QUANTITATIVE STUDIES OF CHRONIC FACILITATION IN HUMAN MOTONEURON POOLS

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Abstract
The reflex responses of the erector spinae muscles to measured pressure stimuli applied to the spinous processes at selected spinal segments were studied. The existence of constant differences in reflex thresholds of segments in different subjects, and from segment to segment and from side to side in the same subject has been confirmed. Low threshold segments showed reflex hyper-excitability to pressure upon the corresponding spinous processes, to pressure upon the spinous processes of distant, high threshold segments, and to impulses from pro-prioceptors associated with positioning, from remote areas of skin and from the higher centers. It is concluded that low threshold segments are those in which a relatively large portion of the motoneurons is maintained in a state of facilitation due to a chronic bombardment by impulses from some unknown source. Presumptive evidence indicates that the facilitating impulses arise from segmentally related structures. Correlation of motor reflex threshold with pain thresholds; susceptibility of supraspinous tissues to minor trauma, and with tissue texture has been demonstrated. This suggests that neurons other than the motoneurons in the low threshold segments may be simultaneously facilitated.