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Notes: Summary In a previous study observations in semithin sections of E-PTA-stained cerebellar cortex of the cat revealed differences in size of synaptic grids between the molecular and granular layer (Van der Want et al. 1984). In addition, synaptic size differences were observed between superficial and deep levels in the molecular layer. The present study was an attempt to analyze synapses in ultrathin sections of the cerebellar cortex with special emphasis on size differences of distinct types of synapses at different levels in the molecular layer. Climbing fibers were identified by means of anterograde transport of 3H-leucine injected in the inferior olive and parallel fibers were identified on account of fine structural criteria. Synaptic profiles were measured semi-automatically in the neuropil of the cerebellar cortex at the supra-Purkinje level and the subpial level. Measurements of the trace- and chordlength were obtained from random sections. The frequency distribution of the true diameters of the synapses was reconstructed with a discrete “unfolding”-procedure. The overall diameter at the superficial level was 390.2±1.5 nm, at the deep level 406.6±1.5 nin. Climbing fibers exhibited mean values of 431.9±4.7 and 461.3±4.1 nm at these levels and parallel fiber terminals mean values of 370.7±2.9 and 395.8±3.0 nm. The frequency distributions showed remarkable and statistically significant differences compared with the overall distributions observed at the superficial and the deep levels respectively. The frequency distributions of synaptic diameters at the superficial and deep levels also differ significantly. The results suggest that synapses are characterized by a specific size which might be related to the region of termination or might be determined by the afferent neuron. This is in agreement with earlier observations in E-PTA treated material.

Notes: Summary The cerebellar projection of the external cuneate nucleus and the adjoining rostral part of the internal cuneate nucleus were investigated by means of anterograde transport of tritiated leucine. The cuneocerebellar tract terminates as mossy fiber rosettes in the granular layer. The termination area has a more or less spherical form with its centre at the ipsilateral side. It comprises the anterior and posterior vermes bilaterally and the ipsilateral hemispheral parts of the anterior and simple lobules, the medial aspect of the ansiform lobule and the paramedian lobule. Within this area the mossy fiber terminals are arranged in continuous sagittal strips, some of them clearly separated from one another. The strips were found in the cerebellar modules A-D. Concomitant bilateral projections to several subdivisions of the inferior olive were found. Some of these provide the anatomical substrate for the simultaneous activation of a number of mossy and climbing fiber zones observed in the anterior lobe following stimulation of different forelimb nerves. No evidence was found for a termination of mossy fiber collaterals in the central cerebellar nuclei.

Notes: Summary The projection of the nucleus reticularis tegmenti pontis and the adjacent tegmental area, to the caudal brain stem and the cerebellum were investigated by means of anterograde transport of tritiated leucine. The nucleus reticularis tegmenti pontis was found to be exclusively connected with the cerebellum. Mossy fiber terminals were absent only from lobule X and most abundant in lobule VII and the hemispheres with a slight contralateral predominance. The paramedian pontine reticular formation projects with bilateral symmetry to the cerebellar lobules VI, VII and the crura I and II, and heavily to the medial aspect of predominantly the ipsilateral reticular formation in the lower brain stem including specific targets as the nucleus reticularis paramedianus, the nucleus prepositus hypoglossi, the nucleus intercalatus, the nucleus of Roller, the nucleus supragenualis and the dorsal cap of the inferior olive. The nucleus vestibularis medialis receives a very weak projection. The connections are discussed in the light of their possible involvement in pathways for the execution of voluntary and reflex eye movements.

Notes: Efferent projections from the lateral reticular nucleus in the rat were investigated with anterograde transport of Phaseolus vulgaris leucoagglutinin. Besides the well known mossy fibre connections to the cerebellar cortex and collaterals to the cerebellar nuclei, a substantial bilateral projection to the lateral vestibular nucleus was found. Terminal arborizations found within this nucleus appeared to detach from the reticulocerebellar fibres in the cerebellar white matter and enter the lateral vestibular nucleus from dorsally. This projection may have functional relevance for the control, by ascending spinal pathways, of the descending lateral vestibulospinal tract.

Notes: This study examines the influence of the cerebellum on the excitability of inferior olivary neurons in the cat. Two major pathways from the cerebellar nuclei to the inferior olive have been investigated by electrophysiological and anatomical techniques. The first, excitatory pathway connects the cerebellar nuclei through nuclei at the mesodiencephalic junction with the inferior olive. The second is the direct, GABAergic, nucleo-olivary pathway. Intra- as well as extracellular recordings obtained in the rostral part of the medial accessory and principal olives revealed that electrical stimulation with a short burst of three pulses delivered at the mesodiencephalic junction results in short-latency activation (4–8 ms) of most olivary neurons. More than half of the units showed, in addition to the short-latency activation, a consistent response with a much longer latency (-180 ms). Many units (66%) that responded to mesodiencephalic stimulation could also be activated by superior cerebellar peduncle stimulation with a similar stimulation paradigm (latency 9–15 ms). However, in such cases consistent long latency responses were only rarely recorded (7%). To distinguish between the effect of the two pathways, both of which are activated by superior cerebellar peduncle stimulation, an electrolytic lesion of the nucleo-olivary fibres was made in the brainstem in six experiments. The effect of this lesion was verified in three cases by retrograde horseradish peroxidase tracing from the rostral inferior olive at the end of the experiment. This time only extracellular recordings were made. Stimulation of the mesodiencephalic junction still resulted in easily activated olivary units which showed an increased probability of firing a long-latency action potential. Stimulation of the superior cerebellar peduncle now resulted in a 50% decrease in probability of activating olivary units in the short-latency range. However, a five-fold increase in the chance of triggering action potentials in the long latency interval was noted, implying that many units reacted only with a long-latency action potential. The results obtained with our experimental paradigm appear enigmatic since it is well established that the nucleo-olivary pathway is GABAergic and thus, by convention, should be inhibitory to the olivary neurons. However, it is possible to explain these results in terms of dynamic coupling of olivary neurons. This concept ascribes an important role to the nucleo-olivary pathway in regulating the degree of electronic coupling between olivary neurons (probably by a shunting mechanism) and as such may be an important instrument in the regulation of synchronous and rhythmic olivary discharges. Thus, lesion of this pathway would be expected to result in coupling of large aggregates of olivary cells. It seems likely that these strongly coupled cell ensembles are more difficult to activate by incoming afferent volleys. However, once activated, the coupled olivary neurons develop an oscillation of the membrane potential which may be conveyed, electronically, to neighbouring neurons and subsequently, during the depolarizing phase of the oscillation, result in a more easily triggered rebound or longlatency response. It is concluded that cerebellar output may not merely inhibit olivary neurons, but also, in conjunction with an excitatory nucleo-mesodiencephalo-olivary circuit, modulate olivary excitability in a rather complex manner.