Vertebral position alters paraspinal muscle spindle responsiveness in the feline spine: effect of positioning duration.

Proprioceptive information from paraspinal tissues including muscle contributes to neuromuscular control of the vertebral column. We investigated whether the history of a vertebra's position can affect signalling from paraspinal muscle spindles. Single unit recordings were obtained from muscle spindle afferents in the L6 dorsal roots of 30 Nembutal-anaesthetized cats. Each afferent's receptive field was in the intact muscles of the low back. The L6 vertebra was controlled using a displacement-controlled feedback motor and was held in each of three different conditioning positions for durations of 0, 2, 4, 6 and 8 s. Conditioning positions (1.0-2.2 mm dorsal and ventral relative to an intermediate position) were based upon the displacement that loaded the L6 vertebra to 50-60% of the cat's body weight. Following conditioning positions that stretched (hold-long) and shortened (hold-short) the spindle, the vertebra was repositioned identically and muscle spindle discharge at rest and to movement was compared with conditioning at the intermediate position. Hold-short conditioning augmented mean resting spindle discharge by +4.1 to +6.2 impulses s(-1); however, the duration of hold-short did not significantly affect this increase (F(4,145) = 0.49, P = 0.74). The increase was maintained at the beginning of vertebral movement but quickly returned to baseline. Conversely, hold-long conditioning significantly diminished mean resting spindle discharge by -2.0 to -16.1 impulses s(-1) (F(4,145) = 11.23, P < 0.001). The relationship between conditioning duration and the diminished resting discharge could be described by a quadratic (F(1,145) = 9.28, P = 0.003) revealing that the effects of positioning history were fully developed within 2 s of conditioning. In addition, 2 s or greater of hold-long conditioning significantly diminished spindle discharge to vertebral movement by -5.7 to -10.0 impulses s(-1) (F(4,145) = 11.0, P < 0.001). These effects of vertebral positioning history may be a mechanism whereby spinal biomechanics interacts with the spine's proprioceptive system to produce acute effects on neuromuscular control of the vertebral column.