The role of the ventrolateral nucleus of the thalamus in the switching of descending influences to motor activity in the rat.

Studies on rats showed that the facilitating influence of preliminary transection of the rubrospinal tract on recovery of motor activity and operant reflexes disrupted by lesioning of the red nucleus was more apparent when lesioning was chemical than when lesioning was electrolytic. This is due to the survival of cerebellothalamic fibers to the ventrolateral nucleus of the thalamus after chemical lesioning of the red nucleus with quinolinic acid. It was also shown that preliminary lesioning of the ventrolateral thalamic nucleus strongly hindered the switching of motor activity under the control of the corticospinal tract in rats subjected to section of the rubrospinal tract and lesioning of the red nucleus.

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.

Compensatory restorative processes and operant reflexes in rats after neurotoxin lesioning of the inferior olive.

Studies on rats showed that complete neurotoxin lesioning of the inferior olive obviated the possibility of developing and restoring previously learned operant balance reflexes. Motor deficit and compensatory-restorative processes in rats treated with 3-acetylpyridine and high section of the dorsolateral funiculus of the spinal cord depended directly on the level of disruption of the inferior olive. Prolonged observation of rats with incomplete lesions to the inferior olive revealed improvements in the compensation of motor lesions and stabilization of operant reflexes.

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.

Synaptic responses of neurons in the parietal associative cortex of the cat to stimulation of the red nucleus.

Acute experiments on anesthetized and immobilized cats using intracellular recording were used to study the responses of neurons in the parietal associative cortex to stimulation of the red nucleus. Efferent neurons of the parietal cortex were identified by antidromal activation on stimulation of the intrinsic nuclei of the pons and motor cortex. Oligo- and polysynaptic EPSP in response to stimulation of the red nucleus were seen. The results are discussed in the light of the morphological organization of the rubrothalamic and cerebellothalamocortical tracts.

Antidromal and synaptic activation of neurons of the associative parietal cortex of the cat brain elicited by spike activity from the intrinsic nuclei of the pons.

Acute experiments were performed on cats with intracellular recording of efferent and unidentified neurons of the anterior suprasylvian and posterior lateral gyri of the parietal cortex, to study the antidromal and synaptic responses to stimulation of the lateral and medial groups of intrinsic nuclei of the pons. Oligo- and polysynaptic components were detected, along with complex EPSP due to convergence of axons from fast- and slow-conducting neurons. Antidromal and synaptic responses were demonstrated in the same parietal cortex neurons, demonstrating a double connection between the intrinsic nuclei of the pons and the associated parietal cortex. The possible pathways of these connections are discussed, along with their features and importance in the functioning of pontocortical connections.

Patterns of inputs to the parietal cortex efferent neurons from the motor cortex and cerebellum in the cat.

Responses of parietal association cortex efferent neurons to motor cortex and cerebellar nuclei stimulation were studied intracellularly in anaesthetized cats. Efferent neurons of the parietal cortex were identified according to their antidromic activation on stimulation of the motor cortex, pontine nuclei proper and red nucleus. Monosynaptic excitatory postsynaptic potentials of ipsilateral anterior suprasylvian and lateral gyri neurons to motor cortex stimulation have been established. Oligo- and polysynaptic excitatory responses of parietal cortex efferent neurons to cerebellar nuclei stimulation have been recorded. Correlation between the latencies of cerebellar-induced excitatory postsynaptic potentials and antidromic invasion of neurons on stimulation of different parietal cortex efferent projections (corticocortical, corticopontine, corticorubral) has been obtained. A similar correlation has been found between the latencies of excitatory postsynaptic potentials evoked on stimulation of one of the cerebellar nuclei and latencies of antidromic invasion induced on stimulation of all studied parietal cortex efferent systems. Feedforward and feedback mechanisms in the input-output organization of parietal association cortex have been discussed.

Electrophysiological evidence for a direct neuronal connection from the motor cortex to the parietal association cortex of the cat.

Neuronal responses of the parietal association cortex to motor cortex stimulation were studied intracellularly in anaesthetized cats. Antidromic responses and monosynaptic excitatory postsynaptic potentials (EPSPs) of ipsilateral anterior suprasylvian and lateral gyri neurons have been established. Oligo- and polysynaptic EPSPs were also recorded. Some cells reacted with both antidromic and orthodromic excitation. It is concluded that, besides the well-known parietal-to-motor cortex projection, there is also a reciprocal link from the motor cortex back to the parietal association cortex.