Jaw reflexes elicited by electrical stimulation of the hard palate of the rat

Abstract

Reflex responses of the oral musculature to electrical stimulation of sensory elements in the hard palate were studied in decerebrate rats. Activity in the suprahyoid muscles, the intrinsic and extrinsic tongue muscles and the jaw-closing muscles was recorded electromyographically; at the same time jaw positions were recorded. Muscle group-activity patterns depended on the place of stimulation and the strength of the stimulus. Stimulation of the antemolar region of the palate evoked:—at threshold stimulation, suprahyoid muscle activity (latency 6–7 ms) which did not result in jaw movement; with stimuli about 1.2–1.5 × threshold, enhancement of suprahyoid muscle activity resulting in a transient jaw opening (latency 15 ms, duration 35 ms) sometimes accompanied by tongue reactions (latency 15 ms, duration 5–10 ms); with stimuli about 1.5–3 × threshold, additional jaw-closing muscle reactions (latencies 5–10 ms and 15–20 ms) sometimes changing the transient jaw opening to a transient jaw closing (latency 15 ms, duration 35 ms). Stimulation of the intermolar region evoked:- at threshold stimulation, two bursts of activity in the intrinsic tongue muscles (latencies 15–20 ms and 40–60 ms) and reactions in the jaw-closing muscles (latency 50–80 ms) resulting in a long-lasting jaw closure; with stimuli about 1.2–1.5 × threshold, a gradual shift from the reactions at threshold to a suprahyoid muscle and jaw-closing muscle response of short latency (6–9 ms), causing fast transient jaw opening followed by prolonged jaw closing; with stimuli about 1.5–3 × threshold, only short latency suprahyoid and jaw-closing muscle reactions. The strongest stimuli at this level abolished the jaw-closing reflex and reinforced the jaw-opening reflex. Stimulation of the intermolar region with two equal stimuli (1.2–1.5 × threshold) and several time intervals showed the following reaction pattern:- At very short intervals (1–7 ms), the jaw-closing reflex was suppressed; intervals larger than 6 ms evoked two bursts in the anterior digastric muscles. At about 12-ms intervals, the second reflex disappeared but reappeared at intervals larger than 30–40 ms, being equal to conditioning value at intervals larger than 60 ms. At stimulus intervals up to 200 ms a second stimulus did not produce a second response of the jaw-closing muscles (long-latency closing reflex).