Behavior of F-response and determination of actively involved motoneurons

Fifteen nerves were examined in 10 healthy subjects, using a collision technique with 2 stimulation pulses of different intensity on the same point. The F-waves which occurred after the antidromic activation of the motor neuron cells by the first supramaximal pulse were blocked on their return pathway by the collision with the depolarization induced by the second pulse. The progressive decrement of the voltage of the second stimulus allowed the transmission of the recurrent discharge along motor fibers with a lower depolarization threshold. The amplitude and the persistence of the F-waves increased (p < 0.05 and p < 0.001 respectively), while their latency decreased (p < 0.01) concurrent with the liberation of additional motor neurons with a lower threshold of depolarization. These findings suggest that the F-wave may be elicited in motoneurons of different depolarization threshold but primarily in larger and faster nerve fibers, with a lower threshold of depolarization.