Cutaneous depression of Ib reflex pathways to motoneurones in man.

Variations in the H-reflex of soleus (Sol), quadriceps (Q) and short head of biceps femoris (Bi) muscles in normal man were used to investigate the effect of volleys in low threshold cutaneous afferents from the ipsilateral limb on transmission of Ib effects from ankle and Q muscles to these different motoneurone (MN) pools. Stimulation of cutaneous afferents from the foot sole and the toes (but not from the thigh, knee or calf), which did not modify the size of the test reflexes when applied alone, strongly depressed Ib reflex pathways to MNs supplying muscles operating at the knee. The very brief central latency of this depression suggests that tactile cutaneous afferents from the foot have oligosynaptic spinal connexions with the interneurones intercalated in the Ib pathways to MNs. The same cutaneous stimuli did not all modify Ib inhibition of Sol MNs from triceps surae. These findings are discussed with regard to the role of the different muscles in human locomotion. It is suggested that during the stance phase of heel bipedal locomotion, the cutaneous depression of Ib reflex pathways to MNs supplying muscles operating at the knee might operate in association with the strong Ia connexions from ankle to knee muscles described in the previous paper.