Characterization of spindle afferents in rat soleus muscle using ramp-and-hold and sinusoidal stretches

The discharge properties of 51 afferents were studied in the rat soleus muscle spindles. Under deep anesthesia using a pentobarbital sodium solution (30 mg/kg), a laminectomy was performed and the right L-4 and L-5 dorsal and ventral roots were transected near their entry into the spinal cord. In situ, the minimal (L-min) muscle length [3 +/- 0.08 (SE) cm] of the soleus was measured at full ankle extension. Unitary potentials from the L-5 dorsal root were recorded in response to ramp-and-hold stretches applied at 3 mm (S3) and 4 mm (S4) amplitudes and four stretch velocities (6, 10, 15, and 30 mm/s), sinusoidal stretches performed at four amplitudes (0.12, 0.25, 0.5, and 1 mm) and six stretch frequencies (0.5, 1, 2, 3, 6, and 10 Hz), and vibrations applied at 50-, 100-, and 150- Hz frequencies. These two kinds of stretches were performed at three different muscle lengths (Lmin+10%, Lmin+15%, and Lmin+20%), whereas vibrations were applied at Lmin+20% muscle length. Conduction velocity of the fibers was calculated but did not allow to discriminate different fiber types. However, the mean conduction velocity of the first fiber group (43.3 +/- 0.8 m/s) was significantly higher than that of the second fiber group (33.9 +/- 0.9 m/s). Three parameters allowed to differentiate the responses of primary and secondary endings: the dynamic index (DI), the discharge during the stretch release from the ramp-and-hold stretches, and the linear range and the vibration sensitivity from sinusoidal stretches. The slope histogram of the linear regression based on the DI and the stretch velocity was clearly bimodal. Therefore the responses were separated into two groups. During the stretch release at a velocity of 3 mm/s, the first response group (n = 26) exhibited a pause, whereas the second (n = 25) did not. The linear range of the second ending group (0.12-1 mm) was broader than that of the first (0.12-0.25 mm). The first ending group showed a higher sensitivity to high-vibration frequencies of small amplitude than the second. In comparison with the literature, we can assert that the first and the second ending groups corresponded to the primary and secondary endings, respectively. In conclusion, our study showed that in rat soleus muscle spindles, it was possible to immediately classify the discharge of Ia and II fibers by using some parameters measured under ramp-and-hold and sinusoidal stretches.