Inhibition of neurones transmitting non-monosynaptic Ia excitation to human wrist flexor motoneurones

Abstract

1. The possibility was investigated that the transmission of the interneuronally mediated Ia excitation to wrist flexor motoneurones described in the companion paper is inhibited by stimulation of afferent fibres. Two techniques were used: (i) the post-stimulus time histogram (PSTH) method for studying changes in firing probability of individual voluntarily activated wrist flexor motor units following various peripheral stimulations; and (ii) the indirect technique of spatial facilitation of the H reflex. 2. In those individual units where stimulation of the median and/or the ulnar nerve evoked a non-monosynaptic excitation, this excitation was reduced when the afferent input was increased. This reduction of the non-monosynaptic Ia excitation was found in 80% of the motor unit recordings, whether the afferent input was increased by increasing the stimulus intensity to one nerve or by using combined stimulation of two nerves. 3. Both group I muscle and low-threshold cutaneous afferents were shown to be able to reduce the non-monosynaptic Ia excitation. 4. The onset of the depression of the excitation was always found within the same 1 ms bin as that of the excitation. 5. Whatever the amount of afferent input (stimulus intensity increased up to motor threshold, spatial and temporal summation, summation of inhibitory effects from different origins), the depression could at the very most suppress the non-monosynaptic Ia excitation: i.e. a trough in the PSTH, reflecting an inhibition at the motoneuronal level, never appeared. In those units in which there was not non-monosynaptic excitation, peripheral stimulation did not evoke any inhibition appearing as a trough in the PSTH either. Hence, inhibition only appeared when there was a non-monosynaptic excitation, and then as a depression of it. 6. On the basis of these findings it is argued that the inhibition is not exerted directly onto motoneurones but acts at a pre-motoneuronal level on the interneurones mediating the non-monosynaptic Ia excitation to motoneurones. 7. Similarly, the homonymous non-monosynaptic Ia facilitation of the flexor carpi radialis H reflex was shown to be reduced by a preceding stimulation applied to the ulnar nerve. It is argued that this result is also compatible with an inhibition of transmission in interneuronal excitatory pathways to motoneurones. 8. It is suggested that the non-monosynaptic excitation of wrist flexor motoneurones in man and the inhibition of this excitation, both evoked by stimulation of low-threshold afferents, could be mediated through a system of cervical propriospinal neurones. Some aspects of the possible role of this system during movement are discussed.