Dynamic responsiveness of lumbar paraspinal muscle spindles during vertebral movement in the cat

Muscle spindles provide essential information for appropriate motor control. In appendicular muscles, much is known about their position and movement sensitivities, but little is known about the axial muscles of the low back. We investigated the dynamic responsiveness of lumbar paraspinal muscle spindle afferents from L(6) dorsal root filaments during constant velocity movement of the L(6) vertebra (the feline has seven lumbar vertebrae) in Nembutal-anesthetized cats. Actuations of 1 mm applied at the L(6) spinous process were delivered at 0.5, 1.0 and 2.0 mm/s. The slow velocity component was measured as the slope of the relationship between displacement during the constant velocity ramp and instantaneous discharge frequency. The quick velocity component was the slope's intercept at zero displacement. The peak component was determined as the highest discharge rates occurring near the end of the ramp compared with control. The slow velocity component over the three increasing velocities was 23.9 (9.9), 21.6 (9.6) and 20.5 (9.5) imp/(s mm) [mean (SD)], respectively. The quick velocity component was 28.4 (8.6), 31.4 (9.8) and 35.8 (10.6) imp/s, respectively. These measures of dynamic responsiveness were at least 5-10 times higher compared with values reported for appendicular muscle spindles. The peak component's velocity sensitivity was 2.9 (imp/s)/(mm/s) [0.2, 5.5, lower, upper 95% confidence interval] similar to that for cervical paraspinal muscles as well as appendicular muscles. Increased dynamic responsiveness of lumbar paraspinal muscle spindles may insure central driving to insure control of intervertebral motion during changes in spinal orientation. It may also contribute to large, rapid and potentially injurious increases in paraspinal muscle activity during sudden and unexpected muscle stretch.