Do joint receptors modulate the motoneuron excitability?

The purpose of this study was to test the effect of joint receptor afferent discharge, during rest and during small excursion movement on kinesthetic sensation and on the excitability of alpha-motoneurons in the ankle joint in normal subjects. Movement kinesthesia was tested using a specially designed test that measured the delay time between the actual and the perceived movement during slow 5 degrees/sec), passive, 20 degrees movement excursion. These data were correlated to a second kinesthetic test using a visual analog scale in which the subject compared movements of the joint being tested and the contralateral control limb joint. The excitability of alpha-motoneurons was tested using soleus H-M recruitment curve with incremented electrical stimuli to the posterior tibial nerve at the back of the knee joint. The H-reflex recovery curve was also tested using double-identical stimuli of increasing interstimulus intervals. Joint receptors were desensitized by iontophoretic application of 3 cc. of 2% xylocaine using 5 mAmp direct current for 30 min, and movement kinesthesia and H-reflexes were tested over time up to 30 min after termination of iontophoresis. Movement kinesthesia was significantly decreased (p less than 0.05) following anesthesia, and the decrease lasted during the time of the experiment. No statistically significant changes were recorded in the H-M recruitment or H-reflex recovery curves. These results indicate that joint receptor afferents may lack the spontaneous activity recorded in other receptors, such as the skin and muscles, and do not provide position sensation at the ankle joint during rest. These results also indicate that joint receptor afferents may inform the central nervous system about movement sensation in the middle range, but this information has minimal effect on the excitability of the motoneurons as measured by H-reflexes.