The Course of Facial Nerve Innervation for the Levator Veli Palatini Muscle KAORU IBUKI, D.D.S., Ph.D. TOKUZO MATSUYA, D.D.S., Ph.D. JUNTARO NISHIO, D.D.S., Ph.D. YASUSHI HAMAMURA, D.D.S. TADASHI MIYAZAKI, D.D.S., D.Med.Sc. Osaka, Japan The present study was designed to determine the motor nerve pathway of the facial nerve to the levator veli palatint muscle. The experiments were carried out on 10 anesthetized rhesus monkeys. Recorded and analyzed were evoked EMG responses of the levator veli palatini and the orbicularis oris muscles by electrical stimulation to both the facial nerve and its branch within the cranium. 4 Muscle action potentials (M-waves) from the two muscles could be recognized on stimulating the facial nerve at the petrosal area of the temporal bone. On stimulating the greater petrosal nerve, M-waves from the levator muscle could be elicited. By cutting off the greater petrosal nerve at the middle cranial fossa, M-waves from the levator muscle completely disappeared on stimulating the facial nerve at the petrosal area. ' Results indicated that the course of the facial nerve for the levator veli palatini muscle is through the greater petrosal nerve. KEY WORDS: Levator veli palatini muscle, Facial nerve, Greater petrosal nerve, Evoked EMG, Rhesus monkey. Since Meckel (1748) reported the possibility gators could not reach specific conclusions of facial nerve innervation to the soft palate, concerning the motor nerve supply to the numerous investigators have discussed the levator, a muscle which is essential to velo- motor nerve supply to the muscles of the pharyngeal closure. Podvinec (1952), Brodal velopharynx. A better understanding of motor (1967), and Sedlackova, et al. (1973) sug- supply to this area is essential to a complete gested that the levator veli palatini muscle understanding of the physiological mecha- had double innervation from the facial nerve nisms on velopharyngeal movements. and branches of the pharyngeal plexus de- Turner (1889), Rethi (1893), Druner rived from the glossopharyngeal and vagus (1896), and Rich (1920) revealed that the nerves. Further, Nishio, et al. (1976a, 1976b), tensor veli palatini muscle was innervated by in studies of evoked EMG and fiberscopic the trigeminal nerve. However, these investi- observations on rhesus monkeys, indicated that the facial nerve innervated the levator The authors are affiliated with the 1st Department of veli palatini muscle and regulated movements Oral & Maxillofacial Surgery, Osaka University Dental similar to those found in man during phona- School, Osaka, Japan. Dr. Ibuki is a staff member, Dr. tion. Matsuya an associate professor, Dr. Nishio a staff mem- There is, however, no general agreement as ber, Dr. Hamamura a staff member, and Dr. Miyazaki a professor. yet on the pathway of the facial nerve to the This article is based on a paper presented at the 3rd levator veli palatini muscle. Futamura (1906) International Congress for Cleft Palate and Other Cran- published a study based on the embryological iofacial Anomalies in Toronto, Canada, June, 1977. findings of human facial muscles and nerves Some of the material in this article has been published in the Journal of Osaka University Dental Society, Vol- and suggested that the stapedius and the lev- ume 22, No. 1, June, 1977. ator veli palatini muscles were developed from 209 210 Cleft Palate Journal, July 1978, Vol. 15 No. 3 the second branchial arch and that the course (1972) and consisting of isonaled platinum of the facial nerve went through the greater wire (Consolodated Reactive Metal Co. Ltd., petrosal nerve. Moritz (1939) suggested that U.S.A.) 50y in diameter with Imm bare tip, the route to the levator muscle passed through was inserted perorally into the levator veli the facial nerve, the chorda tympani, and the palatini muscle to record its action potentials otic ganglion. Nick]! (1950) supported the con- (Fritzel, 1969). The recording electrode of the tention of Moritz that the facial nerve innerv- levator veli palatini muscle was placed 2-3mm ated the levator muscle through the chorda mesial to the hamular process and about tympani. 10mm posterior-superiorly (Dickson, 1974). The present investigation utilized evoked The electrode was also placed percutaneously EMG to determine which branch of the facial in the orbicularis oris muscle as a control. nerve within the cranium innervates the lev- The inter-electrode distance was 2mm in ator veli palatini muscle. all cases. In addition, the tip resistance of the recording electrode was adjusted to 10 # 1 Materials and Methods KQ in normal saline. The experimental animals used in this in- As the second step, the middle cranial fossa vestigation were ten rhesus monkeys, weigh- approach of House (1961) was adapted to ing approximately 3.0 to 8.0 Kg. After the expose the greater petrosal nerve at the pe- trachea was canulated under pentobarbital trosal area of the temporal bone. After appro- sodium anesthesia (35mg/Kg., iv.), the ani- priate skin preparation and draping, an inci- mal was kept under artificial respiration. The sion was made just anterior to the ear at the skull was mounted on the stand of a stereo- level of the zygomatic arch to the top of the taxic instrument in which the ear rods had temporal muscle attachment. A round crani- been converted in order not to rupture the otomy of 5cm in diameter was made superior tympanums of the animals. and anterior to the external auditory canal, As the first step, after trepaning and incis- and the dura was elevated from the floor of ing the dura of the occipital region partial the middle fossa. The fibers of the greater decerebellum was carried out to expose the petrosal nerve, usually found on the under- facial nerve at the petrosal area of the tem- surface of the dura, were separated from the poral bone (Nishio, et al., 1976a). After the dural attachment, and then a bipolar plati- facial nerve was dissected as centrally as pos- num wire electrode was also placed to stimu- sible, a bipolar platinum wire electrode, 2004 late the peripheral stump of the nerve electri- in diameter, was placed on the peripheral cally (Figure 2). The facial and the greater stump of the nerve (Figure 1). A twin needle petrosal nerve were stimulated by square electrode, previously devised by Mimura wave pulses of 0.5msec in duration generated FIGURE 1. Diagram of the stimulation of the facial FIGURE 2. Diagram of the stimulation of the greater nerve. 1. For. Ovale; 2. Greater petrosal nerve; 3. For. petrosal nerve. 1. Bipolar electrode; 2. For. Ovale; 3. lacerum; 4. Facial nerve; 5. Vestibulocochlear nerve; 6. Greater petrosal nerve; 4. For. lacerum; 5. Facial nerve; Bipolar electrode. 6. Vestibulocochlear nerve; 7. For. juglare. Ibuki, et al., FACIAL NERVE INNERVATION 211 by an electrical stimulator (Nihonkohden served from the orbicularis oris muscle. Figure SEN-1101). The minimum intensity of a stim- 4, shows an example of simultaneous recorded ulus necessary to elicit a muscle action poten- muscle action potentials (intensity; 6.5V). tial was defined as threshold. In this experi- Amplitudes and latencies of action potentials ment, the intensity levels of stimulation were at maximum stimulation in the levator mus- converted consecutively from threshold to its cles ranged from 380 to 1100uV and from 1.5 maximum which revealed no further change to 1.9 msec, respectively. in amplitude. However, as the case may be, In Group B, when the peripheral stump of the intensity levels were increased beyond the facial nerve in the petrosal area was stim- maximum stimulation; le., supramaximum ulated, muscle action potentials were recorded stimulation. To prevent spread of electrical from both the levator veli palatini and the current and drying, each nerve was sur- orbicularis oris muscles on the stimulated side rounded by cotton, and bathed in mineral (Figure 5). Amplitudes of action potentials at water. The reactions to the motor nerve stim- maximum stimulation ranged from 320 to uli were ascertained through muscle action 830 uV in the levator and from 560 to 1220uV potentials, M-waves, from the levator and the in the orbicularis oris muscles. Latencies at orbicularis oris muscles. maximum stimulation in the levator and the Hence, the experimental monkeys were di- orbicularis oris muscles ranged from 1.6 to vided into three groups as follows: Group A 1.I9msec and from 1.7 to 2.0msec respectively. (Subjects 1, 2, 3, and 4), stimulating the In Group C, with sustained stimulation, greater petrosal nerve at the upper petrosal intensity of 2.0 V, to the peripheral stump of portion of the temporal bone. Group B (Sub- the facial nerve at the petrosal area, the jects 5, and 6), stimulating the facial nerve just before it entered the internal auditory ol meatus. Group C (Subjects 7, 8, 9, and 10), -A - |I | transecting the greater petrosal nerve at the | i floor of the middle fossa, with sustained stim- | | ulation to the facial nerve. | After the examinations, the monkeys were sacrificed to acertain the positions of the re- cording electrodes in the muscles. Action po- tentials from both the levator and the orbi- cularis oris muscles were simultaneously dis- played on an oscilloscope (Nihonkohden, VC- FIGURE 3. Analysis of evoked EMG. A. Latency; B. 7A) through the R-C coupled preamplifiers Amplitude. and recorded on film when necessary. In gen- eral, intensity levels of thresholds in a study M. orbicularis oris —\4,\A_fi -\\’—_ of evoked EMG are affected by individual 6.5V 1mV variations in experimental monkeys and by <uams —I the damage to the cranial nerves caused by M. levator l10msec operative techniques. Accordingly, the au- FIGURE 4. Examples of muscle action potentials on thors, to eliminate those factors, indicated the stimulating the greater petrosal nerve at the upper part ranges of amplitude and latency at maximum of the temporal bone.