Paper - The spinal accessory nerve in human embryos
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Pearson AA. The spinal accessory nerve in human embryos. (1938) J. Comp. Neurol. 68(2): 243-266.
Pearson AA. The development of the olfactory nerve in man. (1941) J. Comp. Neurol. 199-217.
Pearson AA. The development of the nervus terminali in man. (1941) J. Comp. Neurol. 75: 39-66.
Pearson AA. The trochlear nerve in human fetuses. (1943) J. Comp. Neurol. : 29-43.
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The Spinal Accessory Nerve In Human Embryos
Anthony A. Pearson
Hull Laboratory of Anatomy, The University of Chicago‘
(Accepted for publication September 1, 1937)
This paper is one of a series of studies on the development and analysis of the components of the cranial nerves in human embryos undertaken at the suggestion of Prof. C. Judson Herrick. The study is based on serial sections of embryos cut in several planes and stained by various methods. This Work was facilitated by Prof. G. VV. Bartelmez, Who placed at my disposal several series of human embryos stained With acid carmine and reduced silver, and a number of unstained specimens, and by Dr. Edith Potter of Lying-In Hospital who also contributed several embryos.
The author wishes to thank Dr. G. L. Streeter of the Carnegie Institution of Washington at Baltimore for the privilege of studying a large number of hematoxylin series of human embryos, and Profs. Davenport Hooker and Ira Hogg of the department of anatomy of the University of Pittsburgh for the use of a number of signiﬁcant series of toluidin blue and pyridine silver preparations during a short visit to their laboratories. This material has been supplemented with pyridine silver preparations and a number of series stained with activated copper protargol (Bodian, ’36) made by the author.
This research was aided by a grant to The University of Chicago by the Rockefeller Foundation. Study of the material at the department of anatomy of the University of Pittsburgh was aided by a grant from the Penrose Fund of the American Philosophical Society.
It was found that for the study of ﬁber bundles within the brain, the pyridine silver gave the best results, but the peripheral nervous system was far superior when stained with the activated protargol or a modiﬁed Rogers in which protargol was used (Bartelmez and Hoerr, ’33) and copper added to the protargol as in the former stain. After experimenting with a large number of pig embryos it was found that a mixture of formol (5%), acetic acid (5%), and alcohol (80%) gave the best ﬁxation with the least amount of shrinkage; ﬁxation and shrinkage are factors of vital importance. The results of the staining with the protargol depend largely on the ﬁxation. In embryos, where the tissues contain so large amount of Water and particularly in the very loose tissues around the central nervous system from which the leptomeninges develop, shrinkage often has a marked effect. This very delicate tissue around the brain and spinal cord must be preserved intact with as little shrinkage as possible in order that one may follow the course of the smallest nerve rootlets. To obtain a more thorough embedding the ﬂuids and embryos were aspirated under a vacuum pump to remove all possible air bubbles. The specimens were embedded in methyl-benzoate celloidin before being inﬁltrated with paraffin, and the best results were obtained when the vacuum pump was again used while the specimen was in the last change of hot paraﬁin. Many embryos stained with the best of available methods must be studied intensively before an approximation of the whole story of any one nerve can be learned. This is particularly true of the spinal accessory nerve which is subject to many va_riations.
The author has felt that there is too wide a gap between the approach to the nervous system in gross anatomy and in neurology. It is my opinion that preparations of embryos, old enough for the nervous system to be well formed and in which the complete course of peripheral nerves can be followed from cell of origin to termination in much the same way that Herrick and others worked on functional systems in ﬁshes, would contribute to an understanding of the nervous system as a whole.
Since the literature on the XI cranial nerve is summarized in the recent book by Ariéns Kappers, Huber and Crosby (’36), only such references as are pertinent to this discussion will be mentioned. According to current nomenclature, XI has two parts: a spinal portion, nervus accessorius spinalis, and a cranial portion, nervus accessorius bulbaris. The distribution of the latter is generally included with the vagus. The spinal part is usually considered as purely motor, special visceral efferent; however, its classiﬁcation is still debated. This account is concerned primarily with the spinal portion of this nerve.
The human embryos referred to speciﬁcally in the following description are listed below with the necessary data concerning each:
|table to be formatted
KUMAN EMBBYO No. CROWN-RUHP LENGTH PROBABLE AGE METHOD 0]‘
IN HILLIMETIBS IN WEEKS PREPARATION
1223 Activated protargol 1331 Pyridine silver
11 Activated protargol
19 Pyridine silver 1498 Pyridine silver
15 Activated protargol 1433B Pyridine silver
Human embryos nos. 11 and 15 are in the author ’s collection, no. 19 belongs to the collection at the department of anatomy of the University of Pittsburgh, and the others listed are in the embryological collection at the department of anatomy of The University of Chicago.
List of Abbreviations
A.bas., arteria basilaris
A.vert., arteria vertebralis
bulb.acc.r.gang., bulbar accessory root ganglia
eo., communicating ﬁlament between the trunk of spinal XI and the rudimentary ganglion of C 1
dens cpis., dens epistrophei
f, spinal accessory root ﬁbers
fas.sol., fasciculus solitarius
for., intervertebral foramen between the ﬁrst and second cervical vertebrae
forarnen trans., for-amen transversarium
fun.post., funiculus posterior
gang.C1, ganglion of the ﬁrst cervical nerve
gang.C2, ganglion of the second cervical nerve
gang.jug., ganglion jugulare
gang.N.V, ganglion semilunare
gang.nod., ganglion nodosum
gang.R.acc.sp., spinal accessory root ganglion
gang.sup., ganglion superius
11, highest ﬁlament of the dorsal root of the ﬁrst cervical nerve
h.r.sp.acc., highest root of the spinal accessory nerve
lig.trans.atl., ligamentum transversum atlantis
M.scm., musculus sternocleidomastoideus
M.trap., musculus trapezius
I\T.VII, nervus facialis
N.VIII, nervus acusticus
N.IX, nervus glossopharyngeus
N.X, nervus vagus
N.X et gang., nervus vagus et ganglion jugulare
N.XT, nervus accessorius
N.XI* (ﬁg. 18), on the right, indicates the region where XI descends with X; on the left side, a lower point on XI on the opposite side
N.XII, nervus hypoglossus
n.a., nucleus originis nervi accessorii spinalis
N .acc.sp., nervus accessorius spinalis
N .acc.sp. et gang., spinal accessory nerve and spinal accessory root ganglion
os oce., os occipitale
os temp., os temporale
R.ant.C1, anterior root of the first cervical nerve
R.ant.C2, anterior root of the second cervical nerve
R.ant.C3, anterior root of the third cer~ vical nerve
R.ant.C4, anterior root of the fourth cervical nerve
R.ant.C5, anterior root of the ﬁfth cervical nerve
R.N.acc.sp., root of the spinal accessory nerve
R.post.C1, posterior root of the first cervical nerve
R.post.C2, posterior root of the second cervical nerve
R.post.C3, posterior root of the third cervical nerve
B.post.C-1, posterior root of the fourth cervical nerve
R.post.C5, posterior root of the ﬁfth cervical nerve
Rr.N.XII, roots of nervus hypoglossus
Rr.N.acc.bulb., roots of nervus accessorius bulbaris
Rr.N.acc.sp., roots of spinal accessory nerve
s.tr., sinus transversus
t.sp.XI, trunk of spinal XI
X, cross-cut spinal accessory root ﬁbers
Nucleus of Origin
The nucleus of origin of the spinal accessory nerve is well shown in several series, but particularly well in no. 19. It is easily recognized by its constituent large multipolar cells with their large eccentric nuclei, scanty cytoplasm and prominent nucleoli (ﬁg. 1). The small granules of chromatin of the nuclei stand out in sharp contrast with the large nucleoli. It is thought that in certain of the stages studied, the dense neuropil around the cells at certain levels is responsible for making this cell column stand out more distinctly (ﬁg. 2). The cells of this nucleus are somatic in type, in agreement.
Fig. 1 Cells from the spinal accessory cell column in the upper cervical cord of a human embryo (no. 1433B). Camera lucida drawing. Pyridine silver preparation. X 750,
with Black (’13) who identiﬁed the origin of spinal X1 in the upper two or three cervical segments of the spinal cord of a newborn babe.
This nucleus is located in the lateral part of the ventral horn of the cervical cord. The cells of this nucleus form a column which extends from about the junction of the spinal cord with the medulla obloiigata caudally into the sixth cervical segment of the cord. The nucleus is larger at certain levels, smaller at others, and for a few sections at a time may drop out completely. The type of material on which this description is based is illustrated in ﬁgure 2, and the distribution of the cells and the position of the nucleus in the ventral 110m in ﬁgures 3 to 7.
In the first cervical segment the nucleus is seen in cross sections as a group of about twelve to sixteen cells (fig. 3) in the ventral l1orn, lateral to the medial cell columns. The
Fig. 2 A photomicrograph of the accessory nucleus at the lzecond cervical level of :1. human embryo (no. .19). .l’_vridine silver preparation. X 320.
accessory nucleus becomes somewhat larger and more prominent in the lower and middle portions of the. first cervical segment, but as one traces these cells to the. upper limit of the spinal cord the nucleus becomes smaller, and disappears at about tl1e level of the highest ventral rootlets of the first cervical nerve. In older embryos, where the lowermost decussating ﬁbers of the pyramids may be taken as the lower boundary of the medulla oblongata, one might consider that the spinal accessory nucleus extends for a short distance into the medulla oblongata (Black, ’13). In the material studied it does not appear to be directly continuous with any cell group or column within the brain stem, which is also the case in the newborn babe (Black, ’13). It is not continuous with or in line with the hypoglossal nucleus, as has been described in certain lower animals (Addens, ’33, p. 343; and Beccari, ’22).
Figs.3 to 7 These are drawings of representative cross sections of successive levels of the spinal cord, C1 through C5, of a human embryo (no. 19), illustrating the extent of the accessory nucleus and its position in the ventral horn. The right and left sides of ﬁgures 3 and 4 are from adjacent sections. Pyridine silver preparations. X 40.
In the second cervical segment (ﬁg. 4) this nucleus has much the same relations as in the preceding. The cells of the ventral horn are more densely packed and appear to have crowded the accessory nucleus slightly laterad. It is in the upper cervical segments of the cord that the nerve bundles which form the spinal accessory nerve can be demonstrated most clearly as they leave the nucleus of origin.
In the third cervical segment (ﬁg. 5) the ventral horn has enlarged somewhat, a ventrolateral group having come into the picture, while the accessory nucleus now takes a slightly more dorsolateral position.
From here caudad, this nucleus is less prominent than in the higher cervical levels. In cervical segments four and ﬁve (ﬁgs. 6 and 7) the ventral horn shows further enlargement in that dorsolateral cell columns occupy a position dorsal to the accessory nucleus. Although practically surrounded by other cell columns, the nucleus retains its individuality by deﬁnite morphological characteristics which distinguish it from the other cell groups.
Throughout the extent of this nucleus its position is much the same. The changes in cell grouping about it alter the picture of the ventral horn, but this cell column is almost a straight line, extending into the upper part of the sixth cervical segment. Here the relations are much the same as in the ﬁfth segment, but the cells become fewer and soon drop out completely.
Course of Spinal XI
The ﬁbers arising from the accessory nucleus pass dorsad in loose formation and converge into small bundles. These may pass almost directly dorsad and laterad to the surface of the cord, they may gradually ascend cephalad as they pass dorsad and laterad, or they may pass directly dorsad and then ascend for a certain distance before making their exit from the cord. The bundles which make their exit nearer the ventral funiculus pass more directly toward the lateral surface of the cord thus having a shorter internal course. The rootlets emerging nearer the dorsal funiculus usually pass farther dorsally in a more medial position, ascending in small bundles for a varying distance before turning out. The spinal accessory roots are larger and more numerous in the higher cervical levels than in the lower levels. In fact, on tracing the roots from above down, they become progressively smaller and less numerous until they drop out completely in the ﬁfth or sixth cervical segment of the cord, as would be expected from the distribution of the cells of origin. The ﬁbers arising in the lower part of the accessory cell column tend to ascend farther within the spinal cord than many of the bundles aris— ing higher. However, there are ﬁber bundles that turn out in the lower levels near their cell of origin. The roots of spinal XI do not emerge in a straight line but in a very ir— regular manner, some being much nearer the dorsal funiculus. Often in one section two roots can be seen leaving from the lateral surface of the cord, one being much more dorsal than the other (ﬁg. 3).
On emerging from the cord the rootlets of spinal XI turn cephalad close under the dorsal roots of the cervical nerves. With the addition of higher rootlets, the trunk of XI grows larger as it courses cephalad. This trunk lies close to the dorsal roots where usually there is no intermingling of ﬁber components. However, the relations of the trunk of spinal XI to the ﬁrst cervical dorsal root and ganglion are found to be variable (Streeter, ’04, and others). The dorsal root and ganglion of the ﬁrst cervical segment may be well formed, or they may be either greatly reduced in size or apparently lacking. Spinal XI may have no connection with either the dorsal root or ganglion of C1, or it may be in intimate relation with either or both. Weigner (’01) in adults, Streeter (’04) in human embryos, Windle (’31) in cats and monkeys, and others have described such anastomoses. My observations on the whole agree with the ﬁndings of these investigators. Additional ﬁndings in the embryos at my disposal will be brieﬂy outlined.
In human embryo no. 11 the rootlets forming spinal XI can be traced as they ascend close under the dorsal roots of the upper cervical nerves. Passing in close company with the trunk of XI is an anastomotie branch, with clusters of ganglion cells along it, which passes between the dorsal roots of C1 and C2. The trunk of XI together with the rootlets of XI arising from the first cervical segment merge with the dorsal root of that level as it passes into its ganglion (ﬁgs. 8 and 9, left). The dorsal root ganglion of C1 on one side is partially divided into two parts (ﬁg. 9) which are separated by a constriction in the ganglion. The medial part, which receives XI, lies within the embryonic meninges and is situated slightly cephalad to the more lateral part outside the dura, which last at this time is in the process of differentiation. In this embryo it was not possible to trace the individual fascicles of XI through the ganglion, as its ﬁbers are lost among the ganglion cells. Just above the ganglion of C1 the ﬁbers of XI collect into a compact bundle (ﬁg. 8, left), which bundle constitutes the trunk of XI and passes cephalad to enter the foramen magnum.
Fig. 8 This is a composite semidiagrammatic drawing, illustrating on the left the relation of the higher spinal accessory roots and the first cervical nerve, and the trunk of XI a it arches cephalad and ventrad through the bulbar accessory root ganglia to join X. On the right, an accessory root ganglion is indicated at the approximate point below which the trunk of XI ascends from the ganglion of C1. See ﬁgures 9, 13 and 14. Human embryo no. 11. Approximately X 12.
In embryo no. Template:CE1223 the spinal XI trunk ascends close under the dorsal root of the second cervical nerve. The relation of the two bundles is intimate and it is diﬂicult to determine whether there is an exchange of ﬁbers. Above this XI immediately runs into and through a small ganglion (ﬁg. 10) which is located approximately between the planes of exit of the ﬁrst and second cervical nerves from the vertebral canal. At the lower end of this ganglion a small bundle of ﬁbers is given oil’ which runs ventrad to the ganglion of the second cervical nerve. At the upper end of the ganglion another small bundle of ﬁbers can be traced ventrad and slightly cephalad to the small rudimentary ganglion of the ﬁrst cervical nerve (ﬁg. 11, left). There are apparently no dorsal roots from the ﬁrst cervical segment of the spinal cord. The relations are similar on both sides of this embryo.
In one embryo (no. 15) on one side a dorsal root bundle at the level of the second cervical segment turns cephalad with the trunk of XI, instead of passing to the dorsal root ganglion at that level. These ﬁbers ascend with XI, the two components forming a fairly compact bundle. Just before reaching the level of exit of the ﬁrst cervical nerve, this bundle runs into a small ganglion which lies within the dura jllst dorsal to the dentate ligament and slightly below the exit of the ﬁrst cervical nerve from the vertebral canal. As the ﬁbers pass into and through the ganglion, the ﬁbers of XI tend to gather into small bundles on the medial side of this ganglion (ﬁg. 12, right). At the upper end of the ganglion, XI separates from the dorsal root ﬁbers which pass laterad to join the ventral root of ﬁrst cervical nerve. As far as I could discover there are no dorsal root ﬁbers from the ﬁrst cervical segment of the cord on this side. In the same embryo but on the opposite side, XI adheres closely to the ganglion of C1. The ganglion on this side is small and in much the same position as on the opposite, only it is at the level of the exit of the ﬁrst cervical nerve. Here there is a small dorsal rootlet of C1.
In embryo no. Template:CE1498 the ganglion of C1 is separated into two parts: a medial ganglion within the vertebral canal close to the vertebral artery, and a lateral ganglion outside the dura, just above the neural arch of the ﬁrst cervical vertebra. The trunk of spinal XI passes through the medial ganglion. In this series of preparations the dorsal rootlets of C1 are small and difficult to follow. The highest of these rootlets appear to join the XI trunk above the medial ganglion, which is the case in several other embryos. As the trunk of XI passes through the ganglion, ﬁbers from the trunk can be seen running among the ganglion cells at both the upper and lower ends of the ganglion. It was not possible to determine definitely whether these were dorsal root ﬁbers ascending with XI, but this is very likely the case. In the lateral side of the medial ganglion, ﬁbers collect to pass to the lateral part of the ganglion of C1, located outside the dura.
After examining the course of the spinal accessory nerve in a large number of embryos, one is struck by the variations that occur. But neglecting the many details of numerous specimens, a few general statements can be made as to the usual course of the nerve. The trunk of spinal XI ascends between the dorsal and ventral roots of the upper cervical nerves. The trunk lies close under the dorsal roots, where it usually keeps its integrity up to the level of the ﬁrst cervical segment. Occasionally it is in intimate relation with the dorsal root of the second cervical nerve. It is in relation with the ﬁrst cervical nerve where the variability more often occurs. This is probably largely due to the variability of the dorsal root ganglion of the first cervical nerve. Usually the trunk of XI passes through the ganglion of C1, often joining the ganglion at its junction with its dorsal root. Sometimes the ganglion of C1 is divided into medial and lateral halves, in which case XI may pass through the medial half. Again XI may pass in intimate relation with the dorsal root of C1 and avoid that ganglion, or it may avoid both dorsal root and ganglion of 01. In addition to its relation to the ganglion of 01, there are often ganglionic masses on XI before it reaches the level of the ganglion of C1.
Fig. 9 This is a cross section of the ﬁrst cervical segment of the spinal cord. Note on the left the accessory root joining the dorsal root of C1 at the medial end of the ganglion of C1, and on the right the position of the accessory trunk just above the level of the ganglion of 01. Human embryo no. 11. Activated protargol preparation. X 23.
Fig. 10 Cross section of the vertebral canal just above the level of the dorsal root and ganglion of C2. In this embryo there is a small ganglion on the trunk of XI at this level. Human embryo no. 1223. Activated protargol preparation. X 23.
Fig. 11 The level of this ﬁgure is a little above that of the preceding one. On the left a small communicating ﬁlament is indicated which passes between the very small ganglion of Cl and the trunk of XI. Human embryo no. 12:23. Activated protargol preparation. X 23. N. qcc sp. eh qanq.
Fig. 12 A cross section through the upper cervical cord of a human embryo (no. 15). The right side of the field is just below the level of exit of the first cervical nerve. Note the ganglion within the dura and the absence of a dorsal root. The left side is at the level of the ganglion of the first cervical nerve and shows the more usual relation of XI and C1. Activated protargol preparation. X 23.
Streeter (’04) found that the ganglionic crest of the hindbrain was continuous with that of the spinal cord. Arising from part of this crest is a series of ganglia which extends along XI to the cervical ganglion series. He referred to these as accessory root ganglia and pointed out that they are not to be confused with the precervical ganglion of Froriep. Froriep’s ganglion represents an extra spinal ganglion and, when present, is in relation to the hypoglossal nerve. Streeter considered that the ganglia which often occur on XI between the upper cervical nerves are developed from the spinal crest. It is suggested that the ganglia along the portion of XI that receives bulbar rootlets should be referred to as bulbar accessory root ganglia and those ganglia situated on the spinal portion of this nerve below the bulbar rootlets, as spinal accessory root ganglia (ﬁgs. 10 and 11)
Fig. 13 A cross section taken a little above the level of ﬁgure 9. The left side of the ﬁgure shows the ganglionic Mass which is sometimes present on the trunk of XI just above the ganglion of 01. Lower down in the ﬁeld one will see IX, X and X1 in the relation in which they descend through the jugular foramen. The right side is a little above the level of the left and passes through the bulbar accessory root ganglion and the jugular ganglion. Human embryo no. 11. Activated protargol preparation. X 17.5. '
Fig.1-1 A drawing similar to ﬁgure 13, but at 21 slightly higher plane. On the left note XI arching ventrad to join X. The right field is just above the arch of XI, and bulbar accessory root bundles are present. Human embryo no. 11. Activated protargol preparation. X 17.5.
Along XI, where it passes ccphalad and ventrad in a broad curve to join X, is the series of bulbar accessory root ganglia (figs. 8, 13, 14 and 19). These ganglia form more or less of a sheath around the trunk of XI, which sometimes continues into the cervical region. In a few cases it fuses with the dorsal root ganglion of C1. Ventrally this series is continuous with the jugular ganglion of X (ﬁg. 18), while dorsally it usually becomes smaller and extends for a variable distance along XI. This series of ganglia is located on the portion of the nerve within the cranial cavity, above the point where the nerve turns sharply ventrad toward X after passing through the foramen magnum. Below the level of the bulbar rootlets and at about the level of the highest dorsal rootlets of 01, there is often found a small ganglionic swelling on XI (ﬁgs. 8 and 13). This ganglion is variable in size, sometimes lacking, but is usually found separated from the bulbar root ganglia. In some embryos a separation is indicated only by a constriction, or the ganglion is seen as the continuation of the bulbar accessory root ganglia. Just below this ganglion, XI passes into relation to the dorsal root of C1. In certain embryos there are scattered small ganglia on anastomotic branches between the dorsal roots of the upper cervical nerves, which sometimes are in relation with XI, and often on the trunk of XI itself. In one embryo (no. 1223) where the dorsal root and ganglion of C1 were greatly reduced in size there is a small but well—formed ganglion on XI between the levels of the dorsal root ganglion of C1 and C2 (see p. 253 and ﬁg. 10). In certain embryos the dorsal root ganglion of C1 is apparently shared with spinal XI.
The presence of ganglion cells along the course of XI has suggested to various investigators that this may be a mixed nerve, having a sensory as well as a motor component. Streeter thought that XI being laid down earlier may guide dorsal root ﬁbers out of the expected path and along its own. This appears to be the case in certain embryos. It remains to be determined whether these ganglia along XI usually persist in the human adult and whether XI has a component with cells of origin either in the ganglia mentioned or the ganglion of C1, or both. Windle (’31) found that after section of the peripheral end of XI in the cat, chromatolyzed cells were found on the trunk of XI and a signiﬁcant number in the ganglion of the ﬁrst cervical nerve. Windle and De Lozier (’32) suggested that the sensory component of XI is likely concerned with proprioception. On the basis of Marchi preparations, Hinsey and Corbin (’34) concluded that in the cat, XI contains no proprioceptive ﬁbers with cells of origin in the ﬁrst four cervical dorsal root ganglia. Strecter (’04) has shown that in the development of the vago-accessory complex, the cephalic end becomes predominantly sensory, and the caudal end predominantly motor.
Fig. 17 A sagittal section through the jugular foramen of a human embryo (no. 1498), illustrating the relation of the vagus and accessory nerves. Pyridine silver preparation. Approximately X 50.
Fig. 15 Cross section of the ﬂrst cervical level of the spinal cord of a human embryo (no. 1331). Note the caudal end of the fasciculus solitarius, which extends into the upper cervical region, and the accessory root ﬁbers which run dorsad. The trunk of spinal XI is shown in the left first cervical ganglion. The right and left sides of this ﬁgure are from adjacent sections. Pyridine silver preparation. X 59.
Fig. 16 This drawing is similar to figure 15. It is a. crosssection through the upper part of the third cervical level of a human embryo (no. 19), showing accessory root ﬁbers turning up toward the posterior funiculus. Pyridine silver preparation. X 59.
Fig. 18 This is a composite semischematic drawing showing the origin and course of the spinal accessory nerve in a human embryo (no. 19). The cord is drawn to represent two levels, the left side being lower than the right. On the left, the accessory root ﬁbers are shown ariing from their nucleus of origin and passing out of the cord to join the trunk of the spinal accessory nerve at the second cervical level. A small accessory ﬁlament is drawn turning up toward the posterior funiculus. The upper right half of the ﬁgure is through the ﬁrst cervical level of the spinal cord, and, in addition to the nucleus of origin, shows variations of the intraspinal course of the accessory root ﬁbers. The accessory root ﬁbers, the dorsal root of the ﬁrst cervical nerve and the spinal accessory trunk converge at the medial end of the dorsal root ganglion of that level. The trunk of spinal XI continues cephalad and, as it arches forward passing through the bulbar accessory root ganglion, it is joined by the bulbar accessory root ﬁbers (not shown). On reaching the jugular ganglion, XI descends in close company with the vagus. A XI descends it gradually moves from a dorsal to a lateral position with respect to the vagus. On the left side of the ﬁgure the external branch of XI is shown leaving X and passing to the sternocleidomastoid and trapezius muscles. From pyridine silver preparations. Approximately X 28.
It is interesting to point out that in one embryo (no. 1331), at the level of the ﬁrst cervical segment where the ﬁbers of XI are well impregnated, small fascicles of accessory ﬁbers can be recognized other than those arising from the ventral horn cells (ﬁg. 15). These ﬁbers can be traced out of the accessory
Fig. 19 A reconstruction of the peripheral nerves in a. 6-week human embryo, 17.5 mm. long. His collection embryo FM. Slightly modiﬁed from ﬁgure 12, Streeter (’04). X 11.
rootlets dorsad toward the posterior funiculus and into the region of the caudal end of the fasciculus solitarius, which as it descends takes a. position nearer and nearer the posterior funiculus. As Windle (’31) observed in the cat embryo, these ﬁbers appear to turn caudad with that fasciculus. In our no. 1331, prepared by Cajal’s method, the ﬁbers of the fasciculus solitarius could be followed distinctly into the upper cervical cord. In this embryo the trunk of XI passes through the center of the dorsal root ganglion of 01. In other embryos, Where XI runs through the ganglion of C1 and there are spinal accessory root ganglia, occasional ﬁbers can be seen turning dorsad toward the posterior funiculus (ﬁg. 16). Some of these occur also in the second and third cervical segments. This strongly suggests that at least during development there is in certain embryos a small sensory component in spinal X1.
As the spinal accessory trunk turns ventrad toward the vagus (ﬁgs. 8, 14, 18 and 19) it passes through or under the bulbar accessory root ganglia and is joined by the bulbar accessory root ﬁbers. Together they descend through the jugular foramen close against the dorsal side of the jugular ganglion and the vagus nerve (ﬁg. 13, left). Here ﬁbers of X and XI are medial to the jugular vein and are in close relation (ﬁg. 17), making it difﬁcult to determine in this material what components constitute the external branch of XI (ﬁg. 18) which separates from X at the lower end of the foramen. DuBois and Foley (’36) have shown through degeneration experiments in cats that spinal XI can be traced into the external branch of the accessory, and bulbar XI into the recurrent laryngeal nerve, indicating a deﬁnite peripheral distribution.
As XI descends it gradually creeps around X to a lateral position and then leaves X to pass laterad to the sternocleidomastoid and trapezius muscles. The whole course of spinal XI, from origin to termination, is shown schematically in figure 18.
It is thought that spinal XI is in reality a spinal nerve which is joined by bulbar XI within the cranial cavity, the two parts fusing as they join the vagus. These form a complex which has been difficult to analyze.
While the main component within spinal XI is motor, there is strong evidence that in certain embryos there may be sensory ﬁbers. Windle and De Lozier (’32) have shown evidence in cats that these ﬁbers do not carry painful stimuli but more likely proprioception. The cells of origin of this sensory element are thought to be located either 1) within the ganglia which often occur along the trunk of XI, or 2) in the ganglion of C1 in relation to which spinal XI usually passes, or 3) in both. These sensory fibers may enter the cord with the roots of spinal XI where they then separate from the motor ﬁbers and turn dorsad toward the posterior funiculus, or they may enter with the dorsal root ﬁbers of C1. The majority probably take the latter course.
The nucleus of the spinal accessory in the stages studied appears to be an integral part of the ventral l1orn. Any conclusions drawn from the position of the nucleus should be made cautiously until the earlier embryology of this cell column is more thoroughly understood.
The question as to whether this nerve is visceral or somatic is still debated. This question would seem to resolve itself into the problem as to whether there are elements of branchio— meric origin within the sternocleidomastoid and trapezius muscles, or whether these muscles are entirely of somatic origin. Again the answer will have to come from studies of younger embryos. Phylogenetic evidence is of interest but 11ot conclusive with regard to this problem. This question has been recently reviewed by Addens (’33) who regarded spinal XI as a somatic efferent nerve, and Straus and Howell (’36) who thought this nerve to be of visceral origin. The position of the cells of origin within the ventral horn would cause one to favor the former view, while the manner in which the accessory roots leave the spinal cord, i.e., between the dorsal and ventral roots, would support the latter. Suﬁicient evidence to rule out the one and substantiate the other is still lacking.
- The cells of origin of the spinal portion of the accessory nerve are somatic in type and form a cell column which is an integral part of the ventral horn. The accessory cell column extends from the region of the junction of the spinal cord with the medulla oblongata into the sixth cervical segment.
- The intraspinal course of the accessory rootlets is varia.hle. Some rootlets emerge at the level of origin, others ascend for certain distances within the spinal cord before turning out. The line of emerging accessory roots from the spinal cord is not regular, certain roots being more dorsal than others.
- The course of the spinal accessor_v trunk within the vertebral canal and its relation to the dorsal root and ganglion of the first cervical nerve are variable. Usually the trunk of spinal XI passes through the ganglion of C1. Variations which occur in the material studied are discussed in the text.
- The presence of ganglia along the trunk of spinal XI and the relation of the trunk to the ganglion of 01 suggest the presence of a sensory component. The occasional accessory root ﬁbers which turn up toward the posterior funiculus strengthen this view.
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Cite this page: Hill, M.A. (2019, July 24) Embryology Paper - The spinal accessory nerve in human embryos. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_spinal_accessory_nerve_in_human_embryos
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