Prior work has shown that spinal neurons are capable of discriminating between temporally regular and temporally irregular stimulation. These effects have been observed using an in vivo assay of spinal plasticity based on an instrumental learning task, in which response-contingent leg shock produces an increase in flexion duration. Exposure to temporally regular stimulation (fixed spaced stimulation; FT) promotes learning, and temporally irregular stimulation produces a learning deficit. The experiments in this dissertation were designed to test other properties of fixed spaced shock that promote spinal plasticity and the structure responsible for the FT effect. Experiment 1 focused on the minimum number of stimulations necessary to re-establish the capacity to learn (a component of the “FT effect”), finding that180-360 shocks produced a learning deficit and that additional training (540-900 shocks) allowed learning. Experiment 2 found that shock number, not duration of exposure determined whether the FT effect emerged. Experiment 3 investigated if the FT effect emerges after shock was presented in two sessions separated by 24 hrs, and showed that two bouts of 360 shocks yielded the FT effect. Further, the initial bout of fixed spaced shock had a long-term benefit (Experiment 4). The results of Experiment 5 suggested that omitting shocks from a train of FT stimulation has little effect on the benefit of fixed spaced shock treatment. Experiment 6 replicated this observation, showing that randomly deleting half of the shocks (from a 720 FT shock series) had no effect on learning. Further, this schedule also induces a lasting protective effect, blocking the learning deficit produced by variable spaced shock (Experiment 7). To explore whether a central system or a peripheral filter mediates the FT effect, Experiment 8 challenged spinal neurons by phase shifting the relation between fixed spaced stimulation applied to two dermatomes. The FT effect only emerged when stimuli occurred in an alternating pattern across dermatomes, implying regularity is abstracted by a central system. Experiment 9 surgically isolated central pattern generator (L1-L2) from the portion of the spinal cord that mediates instrumental learning (L4-S2), finding that disrupting the connections between these two regions eliminated the FT effect.

L’accident vasculaire cérébral (AVC) est un problème majeur de santé publique car les troubles fonctionnels associés ont des répercussions délétères sur la qualité de vie des patients. Deux axes de recherche portant sur l’ischémie cérébrale chez le rat sont abordés. Pour le premier, il s’agira d’étudier les modifications de la régulation des réflexes somatiques qui peuvent contribuer à expliquer en partie les déficits fonctionnels. Nos résultats montrent une fatigue précoce au cours d’un exercice isométrique du triceps brachial et une perturbation de l’inhibition du réflexe-H post-exercice. Nous avons précisé que la perturbation de l’activité des voies réflexes est associée à une modification de l’action inhibitrice des afférences musculaires des groupes III et IV pouvant expliquer la fatigue précoce observée. L’axe 2 concerne l’optimisation de l’entrainement d’endurance, stratégie prometteuse pour traiter les déficits. L’objectif est de définir la stratégie d’endurance la plus appropriée en nous focalisant sur l’intensité de l’exercice. Pour cela, les effets des exercices d’intensité modérée (classiquement recommandés) ont été confrontés avec ceux des exercices intermittents de haute intensité (HIT) en analysant la récupération fonctionnelle et la neuroplasticité cérébrale. Nos résultats montrent que l’intensité de travail, déterminée à partir du seuil lactique, est un paramètre crucial car les HIT se révèlent être plus efficaces en termes de récupération de la force, d’aptitude aérobie et de neuroplasticité. Ces travaux ouvrent de nombreuses perspectives où ces 2 axes pourraient se rejoindre en approfondissant les effets des HIT au niveau cérébral et spinal. / Stroke is a major public health issue because associated functional disorders have detrimental impacts on life quality and independence of patients as well as on economic state. This thesis work is focused on 2-research axis concerning cerebral ischemia in rat. The aim of the first axis is to study plasticity at spinal level by investigating the alteration of somatic reflex regulation that could contribute to partially explain functional deficits. Ours results show early fatigue of triceps brachii during an isometric contraction and H-reflex inhibition perturbation post-exercise. We also observed that the disturbance of reflex pathway activity is associated with a decrease of the inhibitory effect induced by muscular metabosensitive afferents that might explain early fatigue and observed functional deficits. The second axis treats about endurance training that is a promising strategy to reduce stroke-induced disorders but remains to improve. In that way, the purpose is to determine which endurance strategy is the most suitable by focusing on exercise intensity. In order to ensure this, the effects of moderate intensity exercise (classically recommended) have been compared to high intensity interval training (HIT) by analysing the functional recovery and cerebral neuroplasticity. Our data show that work intensity, based on lactic threshold, is a critical parameter. Indeed, HIT is more effective for increasing grip strength recovery, aerobic capacity as well as promoting neuroplasticity. The results of these 2 lines of research could be now associated by deepening the HIT effects at cerebral and spinal level that lead to numerous perspectives.