The role of some brain structures in the switching of the descending influences in operantly conditioned rats.

A hypothesis was proposed according to which the switching of descending influences by the corticospinal and corticorubrospinal systems was associated with rubro-olivary projection involvement depending on the context of movement [Kennedy P. R. (1990) Trends Neurosci. 13, 474-479]. Our results confirmed and extended this hypothesis. It was shown that a preliminary transection of the dorsolateral funiculus (containing the rubrospinal tract) accelerated the compensatory rehabilitation process following lesions of the red nucleus and the ventrolateral thalamic nucleus in albino rats with learned instrumental reflexes on equilibrium. A preliminary lesion of the ventrolateral thalamic nucleus considerably hampered the switching process; nevertheless, performance of the reflexes suggested that the switching of cerebellar ascending influences to the cerebral cortex could be completed through other cerebellocortical pathways as well. Comparison of the results of electrolytic and chemical lesions of the red nucleus suggested a similar conclusion. It was established that the conditioning and recovery of already learned instrumental reflexes were impossible after complete neurotoxic destruction of the inferior olive. The data obtained emphasize the role of the inferior olive, ventrolateral thalamic nucleus and red nucleus in the switching of descending influences in operantly motor conditioned rats. Motor deficit and the compensatory rehabilitation process depended on the severity of inferior olive destruction combined with a high transection of the dorsolateral funiculus and a destroyed red nucleus. Long-lasting training improved compensation of motor deficit and stabilized instrumental reflexes to some extent in rats with incomplete destruction of the inferior olive. It has been suggested that these modifications occur because of collateral sprouting in the olivocerebellar system.

Electrophysiological properties of the somatotopic organization of the vestibulospinal system in the frog.

In experiments on the preparation of a frog perfused brain (Rana ridibunda), field and intracellular potentials were recorded from neurons of the vestibular nuclear complex following stimulation of the ipsilateral vestibular nerve and different levels of the spinal cord. Stimulation of the vestibular nerve evoked mono- and polysynaptic excitatory postsynaptic potentials and orthodromic action potentials. In parallel, an antidromic activation of vestibular neurons sending their axons to the labyrinth was recorded. Vestibulospinal neurons sending their axons to the cervical (C neurons) and lumbar (L neurons) enlargements of the spinal cord were identified by their antidromic activation. A rather high conduction velocity along vestibulospinal fibres (mean 15.47 m/s) was observed. A somatotopic arrangement of the vestibulospinal system was established in spite of extremely large overlapping zones for the fore- and hindlimb representations in the vestibular nuclear complex. The hindlimbs were represented more poorly than the forelimbs. Antidromic potentials of C and L neurons were recorded in the medial, descending and with the highest density in the lateral vestibular nuclei (Deiters' nucleus). C neurons were evenly distributed in the other vestibular nuclei studied, while L neurons were located predominantly in the caudal parts of the vestibular nuclear complex. The multiplicity of the origin of the vestibulospinal axons was established. Peculiarities of the functional correlation between the vestibular input and vestibulospinal system are discussed.

Comparison of the effects of electrolytic and chemical destruction of the red nucleus on the compensatory capacity of rats with rubrospinal tract lesions.

Transection of the rubrospinal tract in rats, performed before lesion of the red nucleus, resulted in the facilitated recovery of motor activity and operantly conditioned reflexes. Such facilitation was absent when the red nucleus is lesioned alone. This phenomenon is explained by the switching of descending influences on the corticospinal tract through the participation of the following system: red nucleus--inferior olive--cerebellum--ventrolateral thalamic nucleus--cerebral cortex. The above mentioned facilitating influence on the recovery process was particularly prominent in rats with quinolinic acid-induced lesion of the red nucleus. Under these conditions, the cerebellar ascending fibers to the ventrolateral thalamic nucleus were preserved. Decreased facilitated recovery following electrolytic lesion of the red nucleus suggests the existence of additional cerebello-cortical pathways for the realization of the switching phenomenon.

Organization of afferent projections to the ventral and dorsal regions of the cat lateral vestibular nucleus: an HRP study.

Topical organization of afferent projections to Deiters' nucleus originating from cortical, subcortical, brainstem, and spinal cord structures has been revealed in the cat by microiontophoretic injection of horseradish peroxidase (HRP) into the ventral (NVLV) and dorsal (NVLD) regions of the nucleus and subsequent study of retrograde axonal transport of the enzyme. Differences between afferent inputs to the ventral and dorsal parts of the nucleus, considered as representative of the forelimb and hindlimb regions of the structure, have been observed. The trajectories of labeled fiber systems of the nucleus mentioned have been described. Computer reconstruction of the cat lateral vestibular nucleus (NVL) according the contours drawn from frontal sections of the brain has been carried out.

Comparative characteristic of afferent inputs to the dorsal and ventral regions of the magnocellular part of the cat red nucleus.

The mosaic of topical organization of afferent inputs from some cortical, subcortical, including nucleus caudatus, brainstem and spinal cord structures to the magnocellular part of the cat red nucleus has been revealed by microiontophoretic injection of horseradish peroxidase and subsequent study of retrograde axonal transport of the enzyme. Substantial differences between afferent inputs of dorsal and ventral regions of the red nucleus as representation zones of fore- and hind-limbs in this structure were observed. It was found that while a considerable variety of afferent inputs to the ventral regions of the red nucleus exists, the dorsal region receives information from only a restricted number of structures. The trajectories of retrogradely labelled fibre systems of the red nucleus have also been described.