Effects of Hypodynamia-Hypokinesia on the Muscle Spindle Discharges of Rat Soleus Muscle

Abstract

Falempin. Effects of hypodynamia-hypokinesia on the muscle spindle discharges of rat soleus muscle. J Neurophysiol 89: 3000 -3007, 2003; 10.1152/jn.00875.2002. 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 38 afferent fibers were studied, respectively, in control and hypodynamia-hypokinesia (HH) groups. Under deep anesthesia (pentobarbital, 30 mg/kg), a L 3 -L 6 laminectomy was performed. Unitary potentials from the L 5 dorsal root were recorded in response to ramp-andhold stretches applied at two stretch amplitudes (3 and 4 mm) and four stretch velocities (6, 10, 15, and 30 mm/s) and 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 all parameters measured under ramp-and-hold stretches (except the static sensitivity) were significantly increased and 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.

Kinematics of treadmill locomotion in mice raised in hypergravity

“…An over extended ankle at take off, but more flexed in the main part of the step cycle was also reported in adult suspended rats [28,42]. The ankle extension of suspended rats can be related to changes of paw pressure and muscles spindle stimulation [28,42]. This involves therefore a different mechanism than the one we observed on centrifuged mice.…”

“…They were supposed to show an extensor bias because they constantly adopt a position with the hindlimb extended during their suspension. An over extended ankle at take off, but more flexed in the main part of the step cycle was also reported in adult suspended rats [28,42]. The ankle extension of suspended rats can be related to changes of paw pressure and muscles spindle stimulation [28,42].…”

Physiologically adaptive changes of the L5 afferent neurogram and of the rat soleus EMG activity during 14 days of hindlimb unloading and recovery

“…Indeed, after 14-day HU, we have previously demonstrated in the rat that Ia and II fiber discharges from the soleus during ramp-and-hold (3 and 4·mm) and sinusoidal stretches were increased because of changes in passive mechanical properties of the soleus during HU (De-Doncker et al, 2003). Consequently, the stretches could be better transmitted to the muscle spindles after a HU period and could be related to an increase in the connective tissue, as previously proposed by authors in tenotomized (Hnik and Lessler, 1973) and shortened immobilized muscles (Gioux and Petit, 1993;Maier et al, 1972;Tardieu et al, 1982).…”

An index of spindle efficacy obtained by measuring electroneurographic activity and passive tension in the rat soleus muscle

“…Hnik et al (1977) reported quantitative changes in electroneurograms (ENGs) of the rat soleus nerve after the muscle had been de-efferented at birth. Changes in the spindle afferent discharges of cat muscles due to immobilization have been reported (Gioux and Petit, 1993;Nordstrom et al, 1995) and the Ia and II fibers of rat soleus muscle spindles were recently found to be more sensitive after a period of hypodynamia-hypokinesia (De-Doncker et al, 2003b). These changes in spindle afferent discharge may be due to changes in the spindle structure and/or to changes in the elastic properties of the muscle-tendon elements linked to the muscle spindles.…”