Evidence that low threshold afferents both evoke and depress polysynaptic excitation of wrist flexor motoneurones in man.

The post-stimulus time histogram method was used to measure the time course of the firing probability of voluntarily activated wrist flexors motor units (MUs) after various stimulations. Stimulating the median and the ulnar nerve at an intensity equal to the motor threshold evoked an early increase in firing probability attributed to the monosynaptic Ia EPSP. Reducing the stimulation strength led to the disappearance of this peak and to the appearance of a second peak, whose latency was 3-5 ms longer than that of the monosynaptic peak. It is shown that both group I and cutaneous fibres contribute to the long-latency response but that group I fibres alone are responsible for its onset. Combined stimulation of the same afferents (group I and low threshold cutaneous fibres) reduced the amplitude of the second response. It is argued that this inhibition is exerted onto the interneurones mediating the non-monosynaptic excitation to motoneurones.