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Horseradish peroxidase study of the localization of motoneurons in the accessory nucleus (XI) of the Japanese toad
Neuroscience Letters, 79 (1987) 241 245
241
Elsevier Scientific Publishers Ireland Ltd. NSL 04786
Horseradish peroxidase study of the localization of motoneurons in the accessory nucleus (XI) of the Japanese toad K o h t a r o Takei, Yoshitaka Oka, Masahiko Satou and K a z u o U e d a Zoological Institute, Faculty of Science, University of Tokyo, Tokyo (Japan) (Received 25 April 1987; Revised version received 9 May 1987; Accepted 11 May 1987)
Key words: Localization; Motoneuron; Accessory nucleus; Horseradish peroxidase; Intramuscular injection; Toad It has been conventionally accepted that the anuran accessory nerve (nXI) only innervates the musculus cucullaris (C). However, we have found that the nXI of the Japanese toad innervated the musculus interscapularis (IS) in addition to the C. The motoneurons innervating these muscles were labelled by the intramuscular injection of horseradish peroxidase (HRP). In the accessory nucleus (XI), the motoneurons were somatotopically organized: the C motoneurons were localized more rostrally, while the IS motoneurons more caudally.
In anuran amphibians, several experimental studies have recently confirmed the range of the accessory nucleus (XI) and the morphology of the accessory motoneurons [6, 10, 11, 13, 14]. Although the distribution and localization of the motoneurons innervating the individual muscles in some cranial motor nuclei have thus far been studied [1, 3, 9, 15-18], those within the XI have not. According to Gaupp [4], and Nieuwenhuys and Opdam [8], the accessory nerve (nXI) of the frog is a small branch of the vagus nerve (nX), innervating the musculus cucullaris (C). We have confirmed the innervation to the C through the gross dissection of the Japanese toad. But in addition, we have found that the nXI also innervated the musculus interscapularis (IS). Therefore, we labelled the motoneurons innervating each muscle (the C and the IS), using intramuscular injection of horseradish peroxidase (HRP), after which we examined their distribution. The experimental procedures followed in this experiment were basically similar to those reported previously [15, 16]. Thirteen adult Japanese toads (Bufo japonicus) were anesthetized by injecting 0.25 ml of 10% tricaine methanesulfonate (MS-222) solution per 100 g body weight. Two to 4/ll of 5-10% HRP (Toyobo, Grade I-C) Correspondence: K. Takei, Zoological Institute, Faculty of Science, University of Tokyo, 7-3-I Hongo, Bunkyo-ku, Tokyo 113, Japan. 0304-3940/87/$ 03.50 © 1987 Elsevier Scientific Publishers Ireland Ltd.
242
solution dissolved in saline was injected into the C and the IS using a microliter syringe. Whenever a leakage of H R P was detected from the injected muscle during the surgery, the animal was discarded. After a postoperative survival period of 2 5 days, the animals were deeply reanesthetized, then fixed by transcardial perfusion. The brain, the injected muscles and the muscles adjacent to them were removed. The brains were cut serially into 80-/~m-thick horizontal sections. The sections of the brain were reacted using a slightly modified tetramethylbenzidine (TMB) method [7], and the sections of muscles were reacted using a one-step diaminobenzidine (DAB) method to determine the extent of diffusion of the injected H R P among the neighboring muscles. All the injections used for the data analysis were free of tracer diffusion to the neighboring muscles. Since the labelled motoneurons were found exclusively in the side ipsilateral to the injected muscle, we have simultaneously injected the H R P into each muscle on both sides in most cases. We have already studied the boundary of the XI in the Japanese toad by applying cobaltic lysine to the proximal cut end of the nXI [10, 11]. These results showed that the XI is an elongated cell column that ranges from the slightly caudal level of the obex to the level about 1500~1600/tin posterior to the obex (Fig. IA). Its rostrocaudal extent almost completely overlaps with that of the ventrolateral nucleus (VLN) of the hypoglossal nerve (nXII) and that of the motoneurons supplying the R. thoracicus superior anterior (TSA). Although the XI, the VLN, and the TSA motoneurons are distributed with specific patterns (in the order, from dorsal to ventral, the VLN, the XI, and the TSA motoneurons) and are not intermingled [10, 11], it is hard to distinguish them in the normal histological materials. Therefore, in the illustration (Fig. 1B), the non-labelled motoneurons within the range of the XI inevitably include some VLN motoneurons and TSA motoneurons besides the non-labelled XI motoneurons. Examples of the distribution of labelled motoneurons revealed by the intramuscular injection of H R P which showed the most extensive labelling and the largest number of labelled neurons are shown in Fig. 1B. The C is a slender muscle, and is the only muscle that connects the skull and the shoulder girdle. It originates from the prootic and the squamosal, and inserts on the suprascapula. The IS is a small muscle and hinges the suprascapula and the scapula. It arises from the ventromedial surface of the suprascapula and inserts on the ventral surface of the scapula. The motoneurons innervating the C were distributed ipsilaterally in the rostral part of the XI, covering rostral 2/3 of this nucleus, while the motoneurons innervating the IS were distributed ipsilaterally in the caudal part of the XI, covering caudal 2/3 of this nucleus (Fig. IB). Thus, the motoneurons innervating these two muscles were separately distributed in the rostrocaudal direction within the XI, though there were some overlaps. Furthermore, the distribution pattern of these motoneurons corresponded to the rostrocaudal arrangement of the muscles (somatotopic organization). The rostrocaudal extent of the XI almost completely overlaps with that of the VLN of the nXII (one branch of the second spinal nerve) and the motoneurons supplying the TSA (the remaining branch of the second spinal nerve) in the Japanese toad [10, 11]. In salamanders (family Plethodontidae), it is reported that the spinal accessory
243
o
~
9
~0
)0
nX, nXI
o ~B-'o
"~
C
rq
IS
--
pC
Fig. 1. A: schematic line drawing of the left half of the lower brainstem of the Japanese toad projected on a horizontal plane, showing the boundaries of the motor nuclei. The stippled area indicates the boundary of the accessory nucleus (XI). Arrowhead indicates the level of the obex. B: typical examples of the distribution of labelled motoneurons within the XI revealed by the intramuscular injection of HRP. The left column shows the distribution of the C motoneurons and the right column shows that of the IS motoneurons. Filled circles and open circles indicate HRP-labelled motoneurons and large non-labelled cells within the nucleus respectively. These non-labelled cells include some hypoglossal (VLN) and TSA motoneurons besides the accessory motoneurons (see text). Histograms show the distribution of labelled motoneurons along the rostrocaudal axis of the nucleus (same data shown in the upper diagram). The abscissa indicates the distance (100 gm per division) from the rostral limit of the nucleus. The total length of the abscissa corresponds to the rostrocaudal range of the motor nucleus. This range somewhat varies from toad to toad. The ordinate indicates the number of HRP-labelled motoneurons. Vm, trigeminal motor nucleus; VI, abducens nucleus; VIIm, facial motor nucleus; IXm, glossopharyngeal motor nucleus; Xm, vagus motor nucleus; XI, accessory nucleus; XII (DMN), dorsomedial nucleus of the hypoglossal nerve; nV, trigeminal nerve; nVII, facial nerve; niX, glossopharyngeal nerve; nX, vagus nerve; nXI, accessory nerve; nXII, hypoglossal nerve; r, rostral; c, caudal; m, medial; 1, lateral.
motoneurons
that innervate C and other branchiomeric neck musculature are found
to be intermingled among the motoneurons
o f t h e s e c o n d s p i n a l n e r v e [13, 14]. T h u s ,
the relation of the XI to the spinal motoneuron
column seems to be similar in both
species. A l t h o u g h it h a s b e e n c o n v e n t i o n a l l y d e s c r i b e d t h a t t h e I S is i n n e r v a t e d b y
244
~7
.a,
V
B
O,
g
r ~e
II
A
ZX
c
Fig. 2. Photomicrographs of horizontal sections showing the HRP-labelled motoneurons. Small triangles indicate the midline. Arrows indicate the level of the caudal limit of the XII (DMN) for a reference point (see Fig. IA). Bar= 200/tm; r, rostral; c, caudal. A: HRP-labelled C motoneurons, after the injection of HRP into the left C. In the XI, the labelled motoneurons are localized rostrally. B: HRP-labelled IS motoneurons, after the injection of HRP into the right IS. In the XI, the labelled motoneurons are localized caudally. o n e b r a n c h (R. s c a p u l a r i s ) o f the v a g u s n e r v e [4], it w a s clearly s h o w n f r o m this s t u d y t h a t t h e n X I i n n e r v a t e s n o t o n l y t h e C b u t also the IS. T h e s e t w o m u s c l e s are b e l i e v e d to r e p r e s e n t the t r a p e z i u s c o m p l e x o f o t h e r v e r t e b r a t e s [2, 12]. T h e m o t o n e u r o n s i n n e r v a t i n g the s t e r n o c l e i d o m a s t o i d a n d the t r a p e z i u s m u s c l e s t h a t a r e b e l i e v e d to c o r r e s p o n d to the t r a p e z i u s c o m p l e x , are f o u n d in t h e spinal a c c e s s o r y n u c l e u s a n d the c e r v i c a l spinal s e g m e n t s o f the rat [5]. T h u s , the X I o f a m p h i b i a n s is t h o u g h t to c o r r e s p o n d to the s p i n a l a c c e s s o r y n u c l e u s o f m a m m a l s . T h i s s t u d y was s u p p o r t e d by g r a n t s f r o m t h e M i n i s t r y o f E d u c a t i o n , Science a n d Culture of Japan and from The Mitsubishi Foundation. 1 Barnes, M.D. and Alley, K.E., Maturation and recycling of tfigeminal motoneurons in anuran larvae, J. Comp. Neurol., 218 (1983) 406-414. 2 Black, D., The motor nuclei of the cerebral nerves in phylogeny, a study of the phenomena of neurobiotaxis, lI. Amphibia, J. Comp. Neurol., 28 (1917) 379-427. 3 Fuller, P.M., Afferent and efferent components of the facial nerve in the bullfrog (Rana catesbeiana), J, Morphol., 159 (1979) 245-252. 4 Gaupp, E., Anatomie des Frosches, Vieweg, Braunschweig, 1896-1904. 5 Kitamura, S. and Sakai, A., A study on the localization of the sternocleidomastoid and trapezius motoneurons in the rat by means of the HRP method, Anat. Rec., 202 (1982) 527-536. 6 Matesz, C. and Sz6kely, G., The motor column and sensory projections of the branchial cranial nerves in the frog, J. Comp. Neurol., 178 (1978) 157-176. 7 Mesulam, M.M., Tetramethylbenzidine for horseradish peroxidase neurohistochemistry: A non-carcinogenic blue reaction-product with superior sensitivity for visualizing neural afferents and efferents, J. Histochem, Cytochem., 26 (1978) 106-117.
245 8 Nieuwenhuys, R. and Opdam, P., Structure of the brainstem. In R. Llinfis and W. Precht (Eds.), Frog Neurobiology, Springer, Berlin, 1976, pp. 811-855. 9 0 k a , Y., Localization of motoneurons innervating laryngeal muscles in the Japanese toad, Acta Anat. Nippon, 58 (1983) 271. 10 Oka, Y., Satou, M. and Ueda, K., Morphology and distribution of the motor neurons of the accessory nerve (nXI) in the Japanese toad: a cobaltic lysine study, Brain Res., 400 (1987) 383 388. 11 Oka, Y., Takeuchi, H., Satou, M. and Ueda, K., Cobaltic lysine study of the morphology and distribution of the cranial nerve efferent neurons (motoneurons and preganglionic parasympathetic neurons) and rostral spinal motoneurons in the Japanese toad, J. Comp. Neurol., 259 (1987) 400-423. 12 Romer, A.S., The Vertebrate Body, Saunders, Philadelphia, 1971, Chapter 9, pp. 273-314. 13 Roth, G., Wake, D.B., Wake, M.H. and Retting, G., Distribution of accessory and hypoglossal nerves in the hindbrain and spinal cord of lungless salamanders, family Plethodontidae, Neurosci. Lett., 44 (1984) 53-57. 14 Roth, G. and Wake, D.B., The structure of the brainstem and cervical spinal cord in lungless salamanders (family Plethodontidae) and its relation to feeding, J. Comp. Neurol., 241 (1985) 99-110. 15 Takei, K., Oka, Y., Satou, M. and Ueda, K., Localization of motoneurons involved in the prey-catching behavior in the Japanese toad, Zool. Sci., 1 (1984) 881. 16 Takei, K., Oka, Y., Satou, M. and Ueda, K., Distribution of motoneurons involved in the prey-catching behavior in the Japanese toad, Bufojaponicus, Brain Res., 410 (1987) 395-400. 17 Weerasuriya, A. and Ewert, J.-P., Prey-selective neurons in the toad's optic tectum and sensorimotor interfacing: HRP studies and recording experiments, J. Comp. Physiol., 144 (1981) 429-434. 18 Weerasuriya, A., Snapping in toads: some aspects of sensorimotor interfacing and motor pattern generation. In J.-P. Ewert, R.R. Capranica and D.J. Ingle (Eds.), Advances in Vertebrate Neuroethology, Plenum, New York, 1983, pp. 613~527.
241
Elsevier Scientific Publishers Ireland Ltd. NSL 04786
Horseradish peroxidase study of the localization of motoneurons in the accessory nucleus (XI) of the Japanese toad K o h t a r o Takei, Yoshitaka Oka, Masahiko Satou and K a z u o U e d a Zoological Institute, Faculty of Science, University of Tokyo, Tokyo (Japan) (Received 25 April 1987; Revised version received 9 May 1987; Accepted 11 May 1987)
Key words: Localization; Motoneuron; Accessory nucleus; Horseradish peroxidase; Intramuscular injection; Toad It has been conventionally accepted that the anuran accessory nerve (nXI) only innervates the musculus cucullaris (C). However, we have found that the nXI of the Japanese toad innervated the musculus interscapularis (IS) in addition to the C. The motoneurons innervating these muscles were labelled by the intramuscular injection of horseradish peroxidase (HRP). In the accessory nucleus (XI), the motoneurons were somatotopically organized: the C motoneurons were localized more rostrally, while the IS motoneurons more caudally.
In anuran amphibians, several experimental studies have recently confirmed the range of the accessory nucleus (XI) and the morphology of the accessory motoneurons [6, 10, 11, 13, 14]. Although the distribution and localization of the motoneurons innervating the individual muscles in some cranial motor nuclei have thus far been studied [1, 3, 9, 15-18], those within the XI have not. According to Gaupp [4], and Nieuwenhuys and Opdam [8], the accessory nerve (nXI) of the frog is a small branch of the vagus nerve (nX), innervating the musculus cucullaris (C). We have confirmed the innervation to the C through the gross dissection of the Japanese toad. But in addition, we have found that the nXI also innervated the musculus interscapularis (IS). Therefore, we labelled the motoneurons innervating each muscle (the C and the IS), using intramuscular injection of horseradish peroxidase (HRP), after which we examined their distribution. The experimental procedures followed in this experiment were basically similar to those reported previously [15, 16]. Thirteen adult Japanese toads (Bufo japonicus) were anesthetized by injecting 0.25 ml of 10% tricaine methanesulfonate (MS-222) solution per 100 g body weight. Two to 4/ll of 5-10% HRP (Toyobo, Grade I-C) Correspondence: K. Takei, Zoological Institute, Faculty of Science, University of Tokyo, 7-3-I Hongo, Bunkyo-ku, Tokyo 113, Japan. 0304-3940/87/$ 03.50 © 1987 Elsevier Scientific Publishers Ireland Ltd.
242
solution dissolved in saline was injected into the C and the IS using a microliter syringe. Whenever a leakage of H R P was detected from the injected muscle during the surgery, the animal was discarded. After a postoperative survival period of 2 5 days, the animals were deeply reanesthetized, then fixed by transcardial perfusion. The brain, the injected muscles and the muscles adjacent to them were removed. The brains were cut serially into 80-/~m-thick horizontal sections. The sections of the brain were reacted using a slightly modified tetramethylbenzidine (TMB) method [7], and the sections of muscles were reacted using a one-step diaminobenzidine (DAB) method to determine the extent of diffusion of the injected H R P among the neighboring muscles. All the injections used for the data analysis were free of tracer diffusion to the neighboring muscles. Since the labelled motoneurons were found exclusively in the side ipsilateral to the injected muscle, we have simultaneously injected the H R P into each muscle on both sides in most cases. We have already studied the boundary of the XI in the Japanese toad by applying cobaltic lysine to the proximal cut end of the nXI [10, 11]. These results showed that the XI is an elongated cell column that ranges from the slightly caudal level of the obex to the level about 1500~1600/tin posterior to the obex (Fig. IA). Its rostrocaudal extent almost completely overlaps with that of the ventrolateral nucleus (VLN) of the hypoglossal nerve (nXII) and that of the motoneurons supplying the R. thoracicus superior anterior (TSA). Although the XI, the VLN, and the TSA motoneurons are distributed with specific patterns (in the order, from dorsal to ventral, the VLN, the XI, and the TSA motoneurons) and are not intermingled [10, 11], it is hard to distinguish them in the normal histological materials. Therefore, in the illustration (Fig. 1B), the non-labelled motoneurons within the range of the XI inevitably include some VLN motoneurons and TSA motoneurons besides the non-labelled XI motoneurons. Examples of the distribution of labelled motoneurons revealed by the intramuscular injection of H R P which showed the most extensive labelling and the largest number of labelled neurons are shown in Fig. 1B. The C is a slender muscle, and is the only muscle that connects the skull and the shoulder girdle. It originates from the prootic and the squamosal, and inserts on the suprascapula. The IS is a small muscle and hinges the suprascapula and the scapula. It arises from the ventromedial surface of the suprascapula and inserts on the ventral surface of the scapula. The motoneurons innervating the C were distributed ipsilaterally in the rostral part of the XI, covering rostral 2/3 of this nucleus, while the motoneurons innervating the IS were distributed ipsilaterally in the caudal part of the XI, covering caudal 2/3 of this nucleus (Fig. IB). Thus, the motoneurons innervating these two muscles were separately distributed in the rostrocaudal direction within the XI, though there were some overlaps. Furthermore, the distribution pattern of these motoneurons corresponded to the rostrocaudal arrangement of the muscles (somatotopic organization). The rostrocaudal extent of the XI almost completely overlaps with that of the VLN of the nXII (one branch of the second spinal nerve) and the motoneurons supplying the TSA (the remaining branch of the second spinal nerve) in the Japanese toad [10, 11]. In salamanders (family Plethodontidae), it is reported that the spinal accessory
243
o
~
9
~0
)0
nX, nXI
o ~B-'o
"~
C
rq
IS
--
pC
Fig. 1. A: schematic line drawing of the left half of the lower brainstem of the Japanese toad projected on a horizontal plane, showing the boundaries of the motor nuclei. The stippled area indicates the boundary of the accessory nucleus (XI). Arrowhead indicates the level of the obex. B: typical examples of the distribution of labelled motoneurons within the XI revealed by the intramuscular injection of HRP. The left column shows the distribution of the C motoneurons and the right column shows that of the IS motoneurons. Filled circles and open circles indicate HRP-labelled motoneurons and large non-labelled cells within the nucleus respectively. These non-labelled cells include some hypoglossal (VLN) and TSA motoneurons besides the accessory motoneurons (see text). Histograms show the distribution of labelled motoneurons along the rostrocaudal axis of the nucleus (same data shown in the upper diagram). The abscissa indicates the distance (100 gm per division) from the rostral limit of the nucleus. The total length of the abscissa corresponds to the rostrocaudal range of the motor nucleus. This range somewhat varies from toad to toad. The ordinate indicates the number of HRP-labelled motoneurons. Vm, trigeminal motor nucleus; VI, abducens nucleus; VIIm, facial motor nucleus; IXm, glossopharyngeal motor nucleus; Xm, vagus motor nucleus; XI, accessory nucleus; XII (DMN), dorsomedial nucleus of the hypoglossal nerve; nV, trigeminal nerve; nVII, facial nerve; niX, glossopharyngeal nerve; nX, vagus nerve; nXI, accessory nerve; nXII, hypoglossal nerve; r, rostral; c, caudal; m, medial; 1, lateral.
motoneurons
that innervate C and other branchiomeric neck musculature are found
to be intermingled among the motoneurons
o f t h e s e c o n d s p i n a l n e r v e [13, 14]. T h u s ,
the relation of the XI to the spinal motoneuron
column seems to be similar in both
species. A l t h o u g h it h a s b e e n c o n v e n t i o n a l l y d e s c r i b e d t h a t t h e I S is i n n e r v a t e d b y
244
~7
.a,
V
B
O,
g
r ~e
II
A
ZX
c
Fig. 2. Photomicrographs of horizontal sections showing the HRP-labelled motoneurons. Small triangles indicate the midline. Arrows indicate the level of the caudal limit of the XII (DMN) for a reference point (see Fig. IA). Bar= 200/tm; r, rostral; c, caudal. A: HRP-labelled C motoneurons, after the injection of HRP into the left C. In the XI, the labelled motoneurons are localized rostrally. B: HRP-labelled IS motoneurons, after the injection of HRP into the right IS. In the XI, the labelled motoneurons are localized caudally. o n e b r a n c h (R. s c a p u l a r i s ) o f the v a g u s n e r v e [4], it w a s clearly s h o w n f r o m this s t u d y t h a t t h e n X I i n n e r v a t e s n o t o n l y t h e C b u t also the IS. T h e s e t w o m u s c l e s are b e l i e v e d to r e p r e s e n t the t r a p e z i u s c o m p l e x o f o t h e r v e r t e b r a t e s [2, 12]. T h e m o t o n e u r o n s i n n e r v a t i n g the s t e r n o c l e i d o m a s t o i d a n d the t r a p e z i u s m u s c l e s t h a t a r e b e l i e v e d to c o r r e s p o n d to the t r a p e z i u s c o m p l e x , are f o u n d in t h e spinal a c c e s s o r y n u c l e u s a n d the c e r v i c a l spinal s e g m e n t s o f the rat [5]. T h u s , the X I o f a m p h i b i a n s is t h o u g h t to c o r r e s p o n d to the s p i n a l a c c e s s o r y n u c l e u s o f m a m m a l s . T h i s s t u d y was s u p p o r t e d by g r a n t s f r o m t h e M i n i s t r y o f E d u c a t i o n , Science a n d Culture of Japan and from The Mitsubishi Foundation. 1 Barnes, M.D. and Alley, K.E., Maturation and recycling of tfigeminal motoneurons in anuran larvae, J. Comp. Neurol., 218 (1983) 406-414. 2 Black, D., The motor nuclei of the cerebral nerves in phylogeny, a study of the phenomena of neurobiotaxis, lI. Amphibia, J. Comp. Neurol., 28 (1917) 379-427. 3 Fuller, P.M., Afferent and efferent components of the facial nerve in the bullfrog (Rana catesbeiana), J, Morphol., 159 (1979) 245-252. 4 Gaupp, E., Anatomie des Frosches, Vieweg, Braunschweig, 1896-1904. 5 Kitamura, S. and Sakai, A., A study on the localization of the sternocleidomastoid and trapezius motoneurons in the rat by means of the HRP method, Anat. Rec., 202 (1982) 527-536. 6 Matesz, C. and Sz6kely, G., The motor column and sensory projections of the branchial cranial nerves in the frog, J. Comp. Neurol., 178 (1978) 157-176. 7 Mesulam, M.M., Tetramethylbenzidine for horseradish peroxidase neurohistochemistry: A non-carcinogenic blue reaction-product with superior sensitivity for visualizing neural afferents and efferents, J. Histochem, Cytochem., 26 (1978) 106-117.
245 8 Nieuwenhuys, R. and Opdam, P., Structure of the brainstem. In R. Llinfis and W. Precht (Eds.), Frog Neurobiology, Springer, Berlin, 1976, pp. 811-855. 9 0 k a , Y., Localization of motoneurons innervating laryngeal muscles in the Japanese toad, Acta Anat. Nippon, 58 (1983) 271. 10 Oka, Y., Satou, M. and Ueda, K., Morphology and distribution of the motor neurons of the accessory nerve (nXI) in the Japanese toad: a cobaltic lysine study, Brain Res., 400 (1987) 383 388. 11 Oka, Y., Takeuchi, H., Satou, M. and Ueda, K., Cobaltic lysine study of the morphology and distribution of the cranial nerve efferent neurons (motoneurons and preganglionic parasympathetic neurons) and rostral spinal motoneurons in the Japanese toad, J. Comp. Neurol., 259 (1987) 400-423. 12 Romer, A.S., The Vertebrate Body, Saunders, Philadelphia, 1971, Chapter 9, pp. 273-314. 13 Roth, G., Wake, D.B., Wake, M.H. and Retting, G., Distribution of accessory and hypoglossal nerves in the hindbrain and spinal cord of lungless salamanders, family Plethodontidae, Neurosci. Lett., 44 (1984) 53-57. 14 Roth, G. and Wake, D.B., The structure of the brainstem and cervical spinal cord in lungless salamanders (family Plethodontidae) and its relation to feeding, J. Comp. Neurol., 241 (1985) 99-110. 15 Takei, K., Oka, Y., Satou, M. and Ueda, K., Localization of motoneurons involved in the prey-catching behavior in the Japanese toad, Zool. Sci., 1 (1984) 881. 16 Takei, K., Oka, Y., Satou, M. and Ueda, K., Distribution of motoneurons involved in the prey-catching behavior in the Japanese toad, Bufojaponicus, Brain Res., 410 (1987) 395-400. 17 Weerasuriya, A. and Ewert, J.-P., Prey-selective neurons in the toad's optic tectum and sensorimotor interfacing: HRP studies and recording experiments, J. Comp. Physiol., 144 (1981) 429-434. 18 Weerasuriya, A., Snapping in toads: some aspects of sensorimotor interfacing and motor pattern generation. In J.-P. Ewert, R.R. Capranica and D.J. Ingle (Eds.), Advances in Vertebrate Neuroethology, Plenum, New York, 1983, pp. 613~527.