nucleus interpositus posterior (NIP), from NIA, from the NIA/nucleus lateralis (NL) fusion area and (perhaps) NL. The projection from NIP is similar in size to that provided by the nucleus to the C2 zone in lobule V of the anterior lobe. The findings are discussed, with particular emphasis on their implications for the hypothesis that the cerebellum is divisible into a number of olivo-cortico-nuclear complexes or compartments.

Topographical organisation within the cerebellar nucleocortical projection to the paravermal cortex of lobule Vb/c in the cat

SummaryThe projection from the intracerebellar nuclei to the paravermal (intermediate) cerebellar cortex of lobule Vb/c has been investigated in the cat using a combined electrophysiological and neuroanatomical technique. A small (10–30 nl) injection of WGA-HRP was made into one of the three paravermal zones (c1, c2 or c3) after the mediolateral boundaries of the zones had been delimited on the cerebellar surface by recording climbing fibre field potentials evoked in response to percutaneous stimulation of one or more paws. The distribution of retrogradely labelled cell bodies within the intracerebellar nuclei was compared with the distribution of terminal labelling arising from anterograde transport by cerebellar Purkinje cells. The three paravermal zones displayed marked heterogeneity in their receipt of a projection from the intracerebellar nuclei. The c1 and c3 zones received virtually no such input, although injections in either zone resulted in significant terminal labelling (which was largely restricted to nucleus interpositus anterior). By contrast, the intervening c2 zone received a much heavier nucleocortical input which arose almost exclusively from nucleus interpositus posterior (to which the zone also projected). A sparse contralateral nucleocortical input to the c2 zone was also demonstrated. This arose primarily from nucleus fastigius. It is concluded that the nucleocortical projection to the paravermal cortex of lobule Vb/c displays marked topographical specificity and some functional implications of this are discussed.

Responses of interpositus neurones to nerve stimulation in chloralose anaesthetized cats

Eccles et al. 6 have recently described the responses of neurones of the nucleus interpositus to stimulation of forelimb and hindlimb cutaneous afferents in decerebrate cats. The responses included two phases of excitation associated with cell discharges with latencies of about 6 and 19 msec. These were separated and succeeded by phases of inhibition. The first excitation was attributed to the action of collaterals of cuneo-cerebellar or direct spino-cerebellar axons and the second to collaterals of reticulo-cerebellar and olivo-cerebellar axons. The inhibitions were attributed to the action on the nuclear cells of the impulses from Purkinje cells 5 discharged by the afferent volleys. The present paper describes firing patterns of interpositus neurones following nerve stimulation in cats anaesthetised with a-chloralose. A somewhat surprising finding was that the cells did not always display the high level of spontaneous activity which previous authors have reported for intracerebellar nuclear neurones in unanaesthetised 15, decerebrate 6 or barbiturate anaesthetised s preparations. In addition the responses to nerve stimulation included a hitherto undescribed component consisting of a high-frequency burst of action potentials. The experiments were performed on 17 cats initially anaesthetised with ether or halothane followed by 60 mg/kg of recrystallised a-chloralose dissolved in propane1,2-diol. Some animals were paralysed with gallamine triethiodide and artificially respired. Recordings were usually begun about 7 h after the initial dose of chloralose. The left deep radial (DR), sural (SUR), femoral (FEM), gastrocnemius-soleus (GS) nerves and both superficial radial (SR) nerves were dissected and stimulated with single shocks at a repetition rate of 1/2 sec. Stimulus strengths were usually in the range 1-4 times the threshold of the most excitable fibres in the nerve, though stronger stimuli were occasionally used. Interpositus axons were stimulated in the region of the right red nucleus using a concentric stainless steel electrode. In 7 experiments a similar electrode was placed in the rostral part of the right inferior olivary nucleus.