Excitatory and inhibitory controls of the masseter and temporal muscles elicited from teeth in the rat.
The periodontal mechanism that controls the jaw reflexes was examined in lightly anesthetized rats. Motor-unit activity in the masseter and temporal muscles was recorded electromyographically and pressure stimulation was applied to either an upper incisor or an upper molar. Reflex effects of dental stimulation varied depending on the level of ongoing activity (background activity, BGA) in each motor unit. Incisal or molar stimulation elicited excitatory reflexes in both the masseter and temporal motor units at a low BGA, but inhibitory reflexes in both types of motor unit at a higher BGA. In contrast to these synergistic reflex actions, the reciprocal reflex actions of the two motor units that belonged to the respective muscles occurred when the BGA was intermediate. The results suggest that different patterns of periodontal jaw reflexes may be elicited, depending on the different levels of BGAs. Furthermore, the present reflexes were modified with the site of a stimulated tooth within the dentition. Incisal stimulation produced greater excitation in the masseter motor unit than in the temporal one, and the opposite type of response occurred during molar stimulation. Moreover, smaller-amplitude motor units with a low reflex threshold and larger-amplitude motor units with a higher reflex threshold tended to exhibit excitatory and inhibitory reflexes, respectively.
Yamamura
C
C
Department of Oral Physiology, Niigata University School of Dentistry, Japan.
Shimada
K
K
D000818:Animals; D004576:Electromyography; D005260:Female; D008297:Male; D008406:Masseter Muscle / Q000294:innervation / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D010812:Physical Stimulation; D011312:Pressure; D051381:Rats; D011919:Rats, Inbred Strains; D012018:Reflex / Q000502:physiology; D013703:Temporal Muscle / Q000294:innervation / Q000502:physiology*; D014070:Tooth / Q000502:physiology*