Monosynaptic Ia excitation and recurrent inhibition from quadriceps to ankle flexors and extensors in man.

1. Ia projections and recurrent inhibition from quadriceps to tibialis anterior and soleus motoneurones were investigated in man. 2. Changes in the firing probability of individual voluntarily activated motor units were studied following electrical stimulation of the femoral nerve or quadriceps tendon tap. 3. Femoral nerve stimulation evoked an early increase in the firing probability of tibialis anterior units. This excitation was also evoked by a tendon tap, had a low threshold and its central delay was estimated to be the same as that of the homonymous monosynaptic Ia excitation. These findings strongly suggest that the femoral nerve-induced excitation is Ia in origin and mediated through a monosynaptic pathway. 4. The frequency of heteronymous Ia excitation from quadriceps was about the same to both ankle flexor and extensor units (79 and 70% respectively). 5. In 80% of both tibialis anterior and soleus units the Ia excitation was followed by a decrease in firing probability. This inhibition had a short latency and a long duration (up to 40 ms); it always appeared with the quadriceps reflex discharge and increased with it. These findings suggest that this decrease in firing probability is due to the Renshaw inhibition evoked by the quadriceps motoneurone discharge. 6. Both the Ia excitation and the following inhibition of tibialis anterior and soleus units were also observed when the stimulation was applied to the nerve of the vastus lateralis (a pure knee extensor). 7. The functional significance of these identical projections from quadriceps to both ankle flexor and extensor motoneurones is discussed with regard to the requirements of bipedal stance and gait.