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The parafascicular nucleus (PFN) of the rat, homologous to the human centre médian, is an intralaminar nucleus of the thalamus, classically considered as part of the ascending activating system. We have previously demonstrated that it is also connected to several subcortical nuclei. To obtain a more detailed picture of the connectivity of the PFN, the organization and the topography of the reciprocal parafascicular-telencephalic relationships were studied in both adult and developing rats, using anterograde and retrograde neuronal tracers. In the adult rat, the ascending parafascicular projections were densest to the striatum, dense to the frontal and least dense to cingulate cortex, and were strictly ipsilateral. They displayed a loose topography, with the more medial parafascicular neurons projecting to the medial frontal and cingulate cortex and medial striatum, and the more lateral neurons projecting to the lateral frontal cortex and lateral striatum. All these connections were already present at embryonic day 19. Parafascicular neurons projecting to the telencephalon in adult rats were mostly of the multipolar type, with a few bipolar neurons. In neonatal rats they showed a bipolar morphology at birth; they became mostly multipolar later on, with an increasing complexity of the dendritic arbor up to postnatal day 10. Neurons in the frontal cortex retrogradely labelled from the PFN were more numerous perinatally, and decreased as early as postnatal day 5. The telencephalic connections of the PFN were found to be more discrete and restricted than previously thought, thus suggesting a more specific functional role for the nucleus than cortical recruitment.

The organization of the descending projections from the intralaminar parafascicular nucleus was studied using biocytin as an anterograde tracer in rats. After biocytin injection into the lateral parafascicular nucleus, three bundles of fibers descending throughout the brainstem were seen. Terminal fields were found in several structures, for example the lateral geniculate nucleus, nucleus reticularis thalami, subthalamus, zona incerta, substantia nigra, red nucleus, periaqueductal gray, superior colliculus, reticular formation, raphe nuclei, pontine nuclei, trigeminal complex, and ventral horn of the spinal cord. Different types of labeled terminals (small terminal boutons, en passant varicosities, large claw-like terminals) were observed, particularly in the substantia nigra and reticular formation where the density of terminals was highest. The organization of these extensive descending connections to both motor and sensory structures does not allow functions to be conclusively attributed to the parafascicular nucleus neurons. Further investigations are required to resolve this question.

Thalamocortical projections originating from the parafascicular nucleus were reinvestigated using biocytin or biotylinated dextran amine as anterograde tracers in the rat. After stereotaxic injection of the marker in the lateral part of the parafascicular nucleus, labelled ascending fibres were observed running ipsilaterally to the frontal motor and anterior cingulate areas. Labelled fibres gave rise in layer VI to a plexus of thin ramifications ending in layer V, where sparse boutons en passant and terminaux were seen in close apposition to pyramidal cells. Few retrogradely labelled pyramidal somata, contacted by labelled varicosities, were also observed. Electron microscopy demonstrated the synaptic nature of the labelled contacts, displaying asymmetrical junctions and a round vesicular content. The direct loop parafascicular-motor cortex-parafascicular may be of great functional significance in motor control.

Thalamic projections from the parafascicularis nucleus to the raphe system were studied by means of anterograde techniques, utilizing both biocytin and dextran amine in rats. Both tracers injections in the parafascicularis nucleus resulted in the labeling of descending bundles of fibers running along the brainstem. Labeled terminal fields were found in all the raphe nuclei except the nucleus raphe pallidus. Three different types of labeled terminals (numerous small boutons, less numerous large claw-like terminals, varicosities in close apposition to blood vessel walls) originating from the parafascicular nucleus were present in the raphe system. Ultrastructural data suggest an inhibitory nature for the parafascicular-raphe projections. Our results confirm and extend previous retrograde data by indicating the trajectories, the terminal fields, the fine structure of terminal axonal arborizations and boutons. Based on previous retrograde data and our observations, we conclude that the relationship between the parafascicular nucleus and raphe system is reciprocal and concerns most of the raphe nuclei, suggesting that parafascicularis cell population may be involved in many of the functions ascribed to the raphe system.

The pyramidal tract - past, present and perspective - an introduction to the symposium in honour of Mario Wiesendanger, M.-C. Hepp-Raymond grasping objects - the hand as a pattern recognition device, M. Jeannerod effect of inactivation of the hand representation of the primary and supplementary motor cortical areas on precision grip performance in monkeys, E.M. Rouiller et al balance control during movement - why and how?, J. Massion quantitative analysis of complex movements, A. Pedotti locomotor training in paraplegic patients, V. Dietz motor phenomena during sleep, G. Marini slow cortical potentials developing prior to a self-paced voluntary movement and in a forewarned reaction time task - a study in epileptic patients with implanted electrodes, P. Buser et al transcranial brain stimulation for studying the human motor system, C.W. Hess et al paths of discovery in human motor control - a short historical perspective, M. Wiesendanger.

The role of the nucleus reticularis thalami in spike-wave discharges in rats with genetic absence epilepsy has already been demonstrated. This study further investigated the role of the nucleus reticularis thalami in paroxysmal synchronizations in Sprague-Dawley rats; this strain shows no propensity to epileptic activity. Electroencephalographic patterns were followed in chronically implanted, unrestrained rats. After both electrolytic and chemical unilateral lesions, stereotaxically placed in the anterolateral sectors of this nucleus (verified post mortem), abnormal electroencephalographic rhythms (high-voltage polyspikes and spike-wave complexes) were recorded from the frontoparietal cortex, primarily in the contralateral hemisphere. Stereotyped discharges at 3 Hz developed progressively from multiple spikes within the alpha frequency range through the lengthening of the wave component. The excessive synchronized activity recorded from the intact hemisphere was of greater amplitude and occurred slightly earlier than from the lesioned hemisphere. These EEG patterns were associated with behavioural manifestations closely resembling those seen during absence seizures in humans. Bilateral lesions did not induce paroxysmal activity, both hemispheres being characterized by dominant delta/theta activity without signs of EEG-synchronized sleep. The seizures may thus have been due to disinhibition of the contralateral reticularis nucleus, recently shown to project to the reticularis nucleus of the other side in rats. This working hypothesis is supported by callosal cuts. The results indicate that the reticular neurons exert a control over neocortical paroxysmal activity even in animals which do not present genetic absence epilepsy.

The aim of this study was to extend the anatomical study of the cortico-reticular organization in the monkey by means of microelectrophysiological techniques. Considering the relatively modest projection (see companion paper, Wiesendanger and Wiesendanger, '87), it was surprising to see that over 70% of the investigated LRN neurons were influenced from at least one cortical stimulation site. Many neurons responded, however, with long latencies suggesting an indirect transmission line. In line with the anatomical tracing study, most short-latency responses were obtained from the motor cortex. Postcentral cortex and the SMA were, in general, less effective sites for evoking responses in the LRN. LRN neurons with similar cortical inputs tended to be clustered together suggesting that the corticoreticular projection is discretely organized with an “intermingled somatotopy.” The majority of the 87 tested LRN neurons were not reactive to any peripheral stimulus (33%) or responded only to nociceptive peripheral stimulation (31%). Very large receptive fields were seen in 8% of the units. However, a significant proportion of LRN neurons (10%) had restricted receptive fields and reacted to gentle cutaneous stimuli, and others (17%) responded to discrete passive rotations of one or more joints. There was often a somatotopical correspondence between the peripheral and the cortical inputs. It is concluded that the LRN in monkeys is under the influence of the motor cortex, which, however, may be exerted to a major extent via indirect pathways. The electrophysiological data suggest a discrete rather than a diffuse relationship with the LRN.

Bilateral ibotenic acid lesions of the thalamic medialis dorsalis nucleus in chronically implanted cats produced a significant reduction of slow wave sleep. Rapid eye movement sleep was also affected but in a less significant manner. The lesions were limited to the intermediate portion of the nucleus, which receives projections from the brainstem and prosencephalic structures implicated in sleep mechanisms. These results are consistent with the hypothesis that the medialis dorsalis nucleus plays an important role in the induction and maintenance of sleep.

The injection of an excitotoxin into medialis dorsalis thalamic nuclei (MD) elicited a short-term increase followed by a depression on EEG spindle waves in chronically implanted cats. This biphasic action provides further evidence to the hypothesis that MD plays a crucial role in transferring and inducing spindling on frontal cortex. In addition, retrograde horseradish peroxidase transport from previously lesioned MD labeled subcortical structures such as basal forebrain, anterior hypothalamus, reticular thalamic nucleus, ventral tegmental area, and locus coeruleus.

Bilateral lesions of medialis dorsalis (MD) thalamic nuclei in chronically implanted cats disrupt the sleep-waking cycle by inducing a reduction of both slow-wave and desynchronized sleep and a corresponding increase of wakefulness. Bilateral lesions of the anterior thalamic group produce some postural deficits but no changes in the percentage of sleep and wakefulness. The hypothesis that MD lesions alter the sleep processes by interrupting an anterior forebrain-MD-cortical link has been put forward.

In anesthetized rabbits, electrophysiological experiments were conducted to determine if neurons of the lateral reticular nucleus that modified their activity during masticatory movements project directly into the trigeminal motor nucleus. Of the 125 neurons tested, 50 responded to stimulation of the trigeminal motor nucleus; of these, 22 were antidromically excited. Among this last population, 9 were antidromically driven also by cerebellar stimulation. In these neurons the antidromic response evoked from one structure collided with the response antidromically evoked from the other, thus indicating that both potentials are from the same neuron projecting to the trigeminal motor nucleus and the cerebellum. The remaining 28 neurons responding to stimulation of the trigeminal motor nucleus were orthodromically activated with a wider range of latencies. The possible significance of these findings in the organization of the rhythmic masticatory movements is briefly discussed.

The aim of this work was to reestablish the respiratory function of the paralyzed larynx through reinnervation of the posterior cricoarytenoid (PCA) muscle by phrenic motoneurons. In nine adult cats the adductor branch of the recurrent laryngeal nerve (RLN) of one side was cut and ligated, while the abductor branch was left intact. The whole RLN was then transected lower in the neck and its distal stump anastomosed to the upper branch of the phrenic nerve. Periodical laryngoscopies under ketamine anesthesia assessed that the inspiratory abduction of the paralyzed vocal cord recovered within 45 days to 60 days in all cats. Abduction was caused by reinnervation of the PCA muscle from phrenic motoneurons, as demonstrated by electrophysiological and anatomical (retrograde transport of horseradish peroxidase) testings.