J. Anal. (1979), 129, 1, pp. 69-75 With 3 figures Printed in Great Britain
69
Afferent and efferent myelinated fibres in branches of the avian vagus* A. B. ABDALLA and A. S. KING
Department of Anatomy, Faculty of Veterinary Science, University of Khartoum, Sudan and Department of Veterinary Anatomy, University of Liverpool, England
(Accepted 27 June 1978) INTRODUCTION
The disposition of the myelinated fibres in the vagus and its branches has been extensively studied in mammals (e.g. DuBois & Foley, 1936; Daly & Evans, 1953; Agostoni, Chinnock, Daly & Murray, 1957), but is not very well known in birds. Brown, Molony, King & Cook (1972) gave counts of the myelinated fibres in the left midcervical vagus, the left recurrent nerve, the left pulmono-oesophageal, and the left oesophageal nerves in the domestic fowl. The total numbers of afferent and efferent myelinated fibres in the cervical vagus of the same species of bird were established by Abdalla (1974) and Abdalla & King (1979). This paper gives estimates of the numbers of afferent and efferent myelinated fibres in the vago-glossopharyngeal anastomosis, the cranial cardiac nerve, the recurrent nerve, the pulmono-oesophageal nerve, and the thoracic vagus in the domestic fowl. The term 'thoracic vagus' refers to the portion of the vagus just caudal to the recurrent and pulmono-oesophageal nerves. MATERIALS AND METHODS
Thirteen hens of the Shaver Starcross type 585 (Leahurst Veterinary Field Station) were used in this study. Total counts of myelinated fibres were made in the following nerves: the right and left vago-glossopharyngeal anastomoses, the right cranial cardiac nerve, the right and left recurrent nerves, the left pulmono-oesophageal nerve, and the left thoracic vagus. The study was made in normal birds and in birds previously subjected to either midcervical or intracranial vagotomy. The techniques used for vagotomy, fixation and staining, photography, and counting fibres of various external diameters, have been described in a previous paper (Abdalla & King, 1979). RESULTS
Vago-glossopharyngeal anastomosis The normal nerve contained 1697 myelinated fibres on the right side in bird B4, and 1420 fibres on the left side in bird Bi (Table 1). In the right nerve the largest fibres were 13-14 ,um in diameter, while the largest in the left nerve were 11-12 ,um in diameter (Fig. 1). Following right intracranial vagotomy, the number of surviving myelinated fibres in the right nerve was 680 in bird B13 and 622 in bird B14 (Table 1). These surviving fibres were interpreted as the afferent myelinated component of this * Reprint requests to Professor A. S. King, Department of Veterinary Anatomy, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, Merseyside, England.
0021-8782/79/2828-6210 $02.00 © 1979 Anat. Soc. G.B. & I.
70
A. B. ABDALLA AND A. S. KING
Fig. 1. Normal left vago-glossopharyngeal anastomosis. Some myelinated fibres of large diameter are present, the largest in this field being about 12 ,um in external diameter. The smallest fibres with dark myelin sheaths are about 2 ,um in external diameter. A few fibres with severely disrupted sheaths are present: axon A, concentric sheaths; axon B, broken sheath; axon C, redundant sheath. Axons such as A, B, and C were excluded from all counts. Prepared by the Flemming-Wolter technique of Williams & Wendell-Smith (1960). x 1000. Fig. 2. The normal right recurrent nerve. There are no fibres above 7 ,tm in diameter. Only fibres with distinctly black sheaths were counted: greyish sheaths (thin arrows) were regarded as sheaths of unmyelinated fibres and were thus excluded from the count. An adjustment had to be made for the measurement of irregular fibres (thick arrows); thus oval and crenated fibres were measured by taking the mean of the largest diameters and one at right angles to it. Prepared by the Flemming-Wolter technique of Williams & Wendell-Smith (1960). x 900.
nerve. Since midcervical as well as intracranial vagotomy was carried out in bird B14, the 622 surviving myelinated fibres in this bird must have had their cell bodies in the proximal vagal ganglion (or the distal glossopharyngeal ganglion). This suggests that very few of th- afferent myelinated fibres in this nerve (say 680 minus 622, or about 58 fibres) had their cell bodies in the distal vagal ganglion. It was notable that only about 4 of the largest myelinated fibres (7-14,m) survived intracranial vagotomy. Presumably, therefore, nearly all these large myelinated fibres were efferent.
The cranial cardiac nerve This small nerve was studied on the right side only. In bird Bi it contained about 118 myelinated fibres, 83 % being 4 ,tm or less in diameter (Table 1). Th- number of myelinated fibres which survived in this nerve after midcervical or intracranial vagotomy was 1 4 in birds B10 and B15 (Table 1). Evidently this nerve was almost entirely afferent, the cell bodies being in the distal vagal ganglion. The recurrent nerve In bird BI the normal right recurrent nerve contained 3696 myelinated fibres, while the left nerve in two birds (B5 and B6) contained 2033 and 2159 fibres with a mean of 2096 fibres (Table 1). In neither the right nor the left nerve were there any fibres over 7 ,tm in diameter (Fig. 2). The large majority of fibres were of small diameter. Thus in the right nerve of bird BI about 84 % of the fibres were 4 ,tm or less in diameter; on the left side the fibres of similar diameter formed 80 % and 81 %
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A. B. ABDALLA AND A. S. KING Left
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Fig. 3. Diagram of the cervical vagus and branches, summarizing the approximate numbers of myelinated fibres. For each nerve, the uppermost figure represents the afferent fibres with cell bodies in the proximal vagal ganglion, the middle figure represents the afferent fibres with cell bodies in the distal vagal ganglion, the bottom figure represents the number of efferent fibres, and the figure under the line represents the total count. The number of efferent fibres in each nerve has been estimated by subtracting the number of afferent fibres from the total count. A question mark indicates that a value has been assumed. IX, glossopharyngeal nerve; X, vagus; Anast, vago-glossopharyngeal anastomosis; pulmono-oesoph, pulmono-oesophageal nerve; N, nerve; G, ganglion.
of the total counts in birds B5 and B6. About 1177 myelinated fibres survived midcervical vagotomy on the right side in bird B3, i.e. about 32 % of the total count. A much larger proportion survived in the left nerve of bird BI 1, namely 1526 fibres, or 73 % of the mean total number of 2096 fibres. These results suggest that in the right recurrent about one third, and in the left recurrent nearly three quarters of the myelinated fibres were afferent with cell bodies in the distal vagal ganglion. Following right intracranial vagotomy in bird B15 about 1074 myelinated fibres survived in the right recurrent nerve. This confirms the conclusion from midcervical vagotomy that about one third of the myelinated fibres in the right recurrent nerve are afferent. The pulmono-oesophageal nerve The normal left nerve contained 281 myelinated fibres in bird B7 and 300 fibres in bird B8, the mean being 291. The great majority of these fibres were 5 ,m or less in diameter (Table 1). In the right nerve 155 myelinated fibres
Myelinated fibres of vagal branches
73
survived intracranial vagotomy in bird B15, suggesting that about one half of the myelinated fibres in this small nerve may be afferent. The left thoracic vagus The thoracic vagus contained 2090 myelinated fibres in bird B9 (Table 1), of which the great majority were 5 ,um or less in diameter, its fibre spectrum resembling that of the left recurrent nerve. In bird Bi 1, 976 myelinated fibres in the left thoracic vagus survived midcervical vagotomy; in bird B3, 1695 myelinated fibres in the right thoracic vagus survived midcervical vagotomy. These observations suggest that about half or more of the myelinated fibres of the thoracic vagus are afferent, with their cell bodies in the distal vagal ganglion. DISCUSSION
The vago-glossopharyngeal anastomosis of the domestic fowl contained a number of myelinated fibres of large diameter (up to 14 ,um), as has already been shown by Brown et al. (1972). Our results following intracranial vagotomy indicated that about 60 % of the myelinated fibres in the right vago-glossopharyngeal anastomosis were efferent and 40 % afferent. It was also clear that nearly all the large fibres up to 14 ,um in diameter were efferent, since only a few remained intact after intracranial vagotomy. Our findings are consistent with the suggestion by Hsieh (1951), Watanabe (1960, 1964), Bubien-Waluszewska (1968) and Brown et al. (1972) that the avian vagus supplies efferent fibres to the intrinsic laryngeal muscles through this anastomosis; these efferent fibres are believed to reach the larynx through the laryngeal branch ofthe pharyngeal ramus ofthe glossopharyngeal nerve (Baumel, 1975). Possibly these large fibres in the vago-glossopharyngeal anastomosis of the domestic fowl are homologous to the efferent fibres in the mammalian recurrent nerve. In the cat (Murray, 1957) these fibres reach 18 ,m in diameter; they descend through the cervical vagus and then ascend within the recurrent nerve to innervate the intrinsic muscles of the larynx. In the domestic fowl there are no large myelinated fibres in either the cervical vagus (Abdalla, 1974; Abdalla & King, 1979) or the recurrent nerve, and the latter fails to reach the larynx (Baumel, 1975). The results of fibre counts in the cranial cardiac nerve after vagotomy indicated that the myelinated fibres in this nerve were entirely afferent with cell bodies in the distal vagal ganglion. This is consistent with electrophysiological studies (Fedde, Burger & Kitchell, 1963) which have shown that the cranial cardiac nerve appears to be entirely afferent. The recurrent nerves were markedly asymmetrical. The right nerve contained 3696 myelinated fibres, 43 % more than the left nerve. The results of vagotomy indicated that only about one third of the myelinated fibres in the large right nerve were afferent, whereas about three quarters were afferent in the smaller left nerve. The small pulmono-oesophageal nerve contained only a few myelinated fibres, a mean of 291 fibres being obtained in the left n-erve in two birds. In the right nerve, 155 myelinated fibres were shown by intracranial vagotomy to be afferent. It would seem therefore that this nerve carries a significant component of afferent myelinated fibres. Fedde et al. (1963) have shown electrophysiologically that this nerve is afferent to the lung and efferent to the oesophagus. Comparison of the number of myelinated fibres in the normal thoracic vagus with the number surviving vagotomy indicated that half or more of the myelinated fibres in this nerve were afferent with their cell bodies in the distal vagal ganglion.
74
A. B. ABDALLA AND A. S. KING
The left thoracic vagus contained only 2090 myelinated fibres, compared with 8535 in the left midcervical vagus (Abdalla & King, 1979). In the branches of the left vagus the mean number was 2096 in the left recurrent nerve, and 291 in the left pulmono-oesophageal nerve. The sum of these and the left thoracic vagus was only 4477. Thus there appeared to be a substantial loss of myelinated fibres as the nerve proceeded to the thoracic cavity. Admittedly not all the relevant branches of the left vagus have been included. For example the left cranial cardiac nerve was omitted, but this is a small nerve (with only 118 myelinated fibres on the right side). Vagal branches to the thyroid, parathyroid, and ultimobranchial glands, and to the carotid body, were also not counted, but all of these too are small nerves. A more likely explanation of the disparity between the total numbers is that some of the myelinated fibres in the cervical vagus lose their myelin sheaths in their passage caudally, as in the mammalian vagus (Gaskell, 1886; Chase & Ranson, 1914; Hoffman & Kuntz, 1957; Iggo, 1958). On the basis of the observations reported here and those given by Abdalla & King (1979), the approximate disposition of the myelinated fibres in the avian vagus and its main branches can be summarised as in Figure 3. It is important to recognise, however, that these estimates of the numbers of fibres do not take into account the possible presence of sympathetic fibres (see Abdalla & King, 1979). SUMMARY
The numbers of afferent and efferent myelinated fibres in the branches of the vagus nerve in the domestic fowl were studied. The vago-glossopharyngeal anastomosis contained large fibres (up to 14,um), the majority of which were efferent. The right recurrent nerve contained more fibres than the left one; in the right recurrent nerve about one third of the myelinated fibres were afferent. Almost all the myelinated fibres in the right cranial cardiac nerve were afferent. About half of those in the pulmono-oesophageal nerve and in the thoracic vagus were afferent. We gratefully thank Dr V. M. Molony for devising and performing intracranial vagotomy, Julie Henry for helpful support with histology, and M. Goldberg and J. Geary for much expert assistance with photography, We also gratefully acknowledge the financial support from the University of Khartoum which made our cooperation possible. REFERENCES ABDALLA, A. B. (1974). Studies on the fibre composition of the vagus nerve of Gallus gallus domesticus. Ph.D. Thesis, University of Liverpool. ABDALLA, A. B. & KING, A. S. (1979). The myelinated fibres of the avian cervical vagus. Journal of Anatomy 128, 135-142. AGOSTONI, E., CHINNOCK, J. E., DALY, M. DE BURGH & MURRAY, J. G. (1957). Functional and histological studies of the vagus nerve and its branches to the heart, lungs and abdominal viscera in the cat. Journal of Physiology 135, 182-205. BAUMEL, J. J. (1975). Aves nervous system. In Sisson and Grossman's The Anatomy of the Domestic Animals (ed. R. Getty), p. 2033, 2035. Philadelphia, London, Toronto: W. B. Saunders Co. BROWN, C. M., MOLONY, V., KING, A. S. & COOK, R. D. (1972). Fibre size and conduction velocity in the vagus of the domestic fowl (Gallus domesticus). Acta anatomica 83, 451-460. BUBIEN-WALUSZEWSKA, A. (1968). Le groupe caudale des nerfs craniens de la poule domestique (Gallus domesticus). Acta anatomica 69, 445-457. CHASE, M. R. & RANSON, S. W. (1914). The structure of the roots, trunks and branches of the vagus nerve. Journal of Comparative Neurology 24, 31-60.
Myelinatedfibres of vagal branches
75
DALY, M. DE BURGH & EVANS, D. H. L. (1953). Functional and histological changes in the vagus nerve of the cat after degenerative section at various levels. Journal of Phtysiology 120, 579-595. DuBois, F. S. & FOLEY, J. 0. (1936). Experimental studies on the vagus and spinal accessory nerves in the cat. Anatomical Record 64, 285-307. FEDDE, M. R., BURGER, R. E. & KITCHELL, R. L. (1963). Localization of vagal afferents involved in the maintenance of normal avian respiration. Poultry Science 42, 1224-1236. GASKELL, W. H. (1886). On the structure, distribution and function of the nerves which innervate the visceral and vascular system. Journal of Physiology 7, 1-80. HOFFMAN, H. H. & KUNTZ, A. (1957). Vagus nerve components. Anatomical Record 127, 551-567. HSIEH, T. M. (1951). Sympathetic and parasympathetic nervous systems of the fowl. Ph.D. Thesis, University of Edinburgh. IGGO, A. (1958). The electrophysiological identification of single nerve fibres with particular reference to the slowest conducting vagal afferent fibres in the cat. Journal of Physiology 142, 110-126. MURRAY, J. G. (1957). Innervation of the intrinsic muscles of the cat's larynx by the recurrent laryngeal nerve: a unimodal nerve. Journal of Physiology 135, 206-212. WATANABE, T. (1960). Comparative and topographical anatomy of the fowl. VII: on the peripheral course of the vagus in the fowl. Japanese Journal of Veterinary Science 22, 152-154. WATANABE, T. (1964). Comparative and topographical anatomy of the fowl. XVII: Peripheral courses of the hypoglossal, accessory and glossopharyngeal nerves. Japanese Journal of Veterinary Science 26, 256-258. WILLIAMS, P. L. & WENDELL-SMITH, C. P. (1960). The use of fixed and stained sections in quantitative studies of peripheral nerve. Quarterly Journal of Microscopical Science 101, 43-54.
69
Afferent and efferent myelinated fibres in branches of the avian vagus* A. B. ABDALLA and A. S. KING
Department of Anatomy, Faculty of Veterinary Science, University of Khartoum, Sudan and Department of Veterinary Anatomy, University of Liverpool, England
(Accepted 27 June 1978) INTRODUCTION
The disposition of the myelinated fibres in the vagus and its branches has been extensively studied in mammals (e.g. DuBois & Foley, 1936; Daly & Evans, 1953; Agostoni, Chinnock, Daly & Murray, 1957), but is not very well known in birds. Brown, Molony, King & Cook (1972) gave counts of the myelinated fibres in the left midcervical vagus, the left recurrent nerve, the left pulmono-oesophageal, and the left oesophageal nerves in the domestic fowl. The total numbers of afferent and efferent myelinated fibres in the cervical vagus of the same species of bird were established by Abdalla (1974) and Abdalla & King (1979). This paper gives estimates of the numbers of afferent and efferent myelinated fibres in the vago-glossopharyngeal anastomosis, the cranial cardiac nerve, the recurrent nerve, the pulmono-oesophageal nerve, and the thoracic vagus in the domestic fowl. The term 'thoracic vagus' refers to the portion of the vagus just caudal to the recurrent and pulmono-oesophageal nerves. MATERIALS AND METHODS
Thirteen hens of the Shaver Starcross type 585 (Leahurst Veterinary Field Station) were used in this study. Total counts of myelinated fibres were made in the following nerves: the right and left vago-glossopharyngeal anastomoses, the right cranial cardiac nerve, the right and left recurrent nerves, the left pulmono-oesophageal nerve, and the left thoracic vagus. The study was made in normal birds and in birds previously subjected to either midcervical or intracranial vagotomy. The techniques used for vagotomy, fixation and staining, photography, and counting fibres of various external diameters, have been described in a previous paper (Abdalla & King, 1979). RESULTS
Vago-glossopharyngeal anastomosis The normal nerve contained 1697 myelinated fibres on the right side in bird B4, and 1420 fibres on the left side in bird Bi (Table 1). In the right nerve the largest fibres were 13-14 ,um in diameter, while the largest in the left nerve were 11-12 ,um in diameter (Fig. 1). Following right intracranial vagotomy, the number of surviving myelinated fibres in the right nerve was 680 in bird B13 and 622 in bird B14 (Table 1). These surviving fibres were interpreted as the afferent myelinated component of this * Reprint requests to Professor A. S. King, Department of Veterinary Anatomy, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, Merseyside, England.
0021-8782/79/2828-6210 $02.00 © 1979 Anat. Soc. G.B. & I.
70
A. B. ABDALLA AND A. S. KING
Fig. 1. Normal left vago-glossopharyngeal anastomosis. Some myelinated fibres of large diameter are present, the largest in this field being about 12 ,um in external diameter. The smallest fibres with dark myelin sheaths are about 2 ,um in external diameter. A few fibres with severely disrupted sheaths are present: axon A, concentric sheaths; axon B, broken sheath; axon C, redundant sheath. Axons such as A, B, and C were excluded from all counts. Prepared by the Flemming-Wolter technique of Williams & Wendell-Smith (1960). x 1000. Fig. 2. The normal right recurrent nerve. There are no fibres above 7 ,tm in diameter. Only fibres with distinctly black sheaths were counted: greyish sheaths (thin arrows) were regarded as sheaths of unmyelinated fibres and were thus excluded from the count. An adjustment had to be made for the measurement of irregular fibres (thick arrows); thus oval and crenated fibres were measured by taking the mean of the largest diameters and one at right angles to it. Prepared by the Flemming-Wolter technique of Williams & Wendell-Smith (1960). x 900.
nerve. Since midcervical as well as intracranial vagotomy was carried out in bird B14, the 622 surviving myelinated fibres in this bird must have had their cell bodies in the proximal vagal ganglion (or the distal glossopharyngeal ganglion). This suggests that very few of th- afferent myelinated fibres in this nerve (say 680 minus 622, or about 58 fibres) had their cell bodies in the distal vagal ganglion. It was notable that only about 4 of the largest myelinated fibres (7-14,m) survived intracranial vagotomy. Presumably, therefore, nearly all these large myelinated fibres were efferent.
The cranial cardiac nerve This small nerve was studied on the right side only. In bird Bi it contained about 118 myelinated fibres, 83 % being 4 ,tm or less in diameter (Table 1). Th- number of myelinated fibres which survived in this nerve after midcervical or intracranial vagotomy was 1 4 in birds B10 and B15 (Table 1). Evidently this nerve was almost entirely afferent, the cell bodies being in the distal vagal ganglion. The recurrent nerve In bird BI the normal right recurrent nerve contained 3696 myelinated fibres, while the left nerve in two birds (B5 and B6) contained 2033 and 2159 fibres with a mean of 2096 fibres (Table 1). In neither the right nor the left nerve were there any fibres over 7 ,tm in diameter (Fig. 2). The large majority of fibres were of small diameter. Thus in the right nerve of bird BI about 84 % of the fibres were 4 ,tm or less in diameter; on the left side the fibres of similar diameter formed 80 % and 81 %
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A. B. ABDALLA AND A. S. KING Left
Right Proximal vagal G. Distal IXth G
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X
X
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1420
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Fig. 3. Diagram of the cervical vagus and branches, summarizing the approximate numbers of myelinated fibres. For each nerve, the uppermost figure represents the afferent fibres with cell bodies in the proximal vagal ganglion, the middle figure represents the afferent fibres with cell bodies in the distal vagal ganglion, the bottom figure represents the number of efferent fibres, and the figure under the line represents the total count. The number of efferent fibres in each nerve has been estimated by subtracting the number of afferent fibres from the total count. A question mark indicates that a value has been assumed. IX, glossopharyngeal nerve; X, vagus; Anast, vago-glossopharyngeal anastomosis; pulmono-oesoph, pulmono-oesophageal nerve; N, nerve; G, ganglion.
of the total counts in birds B5 and B6. About 1177 myelinated fibres survived midcervical vagotomy on the right side in bird B3, i.e. about 32 % of the total count. A much larger proportion survived in the left nerve of bird BI 1, namely 1526 fibres, or 73 % of the mean total number of 2096 fibres. These results suggest that in the right recurrent about one third, and in the left recurrent nearly three quarters of the myelinated fibres were afferent with cell bodies in the distal vagal ganglion. Following right intracranial vagotomy in bird B15 about 1074 myelinated fibres survived in the right recurrent nerve. This confirms the conclusion from midcervical vagotomy that about one third of the myelinated fibres in the right recurrent nerve are afferent. The pulmono-oesophageal nerve The normal left nerve contained 281 myelinated fibres in bird B7 and 300 fibres in bird B8, the mean being 291. The great majority of these fibres were 5 ,m or less in diameter (Table 1). In the right nerve 155 myelinated fibres
Myelinated fibres of vagal branches
73
survived intracranial vagotomy in bird B15, suggesting that about one half of the myelinated fibres in this small nerve may be afferent. The left thoracic vagus The thoracic vagus contained 2090 myelinated fibres in bird B9 (Table 1), of which the great majority were 5 ,um or less in diameter, its fibre spectrum resembling that of the left recurrent nerve. In bird Bi 1, 976 myelinated fibres in the left thoracic vagus survived midcervical vagotomy; in bird B3, 1695 myelinated fibres in the right thoracic vagus survived midcervical vagotomy. These observations suggest that about half or more of the myelinated fibres of the thoracic vagus are afferent, with their cell bodies in the distal vagal ganglion. DISCUSSION
The vago-glossopharyngeal anastomosis of the domestic fowl contained a number of myelinated fibres of large diameter (up to 14 ,um), as has already been shown by Brown et al. (1972). Our results following intracranial vagotomy indicated that about 60 % of the myelinated fibres in the right vago-glossopharyngeal anastomosis were efferent and 40 % afferent. It was also clear that nearly all the large fibres up to 14 ,um in diameter were efferent, since only a few remained intact after intracranial vagotomy. Our findings are consistent with the suggestion by Hsieh (1951), Watanabe (1960, 1964), Bubien-Waluszewska (1968) and Brown et al. (1972) that the avian vagus supplies efferent fibres to the intrinsic laryngeal muscles through this anastomosis; these efferent fibres are believed to reach the larynx through the laryngeal branch ofthe pharyngeal ramus ofthe glossopharyngeal nerve (Baumel, 1975). Possibly these large fibres in the vago-glossopharyngeal anastomosis of the domestic fowl are homologous to the efferent fibres in the mammalian recurrent nerve. In the cat (Murray, 1957) these fibres reach 18 ,m in diameter; they descend through the cervical vagus and then ascend within the recurrent nerve to innervate the intrinsic muscles of the larynx. In the domestic fowl there are no large myelinated fibres in either the cervical vagus (Abdalla, 1974; Abdalla & King, 1979) or the recurrent nerve, and the latter fails to reach the larynx (Baumel, 1975). The results of fibre counts in the cranial cardiac nerve after vagotomy indicated that the myelinated fibres in this nerve were entirely afferent with cell bodies in the distal vagal ganglion. This is consistent with electrophysiological studies (Fedde, Burger & Kitchell, 1963) which have shown that the cranial cardiac nerve appears to be entirely afferent. The recurrent nerves were markedly asymmetrical. The right nerve contained 3696 myelinated fibres, 43 % more than the left nerve. The results of vagotomy indicated that only about one third of the myelinated fibres in the large right nerve were afferent, whereas about three quarters were afferent in the smaller left nerve. The small pulmono-oesophageal nerve contained only a few myelinated fibres, a mean of 291 fibres being obtained in the left n-erve in two birds. In the right nerve, 155 myelinated fibres were shown by intracranial vagotomy to be afferent. It would seem therefore that this nerve carries a significant component of afferent myelinated fibres. Fedde et al. (1963) have shown electrophysiologically that this nerve is afferent to the lung and efferent to the oesophagus. Comparison of the number of myelinated fibres in the normal thoracic vagus with the number surviving vagotomy indicated that half or more of the myelinated fibres in this nerve were afferent with their cell bodies in the distal vagal ganglion.
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A. B. ABDALLA AND A. S. KING
The left thoracic vagus contained only 2090 myelinated fibres, compared with 8535 in the left midcervical vagus (Abdalla & King, 1979). In the branches of the left vagus the mean number was 2096 in the left recurrent nerve, and 291 in the left pulmono-oesophageal nerve. The sum of these and the left thoracic vagus was only 4477. Thus there appeared to be a substantial loss of myelinated fibres as the nerve proceeded to the thoracic cavity. Admittedly not all the relevant branches of the left vagus have been included. For example the left cranial cardiac nerve was omitted, but this is a small nerve (with only 118 myelinated fibres on the right side). Vagal branches to the thyroid, parathyroid, and ultimobranchial glands, and to the carotid body, were also not counted, but all of these too are small nerves. A more likely explanation of the disparity between the total numbers is that some of the myelinated fibres in the cervical vagus lose their myelin sheaths in their passage caudally, as in the mammalian vagus (Gaskell, 1886; Chase & Ranson, 1914; Hoffman & Kuntz, 1957; Iggo, 1958). On the basis of the observations reported here and those given by Abdalla & King (1979), the approximate disposition of the myelinated fibres in the avian vagus and its main branches can be summarised as in Figure 3. It is important to recognise, however, that these estimates of the numbers of fibres do not take into account the possible presence of sympathetic fibres (see Abdalla & King, 1979). SUMMARY
The numbers of afferent and efferent myelinated fibres in the branches of the vagus nerve in the domestic fowl were studied. The vago-glossopharyngeal anastomosis contained large fibres (up to 14,um), the majority of which were efferent. The right recurrent nerve contained more fibres than the left one; in the right recurrent nerve about one third of the myelinated fibres were afferent. Almost all the myelinated fibres in the right cranial cardiac nerve were afferent. About half of those in the pulmono-oesophageal nerve and in the thoracic vagus were afferent. We gratefully thank Dr V. M. Molony for devising and performing intracranial vagotomy, Julie Henry for helpful support with histology, and M. Goldberg and J. Geary for much expert assistance with photography, We also gratefully acknowledge the financial support from the University of Khartoum which made our cooperation possible. REFERENCES ABDALLA, A. B. (1974). Studies on the fibre composition of the vagus nerve of Gallus gallus domesticus. Ph.D. Thesis, University of Liverpool. ABDALLA, A. B. & KING, A. S. (1979). The myelinated fibres of the avian cervical vagus. Journal of Anatomy 128, 135-142. AGOSTONI, E., CHINNOCK, J. E., DALY, M. DE BURGH & MURRAY, J. G. (1957). Functional and histological studies of the vagus nerve and its branches to the heart, lungs and abdominal viscera in the cat. Journal of Physiology 135, 182-205. BAUMEL, J. J. (1975). Aves nervous system. In Sisson and Grossman's The Anatomy of the Domestic Animals (ed. R. Getty), p. 2033, 2035. Philadelphia, London, Toronto: W. B. Saunders Co. BROWN, C. M., MOLONY, V., KING, A. S. & COOK, R. D. (1972). Fibre size and conduction velocity in the vagus of the domestic fowl (Gallus domesticus). Acta anatomica 83, 451-460. BUBIEN-WALUSZEWSKA, A. (1968). Le groupe caudale des nerfs craniens de la poule domestique (Gallus domesticus). Acta anatomica 69, 445-457. CHASE, M. R. & RANSON, S. W. (1914). The structure of the roots, trunks and branches of the vagus nerve. Journal of Comparative Neurology 24, 31-60.
Myelinatedfibres of vagal branches
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DALY, M. DE BURGH & EVANS, D. H. L. (1953). Functional and histological changes in the vagus nerve of the cat after degenerative section at various levels. Journal of Phtysiology 120, 579-595. DuBois, F. S. & FOLEY, J. 0. (1936). Experimental studies on the vagus and spinal accessory nerves in the cat. Anatomical Record 64, 285-307. FEDDE, M. R., BURGER, R. E. & KITCHELL, R. L. (1963). Localization of vagal afferents involved in the maintenance of normal avian respiration. Poultry Science 42, 1224-1236. GASKELL, W. H. (1886). On the structure, distribution and function of the nerves which innervate the visceral and vascular system. Journal of Physiology 7, 1-80. HOFFMAN, H. H. & KUNTZ, A. (1957). Vagus nerve components. Anatomical Record 127, 551-567. HSIEH, T. M. (1951). Sympathetic and parasympathetic nervous systems of the fowl. Ph.D. Thesis, University of Edinburgh. IGGO, A. (1958). The electrophysiological identification of single nerve fibres with particular reference to the slowest conducting vagal afferent fibres in the cat. Journal of Physiology 142, 110-126. MURRAY, J. G. (1957). Innervation of the intrinsic muscles of the cat's larynx by the recurrent laryngeal nerve: a unimodal nerve. Journal of Physiology 135, 206-212. WATANABE, T. (1960). Comparative and topographical anatomy of the fowl. VII: on the peripheral course of the vagus in the fowl. Japanese Journal of Veterinary Science 22, 152-154. WATANABE, T. (1964). Comparative and topographical anatomy of the fowl. XVII: Peripheral courses of the hypoglossal, accessory and glossopharyngeal nerves. Japanese Journal of Veterinary Science 26, 256-258. WILLIAMS, P. L. & WENDELL-SMITH, C. P. (1960). The use of fixed and stained sections in quantitative studies of peripheral nerve. Quarterly Journal of Microscopical Science 101, 43-54.
