Effect of spinal cord injury and its lesion level on stretch reflex modulation by cold stimulation in humans

Objective: To determine how short-latency stretch reflex amplitude in the soleus muscle is modulated by cold stimulation in able-bodied individuals and individuals with complete spinal cord injury. Methods: An initial 100-s baseline period was followed by 50-s cold stimulation periods. Stretch reflex of the right soleus muscle was elicited for 10-s intervals, while cold stimulation was applied to the left thigh. Results: Peak-to-peak amplitude of the stretch reflex increased significantly during cold stimulation up to 127 ± 21% of the baseline in the able-bodied group (n= 9, P< 0.01). Similarly, stretch reflex increased up to 125 ± 11% in a group with injury level at or below thoracic 10 (n= 4), although this increase was not significant. On the other hand, stretch reflex decreased significantly down to 78 ± 20% in a group with injury level at or above thoracic 6 (n= 8, P< 0.05). Conclusions: Effect of afferent inputs induced by cold stimulation on stretch reflex modulation is different depending on the extent of central nervous systems participating in the modulation. Significance: Our findings provide a better understanding of some basic changes in afferent-efferent spinal reflex pathways which are probably not monosynaptic in nature.