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1 Laboratoire de Plasticit Neuromusculaire, Universite des Sciences et Technologies de Lille, Villeneuve d'Ascq, France
* To whom correspondence should be addressed. E-mail: Neuromus{at}univ-lille1.fr.
The aim of this study was to determine whether Ia and II fiber discharges of soleus muscle spindles were modified after a 14-day period of hypodynamia (absence of weight bearing) and hypokinesia (reduction of motor activity). Fifty-one and thirty-eight afferent fibers were respectively studied in control and hypodynamia-hypokinesia (HH) groups. Under deep anaesthesia (pentobarbital, 30 mg.kg-1), a L3-L6 laminectomy was performed. Unitary potentials from the L5 dorsal root were recorded in response : 1) to ramp-and-hold stretches applied at two stretch amplitudes (3 mm and 4 mm) and four stretch velocities (6, 10, 15, and 30 mm.s-1) and 2) to sinusoidal stretches applied at four stretch amplitudes (0.12, 0.25, 0.5, and 1 mm) and six stretch frequencies (0.5, 1, 2, 3, 6, and 10 Hz). In both animal groups, the Ia fibers showed higher dynamic index values, smaller linear range, and higher vibration sensitivity than the II fibers. They also exhibited a pause in their discharges during the stretch release contrary to II fibers which displayed no pause in their responses. After HH, our results showed that for both fiber types : 1) all parameters measured under ramp-and-hold stretches (except the static sensitivity) were significantly increased, and 2) under sinusoidal stretches, the vibration sensitivity increased, and the response amplitude only increased at 0.12 mm stretch amplitude. The linear range of Ia afferents was limited to 0.12 mm, whereas it was unchanged for the II fibers. After HH, the stretches could be better transmitted to the muscle spindles, probably resulting from changes in passive mechanical properties of the soleus.
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