Monosynaptic connections between neurons of trigeminal mesencephalic nucleus and jaw-closing motoneurons in the rat: an intracellular horseradish peroxidase labelling study

267

Brain Research, 559 (1991) 267-275 (~) 1991 Elsevier Science Publishers B.V. All rights reserved. 0006-8993/911503.50 ADONIS 0006899391169970

BRES 16997

Monosynaptic connections between neurons of trigeminal mesencephalic nucleus and jaw-closing motoneurons in the rat: intracellular horseradish peroxidase labelling study

an

Pifu Luo and Jishuo Li Department of Anatomy, Fourth Military Medical University, Xi'an 710032, People's Republic of China

(Accepted 30 April 1991) Key words: Monosynaptic connection; Jaw stretch reflex; Jaw-closing motoneuron; Intracellular horseradish perioxidase injection; Rat

In order to confirm the monosynaptic connections of muscle spindle-mediated jaw stretch reflexes, 8 neurons of trigeminal mesencephalic nucleus innervating masseteric muscle spindles were identified electrophysiologicallyand stained intracellularly with horseradish peroxidase. These axon terminals projected to ipsilateral dorsal and dorsolateral divisions of trigeminal motor nucleus and extensive premotor areas. Under electron microscope, labeled terminals made monosynaptic contacts predominantly with dendrites in the jaw-closingmotoneuron pools. One labeled and many non-labeled terminals were frequently observed to converge simultaneously on one dendrite in the area. However, it was of particular interest that 28% of the labeled terminals constituted the intermediate component of axo-axodendritic synaptic triads. The present study confirmed, for the first time, monosynaptic connections between jaw-closing muscle spindle afferents and jaw-closing motoneurons. These findings also provided ultrastructural evidence for the monosynaptic excitation of muscle spindle-mediated jaw stretch reflexes which received presynaptic and postsynaptic inhibitions of the premotor neurons from other sources. INTRODUCTION Electrophysiological and anatomical studies indicated that jaw-dosing muscles are richly supplied with muscle spindles while jaw-opening muscles are not 3'7'9'1°'14'25. This remarkable asymmetry of distribution of the muscle spindles between the jaw muscle groups reflects that the information from the jaw-closing muscle spindles is important for maintenance of the masticatory muscle tone and coordination of the jaw stretch reflexes 2'4-6'1°12,14,27 A variety of anatomical studies have shown that the cell bodies of masseteric muscle spindle afferents were located in the trigeminal mesencephlic nucleus (Vine) s' 9,17,24,27,29. Further investigations in animals of various species by using different methods have demonstrated that the Vme neurons innervating the jaw-dosing muscle spindles send their central axons directly to a restricted division of the ipsilateral trigeminal motor nucleus (Vmo) 8'9'17'19'29'31 where the jaw-closing motoneurons were located 21. In addition, a large number of central axons were found to end extensively in the premotor n e u r o n a r e a s 3's'9"17"19'24'29'31. Considerable data indicated that the neurons of premotor neuron areas contributed axon collaterals to the Vmo 22'3°. These stud-

ies raised the possibility of a monosynaptic pathway between the Vme and jaw-closing motoneurons and polysynaptic pathway via extensive premotor neurons. Electrophysiological investigations have shown that phasic stretch of jaw-closing muscles or stimulation of the Vme might produce monosynaptic excitatory postsynaptic potentials (EPSPs) in jaw-closing motoneurons 2'4' 5,7,27, while stimulation of the intraoral structures or direct stimulation of the premotor areas might produce inhibitory postsynaptic potentials (IPSPs) in jaw-dosing motoneurons 4's'7'11-15. Physiologists further presumed that two kinds of inhibitory mechanisms, presynaptic and postsynaptic, were involoved to function on the jaw-dosing motoneurons 3-5,14. There remain, however, significant lacunae in our knowledge of ultrastructural evidence for the anatomical and electrophysiological conclusions and postulations. In order to provide direct electron microscopic evidence for the jaw-dosing muscle spindle-mediated jaw stretch reflexes and modulations of these jaw stretch reflexes by the premotor neurons, the present study was an attempt to explore the synaptic connections and convergent relationships among the Vme neurons, jaw-closing motoneurons and the premotor neurons by using a method of intracellular injection of horseradish peroxidase (HRP).

Correspondence: P. Luo, Department of Anatomy, Fourth Military Medical University, Xi'an 710032, People's Republic of China.

.....

"!!~i~i

~iiii~ii~ ¸¸

269

Fig. 2. Electron micrograph of a typical HRP intracellularly labeled soma of unipolar Vme neuron; note that the HRP labeled Vine neuron has a round nucleus with centrally located nucleolus. Bar = 10/~m.

MATERIALS AND METHODS Experiments were performed on 8 adult Sprague-Dawley rats weighing 200-250 g. The animals were anesthetized with sodium pentobarbital (40 mg/kg i.p.), secured in a stereotaxic beadholder, paralyzed with gallamine trithiodide (50 mg/kg b.wt.) and artificially ventilated. The masseter nerve branches of one side were dissected out and placed on a stimulation silver hook. Glass micro* electrodes with 1-2/~m tip diameter containing 4-8% HRP (Sigma VI) dissolved in 0.05 M Tris buffer with 0.25 M KCI (pH 7.6) were used for intracellular recording and staining. The Vme neurons innervating the jaw-closing muscle spindles were identified by stimulation of masseter nerve branches (the masseter muscle branch, n = 4; temporal muscle branch, n = 2; and medial pterygoid muscle branch, n = 2) respectively, and HRP was electrophoretically injected into the cell bodies. The details of the procedure have been published elsewhere 17. After a postoperative survival of 10-12 h, the animals were reanesthetized deeply and perfused through the ascending aorta with saline, followed by 300 ml of fixative (1.0% paraformaldehyde and 1.25% glutaraldehyde in 0.1 M phosphate buffer, pH 7.4). The braiustem was removed and sectioned transversely at 50-60/~m with a vibratome. The sections were processed for HRP histochemical reaction with diaminobenzidine (DAB) according to Adams' cobalt intensification method 1. Microphotographs of the labeled

cell bodies and terminals were taken with bright-field illumination of a light microscope (Olympus). Electron microscopic experiments were carried out on 4 rats. After HRP histochemical reaction, the sections were postfixed in 0.1 M phosphate-buffered 1% osmium tetroxide (pH 6.0) for 1 h, section-stained with saturated uranyl acetate for 8-10 h (4 °C), dehydrated in graded alcohol and propylene oxide series, and plateembedded in Epon. After the embedding sections were microphotographed, the labeled cell bodies in the Vme and terminals in the dorsolateral division of the Vmo were taken and re-embedded on a pyramid for ultrathin sectioning. The ultrathin sections were observed with electron microscope (Hitachi H-300).

RESULTS I n t h e light m i c r o s c o p i c study, 8 V m e n e u r o n s innerv a t i n g t h e j a w - c l o s i n g m u s c l e spindles w e r e successfully stained. T h e l a b e l e d n e u r o n s w e r e c h a r a c t e r i z e d by ovalshaped unipolar or pseudounipolar perikarya, 30-40/~m in size, d i s t r i b u t e d in b o t h t h e rostral p a r t (Fig. 1 A , B ) a n d t h e c a u d a l p a r t (Fig. 1 C , D , E ) o f t h e V m e . T h e cen-

¢e--

Fig. 1. Light photomicrographs showing intracellularly HRP-labeled Vme neurons, which were identified by stimulation of masseteric nerve. Cell bodies of the Vme unipolar or pseudo*unipolar neurons in rostral part (A,B) and caudal part (C,D,E) of the Vme. F: a cluster of labeled axon terminals projecting to the do~olateral division of the Vmo (Vmo DL). G: Y-shaped terminal with beaded boutous en passant in dorsolateral division of the Vmo. H: labeled axon terminals distributed in dorsal (Vmo D) and dorsolateral divisions (Vmo DL) of the Vmo. Photomicrographs of D, E and F were taken after osmication. Bars: A - E = 30/~m, F - G = 50/~m, and H -- 100/~m.

270

Fig. 3. Electron micrographs of the intracellular HRP-labeled cell body of a pseudounipolar Vme neuron(A); myelinated axon collaterals (B,C) and clusters of axon boutons (D,E) in the jaw-closing motoneuron pools of the dorsolateral division of the Vmo. Bars: A = 10/~m, B-C = 1 ~m and D-if, = 2/zm.

Fig. 4. Electron micrographs showing labeled axon boutons of the Vine neurons making synaptic contacts with large and medium-sized dendrites (A,B,D,F) and small-sized dendrite or dendritic spine (C) and also occasionally with unlabeled axon boutons (E) in the jaw-closing motoneuron pools. Arrowheads indicate obviously asymmetric synapses. Note that in D and F the labeled axon boutons (A1) which, together with many unlabeled axon boutons (A2-A7, containing ellipsoid, flat or pleomorphous synaptic vesicles), simnltaneonsly converge on the medium-sized dendrite in the jaw-closing motoneuron pools, thus constituting a synaptic complex. Glia processes (asterisks) were observed surrounding the synaptic complex in F. Both asymmetric synapses (A3 with Den in D, and A4, A6 with Den in F) were observed within the synaptic complex. In A, B, and E, the synaptic vesicles of labeled boutons are obscure due to heavy staining by HRP reaction product. Bars: A - F = 1/zm.

272 tral axons of the labeled neurons end predominantly in the dorsal and dorsolateral divisions of the ipsilateral Vmo (Fig. 1H). The axon terminals were Y-shaped and/or of irregular thready shape with beaded boutons en passant (Fig. 1F, G,H). A large number of central axons and terminals also distributed extensively in the premotor areas, such as supratrigeminal nucleus (Vsup), intertrigeminal nucleus, juxta-trigeminal zone, dorsomedial part of the principal sensory trigeminal nucleus, dorsomedial part of the pars oralis of spinal trigeminal nucleus, and parvocellular part of the reticular formation. In the electron microscopic study, the intracellular H R P reaction product was highly electron-dense and granular, and was homogeneously distributed in the perikarya of the Vme neurons (Figs. 2 and 3A), axons (Fig. 3B,C) and beaded terminal boutons en passant (Fig. 3D,E). The ovoid cell body contained a round nucleus (Figs. 2 and 3A). In the neuropil among the jawclosing motoneurons, the labeled terminals were easily identified due to the highly electron-dense and finely granular H R P reaction products. The labeled terminals had a long axis ranging from 1.0 to 6.0 a m which contained either round or oval synaptic vesicles (Figs. 4 and 5). These axon boutons synapsed predominantly upon large to medium-sized dendrites in the jaw-closing motoneuron pools (Figs. 4A,B,D,F and 5A,B,D,E,F). They also synapsed occasionally upon small-sized dendrites or spine-like protrusions (Figs. 4C and 5C). It was observed that some labeled and unlabeled axon boutons synapsed or contacted simultaneously with one medium-sized dendrite in jaw-closing motoneuron pools. They formed a synaptic complex (Fig. 4D,F). The dendrite was the central component of the synaptic complex. The unlabeled axon boutons in the synaptic complex contained round, flat or pleomorphous synaptic vesicles and formed symmetric or asymmetric synaptic contacts with the central dendrite. The labeled axon boutons were presynaptic both to the dendritic shafts outside the synaptic complex and to the central dendrite of the synaptic complex in the jaw-closing motoneuron pools. A small number of labeled axon boutons, as presynaptic components, made axo-axosynaptic contacts with unlabeled axon terminals (Fig. 4E).

It is of particular interest to notice that 40 of 143 (28%) labeled axon boutons were, as intermediate components, sandwiched between unlabeled axon boutons and dendrites in the neuropil of the jaw-closing motoneurons (Fig. 5). These axon boutons formed a typical ' sandwich-like', axo-axodendritic synaptic triad (Fig. 5A,D), and were postsynaptic to unlabeled axon boutons, but presynaptic to dendrites. The unlabeled axon boutons contained oval or pteomorphous synaptic vesicles. Some large dense-cored vesicles were occasionally encountered (Fig. 5B,E). In these axo-axodendritic synaptic triads, axo-axonal synaptic contacts were symmetric, while axodendritic synapses were asymmetric. DISCUSSION Using the method of intracellular injection of HRP, the present study, for the first time, provides ultrastructural evidence of monosynaptic connections between the Vme neurons innervating jaw-closing muscle spindles and jaw-closing motoneurons. Since Szent~igothai proposed that the jaw stretch reflex is monosynaptic on the basis of anatomical methods32; a series of anatomical investigations showed that the jaw-closing muscle spindles afferents via the central axons of the Vme neurons project predominantly to their 'own' jaw-closing motoneurons in the Vmo by the methods of transganglionic H R P labelling 24'31, autoradiography 19 and intracellular or intra-axonal H R P injection 8'9"17"29. It was generally accepted that in jaw stretch reflexes, a monosynaptic pathway does exist. Although electrophysiological studies 2'5'12'14 indicated that there are monosynaptic EPSPs in the jaw-closing motoneurons during stimulation of masseteric nerve or Vme, no ultrastructural evidence was reported to support this monosynaptic pathway, The present result goes a step further to confirm these anatomical and electrophysiological studies for the jaw-closing muscle spindle-mediated, monosynaptic jaw stretch reflexes. Attention is drawn by the fact that the labeled axon boutons of Vme neurons innervating jaw-closing muscle spindles synapses merely upon the dendritic shafts (especially large and middle-sized dendrites) of jaw-closing motoneurons. A majority of these axodendritic synapses

Fig. 5. Electron micrographs showing axo-axodendritic synaptic triads constituted by labeled axon boutons (A1, as middle component, with round vesicles), unlabeled axon boutons (A2, with oval and pleomorphous vesicles) and dendrites (Den) in the jaw-closing motoneuron pools of the dorsolateral division of the Vmo. Note that in D these form a typical 'Sandwich-like' synaptic triad; a small part of the labeled bouton is separated by a dendritic spine (S). White arrows indicate symmetric axo-axonai synapses between A2 and A1; black arrowheads indicate asymmetric axodendritic synapses between A1 and Den. In E there are dense bodies (black arrowheads) beneath the postsynaptic density in the cytoplasm of the dendrite, and large dense-cored vesicles (open arrowheads) are observed in B and E. In F, A1 makes synaptic junctions simultaneously with large-sized dendrite (Den1, black arrowheads), and small-sized dendrite or dendritic spine (D2, black arrowheads). Bars: A-F = 1 /~m.

~P

274 were asymmetric. However, synaptic contacts on the soma of jaw-closing motoneurons were not encountered. These findings are consistent with the results of Appenteng et al. 2 and Chandler et al. 4. They inferred respectively that the excitatory synapses from the spindle units in the Vme to the jaw-closing motoneurons are predominantly located on the dendrites of jaw-closing motoneurons on the basis of electrophysiological measurement, Chandler 6 suggested, furthermore, that an excitatory amino acid, glutamate or glutamate-like substance, is responsible for the synaptic transmission between the Vme afferents and jaw-closing motoneurons. One striking property of this study is that identification for the first time of 28% labeled axon boutons of the Vme neurons which, together with non-labeled axon boutons and dendrites in the jaw-closing motoneuron pools, constituted axo-axodendritic synaptic triads. It is also found that these labeled and unlabeled axon boutons made synaptic contacts simultaneously with the dendrites of jaw-closing motoneurons. These formed synaptic complexes. This complex convergent relationship suggests a complicated function of integration in the jawdosing motoneuron pools. Even though it is impossible to determine the origins of the unlabeled terminals in the present study, a majority of them were presumed to come from the premotor areas near the V m o on the basis of previous studies, and in fact, the premotor neurons projecting to the V m o were considered as interneurons which were located in different nearby regions 13'15' 16,20,22,26,28,30,33. Most of these interneurons, such as the

nisms, the presynaptic inhibition exerted on central terminals of Ia muscle afferents is responsible for the phasic suppression of trigeminal monosynaptic reflexes. In the postsynaptic mechanism, inhibitory inputs from the premotor neurons to the jaw-closing motoneurons are distributed predominantly on dendritic components. The present ultrastructural finding that the labeled terminals from the Vme neurons, together with unlabeled terminals assumed to come from the premotor neurons, and dendrites of jaw-closing motoneurons constituted axoaxodendritic synaptic triads or synaptic complexes, provides strong morphological evidence for these two kinds of inhibitory synaptic mechanisms. It suggests that jawclosing muscle spindle-mediated jaw stretch reflexes are modified by presynaptic and postsynaptic inhibitions from the premotor neurons. The inhibition of jaw stretch reflexes by premotor neurons, also receives modulation, mono- or polysynaptic, from amygdala complex 26'33, masticatory cortical area, orbital and frontal cortex a°. It is surprisingly coincidental that the premotor neurons projecting to jaw-closing motoneurons also received projections of the Vme neurons innervating jaw-closing muscle spindles s'17'24'31. Furthermore, difference between the axon terminating patterns of the Vme neurons in these premotor areas and in the Vme has been reported previously 17. Therefore, it is reasonable to suggest that there are various polysynaptic pathways between the Vme and V m o via premotor neurons to modulate and/or coordinate a series of masticatory reflexes.

supratrigeminal (Vsup) neurons are obviously inhibitory to jaw-closing motoneurons 13"15'26. It was reported that 90% of the Vsup neurons are G A B A e r g i c 18'23. Chandler et al. indicated that the premotor neurons inhibit the jaw-closing motoneurons through presynaptic and postsynaptic mechanisms 4. In the presynaptic mecha-

Acknowledgements. The authors wish to thank Yoshiki Takeuchi, Professor in the Department of Anatomy, Kagawa Medical School, Japan, for his review, comments and editorial help of the manuscript. We also wish to thank Professor Huimin Li for his helpful suggestions and language correction of the manuscript.

REFERENCES 1 Adams, J.C., Technical considerations on the use of horseradish peroxidase as a neural marker, Neuroscience, 2 (1977) 141145. 2 Appenteng, K., O'Donovan, M.J., Somjen, G., Stephens, I.A. and Taylor, A., The projection of jaw elevator muscle spindle afferents to fifth nerve motoneurons in the cat, J. Physiol., 279 (1978) 409-423. 3 Appcnteng, K., Donga, R. and Williams, R.G., Morphological and electrophysiological determination of the projections of jaw-elevator muscle spindle afferents in rats, J. Physiol., 369 (1985) 93-113. 4 Chandler, S H., Chase, M.H. and Nakamura, Y., Intracellular analysis of synaptic mechanisms controlling trigeminal motoneurun activity during sleep and wakefulness, J. Neurophysiol., 44 (1980) 359-371. 5 Chandler, S.H. and Goldberg, L.J., Intracellular analysis of synaptic mechanisms controlling spontaneous and cortically induced rhythmical jaw movements in the guinea pig, J. Neuro-

physiol., 48 (1982) 126-130. 6 Chandler, S.H., Evidence for exieitatory amino add trammission between meseneephalic nucleus of V afferents and jawclosing motoneurons in the guinea pig, Brain Research, 477 (1989) 252-264. 7 Cody EW.J., Harrison, L.M. and Taylor, A., Analysis of activity of muscle spindles of the jaw-closing muscles during normal movements in the cat, J. Physiol., 253 (1975) 565-582. 8 Dacey, D.M., Axon morphology of meseneephalie trigeminal neurons in a snake, Thamnophis sirtalis, J. Comp. Neurol., 204 (1982) 268-279. 9 Dessemd, D. and Taylor, A., Morphology of jaw-muscle spindie afferents in the rat, J. Comp. NeuroL, 282 (1989) 389-403. 10 Dubbeldam, J.L., Stein, R.B. and Capaday, C., The role of muscle spindles: a functional-morphological view, Trends in Neurosci., 12 (1989) 93-94. 11 Finoeehio, D.V. and Luschei, E.S., Characteristics of complex voluntary mandibular movements in the monkey before and after destruction of most jaw muscle spindle afferents, J. Voice, 2 (1988) 279-290.

275 12 Goldberg, L.J., Chandler, S.H. and "Pal, M., Relationship between jaw movements and trigeminal motoneuron membranepotential fluctuations during cortically induced rhythmical jaw movements in the guinea pig, J. Neurophysiol., 48 (1982) 110125. 13 Jerge, C.R., The function of the nucleus supratrigeminalis, J. Neurophysiol., 26 (1963) 393--402. 14 Kidokoro, Y., Kubota, Y., Shuto, S. and Sumino, R., Reflex organization of cat masticatory muscles, J. Neurophysiol., 31 : (1968)695-708. 15 Kidokoro, Y., Kubota, Y., Shuto, S. and Sumino, R., Possible interneurous responsible for reflex inhibition of motoneurons of jaw-dosing muscles from the inferior dental nerve, J. NeurophysioL, 31 (1968) 709-716. 16 Landgren, S., Olsson, K.A. and Westberg, K.G., Bulbar neuroues with axonal projections to the trigeminal motor nucleus in the cat, Exp. Brain Res., 65 (1986) 98-111. 17 Luo, P.E, Wang, B.R., Peng, Z.Z. and Li, J.S., The morphological characteristics and terminating patterns of masseteric neurons of the meseucephalic trigeminal nucleus in the rat: an intracellular horseradish peroxidase labeling study, J. Comp. Neurol., 303 (1991) 286-299. 18 Luo, P.E and Li, J.S., The distribution of GABAergic neurons in the trigeminal sensory nuclear complex of the rat, Chinese J. Anat., 13 (1990) 178-179. 19 Lusehei, E.S., Central projections of the mesencephalie nucleus of the fifth nerve: an autoradiographic study, J. Comp. NeuroL, 263 (1987) 137-145. 20 Mishima, K., Sasamoto, K. and Ohta, M., Amygdaloid or cortical facilitation of antidromic activity of trigeminal motoneuroils in the rat, Comp. Biochem. Physiol., 73A (1982) 355-359. 21 Mizuno, N., Konishi, A. and Sato, M., Localization of masticatory motoneurons in the cat and rat by means of retrograde axonal transport of horseradish peroxidase, J. Comp. Neurol., 164 (1975) 105-116. 22 Mizuno, N., Yasui, Y., Nomura, S., Itoh, K., Konishi, A., Takada, M. and Kudo, M., A light and electron microscopic study of premotor neurons for the trigeminal motor nucleus, J. Comp. Neurol., 215 (1983) 290-298. 23 Mugnaini, E. and Oertel, W.H., An arias of the distribution of GABAergic neurons and ~terminals in the rat CNS as revealed by GAD immunohistoehemistry. In A. Bj6rklund and T. H6kfelt

(Eds.), Handbook of Chemical Neuroanatomy, Vol. 4, Elsevier, Amsterdam, 1985, pp. 480-485. 24 Nomura, S. and Mizuno, N., Differential distribution of cell bodies and central axons of mesencephalic trigeminal nucleus neurons supplying the jaw-dosing muscles and periodontal tissue: a transgangiionic tracer study in the cat, Brain Research, 359 (1985) 311-319. 25 Nozaki, S., Iriki, A. and Nakamura, Y., Tdgeminal mesencephalic neurons innervating functionally identified muscle spindles and involved in the monosynaptic stretch reflex of lateral pterygoid muscle of the guinea pig, J. Comp. Neurol., 236 (1985) 106-120. 26 Ohta, M. and Moriyama, Y., Supratrigeminal neurons mediate the shortest, disynaptic pathway from the central amygdaloid nucleus to the contralateral trigeminal motoneurons in the rat, Comp. Biochem. Physiol., 83A (1986) 633-641. 27 Passatore, M., Lucchi, M.L., Filippi, G.M., Manni, E. and Bortolami, R., Localization of neurons innervating masticatory muscle spindle and periodontal receptors in the mesencephalic trigeminal nucleus and their reflex actions, Arch. ltal. Biol., 121 (1983) 117-130. 28 Rokx, J.T.M., Van Willigen, J.D. and Juch, P.J.W., Bilateral bralnstem connections of the rat supratrigeminai region, Acta Anat., 127 (1986) 16-21. 29 Shigenaga, Y., Mitsuhiro, Y., Yoshida, A., Cao, C.Q. and Tsuru, K., Morphology of single mesencephalic trigeminal neurons innervating masseter muscle of the cat, Brain Research, 445 (1988) 392-399. 30 Shigenaga, Y., Yoshida, A., Mitsuhiro, Y., Tsurn, K. and Doe, K., Morphological and functional properties of trigeminal nudeus oralis neurons projecting to the trigeminal motor nucleus of the cat, Brain Research, 461 (1988) 143-149. 31 Shigenaga, Y., Sera, M., Nishimori, T., Suemune, S., Nishimura, M., Yoshida, A. and Tsuru, K., The central projection of masticatory afferent fibers to the trigeminal sensory nuclear complex and upper cervical spinal cord, J. Comp. Neurol., 268 (1988) 489-507. 32 Szent~tgothai, J., Anatomical considerations of monosynaptic reflex arcs, J. Neurophysiol., 11 (1948) 445-454. 33 Takeuchi, Y., Satoda, T. and Matsushima, R., Amygdaloid projections to commissural interneurons for masticatory motoneutons, Brain Res. Bull., 21 (1988) 123-127.