Interaction of excitatory and inhibitory inputs to motoneurons in man.

Two separate paradigms were used to study the interaction of excitatory and inhibitory inputs to the motoneuron pools of lower leg muscles in human subjects. During inhibition of tibialis anterior muscle from Ia soleus afferents, the probability of firing of the higher threshold unit decreased more than that of the lower theshold unit in 14 out of 18 pairs. When soleus motor units were recruited by tendon vibration reflex and the subject voluntarily inhibited one unit, the higher threshold unit was silenced first in 55 pairs, whereas the lower threshold unit ceased firing first in 29 pairs, and 16 showed inconsistencies. It is concluded that although under standard conditions there is orderly derecruitment of units which is the reverse of recruitment, vibration of the intact human tendon brings about synaptic effects in the spinal cord which affect the normal patterns of recruitment and derecruitment of motor units.