JAW MOVEMENT-RELATED PRIMARY SOMATOSENSORY CORTICAL AREA IN THE RAT K. UCHINO, a,b  K. HIGASHIYAMA, a  T. KATO, a T. HAQUE, a F. SATO, a A. TOMITA, a K. TSUTSUMI, a M. MORITANI, a K. YAMAMURA c AND A. YOSHIDA a * a Department of Oral Anatomy and Neurobiology, Osaka University Graduate School of Dentistry, Suita, Osaka 565-0871, Japan b Department of Acupuncture, Takarazuka University of Medical and Health Care, Takarazuka, Hyogo 666-0162, Japan c Division of Oral Physiology, Niigata University Graduate School of Medical and Dental Sciences, Niigata 951-8514, Japan Abstract—It has anatomically been revealed that the rostral part of the rat primary somatosensory cortex (S1) directly projects to the dorsal part of the trigeminal oral subnucleus (dorVo) and the dorsal part of juxtatrigeminal region (dorVjuxt), and that the dorVo and dorVjuxt contain premot- oneurons projecting directly to the jaw-opening or jaw-clos- ing motoneurons in the trigeminal motor nucleus (Vmo). However, little is known about how the rostral S1 regulates jaw movements in relation to its corticofugal projections. To address this issue, we performed intracortical microsti- mulation of the rat rostral S1 by monitoring jaw movements and electromyographic (EMG) activities. We for the first time found that low-frequency long-train stimulation of the rostral S1 induced single sustained opening of the jaw with elevated EMG activities of the anterior digastric muscles (jaw- opener). The effective sites for the low-frequency long-train stimulation overlapped the S1 sites where traditional high- frequency short-train stimulation was effective to induce single twitch-like jaw movement. We also found that the effective sites for the two kinds of train stimuli were included in the rostral S1 area, which has previously been identified to send direct projections to the dorVo or the dorVjuxt. Specif- ically, the most effective stimulation sites for the two kinds of train stimuli were located in the rostralmost part of S1 which has been reported to emanate strong direct projections to the dorVjuxt but less to the dorVo. Therefore, the present study suggests that the rat rostral S1, especially its rostral- most part, plays an important role in controlling jaw move- ments by activation of direct descending projections from the rostral S1 to the trigeminal premotoneuron pools, espe- cially to the dorVjuxt. Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: Primary somatosensory cortex, SI, Intracortical microstimulation, Corticofugal, Trigeminal motoneuron. INTRODUCTION Ferrier (1876) provided the first evidence that electrical stimulation of the cerebral cortex evokes jaw movements in the dog. Since then, intracortical microstimulation tech- niques were widely used to identify jaw movement-related cortical areas in several species (monkey, Vogt and Vogt, 1919, 1926; Walker and Green, 1938; cat, Iwata et al., 1985, 1990; rabbit, Lund et al., 1984; guinea pig, Iriki et al., 1987; rat, Donoghue and Wise, 1982; Neafsey et al., 1986). Based on earlier anatomical studies, direct projections from the cerebral cortex to the cranial nerve motor nuclei including the trigeminal motor nucleus (Vmo) are rare in various species (human, Kuypers, 1958a; Schoen, 1969; monkey, Kuypers, 1958b, 1960; Kuypers and Lawrence, 1967; cat, Rossi and Brodal, 1956; Walberg, 1957; rat, Valverde, 1962; Ohta et al., 1989). Therefore, the motor information from the jaw movement-related cortical areas to jaw-closing and jaw- opening motoneurons in the Vmo is considered to be di- or multi-synaptically conveyed thorough the jaw-closing premotoneurons (interneurons projecting directly to the jaw-closing component of the Vmo) and jaw-opening premotoneurons (those to the jaw-opening component), respectively (Olsson and Landgren, 1980; Lund et al., 1984; Olsson et al., 1986a). Our previous morphological study in the rat (Yoshida et al., 2009) has revealed that the intertrigeminal region (reticular formation between the Vmo and the trigeminal principal nucleus) mainly includes jaw-closing premoto- neurons, whereas the reticular formation medial to the Vmo mainly contains jaw-opening premotoneurons. By contrast, the dorsal part of the trigeminal oral subnu- cleus (dorVo) and the dorsal part of the juxtatrigeminal region (dorVjuxt; lateral reticular formation medial to the trigeminal spinal nucleus) contain both the jaw-clos- ing and jaw-opening premotoneurons. This morphologi- cal study (Yoshida et al., 2009) has also shown that the rostral part of the lateral agranular cortex (Agl; homologous to the primary motor cortex of the primate; see Neafsey et al., 1986) sends direct projections to the http://dx.doi.org/10.1016/j.neuroscience.2014.09.072 0306-4522/Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. * Corresponding author. Address: Department of Oral Anatomy and Neurobiology, Osaka University Graduate School of Dentistry, 1-8 Yamadaoka, Suita, Osaka 565-0871, Japan. Tel: +81-6-6879-2877; fax: +81-6-6879-2880. E-mail address: yoshida@dent.osaka-u.ac.jp (A. Yoshida).   The first two authors equally contribute to this study. Abbreviations: Agl, lateral agranular cortex; c-Dig, contralateral Dig; c-Mas, contralateral Mas; Dig, digastric; dorVo, dorsal part of the trigeminal oral subnucleus; dorVjuxt, dorsal part of juxtatrigeminal region; EMG, electromyography; HRP, horseradish peroxidase; i-Dig, ipsilateral Dig; i-Mas, ipsilateral Mas; Mas, masseter; S1, primary somatosensory cortex; vjuxt-S1, S1 area projecting to the dorVjuxt; Vmo, trigeminal motor nucleus; vo-S1, S1 area projecting to the dorVo. Neuroscience 284 (2015) 55–64 55