Using vertebral movement and intact paraspinal muscles to determine the distribution of intrafusal fiber innervation of muscle spindle afferents in the anesthetized cat

Abstract

Increasing our knowledge regarding intrafusal fiber distribution and physiology of paraspinal proprioceptors may provide key insights regarding proprioceptive deficits in trunk control associated with low back pain and lead to more effective clinical intervention. The use of vertebral movement as a means to reliably stretch paraspinal muscles would greatly facilitate physiological study of paraspinal muscle proprioceptors where muscle tendon isolation is either very difficult or impossible. The effects of succinylcholine (SCh) on 194 muscle spindle afferents from lumbar longissimus or multifidus muscles in response to computer-controlled, ramp-and-hold movements of the L(6) vertebra were investigated in anesthetized cats. Paraspinal muscles were stretched by moving the L(6) vertebra 1.5-1.7mm in the dorsal-ventral direction. Initial frequency (IF), dynamic difference (DD), their changes (∆) following SCh injection (100-400μgkg(-1)), and post-SCh dynamic difference (SChDD) were measured. Muscle spindle intrafusal fiber terminations were classified as primary or secondary fibers as well as bag(1) (b(1)c), bag(2) (b(2)c), b(1)b(2)c, or chain (c) fibers. Intrafusal fiber subpopulations were distinguished using logarithmic transformation of SChDD and ∆IF distributions as established by previous investigators. Increases in DD indicate strength of b(1)c influence while increases in IF indicate strength of b(2)c influence. Out of 194 afferents, 46.9% of afferents terminated on b(2)c fibers, 46.4% on b(1)b(2)c fibers, 1% on b(1)c fibers, and 5.7% terminated on c fibers. Based on these intrafusal fiber subpopulation distributions, controlled vertebral movement can effectively substitute for direct tendon stretch and allow further investigation of paraspinal proprioceptors in this anatomically complex body region.