EXPERIMENTAL NEUROLOGY 58, loi?- (1978) Intracellular Recording in Trigeminal Motoneurons of the Anesthetized Guinea Pig during Rhythmic Jaw Movements LOUIS J. GOLDBERG AND MICHAEL TAL l Departments of Oral Biology and Anatomy a.4 Dental Research Institute, Schools of Dentistry and Medicine, University of C3alifornia, Los Angeles, California 90024 Received July 28, 1977 Intracellular recordings were obtained from guinea pig trigeminal moto- neurons during rhythmic jaw movements. The cells were identified by stim- ulation of the trigeminal mesencephalic nucleus which evokes excitatory postsynaptic potentials and spikes in jaw-closer motoneurons. The anesthe- tized guinea pig in the stereotaxic apparatus demonstrated spontaneous rhythmic jaw movements which were characterized by hyperpolarization of jaw-closer motoneurons, occurring concurrently with digastric muscle ex- citation during the jaw-opening phase of the cycle. The anesthetized guinea pig could also be induced to rhythmically clench and release a stick placed between the molar teeth. During this behavior intracellular recordings in jaw- closer motoneurons revealed a rapid depolarization leading to bursts of action potentials following the hyperpolarization which occurred during the jaw- opening phase of the cycle. The results demonstrate the feasibility of intra- cellular recording in guinea pig trigeminal motoneurons during rhythmic jaw movements. It was also shown that during the opening phase of the rhythmic jaw movement cycle there is a pronounced hyperpolarization present in the membrane potential of jaw-closer motoneurons. Resolution of the problem of central vs. peripheral origin of this hyperpolarization is significant for our understanding of the motor control of the jaw. INTRODUCTION Intracellular recordings in trigeminal motoneurons have previously been obtained in anesthetized or decerebrate cats immobilized with neuro- Abbreviations : EMG-electromyogram; EPSP-excitatory postsynaptic potential ; im-intramuscular ; iv-intravenous. 1 This research was supported in whole by the National Institute of Dental Research, National Institutes of Health, Research Grant ROl DE 4166. 102 0014-4886/78/0581-0102$02.00/0 Copyright 0 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.