AN ANALYSIS OF THE VARIABILITY OF SPINAL REFLEX THRESHOLDS

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Abstract
The reflex contraction response, m the spinal extensors, to measured moving pressure stimuli applied to the thoracic, lumbar and 1st sacral spinous processes was detd. Hypodermic needle electrodes in the muscle fed dual channel amplifiers and contraction was noted by electromyograms from the right and left sides near the spinous process receiving the stimulus. The stimulator was a pressure meter, the tip being a grooved lucite block to straddle a single vertebral spinous process. An eccentric moved the tip 15 mm. horizontally, in the long axis of the body, at the pressure selected. Pressure gradations were 1-7 kg. inclusive. The stimulus required to cause contraction was considered to be "the threshold"; this was detd. at each spinous process from 1T to 1S, inclusive, 3 times on each of 30 "normal" [male] subjects. Five fundamental points were observed in these data: A, the threshold at a given segment remains at approx. the same level when the subject''s physical condition is grossly unchanged; B, all subjects do not have the same threshold at each segment; C, the right and left sides do not respond similarly to midline MP; D, all segments do not have the same thresholds; and E. reflex activity frequently results from a stimulus which is beneath the pain level and is not abolished by blocking skin receptors. The data indicate that the different thresholds are due to both peripheral and central factors. The different thresholds seen might be due to different degrees of postural instability and indicate a variable ability of joints to cope with extrinsic and intrinsic conditions.

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