Jpn. J. Oral Biol., 33: 592-599, 1991. ORIGINAL Jaw and orofacial motor representation in cat orbital gyrus Yoshiyuki Tsuboi, Koichi Iwata, Hiroyuki Muramatsu, Junichi Yagi, Yuji Inomata, Reo Kikuta, Keiji Yoshii and Rhyuji Sumino Department of Physiology, Nihon University. School of Dentistry, 1-8-13 Kandasurugadai, Chiyoda-ku, Tokyo 101, Japan (Director: Prof. Rhyuji Sumino) Accepted for publication: June•k 20, 1991•l Key words: motor effect/trigeminal nerve/orbital gyrus/ICMS/cat Abstract: Jaw and orofacial motor representation in the orbital cortex was studied by intracortical microstimulation (ICMS) in lightly anesthetized cats. ICMS of the posterior portion of the the orbital gyrus produced movements of facial muscles, whereas stimulation of the anterior portion of the orbital gyrus produced more generalized movement of facial, jaw and tongue muscles. The region producing jaw movements was more restricted than the regions producing tongue and facial movements. Repe- titive stimulation of the anterior orbital gyrus produced either rhythmic jaw movements or sustained jaw opening. Cytoarchitectonically, the posterior portion of the orbital gyrus was restricted to area 43 and the anterior portion to areas 43 and 6 aƒÀ. present study, the function of the orbital gyrus Introduction was studied by detailed topographic mapping The orbital cortex in the cat has been ter- of the effects produced by ICMS and the med the cortical masticatory area because results correlated with the cytoarchitectonic criteria of Hassler and Muhs-Clement7). repetitive stimulation of its anterior part pro- duces rhythmic jaw and tongue move- Materials and Methods ments1-3). The axons of cells in this cortical region may project to a "pattern generator" Experiments were performed on 16 cats in the rhythmic generation of masticatory anesthetized initially with Ketamine HCL (50 jaw movements2,4). However, it was shown mg/kg i. m.). During surgery, anesthesia that intracortical microstimulation (ICMS) of was maintained with a mixture of halothane, the same area produced not only movements nitrous oxide, and oxygen and anesthetic level of the jaw and tongue but also of the facial was maintained throughout the experiment muscles suggesting that the orbital cortex of by intramuscular infusion of Ketamine HCl the cat was involved in both orofacial and (20 mg/kg/h). Additionally, incision sites masticatory motor function5). Earlier stud- were infiltrated with a 2% lidocaine HCl es1-3,6) did not determine the most effectivei solution. Heart rate was continually mon- sites which evoked jaw and orofacial move- itored by recordings of EKG. Gas anesthesia. ments because the stimulation electrodes were was discontinued after the surgery. Anes- placed on the surface of the cortex. In the thetic level was judged by miosis, animal's Y. Tsuboi et al.: Jaw and orofacial motor representation in cat orbital gyrus 593 movement and heart rate. When animal's appropriate. EMG activities of each muscle pupil quickly responded to light presentation and minimum latencies to contraction were or animal made gross non-reflexive move- determined by using suprathreshold current to ment or the heart beat rate was changed evoke an EMG response. When jaw and during electrical stimulation of the cortex , tongue muscle contractions were elicited, re- additional ketamine HC1 (20 mg/kg) was in- petitive stimulation at a frequency of 30 Hz jected i. m. Body temperature was continu- (stimulus current less than 30 ƒÊA, 0.2 ms pulse ously monitored with a rectal probe and main- width) was applied to see if it produced sus- tained between 37•Ž and 39•Ž with a heat- tained jaw opening or rhythmic jaw move- inguAd. The animal's head was fixed rigidly ment. Jaw and orofacial muscle contractions to a head-holder by means of skull screws were analyzed and lesions were placed in and dental acrylic cement. The left eyeball selected penetrations at the end of each ex- and the marginal region of the maxillary periment byuAssing direct current (10 ƒÊA, 10 s) bone were removed to give access for the through the electrodes to facilitate histolo- electrodes to the orbital cortex. The skull gical examination. over the coronal and orbital gyri was re- moved and an acrylic resin chamber was insta- Results lled over the opening. The cisterna magna was then opened to allow cerebrospinal fluid 1. Cortical representation of the jaw and to drain away and to prevent swelling of the orofacial motor zones brain. The acrylic chamber was filled with Fig. 1 shows sample penetration tracks from mineral oil after removing the dura from the one cat in which movements of jaw, tongue orbital and coronal gyri. Electrode penetra- and face were produced by ICMS of less than tions were placed 0.5 mm or less auArt in the 30 ƒÊA (Eyelid lowering was analyzed only orbital cortex and were made into the orbital on the contralateral side because the left eye- gyrus and cortical area extending to the pre- ball had been removed for installation of the sylvian sulcus. Each penetration was made acrylic chamber). ICMS delivered to the an- as near perpendicular to the cortical surface terioruArt of the orbital gyrus evoked eyelid as possible and was observed through a mi- lowering, movements of vibrissae, lower lip, croscope and photographed. jaw and tongue, whereas only eyelid lowering Each grass-coated tungsten microelectrode was observed after ICMS of the posterior (10-20 pm of tip diameter) was advanced rt (Fig. 1 B and C). Thus the uAanterior through the cortex in 200 pm steps and ICMS portion of the orbital gyrus was termed the "jaw (twelve 0.2 ms cathodal pulses,<30 ƒÊA, 300 , tongue and facial motor zone" (Fig. 1 Hz with 40 ms train duration) applied after Ca) and the posterior portion, the "facial each step, according to Asanuma's tech- motor zone" (Fig. 1 Cb). Stimulation of niques8,9). We accumulated the effective points the posterior portion of the orbital gyrus (tracks activated by ICMS of less than 30 ƒÊA, on 9 and 10) produced eyelid lowering. In the basis of Sessle and Wiesendanger's cri- addition to eyelid lowering, movements of the teria16). They have shown that ICMS of less vibrissae and lower lip were evoked from than 30 ƒÊA upper limit defines an excitable tracks 7 and 8. Movement of facial muscles region of the limb motor cortex, which con- and jaw opening were produced from stimula- tains nearly all physiologically as well as ana- tion in track 6 and of the vibrissae, lower tomically identified cortico-spinal projection lip and tongue from stimulation in tracks 3 elements16). Once a muscle movement had and 4. Stimulation of the most anterioruArt been identified by two independent observers, of the orbital gyrus (tracks 1 and 2) evoked the fine muscle contractions were examined movements of the lower lip and tongue. both visually and byuAluAtion. When Jaw movements were obtained from ICMS threshold intensities less than 10 ƒÊA were of an area just adjacent to the anterior end of observed, bipolar wire electrodes were placed orbital sulcus (tracks 5 and 6). These results acutely in jaw, tongue and facial muscles, as demonstrated that the jaw, tongue and oro- 594 Jpn. J. Oral Biol., 33: 592-599, 1991. A B C D T4 T6 T10 Fig. 1 Tracks represented on the cortical surface from which movements of the jaw and orofacial muscles were evoked by ICMS in one cat. A=photograph of the cortical surface of the orbital gyrus and adjacent region of the cat frontal cortex. B=table indicating the penetration tracks and threshold intensities for evoking movements on jaw and orofacial regions. Penetration tracks are numbered from 1 to 10 along the abscissa. The numbers in each frame indicate the stimulus threshold (in ) for producing each movement. eye l.=eyelid lowering, vibrissae=movement of vibrissae,uA lower lip=movement of lower lip, j. o.=jaw opening, j. c.=jaw closing, tongue=tongue move- ment. C=schematic drawing of the orbital gyrus. The two regions of the orbital gyrus (a, b) are seuArated by a dotted line; the line between a and b is vertical to the orbital sulcus. D= sample records of EMG activites of jaw and orofacial muscles at tracks 4, 6 and 10. Note that tracks 1 to 8 are located in the anterior portion of the orbitalgyrus (a) and tracks 9 and 10 are in the posterior portion (b). COR=coronal sulcus, ORB=orbital sulcus, PRESYL=presylvian sulcus, RHIN=rhinal sulcus, cl=contralateral lower lip, il=ipsilateral lower lip, cv=contralateral vibrissae, iv=ipsilateral vibrissae, e=eye lid lowering, t=tongue movement, c. dig=contralateral digastric muscle, i. dig=ipsilateral digastric muscle. facial motor zones were represented on the obtained from 155 tracks were as follows: 33.5 orbital gyrus along the orbital sulcus. Further- 7.5 ms for facial muscles (Fig. 2 A); 31.5•} •} more, it can be seen that the largest repre- 8.1 ms for tongue muscle (Fig. 2 B); 25.5•} sentation within this effective region was 4.4 ms for masticatory muscles (Fig. 2 C). devoted to the facial musculature, as described These results indicate that latencies for EMG for the conscious monkey10). In the present activities of facial and tongue muscles were study, the orbital gyrus was seuArated into statistically longer than those of masticatory two different jaw, tongue and orofacial motor muscles (p<0.01: Mann-Whitney U test) zones (shown as a and b in Fig. 1 C). Sam- and that there were no differences between ple records of EMG activities evoked by sti- facial and tongue muscles. mulation of each of these zones are shown 2. Intracortical representation of the jaw in Fig. 1 D. The mean latencies (•}standard and orofacial motor zones deviation) of EMG activities following ICMS In the present study, a total of 571 loci Y.