Time course for the development of muscle history in lumbar paraspinal muscle spindles arising from changes in vertebral position

Background contextIn neutral spinal postures with low loading moments, the lumbar spine is not inherently stable. Small compromises in paraspinal muscle activity may affect lumbar spinal biomechanics. Proprioceptive feedback from muscle spindles is considered important for control of muscle activity. Because skeletal muscle and muscle spindles are thixotropic, their length history changes their physical properties. The present study explores a mechanism that can affect the responsiveness of paraspinal muscle spindles in the lumbar spine.PurposeThis study has the following two aims: to extend our previous findings demonstrating the history-dependent effects of vertebral position on the responsiveness of lumbar paraspinal muscle spindles and to determine the time course for these effects. Based on previous studies, if a cross-bridge mechanism underlies these thixotropic effects, then the relationship between the magnitude of spindle discharge and the duration of the vertebral position will be one of exponential decay or growth.Study design/settingA neurophysiological study using the lumbar spine in a feline model.MethodsThe discharge from individual muscle spindle afferents innervating lumbar paraspinal muscles in response to the duration and direction of vertebral position was obtained from teased filaments in the L6 dorsal roots of 30 Nembutal-anesthetized cats. 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, 0.5, 1, 1.5, and 2 seconds. Two of the conditioning positions stretched or shortened the lumbar muscles relative to an intermediate conditioning position. Conditioning positions for all cats ranged from 0.9 to 2.0 mm dorsal- and ventralward relative to the intermediate position. These magnitudes were determined based on the displacement that loaded the L6 vertebra to 50% to 60% of the cat's body weight. Conditioning was thought to simulate a motion segment's position that might be passively maintained because of fixation, external load, a prolonged posture, or structural change. After 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 having conditioned at the intermediate position.ResultsLumbar vertebral positions maintained for less than 2 seconds were capable of evoking different discharge rates from lumbar paraspinal muscle spindles despite the vertebra having been returned to an identical locations. Both resting spindle discharge and their responsiveness to movement were altered. Conditioning vertebral positions that stretched the spindles decreased spindle activity and positions that unloaded the spindles increased spindle activity on returning the vertebra to its identical original (intermediate) position. The magnitude of these effects increased as conditioning duration increased to 2 seconds. These effects developed with a time course following a first-order exponential reaching a maximal value after approximately 4 seconds of history. The time constant for a hold-short history was 2.6 seconds and for a hold-long history was approximately half of that at 1.1 seconds.ConclusionsThixotropic contributions to the responsiveness of muscle spindles in the low back are caused by the rapid, spontaneous formation of stable crossbridges. These sensory alterations because of vertebral history would represent a proprioceptive input not necessarily representative of the current state of intersegmental positioning. As such, they would constitute a source of inaccurate sensory feedback. Examples are presented suggesting ways in which this novel finding may affect spinal physiology.