Spatial distribution of the vestibulospinal neurons in the frog vestibular nuclei.

In experiments on the preparation of a frog perfused brain (Rana ridibunda), intracellular potentials were recorded from neurons of the vestibular nuclei following stimulation of the vestibular nerve and the spinal cord. The vestibulospinal neurons were identified on the basis of excitatory postsynaptic potentials evoked by the stimulation of the ipsilateral vestibular nerve and antidromic activation from the stimulation of the cervical and lumbar enlargements of the spinal cord. The cells that could be activated antidromically only by cervical cord stimulation have been designated as C cells, and the cells that could also be activated antidromically as a result of lumbar stimulation have been termed L cells. The average conduction velocity determined for C neurons was 10.67 m/s and for L neurons 15.84 m/s. The ratio of C and L neurons over the vestibular nuclear complex was very similar to each other: 52% C neurons and 48% L neurons. The majority of both types of neurons were localized in the lateral vestibular nucleus (58.6%), to a lesser extent in the descending vestibular nucleus (30.7%) and very little in the medial vestibular nucleus (10.6%). In the lateral vestibular nucleus, C neurons prevailed in the caudal part of the nucleus and L neurons prevailed in the rostral part. By contrast, in the descending and medial vestibular nuclei there was a gradual increase of C and L cells quantitatively from the rostral to the caudal part. Fast and slow cells were detected among the vestibulospinal neurons. The fast neurons of L cells did not prevail greatly over the slow ones, whereas the slow neurons of C cells prevailed comparatively largely over the fast neurons. Thus, it became possible to reconstruct the spatial distribution of the identified vestibulospinal neurons. The results of spatial distribution of C and L vestibulospinal neurons in the frogs failed to conform to definite somatotopy, which is characteristic of mammalian vestibular nuclei. The results of this study have confirmed an earlier assumption that C and L neurons in the frog's vestibular nuclei as a source of vestibulospinal fibers, are scattered separately or more frequently in groups, so that they establish a 'patch-like' somatotopy and do not form a distinctly designed field as in mammals.

Time-dependent effects of pyramidotomy in the operantly conditioned rats.

The effect of unilateral transection of bulbar pyramid on operantly conditioning in rats, have been shown to be in direct relationship with the time of its execution. Stable instrumental reflexes reappeared on average 3.9 days after the transection of the bulbar pyramid. However, preliminary transection of the pyramid led to the prolongation of stabilization of operant conditioning after an average of 16.5 days. These findings are considered to be in view of the mechanisms of switching of descending influences of corticospinal and cortico--rubrospinal systems.

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.

Enhanced behavioral recovery from sensorimotor cortex lesions after pyramidotomy in adult rats.

Unilateral transection of the bulbar pyramid, performed before the ablation of the ipsilateral sensorimotor cortex, has been shown to facilitate the recovery of operantly conditioned reflexes and compensatory processes in rats. Such enhanced behavioral recovery was absent when only the sensorimotor cortex was ablated. This phenomenon is explained by the switching of motor activity under the control of the cortico-rubrospinal system. Switching of the descending influences is accomplished through the following loop: cortico-rubral projection-red nucleus-inferior olive-cerebellum-thalamus-cerebral cortex. This suggests that a preliminary lesion of the peripheral part of the system, represented by a descending spinal projection, facilitates the recovery processes to develop during the subsequent destruction of its central part.

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.

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.

Neuronal mechanisms of interaction of Deiters nucleus with the cerebral cortex.

The effects of stimulation of the vestibular nerve and five different cerebral cortex areas on the neuronal activity of the lateral vestibular nucleus of Deiters were studied. Stimulation of the cerebral cortex is shown to lead to antidromic and synaptic activation of Deiters neurons. The synaptic potentials of Deiters neurons evoked from the cerebral cortex were of mono- and polysynaptic origin. In particular, stimulation of the cerebral cortex evoked in Deiters neurons mono- and polysynaptic excitatory postsynaptic potentials. Collaterals of vestibulospinal neurons reaching different cortex fields as well as convergence of influences from these cortex fields on Deiters neurons were revealed. Inhibitory effects of the cerebral cortex on Deiters neurons were of polysynaptic origin and occurred rarely. The topical correlation between Deiters nucleus and different areas of the cerebral cortex was found. The peculiarities and functional significance of the effects obtained are discussed.

Antidromic and synaptic activation of Deiters' neurons induced by stimulation of red nucleus in the cat.

Antidromic and orthodromic action potentials of neurons located in the lateral vestibular nucleus of Deiters' evoked by stimulation of red nucleus were studied in anaesthetized cats. Vestibulospinal neurons were identified by stimulation of the lateral vestibulospinal tract. The 'second-order' vestibular neurons were revealed by mean of stimulation of the ipsilateral VIIIth nerve. Stimulation of the red nucleus is shown to lead mainly to antidromic, as well as mono-, oligo- and polysynaptic activation of Deiters' neurons. Not any inhibitory reaction was observed in vestibular neurons in response to stimulation of the red nucleus. Ascending axon collaterals of the vestibulospinal neurons to this brainstem structure were revealed. The peculiarities and functional significance of the effects mentioned are discussed.

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.

Complex composition of synaptic potentials of the rubrospinal neurons to corticofugal impulses.

Complex, multicomponent excitatory postsynaptic potentials (EPSPs) of the red nucleus rubrospinal neurons evoked by stimulation of the sensorimotor cortex and associative field of the parietal cortex were studied in acute experiments on pentobarbitalized cats by the intracellular recording technique. Complex cortical EPSPs were recorded in two-thirds of the studied number of neurons. Monosynaptic nature of the first two components of EPSPs evoked by corticofugal impulses propagating at an average velocity of 18.5 ms and 7.5 ms was revealed. Participation of the corticospinal input in the genesis of EPSPs mentioned was revealed by selective activation of corticospinal fibers on the level of medullary pyramids as well as by studying peculiarities of their interaction with effects of cortical stimulation. It was shown that both corticospinal and corticorubral (predominantly slow conducting) neurons participate in the genesis of the first two components of complex EPSPs.

Synaptic mechanisms of interaction between Deiters' nucleus and the nuclei of some cranial nerves.

The effects of stimulation of the vestibular nerve, spinal trigeminal nucleus, facial and hypoglossal nuclei of the cranial nerves on the neuronal activity in the lateral vestibular nucleus of Deiters were studied in cats anaesthetized with pentobarbitone. Stimulation of these nuclei was found to produce antidromic and synaptic activation of Deiters' neurons. Descending axon collaterals of the vestibular neurons to these brainstem structures were revealed. Stimulation of the VIIIth nerve, spinal trigeminal and facial nuclei evoked mono- and polysynaptic excitatory postsynaptic potentials in Deiters' neurons. Stimulation of the spinal trigeminal nucleus evoked mono- and polysynaptic inhibitory postsynaptic potentials and disfacilitation in Deiters' neurons. In some vestibular neurons inhibitory postsynaptic potentials were also evoked by stimulation of the nucleus hypoglossus. Convergence of influences from these structures on Deiters' neurons was shown to exist. The peculiarities and functional significance of the effects mentioned are discussed.

Electrophysiological peculiarities of cortical inputs to the cat red nucleus.

Complex multicomponent EPSPs of the red nucleus rubro-spinal neurons evoked by stimulation of the sensorimotor cortex and associative fields of the parietal cortex were studied in acute pentobarbitalized cats by intracellular recording technique. Complex cortical EPSPs were recorded in 2/3 of the neurons studied. Components of the EPSPs in question were distinguished by using stimulation of various frequency and intensity. The first component of the EPSPs appearing at the lowest threshold was found to have a short and stable latency, stable rising time for depolarization and was able to follow high frequencies of stimulation. The second component was more variable, although in some EPSPs it too had a short latency, was stable enough and, like the first component could be classified as monosynaptic. The complex character of the EPSPs recorded persisted after the removal of the cerebral gray and was observed when stimulating the white matter so excluding its cortical origin. The first two components of the EPSP were evoked by corticofugal impulsation propagating at an average velocity of 18.5 m/s and 7.5 m/s being supposedly the result of activation of the slow-conducting pyramidal and cortico-rubral neurons. In some rubro-spinal neurons they were characterized by a fast rising phase being apparently an electrophysiological manifestation of the activation of axosomatic synapses.

Functional properties of the cerebellorubral synapses in the cat.

Facilitation of cerebellorubral transmission was illustrated in nembutalized cats on paired stimulation of the cerebellar nucleus interpositus by an example of monosynaptic EPSPs of the rubrospinal neurons. It was found that the facilitation stated is not determined by the change in the presynaptic volley of impulses. There is reason to believe that the facilitation is determined by specific features of functioning of the cerebellar synapses on the red nucleus neurons.

Contribution of glia and neurons to the surface-negative potentials of the cerebral cortex during its electrical stimulation.

In 20 cats anaesthetized with pentobarbital the suprasylvian gyrus was stimulated by single stimuli or by trains of 50 s stimuli and the potentials from the cortical surface and the intracellular potentials from glial and nerve cells were recorded. Glial cells were identified according to conventional electrophysiological criteria: the absence of action potentials and postsynaptic potentials; slow depolarization in response to electrical stimulation. The slow negativity of direct response to a single stimulus is similar in shape and time course to the depolarization of the cortical glial cells and is unlike the hyperpolarization of the cortical neurons. Quantitative analysis showed that the basic part of the slow negativity is the glial component, whereas the neuronal component--inhibitory postsynaptic potential--plays a much lesser role. The negative shift of the potential on the cortical surface evoked by its high-frequency stimulation is similar in shape and time course to the depolarization shift of the membrane potential of the cortical glial cells (the mean value and standard error of time to peak for glial depolarization were 567.6 +/- 26.8 ms and 427 +/- 24 ms for negative shift of potential). (The results are based on recordings from 37 cells.) The negative shift decays much quicker; it is not similar in shape and time course to the hyperpolarization shift of the neuronal membrane potentials (the mean value and standard error of time to peak for inhibitory postsynaptic potential was 44.9 +/- 4.5 ms). According to the quantitative analysis, the negative shift of the potential reflects mainly the depolarization of the cortical glial cells. The contribution of the hyperpolarization of neurons to the surface-negative shift can be distinctly observed during the first 0.2-0.3 s of stimulation. It is supposed that accumulation of K+ ions in intercellular clefts results in depolarization of glial syncytium, which is reflected on the cortical surface as a slow negativity and a negative shift of the potential.

Mechanisms regulating the activity of facial nucleus motoneurons--III. Synaptic influences from the cerebral cortex and subcortical structures.

Peculiarities of synaptic processes in facial motoneurons evoked by stimulation of various regions of the cerebral cortex and subcortical structures were studied in acute experiments on cats by intracellular recording technique. Stimulation of the motor cortex as well as gyrus proreus and pyramidal tract was shown to evoke polysynaptic excitatory and inhibitory postsynaptic potentials in facial motoneurons. Stimulation of the lateral hypothalamus produced exclusively excitatory polysynaptic effects. It was also found that stimulation of the head of nucleus caudatus and globus pallidus evokes a polysynaptic activation in facial motoneurons, while stimulation of nucleus amygdala centralis leads to mono- and polysynaptic excitation of these neurons. Convergence of the above effects on the same motoneurons is shown to exist. Possible pathways and mechanisms of descending influences on the activity of facial motoneurons is discussed.