Contribution du mécanisme d'inhibition présynaptique à l'induction de réactions posturales efficaces suite à une perturbation d'équilibre
Falls are a significant problem among the elderly or persons with a neurological impairment, and reflect a deficit in the nervous mechanisms underlying postural control. Previous research shows that the integration of sensory feedback is a crucial component of postural control and that presynaptic inhibition (PSI) plays an important role in controlling the sensory processing of information. The aim of this study was to identify the contribution of PSI to the induction of effective postural responses following an unexpected balance perturbation. We hypothesized that a decrease in PSI would contribute to the induction of these responses by increasing the influence of sensory feedback onto spinal networks during the perturbation. First we assessed the level of spinal excitability during perturbations, using the soleus H-reflex. Results show that spinal excitability is modulated according to the direction of the perturbation (forward and backward tilts) and that this modulation occurs 75 and 100 ms after tilt-onset in all subjects (p<0.05). To further estimate changes in PSI, spatial facilitation techniques were used. PSI was shown to decrease in both perturbation directions shortly after tilt onset at 75 and 100 ms (p<0.05), suggesting an increase in sensory transmission in the spinal cord. These observations suggest that sensory feedback is critical for the induction of effective postural responses and that impaired sensory transmission or integration, due to CNS lesions or ageing, may lead to certain balance deficits. Identifying patients with such impairments may improve fall risk-assessment and prevention.
Presynaptic inhibition H-reflex postural responses risk of falls balance