Presynaptic inhibition of jaw-opening reflex by high threshold afferents from the masseter muscle of the cat

Abstract

Summary (1) The effects were studied of masseteric nerve stimulation upon the excitability of the central terminals of lingual primary afferent fibers in cats, using Wall's method, and upon lingually evoked potentials in the trigeminal spinal nucleus as well as upon the linguo-digastric reflex. (2) Stimulation of the masseteric nerve with intensities of more than 1.5 times threshold induced an increase in amplitude of antidromic spikes (conduction velocity, 50–66 m/sec) directly evoked by stimulation of preterminal fibers of the lingual nerve in the trigeminal nucleus oralis. This increase in excitability of preterminal fibers started around 15 msec after conditioning stimulation of the masseteric nerve, reached its peak after 30–50 msec, and returned to the control level after 100–150 msec. The effect was increased by raising the stimulus intensity in the supramaximal range for low threshold fibers in the masseteric nerve (conduction velocity, 96 m/sec in average). (3) Partial transection of the brain stem at the obex level, including the trigeminal spinal nucleus and tract, as well as application of picrotoxin reduced or abolished the facilitation of the antidromic spike. (4) Selective section of the trigeminal motor root at the semilunar ganglion abolished the observed effects of masseteric nerve stimulation so long as the intensity was kept below 5 times the nerve threshold, but the effect of stronger stimulation persisted. (5) Responses of second order neurons in the spinal nucleus (nucleus oralis) to lingual nerve stimulation were depressed, and the linguo-digastric reflex was diminished with increase in excitability of lingual preterminal fibers. (6) It is concluded that group II and III afferents in the masseteric nerve induce preterminal depolarization in the mechanoreceptor fibers of the lingual nerve by intervention of the trigeminal nucleus caudalis, and that they exert a presynaptic inhibitory action on the jaw-opening reflex.