Response of muscle proprioceptors to spinal manipulative-like loads in the anesthetized cat

The mechanisms underlying the benefits of spinal manipulation are not well understood. Neurophysiological mechanisms likely mediate its effects, at least in part, yet we know little about how the nervous system is affected by spinal manipulation. The purpose of the present study was to determine whether muscle spindles and Golgi tendon organs in paraspinal muscles respond to a mechanical load whose force-time profile is similar to that of a spinal manipulation.

Experiments were performed on 10 anesthetized adult cats. The L6 dorsal root was isolated for electrophysiological recordings while the L6-L7 vertebrae and associated paraspinal tissues on one side of the vertebral column were left intact. Single unit recordings were obtained from 5 muscle spindles, 4 Golgi tendon organs, and 1 presumed Pacinian corpuscle afferent with receptive fields in paraspinal muscles. Loads were applied at the spinous process of the L6 vertebra through use of an electronic feedback control system. The load simulated the force-time profile of a spinal manipulation. Loads were applied in compressive and distractive directions and at 2 different angles (0 degrees and 45 degrees) with respect to the long axis of the vertebral column.

Golgi tendon organ afferent discharge frequency increased more to the impulse than to the preload during 13 of 15 spinal manipulations. Generally, the 4 Golgi tendon organ afferents became silent immediately at the end of each impulse. Similarly, muscle spindle discharge frequency increased more to the impulse than to the preload during 10 of 16 manipulations. Distractive manipulations loaded the spindles more effectively than compressive manipulations. After 7 of these 10 manipulations, muscle spindles became silent for 1.3 +/- 0.6 seconds (range, 0.1-4.3 seconds). Six of the 16 manipulations unloaded the muscle spindles. A presumed Pacinian corpuscle responded to the impulse of a manipulative-like load but not to loads with a slower force-time profile.

The data suggest that the high-velocity, short-duration load delivered during the impulse of a spinal manipulation can stimulate muscle spindles and Golgi tendon organs more than the preload. The physiologically relevant portion of the manipulation may relate to its ability to increase as well as decrease the discharge of muscle proprioceptors. In addition, the preload, even in the absence of the impulse, can change the discharge of paraspinal muscle spindles. Loading of the vertebral column during a sham manipulation may affect the discharge of paraspinal proprioceptors.