Cousins_1998_Physiol.Behav_64_153

Rats were implanted with fine-wire electromyograph (EMG) electrodes and were videotaped to identify the local frequency characteristics and muscle activity associated with tacrine-induced tremulous jaw movements. All rats received intraperitoneal injections of 2.5 mg/kg tacrine. The videotape sessions were played back in slow motion (i.e., one-sixth normal speed), and an observer entered each jaw movement into a computer program that recalculated the interresponse time and the local frequency (in hertz) for each movement within a burst. Analyses of the distribution of frequencies showed that the peak frequency of jaw movements was in the 3- to 5-Hz frequency range, with an average frequency of 4.0 Hz. EMG electrodes were implanted into three jaw muscles: temporalis, anterior belly of digastricus, and masseter. Tremulous jaw movements were not accompanied by consistent changes in masseter activity. The anterior belly of digastricus showed bursts of EMG activity during some jaw movements, although the temporal relation between jaw movements and EMG activity was somewhat inconsistent. The muscle that showed activity most closely related to tremulous jaw movements was the temporalis. During bursts of jaw movements, temporalis muscles across several different rats showed bursts of EMG activity. Sections of videotape corresponding to bursts of EMG activity were reanalyzed by freeze-frame examination of the tape; typically, the temporalis showed a burst for each jaw movement, with the burst of activity occurring during the jaw-closing phase and the transition between jaw closing and opening. These results indicate that the local frequency of tremulous jaw movements is within the 3- to 7-Hz frequency that is typically associated with parkinsonian tremor. Moreover, the EMG data suggest that temporalis is a major contributor to the muscle activity that underlies tremulous jaw movements.