Participation of mono- and polysynaptic transmission during tonic activation of the stretch reflex arcs

Abstract

Through intracellular observations of the cat spinal motoneuron both mono-and polysynaptic reflex arcs were shown to participate in repetitive activation of stretch reflex. Experiments were performed on 25 cats anesthetized by intraperitoneal (i.p.) injection of 3 cc/kg of a mixture of 1% chloralose and 10% urethane. Driving of the motoneuronal discharges accomplished by electric stimulation (16-100 Hz) of the gastrocnemius lateralis and medialis nerves. Repeated electric stimulation of muscle nerves elicited monosynaptic ‘vibratory’ EPSPs and a polysynaptic ‘augmenting’ EPSP in the motoneuron. The firing of the motoneuron occurred when a temporal summation of the ‘vibratory’ EPSPs was sufficiently obtained. The discharge frequency of motoneuron, MNf, was expressed as a product of electrostimulus frequency, Sf, and reciprocal of an integer, n, i. e. MNf=(1/n)Sf. An increase in the stimulus intensity resulted in a diminution of the vibratory EPSP and a remarkable increase of n. This was assumed to be due to an inhibitory process produced by the Group II fibers. The minimum value of integer n was always a determinant factor for the maximum value of MNf which was referred to as the ‘preferred’ frequency of the motoneuron. Long-lasting electric stimulation of muscle nerves was followed by a more remarkable recruitment of the augmenting EPSP which made the motoneuron attain its critical threshold of firing. Motoneuronal spikes elicited by such a large augmenting EPSP were not locked to the Ia impulses. Unlocked spikes were observed only in a later stage of electric stimulation of Ia fibers. © 1975, PHYSIOLOGICAL SOCIETY OF JAPAN. All rights reserved.