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Location of the motoneurons innervating the transverse mandibular muscle in the guinea pig
Neuroscience Letters, 116 (1990) 6469 Elsevier Scientific Publishers Ireland Ltd.
64
NSL 07040
Location of the motoneurons innervating the transverse mandibular muscle in the guinea pig Luis A.G. Segade Department of Morphological Sciences, Faculty of Medicine, University of Santiago de Compostela, Santiago de Compostela (Spain) (Received 21 February 1990; Revised version received 9 April 1990; Accepted 11 April 1990)
Key words.- Transverse mandibular muscle; Trigeminal motor nucleus; Mylohyoid nerve; Guinea pig Motoneurons supplying the transverse mandibular muscle (TMM) in the guinea pig have been traced by injecting wheat germ agglutinin-borseradish peroxidase (WGA-HRP) in the TMM, and after applying HRP to the mylohyoid nerve. The TMM is bilaterally innervated by 22-36 motoneurons in each trigeminal motor nucleus, forming a column located ventrolaterally along the entire length of the superficial masseter motoneuron group. The axons are incorporated to the mylohyoid nerve. The location, the axon pathways in the brainstem and the pattern of the dentritic tree suggest that in the guinea pig the TMM motoneurons are involved in the masticatory movements in coordination with other jaw-closing muscles.
Topographic organization of masticatory motoneurons has been examined by retrograde transport of horseradish peroxidase (HRP) in the cat [1, 3, 8, 9], rat [2, 5, 6, 8, 9, 13, 16], rabbit [7], makaque monkey [10] and guinea pig [8, 12, 19, 20]. The pattern of myotopical arrangement in the trigeminal motor nucleus (TMN) has also been confirmed in the rat by fluorescent tracers [13, 14] as well as by electrophysiological methods [2, 16]. According to these investigations, the motoneurons innervating the jaw-closing muscles are located in the dorsolateral division (Vdl) of the TMN, the jaw-opening motoneurons supplying the mylohyoid (MYM) and the anterior belly of the digastric (ABDM) muscles are represented in the ventromedial division (Vvm) of the TMN, while the jaw-opening motoneurons of the posterior belly of the digastric are located in the accessory facial nucleus [3, 8]. None of the above studies have investigated the localization of the motoneurons supplying the transverse mandibular muscle (TMM). This small muscle, located caudal to the mandibular symphysis, is present in some mammals, such as ongulates [15], rodents [4, 15, 21] and insectivores [15]. Using HRP, free or conjugated with wheat germ agglutinin (WGA), the present study shows the location of the perikarya and the axon pathways of the TMM motoneurons in the guinea pig. Correspondence." L.A.G. Segade, Department of Morphological Sciences, Faculty of Medicine, E-15705 Santiago de Compostela, Spain. 0304-3940/90/$ 03.50 ~ 1990 Elsevier Scientific Publishers Ireland Ltd.
66
(Serva) dissolved
in distilled
water containing
the ABDM, and in the 2 remaining da1 part of the MYM.
After a postoperative
survival
animals
2% dimethylsulfoxide
was injected in
the same volume was injected
time of 3642
in the cau-
h, in the case of the mylohyoid
nerve,
or 24-36 h, in the other experiments, the animals were deeply anesthetized and perfused through the ascending aorta. Perfusion was begun by washing out the vascular system with a physiological saline solution warmed to 37°C and containing 10,000 UI of heparin
and 0.125% of amyl nitrite,
followed
by 40&600
ml of 2.55~ glutaralde-
hyde in 0.1 M phosphate buffer of pH 7.2, and finally with 300 ml of the same fixative solution containing 5% sucrose. The brainstem was quickly removed EL.n the skull, suitably blocked, postfixed at 24°C for 3-8 h in the last fixative solution, and transferred at the same temperature for l-6 h to 0.1 M phosphate buffer of pH 7.2 containing 5% sucrose. The brainstem was cut in 100 ,um transverse sections in a vibratome, collected in distilled water and processed immediately using a modified tetramethylbenzidine/sodium nitroprusside method [ 171. Every fourth section was counterstained with 0.1% pyronin in acetate buffer of pH 3.3. The slides were examined by brightfield and darkfield microscopy. In the guinea pigs in which l-2 ,~l of HRP was injected in the TMM, a small number of heavily labeled neurons were observed bilaterally in the Vdl, forming a column which extended ventrolaterally along the entire length of the superficial masseter neuronai cluster (Figs. la-c and 2a-b) 2. In one of these guinea pigs weakly labeled neurons were bilaterally detected in the Vvm, this might be due to diffusion of the tracer from the TMM to the mylohyoid muscles during the survival period. In the guinea pig in which the mylohyoid nerve was sectioned at the point indicated by Fig. 1D (double arrowhead), neurons were labeled in the contralateral TMN but none in the ipsilateral. When the injected volume was reduced to 0.24.3 pm, HRPbearing neurons were only found in the ipsilateral TMN. The number of labeled neurons in each animal vary between 22 and 36 in each TMN. After HRP application to the severed mylohyoid nerve, labeled cells were observed in the ipsilateral Vvm (Fig. 2c-d). 24-35 HRP-bearing cells were also detected lateroventral to the ipsilaterai superficial masseter motoneuronal group. The position of this small group of neurons is similar to that found when the tracer was administered in the TMM. After injecting HRP in the MYM and ABDM, labeled neurons were only detected in the Vvm. This confirms that the labeled neurons which were found lateroventral to the superficial masseter neurons only innervate the TMM, and that their axons are incorporated to the mylohyoid nerve. In our animals the TMM motoneuron axons are directly incorporated to the trigeminal motor roots in the brainstem (Fig le, 2a7ndash;b) but unlike the ABDM and MYM motoneuron axons they do not form a genu, such as has been described in the cat [ 1 I] and rat [5]. A group of dendrites branch out among the trigeminal motor roots towards the intertrigeminal region, whilst the others fan out towards the region of medial pterygoid motoneuron cluster and the ventral surrounding area (Figs. Id and 2b). As far as we are aware the location
of TMM motoneurons
has as yet to be reported
67
tl
j" t /
/
/ 7
!
,,d
'
t¢"
/
Fig. 2. a,b: labeled motoneurons and axons (arrowed) after injecting W G A - H R P in the T M M . The neuron shown in b is represented in Fig. ld. c: labeled axons and motoneurons in the VM after H R P application to the mylohyoid nerve (the arrow shows 2 labeled T M M motoneurons), d: T M M motoneurons labeled at the middle level of the T M N after H R P application to the mylohyoid nerve (the arrows shows 4 labeled neurons in the rostral VM division o f the TMN). For abbreviations see Fig. I. Bars = 100/am in a and b, and 200/am in c and d.
68 in m a m m a l s . In Fig. 9 o f the w o r k o f J a c q u i n et al. [5] a few labeled n e u r o n s are represented in the ventral a r e a o f the Dvl after H R P a p p l i c a t i o n to the m y l o h y o i d nerve o f the rat. H o w e v e r , these n e u r o n s are n o t i n t e r p r e t e d as the nerve cells innervating the T M M o f the rat. F u r t h e r investigations m a y be necessary to clarify this possibility. The l o c a t i o n o f T M M m o t o n e u r o n s a n d the central a n d p e r i p h e r a l p a t h w a y s o f their a x o n s d e m o n s t r a t e that t h o u g h in the guinea pig the T M M is i n n e r v a t e d by m o t o r fibers r u n n i n g in the p o s t e r i o r t r u n k o f the trigeminal m a n d i b u l a r division, the a b o v e muscle is t o t a l l y i n d e p e n d e n t o f the A B D M a n d M Y M . As in the superficial masseter m o t o n e u r o n s o f the cat [18], in the guinea pig s o m e dendrites o f T M M m o t o n e u r o n s b r a n c h o u t t o w a r d s the intertrigeminal region a m o n g the trigeminal m o t o r roots. H o w e v e r , the o t h e r T M M d e n d r i t e s fan o u t t o w a r d s the ventral reticular region s u r r o u n d i n g the T M N , whereas the dendritic tree o f the superficial masseter m o t o n e u r o n s in the cat is practically limited to the T M N [18, 23]. T h e t o p o g r a phy a n d dendritic tree o f the T M M m o t o n e u r o n s suggest i n v o l v e m e n t o f the T M M in m a s t i c a t o r y m o v e m e n t s in c o o r d i n a t i o n with o t h e r j a w - c l o s i n g muscles. As Weijs a n d D a n t u m a have p o i n t e d out in the rat [22], the T M M is p r o b a b l y responsible o f a small transverse force in m a n d i b u l a r symphysis. F u t u r e e l e c t r o p h y s i o l o g i c a l studies in the guinea pig are required to u n d e r s t a n d the role o f the T M M a n d its m o t o n e u rons in the m a s t i c a t o r y process. The T M M is a sole muscle. Therefore, the bilateral i n n e r v a t i o n o b s e r v e d in o u r m a t e r i a l was to be expected. The exclusively ipsilateral i n n e r v a t i o n f o u n d in the guinea pigs, in which the v o l u m e o f the tracer was r e d u c e d to 0 . 2 - 0 . 3 / t l , m a y indicate that the m o t o r a x o n s o f b o t h sides d o n o t cross the midline. This w o r k was s u p p o r t e d by G r a n t s PB85-217 f r o m C A I C Y T ( M i n i s t r y o f E d u c a tion a n d Science o f Spain) a n d 6090275610 f r o m X u n t a de G a l i c i a (Spain). 1 Batini, C., Buisseret-Delmas, C. and Corvisier, J., Horseradish peroxidase localization of masticatory muscle motoneurons in cat, J. Physiol. (Paris), 72 (1976) 301-309. 2 De Santis, M., Limwongse, V. and Rigamonti, D., Somatotopy in the trigeminal motor nucleus of the rat: field potentials recorded in the neuron pool after retrograde transport of horseradish peroxidase, Neurosci. Left., 10 (1978) 95 98. 3 Grant, K., Guegan, M. and Horcholle-Bossavit, G., The anatomical relationship of the retractor bulbi and posterior digastric motoneurones to the abducens and facial nuclei in the cat, Arch. ltal. Biol., 119 (1981) 195-207. 4 Greene, E.C., Anatomy of the Rat, Hafner, New York, 1963, 370 pp. 5 Jacquin, M.F., Rhoades, R.W., Enfiejian, H.L. and Egger, M.D., Organization and morphology of masticatory neurons in the rat: a retrograde HRP study, J. Comp. Neurol., 218 (1983) 239 256. 6 Limwongse, V. and De Santis, M., Cell body locations and axonal pathways of neurons innervating muscles of mastication in the rat, Am. J. Anat., 149 (1977) 477-488. 7 Matsuda, K., Uemura, M., Kume, M., Matsushima, R. and Mizuno, N., Topographical representation of masticatory muscles in the motor trigeminal nucleus in the rabbit: a HRP study, Neurosci. Lett., 8 (1978) 1-4. 8 Matsuda, K., Uemura, M., Takeuchi, Y., Kume, M., Matsushima, R. and Mizuno, N., Localization of motoneurons innervating the posterior belly of the digastric muscle: a comparative anatomical study by the HRP method, Neurosci. Lett., 12 (1979) 4752.
69 9 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) 105116. 10 Mizuno, N., Matsuda, K., Iwahori, N., Uemura-Sumi, M., Kume, M. and Matsushima, K., Representation of the masticatory muscles in the motor trigeminal nucleus of the macaque monkey, Neurosci. Lett., 21 (1981) 19 22. 11 Nomura, S. and Mizuno, N., Axonal trajectories of masticatory motoneurons: a genu formation of axons of jaw-opening motoneurons in the cat, Neurosci. Lett., 37 (1983) 11 15. 12 Nozaki, S., Iriki, A. and Nakamura, Y., Trigeminal mesencephalic neurons innervating functionally identified muscle spindles and involved in the monosynaptic stretch reflex of the lateral pterygoid muscle of the guinea pig, J. Comp. Neurol., 236 (1985) 106-120. 13 Rokx, J.T.M., Jiich, P.J.W. and Van Willigen, J.D., On the bilateral innervation of masticatory muscles: a study with retrograde tracers, J. Anat., 140 (1985) 237-243. 14 Rokx, J.T.M. and Van Willigen, J.D., Arrangement of supramandibular and suprahyoid motoneurons in the rat: a fluorescent tracer study, Acta Anat., 125 (1985) 158 162. 15 Saban, R., Musculature de la t6te. In P.P. Grass6 (Ed.), Trait6 de Zoologie, Anatomie, Systefiaatique, Biologie, Masson, Paris, Tome XVI, Fascicule II, 1968, pp. 229472. 16 Sasamoto, K., Motor nuclear representation of masticatory muscles in the rat, Jpn. J. Physiol., 29 (1979) 739-747. 17 Segade, L.A.G., Sufi.rez-Quintanilla, J. and Cobos, M.A.R., Contralateral projections of trigeminal mandibular primary afferents in the guinea pig as seen by transganglionic transport of horseradish peroxidase, Brain Res., 506 (1990) 267 280. 18 Shigenaga, Y., Yoshida, A., Tsuru, K., Mitsuhiro, Y., Otani, K. and Cao, C.Q., Physiological and morphological characteristics of cat masticatory motoneurons-intracellular injection of HRP, Brain Res., 461 (1988) 238-256. 19 Tal, M., Representation of some masticatory muscles in the trigeminal motor nucleus of the guinea pig, Exp. Neurol., 70 (1980) 726-730. 20 Uemura-Sumi, M., Takahashi, O., Matsushima, R., Takata, M., Yasui, Y. and Mizuno, N., Localization of masticatory motoneurons in the trigeminal motor nucleus of the guinea pig, Neurosci. Lett., 29 (1982) 219-224. 21 Weijs, W.A., Morphology of the muscles of mastication in the albino rat, Rattus norvegicus (Berkenhout, 1769), Acta Morphol. Neerl. Scand., 11 (1973) 321-340. 22 Weijs, W.A. and Dantuma, R., Electromyography and mechanics of mastication in the albino rat, J. Morphol., 146 (1975) 1 33. 23 Yoshida, A. Tsuru, K., Mitsuhiro, Y., Otani, K. and Shigenaga, Y., Morphology of masticatory motoneurons stained intracellularly with horseradish peroxidase, Brain Res., 416 (1987) 393~,01.
64
NSL 07040
Location of the motoneurons innervating the transverse mandibular muscle in the guinea pig Luis A.G. Segade Department of Morphological Sciences, Faculty of Medicine, University of Santiago de Compostela, Santiago de Compostela (Spain) (Received 21 February 1990; Revised version received 9 April 1990; Accepted 11 April 1990)
Key words.- Transverse mandibular muscle; Trigeminal motor nucleus; Mylohyoid nerve; Guinea pig Motoneurons supplying the transverse mandibular muscle (TMM) in the guinea pig have been traced by injecting wheat germ agglutinin-borseradish peroxidase (WGA-HRP) in the TMM, and after applying HRP to the mylohyoid nerve. The TMM is bilaterally innervated by 22-36 motoneurons in each trigeminal motor nucleus, forming a column located ventrolaterally along the entire length of the superficial masseter motoneuron group. The axons are incorporated to the mylohyoid nerve. The location, the axon pathways in the brainstem and the pattern of the dentritic tree suggest that in the guinea pig the TMM motoneurons are involved in the masticatory movements in coordination with other jaw-closing muscles.
Topographic organization of masticatory motoneurons has been examined by retrograde transport of horseradish peroxidase (HRP) in the cat [1, 3, 8, 9], rat [2, 5, 6, 8, 9, 13, 16], rabbit [7], makaque monkey [10] and guinea pig [8, 12, 19, 20]. The pattern of myotopical arrangement in the trigeminal motor nucleus (TMN) has also been confirmed in the rat by fluorescent tracers [13, 14] as well as by electrophysiological methods [2, 16]. According to these investigations, the motoneurons innervating the jaw-closing muscles are located in the dorsolateral division (Vdl) of the TMN, the jaw-opening motoneurons supplying the mylohyoid (MYM) and the anterior belly of the digastric (ABDM) muscles are represented in the ventromedial division (Vvm) of the TMN, while the jaw-opening motoneurons of the posterior belly of the digastric are located in the accessory facial nucleus [3, 8]. None of the above studies have investigated the localization of the motoneurons supplying the transverse mandibular muscle (TMM). This small muscle, located caudal to the mandibular symphysis, is present in some mammals, such as ongulates [15], rodents [4, 15, 21] and insectivores [15]. Using HRP, free or conjugated with wheat germ agglutinin (WGA), the present study shows the location of the perikarya and the axon pathways of the TMM motoneurons in the guinea pig. Correspondence." L.A.G. Segade, Department of Morphological Sciences, Faculty of Medicine, E-15705 Santiago de Compostela, Spain. 0304-3940/90/$ 03.50 ~ 1990 Elsevier Scientific Publishers Ireland Ltd.
66
(Serva) dissolved
in distilled
water containing
the ABDM, and in the 2 remaining da1 part of the MYM.
After a postoperative
survival
animals
2% dimethylsulfoxide
was injected in
the same volume was injected
time of 3642
in the cau-
h, in the case of the mylohyoid
nerve,
or 24-36 h, in the other experiments, the animals were deeply anesthetized and perfused through the ascending aorta. Perfusion was begun by washing out the vascular system with a physiological saline solution warmed to 37°C and containing 10,000 UI of heparin
and 0.125% of amyl nitrite,
followed
by 40&600
ml of 2.55~ glutaralde-
hyde in 0.1 M phosphate buffer of pH 7.2, and finally with 300 ml of the same fixative solution containing 5% sucrose. The brainstem was quickly removed EL.n the skull, suitably blocked, postfixed at 24°C for 3-8 h in the last fixative solution, and transferred at the same temperature for l-6 h to 0.1 M phosphate buffer of pH 7.2 containing 5% sucrose. The brainstem was cut in 100 ,um transverse sections in a vibratome, collected in distilled water and processed immediately using a modified tetramethylbenzidine/sodium nitroprusside method [ 171. Every fourth section was counterstained with 0.1% pyronin in acetate buffer of pH 3.3. The slides were examined by brightfield and darkfield microscopy. In the guinea pigs in which l-2 ,~l of HRP was injected in the TMM, a small number of heavily labeled neurons were observed bilaterally in the Vdl, forming a column which extended ventrolaterally along the entire length of the superficial masseter neuronai cluster (Figs. la-c and 2a-b) 2. In one of these guinea pigs weakly labeled neurons were bilaterally detected in the Vvm, this might be due to diffusion of the tracer from the TMM to the mylohyoid muscles during the survival period. In the guinea pig in which the mylohyoid nerve was sectioned at the point indicated by Fig. 1D (double arrowhead), neurons were labeled in the contralateral TMN but none in the ipsilateral. When the injected volume was reduced to 0.24.3 pm, HRPbearing neurons were only found in the ipsilateral TMN. The number of labeled neurons in each animal vary between 22 and 36 in each TMN. After HRP application to the severed mylohyoid nerve, labeled cells were observed in the ipsilateral Vvm (Fig. 2c-d). 24-35 HRP-bearing cells were also detected lateroventral to the ipsilaterai superficial masseter motoneuronal group. The position of this small group of neurons is similar to that found when the tracer was administered in the TMM. After injecting HRP in the MYM and ABDM, labeled neurons were only detected in the Vvm. This confirms that the labeled neurons which were found lateroventral to the superficial masseter neurons only innervate the TMM, and that their axons are incorporated to the mylohyoid nerve. In our animals the TMM motoneuron axons are directly incorporated to the trigeminal motor roots in the brainstem (Fig le, 2a7ndash;b) but unlike the ABDM and MYM motoneuron axons they do not form a genu, such as has been described in the cat [ 1 I] and rat [5]. A group of dendrites branch out among the trigeminal motor roots towards the intertrigeminal region, whilst the others fan out towards the region of medial pterygoid motoneuron cluster and the ventral surrounding area (Figs. Id and 2b). As far as we are aware the location
of TMM motoneurons
has as yet to be reported
67
tl
j" t /
/
/ 7
!
,,d
'
t¢"
/
Fig. 2. a,b: labeled motoneurons and axons (arrowed) after injecting W G A - H R P in the T M M . The neuron shown in b is represented in Fig. ld. c: labeled axons and motoneurons in the VM after H R P application to the mylohyoid nerve (the arrow shows 2 labeled T M M motoneurons), d: T M M motoneurons labeled at the middle level of the T M N after H R P application to the mylohyoid nerve (the arrows shows 4 labeled neurons in the rostral VM division o f the TMN). For abbreviations see Fig. I. Bars = 100/am in a and b, and 200/am in c and d.
68 in m a m m a l s . In Fig. 9 o f the w o r k o f J a c q u i n et al. [5] a few labeled n e u r o n s are represented in the ventral a r e a o f the Dvl after H R P a p p l i c a t i o n to the m y l o h y o i d nerve o f the rat. H o w e v e r , these n e u r o n s are n o t i n t e r p r e t e d as the nerve cells innervating the T M M o f the rat. F u r t h e r investigations m a y be necessary to clarify this possibility. The l o c a t i o n o f T M M m o t o n e u r o n s a n d the central a n d p e r i p h e r a l p a t h w a y s o f their a x o n s d e m o n s t r a t e that t h o u g h in the guinea pig the T M M is i n n e r v a t e d by m o t o r fibers r u n n i n g in the p o s t e r i o r t r u n k o f the trigeminal m a n d i b u l a r division, the a b o v e muscle is t o t a l l y i n d e p e n d e n t o f the A B D M a n d M Y M . As in the superficial masseter m o t o n e u r o n s o f the cat [18], in the guinea pig s o m e dendrites o f T M M m o t o n e u r o n s b r a n c h o u t t o w a r d s the intertrigeminal region a m o n g the trigeminal m o t o r roots. H o w e v e r , the o t h e r T M M d e n d r i t e s fan o u t t o w a r d s the ventral reticular region s u r r o u n d i n g the T M N , whereas the dendritic tree o f the superficial masseter m o t o n e u r o n s in the cat is practically limited to the T M N [18, 23]. T h e t o p o g r a phy a n d dendritic tree o f the T M M m o t o n e u r o n s suggest i n v o l v e m e n t o f the T M M in m a s t i c a t o r y m o v e m e n t s in c o o r d i n a t i o n with o t h e r j a w - c l o s i n g muscles. As Weijs a n d D a n t u m a have p o i n t e d out in the rat [22], the T M M is p r o b a b l y responsible o f a small transverse force in m a n d i b u l a r symphysis. F u t u r e e l e c t r o p h y s i o l o g i c a l studies in the guinea pig are required to u n d e r s t a n d the role o f the T M M a n d its m o t o n e u rons in the m a s t i c a t o r y process. The T M M is a sole muscle. Therefore, the bilateral i n n e r v a t i o n o b s e r v e d in o u r m a t e r i a l was to be expected. The exclusively ipsilateral i n n e r v a t i o n f o u n d in the guinea pigs, in which the v o l u m e o f the tracer was r e d u c e d to 0 . 2 - 0 . 3 / t l , m a y indicate that the m o t o r a x o n s o f b o t h sides d o n o t cross the midline. This w o r k was s u p p o r t e d by G r a n t s PB85-217 f r o m C A I C Y T ( M i n i s t r y o f E d u c a tion a n d Science o f Spain) a n d 6090275610 f r o m X u n t a de G a l i c i a (Spain). 1 Batini, C., Buisseret-Delmas, C. and Corvisier, J., Horseradish peroxidase localization of masticatory muscle motoneurons in cat, J. Physiol. (Paris), 72 (1976) 301-309. 2 De Santis, M., Limwongse, V. and Rigamonti, D., Somatotopy in the trigeminal motor nucleus of the rat: field potentials recorded in the neuron pool after retrograde transport of horseradish peroxidase, Neurosci. Left., 10 (1978) 95 98. 3 Grant, K., Guegan, M. and Horcholle-Bossavit, G., The anatomical relationship of the retractor bulbi and posterior digastric motoneurones to the abducens and facial nuclei in the cat, Arch. ltal. Biol., 119 (1981) 195-207. 4 Greene, E.C., Anatomy of the Rat, Hafner, New York, 1963, 370 pp. 5 Jacquin, M.F., Rhoades, R.W., Enfiejian, H.L. and Egger, M.D., Organization and morphology of masticatory neurons in the rat: a retrograde HRP study, J. Comp. Neurol., 218 (1983) 239 256. 6 Limwongse, V. and De Santis, M., Cell body locations and axonal pathways of neurons innervating muscles of mastication in the rat, Am. J. Anat., 149 (1977) 477-488. 7 Matsuda, K., Uemura, M., Kume, M., Matsushima, R. and Mizuno, N., Topographical representation of masticatory muscles in the motor trigeminal nucleus in the rabbit: a HRP study, Neurosci. Lett., 8 (1978) 1-4. 8 Matsuda, K., Uemura, M., Takeuchi, Y., Kume, M., Matsushima, R. and Mizuno, N., Localization of motoneurons innervating the posterior belly of the digastric muscle: a comparative anatomical study by the HRP method, Neurosci. Lett., 12 (1979) 4752.
69 9 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) 105116. 10 Mizuno, N., Matsuda, K., Iwahori, N., Uemura-Sumi, M., Kume, M. and Matsushima, K., Representation of the masticatory muscles in the motor trigeminal nucleus of the macaque monkey, Neurosci. Lett., 21 (1981) 19 22. 11 Nomura, S. and Mizuno, N., Axonal trajectories of masticatory motoneurons: a genu formation of axons of jaw-opening motoneurons in the cat, Neurosci. Lett., 37 (1983) 11 15. 12 Nozaki, S., Iriki, A. and Nakamura, Y., Trigeminal mesencephalic neurons innervating functionally identified muscle spindles and involved in the monosynaptic stretch reflex of the lateral pterygoid muscle of the guinea pig, J. Comp. Neurol., 236 (1985) 106-120. 13 Rokx, J.T.M., Jiich, P.J.W. and Van Willigen, J.D., On the bilateral innervation of masticatory muscles: a study with retrograde tracers, J. Anat., 140 (1985) 237-243. 14 Rokx, J.T.M. and Van Willigen, J.D., Arrangement of supramandibular and suprahyoid motoneurons in the rat: a fluorescent tracer study, Acta Anat., 125 (1985) 158 162. 15 Saban, R., Musculature de la t6te. In P.P. Grass6 (Ed.), Trait6 de Zoologie, Anatomie, Systefiaatique, Biologie, Masson, Paris, Tome XVI, Fascicule II, 1968, pp. 229472. 16 Sasamoto, K., Motor nuclear representation of masticatory muscles in the rat, Jpn. J. Physiol., 29 (1979) 739-747. 17 Segade, L.A.G., Sufi.rez-Quintanilla, J. and Cobos, M.A.R., Contralateral projections of trigeminal mandibular primary afferents in the guinea pig as seen by transganglionic transport of horseradish peroxidase, Brain Res., 506 (1990) 267 280. 18 Shigenaga, Y., Yoshida, A., Tsuru, K., Mitsuhiro, Y., Otani, K. and Cao, C.Q., Physiological and morphological characteristics of cat masticatory motoneurons-intracellular injection of HRP, Brain Res., 461 (1988) 238-256. 19 Tal, M., Representation of some masticatory muscles in the trigeminal motor nucleus of the guinea pig, Exp. Neurol., 70 (1980) 726-730. 20 Uemura-Sumi, M., Takahashi, O., Matsushima, R., Takata, M., Yasui, Y. and Mizuno, N., Localization of masticatory motoneurons in the trigeminal motor nucleus of the guinea pig, Neurosci. Lett., 29 (1982) 219-224. 21 Weijs, W.A., Morphology of the muscles of mastication in the albino rat, Rattus norvegicus (Berkenhout, 1769), Acta Morphol. Neerl. Scand., 11 (1973) 321-340. 22 Weijs, W.A. and Dantuma, R., Electromyography and mechanics of mastication in the albino rat, J. Morphol., 146 (1975) 1 33. 23 Yoshida, A. Tsuru, K., Mitsuhiro, Y., Otani, K. and Shigenaga, Y., Morphology of masticatory motoneurons stained intracellularly with horseradish peroxidase, Brain Res., 416 (1987) 393~,01.