Difference between revisions of "Paper - The trochlear nerve in human fetuses"

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Loyola University School of Medicine, Chicago, Illinois
 
Loyola University School of Medicine, Chicago, Illinois
  
TEN FIGURES (1943)  
+
Ten Figures (1943)
  
INTRODUCTION
+
==Introduction==
  
The early development of the trochlear nerve in mammalian
+
The early development of the trochlear nerve in mammalian embryos has been described by Windle (’32 a, ’32 b), Zahajszky ( ’40), and others. The discussion will be limited to the later development of the trochlear nerve and its relation to the trigeminal nerve.‘ This study is based on serial sections of human embryos and fetuses stained with various silver methods. The whole heads of the smaller fetuses and em— bryos were sectioned and studied; however, only the brains of the larger fetuses were prepared. Most of the material used in this study has been listed in previous papers (Pearson, ’41 a, ’41 b) and only necessary data will be given here.
embryos has been described by Windle (’32 a, ’32 b),
 
Zahajszky ( ’40), and others. The discussion will be limited to
 
the later development of the trochlear nerve and its relation
 
to the trigeminal nerve.‘ This study is based on serial sections
 
of human embryos and fetuses stained with various silver
 
methods. The whole heads of the smaller fetuses and em—
 
bryos were sectioned and studied; however, only the brains of
 
the larger fetuses were prepared. Most of the material used
 
in this study has been listed in previous papers (Pearson,
 
’41 a, ’41 b) and only necessary data will be given here.
 
  
NUCLEUS OF ORIGIN
+
==Nucleus Of Origin==
  
The nucleus of the trochlear nerve is easily recognized in
+
The nucleus of the trochlear nerve is easily recognized in older human embryos and fetuses as a large mass of multipolar neurons located just caudal to the nucleus of the oculo— motor nerve. There is no wide interval between the nuclei of the third and fourth cranial nerves. A distinct thinning out of the cells of the motor column, however, indicates a boundary between the nuclei of these two nerves. The trochlear nucleus is in line with the lateral groups of cells of the oculomotor nucleus. It is located i11 its characteristic position, dorsal to and partly imbedded in the medial longitudinal fasciculus.
older human embryos and fetuses as a large mass of multipolar neurons located just caudal to the nucleus of the oculo—
 
motor nerve. There is no wide interval between the nuclei of
 
the third and fourth cranial nerves. A distinct thinning out
 
of the cells of the motor column, however, indicates a boundary
 
between the nuclei of these two nerves. The trochlear nucleus
 
is in line with the lateral groups of cells of the oculomotor
 
nucleus. It is located i11 its characteristic position, dorsal to
 
and partly imbedded in the medial longitudinal fasciculus.
 
  
The nucleus of the trochlear nerve is formed by one main
 
  
mass of cells and sometimes by an additional smaller mass of cells which lies farther caudad (figs. 1, 2, 3). The caudal group
+
The nucleus of the trochlear nerve is formed by one main mass of cells and sometimes by an additional smaller mass of cells which lies farther caudad (figs. 1, 2, 3). The caudal group of cells may be bilateral or unilateral. VVhen it is unilateral, it may occur on either the right _or the left side.
of cells may be bilateral or unilateral. VVhen it is unilateral,
 
it may occur on either the right _or the left side.
 
  
This subdivision of the trochlear nucleus into a larger
 
cephalic portion and a smaller caudal portion has been observed by several investigators (Sabin, ’O1; Tsuchida, ’06;
 
van Valkenburg, ’12; and Sano, ’41). Tsuchida reported a subdivision in the trochlear nucleus in some 20 to 30% of the
 
brainstems studied, this differentiation occurring in most of
 
the specimens on only one side. In the material studied‘ by
 
the author the percentages were somewhat higher. The caudal
 
division of the trochlear nucleus occurred nearly as often on both sides as on only one side. When the caudal division was
 
present on one side only, it was found to occur about as frequently on the right side as on the left side.
 
  
 +
This subdivision of the trochlear nucleus into a larger cephalic portion and a smaller caudal portion has been observed by several investigators (Sabin, ’O1; Tsuchida, ’06; van Valkenburg, ’12; and Sano, ’41). Tsuchida reported a subdivision in the trochlear nucleus in some 20 to 30% of the brainstems studied, this differentiation occurring in most of the specimens on only one side. In the material studied‘ by the author the percentages were somewhat higher. The caudal division of the trochlear nucleus occurred nearly as often on both sides as on only one side. When the caudal division was present on one side only, it was found to occur about as frequently on the right side as on the left side.
  
  
Line 74: Line 46:
 
cer., cerebellum
 
cer., cerebellum
  
c.mes.V., cells of the mesencephalic root
+
c.mes.V., cells of the mesencephalic root /of V
/of V
 
  
dcc.N.IV, decussation of the trochlear
+
dcc.N.IV, decussation of the trochlear nerve
nerve
 
  
 
f.1.m., fasciculus longitudinalis medialis
 
f.1.m., fasciculus longitudinalis medialis
Line 88: Line 58:
 
med.ob1., medulla oblongata
 
med.ob1., medulla oblongata
  
me.root V, mesencephalic root of the
+
me.root V, mesencephalic root of the trigeminal nerve
trigeminal nerve
 
  
mes.V et nu., mesencephalic root of V
+
mes.V et nu., mesencephalic root of V and nucleus
and nucleus
 
  
 
N.III, nervus oculomotorius
 
N.III, nervus oculomotorius
Line 102: Line 70:
 
nu.IV, nucleus nervi trochlearis
 
nu.IV, nucleus nervi trochlearis
  
nu.IV p.caud., nucleus trochlearis (pars
+
nu.IV p.caud., nucleus trochlearis (pars caudali)
caudali)
 
  
 
V., or vent., ventriculus
 
V., or vent., ventriculus
  
X, 3. group of cells in the path of the
+
X, 3. group of cells in the path of the trochlear nerve
trochlear nerve
 
  
 
  
Fig. 1 A agittal section through the brainstem of a 29-mm. human fetus
+
Fig. 1 A agittal section through the brainstem of a 29-mm. human fetus (no. 29). Note the rostral and caudal divisions of the trochlear nucleus. Copper protargol preparation (section 16-1-2). X 12.
(no. 29). Note the rostral and caudal divisions of the trochlear nucleus. Copper protargol preparation (section 16-1-2). X 12.
 
  
  
 +
Fig. 2 A transverse section through the mesencephalon of a 5-month-old fetus (no. 64) at the level of the main nucleus of the trochlear nerve. In the left nucleus of the IV nerve there are two cells of the mesencephalic V type. Pyridine silver preparation (section 29-3-4). X 24.
  
Fig. 2 A transverse section through the mesencephalon of a 5-month-old fetus
 
(no. 64) at the level of the main nucleus of the trochlear nerve. In the left nucleus
 
of the IV nerve there are two cells of the mesencephalic V type. Pyridine silver
 
preparation (section 29-3-4). X 24.
 
  
 +
In one human fetus of about 5 months of age (no. 63) the trochlear nucleus on each side was a single mass of cells. In another fetus (no. 64) of about the same age, there was a caudal division of the trochlear nucleus on each side (fig. 3). This caudal division was a small group of darkly staining Inultipolar neurons. It was located about 1 mm. caudal to the
  
  
In one human fetus of about 5 months of age (no. 63) the
+
Fig. 3 A transverse section through the mesencephalon of a 5-month-old fetus (no. 64) at the level of the caudal division of the trochlear nucleus. Pyridine silver preparation (section 24-3-3). X 24.
trochlear nucleus on each side was a single mass of cells. In another fetus (no. 64) of about the same age, there was a
 
caudal division of the trochlear nucleus on each side (fig. 3).
 
This caudal division was a small group of darkly staining
 
Inultipolar neurons. It was located about 1 mm. caudal to the
 
  
+
main nucleus and was approximately 0.2 mm. in length. This caudal nucleus of the trochlear nerve was slightly smaller on the right side than on the left. There are about sixty cells in the caudal nucleus on the right side and about eighty-three cells on the left side. This is only an approximation, as the cells were closely packed and it was difficult to make an exact count. In the fetus which possessed only the main nucleus of the trochlear nerve it is interesting to note that this main nucleus measured slightly more than 1 mm. in length and was quite symmetrical. In the other fetus of this same age, tl1e main nucleus measured less than 1 mm. in length. On the right side, the main nucleus was about 0.9 mm. in length and the caudal division contained about sixty cells. The main nucleus of the trochlear nerve on the left side measured about 0.6 mm. and the caudal division contained about eighty—three cells. The cross sectional area of the main nuclei of the trochlear nerve in both fetuses appeared to be approximately equal. These two fetuses are thought to be fair examples of the cellular arrangement of the trochlear nerve nuclei. This suggests that the size of the main nucleus is in inverse proportion to the size of the caudal division. There is no indication that the fiber connections of the caudal division are any different from those of the main nucleus of the trochlear nerve nor is there any apparent physiological significance to the separation of the cells of the trochlear nerve into groups.
  
Fig. 3 A transverse section through the mesencephalon of a 5-month-old fetus
+
A few cells of the mesencephalic V type were observed in the main nucleus of the trochlear nerve (fig. 2, left). These cells are easily distinguished from the usual multipolar neurons of the trochlear nucleus by their shape and size. Weinbe1'g' (’28) has made a similar observation.
(no. 64) at the level of the caudal division of the trochlear nucleus. Pyridine silver
 
preparation (section 24-3-3). X 24.
 
  
main nucleus and was approximately 0.2 mm. in length. This
+
In certain embryos, scattered cells have been observed along the course of the trochlear nerve central to its" decussation (fig. 4). These cells are thought to resemble neuroblasts, although their exact character is diflicult to determine. Occasionally, such cells form a distinct cluster. In one 3-month-old human fetus, such a group was found in the central course of the trochlear nerve on each side (fig. 5). This group is located dorsolateral to the medial longitudinal fasciculus and at a plane slightly caudal to the level of the trochlear nuclei. Some of the cells appear to be bipolar, although their exact shapes are not distinct. Sabin (’01) has also reported the presence of groups of cells along the root fibers of the trochlear nerve.
caudal nucleus of the trochlear nerve was slightly smaller on
 
the right side than on the left. There are about sixty cells
 
in the caudal nucleus on the right side and about eighty-three
 
cells on the left side. This is only an approximation, as the
 
cells were closely packed and it was difficult to make an exact count. In the fetus which possessed only the main nucleus
 
of the trochlear nerve it is interesting to note that this main
 
nucleus measured slightly more than 1 mm. in length and was
 
quite symmetrical. In the other fetus of this same age, tl1e
 
main nucleus measured less than 1 mm. in length. On the right
 
side, the main nucleus was about 0.9 mm. in length and the
 
caudal division contained about sixty cells. The main nucleus
 
of the trochlear nerve on the left side measured about 0.6 mm.
 
and the caudal division contained about eighty—three cells.
 
The cross sectional area of the main nuclei of the trochlear
 
nerve in both fetuses appeared to be approximately equal.
 
These two fetuses are thought to be fair examples of the
 
cellular arrangement of the trochlear nerve nuclei. This suggests that the size of the main nucleus is in inverse proportion
 
to the size of the caudal division. There is no indication that
 
the fiber connections of the caudal division are any different
 
from those of the main nucleus of the trochlear nerve nor is
 
there any apparent physiological significance to the separation
 
of the cells of the trochlear nerve into groups.
 
  
A few cells of the mesencephalic V type were observed in the
 
main nucleus of the trochlear nerve (fig. 2, left). These cells
 
are easily distinguished from the usual multipolar neurons
 
of the trochlear nucleus by their shape and size. Weinbe1'g'
 
(’28) has made a similar observation.
 
  
In certain embryos, scattered cells have been observed along
+
Fig. 4 A sagittal section through the brainstem of a 17-mm. human embryo (no. 16). Scattered cells are shown along the course of the trochlear nerve. Pyridine silver preparation (8—3~2). X 75.
the course of the trochlear nerve central to its" decussation
 
(fig. 4). These cells are thought to resemble neuroblasts, although their exact character is diflicult to determine. Occasionally, such cells form a distinct cluster. In one 3-month-old
 
human fetus, such a group was found in the central course
 
of the trochlear nerve on each side (fig. 5). This group is
 
located dorsolateral to the medial longitudinal fasciculus and
 
at a plane slightly caudal to the level of the trochlear nuclei.
 
Some of the cells appear to be bipolar, although their exact
 
shapes are not distinct. Sabin (’01) has also reported the
 
presence of groups of cells along the root fibers of the trochlear
 
nerve.
 
 
 
 
 
 
 
Fig. 4 A sagittal section through the brainstem of a 17-mm. human embryo
 
(no. 16). Scattered cells are shown along the course of the trochlear nerve.
 
Pyridine silver preparation (8—3~2). X 75.
 
  
 
cm. car,
 
cm. car,
  
 
 
Fig. 5 A transverse section through the mesencephalon of 9. 63-mm. human
 
fetus (no. 57). A small cluster of cells is hown in the path of the IV nerve.
 
Copper protargol preparation (section 48—2—3). X 14.
 
 
 
THE COURSE OF THE NERVE
 
  
Fibers of the trochlear nerve are collected into several small
+
Fig. 5 A transverse section through the mesencephalon of 9. 63-mm. human fetus (no. 57). A small cluster of cells is hown in the path of the IV nerve. Copper protargol preparation (section 48—2—3). X 14.
bundles along the dorsolateral border of the trochlear nucleus
 
toward its caudal end. When a caudal division of the trochlear
 
nucleus is present an additional bundle of fibers arises here.
 
This bundle soon joins the larger bundles of the trochlear
 
nerve. The fibers of the trochlear nerve follow an oblique
 
course through the periphery of the central gray which surrounds the cerebral aqueduct. At first the fibers pass dorsolaterad. On reaching the lateral part of the central gray, the
 
course of the trochlear nerve fibers is directed caudad and
 
dorsad. The caudal course of the nerve is attributed in part
 
to the shifting forward of the cells of origin during development (Ariens Kappers, Huber, and Crosby, ’36). The fibers
 
of the trochlear nerve runalmost parallel to the mesencephalic
 
root of the trigeminal nerve for a short distance. As the fibers
 
of the trochlear nerve approach the anterior medullary velum,
 
they begin to turn toward the midline. Here they lie almost
 
perpendicular to the fibers of the mesencephalic root of the
 
trigeminal nerve. As the midline is approached, the fibers of
 
the trochlear nerve can then be traced into the ventral part of
 
the decussation of that nerve. After they pass to the other
 
side, the fibers of the trochlear nerve emerge from the decussation and make their exit from the dorsolateral surface of the
 
brainstem. No trochlear fibers were observed which failed to
 
decussa.te as van Gehuchten (’03) noted in the rat and rabbit.
 
' The course of the trochlear nerve within the brain lies deep,
 
but close to the mesencephalic root of the trigeminal nerve and
 
the nucleus of that root. It has been suggested by several
 
authors that there are connectionsxbetween the mesencephalic
 
root of the trigeminal nerve and the trochlear nerve (Freeman,
 
’27, Weinberg, ’28 and others). Such a connection has been
 
difficult to demonstrate or rule out. There are cells of the
 
  
mesencephalic V type, however, which are thought to be '
 
  
definitely related to the trochlear nerve. Cells of this type
+
==The Course of the Nerve==
occasionally occur -in the trochlear nucleus (fig. 2), as already
 
mentioned. There are cells of the mesencephalic V type in the
 
region lateral to the anterior medullary velum. Some of these cells appear to be connected with the trochlcar nerve. An
 
occasional cell may be found directly in the course of this
 
nerve (fig. 7). These cells may appear singly or in clusters.
 
Occasionally, a process of one of these cells may be followed
 
directly among the fibers of the trochlear nerve (fig. 8). A
 
few scattered cells of the mesencephalic V type may be found
 
in the anterior medullary Velum. An example of one of these
 
cells is shown in figure 9. A bipolar cell is shown in the anterior medullary velum lying; ventral to the fibers of the
 
trochlear nerve. A process from either end of the cell extends
 
out among the fibers of the trochlear nerve. A similar cell
 
was described by Golgi (1893). V-*'indle (’32 a) described a
 
cell which sent one branch into the trochlear nerve and
 
another branch into the mesencephalic tract of a cat.
 
  
 +
Fibers of the trochlear nerve are collected into several small bundles along the dorsolateral border of the trochlear nucleus toward its caudal end. When a caudal division of the trochlear nucleus is present an additional bundle of fibers arises here. This bundle soon joins the larger bundles of the trochlear nerve. The fibers of the trochlear nerve follow an oblique course through the periphery of the central gray which surrounds the cerebral aqueduct. At first the fibers pass dorsolaterad. On reaching the lateral part of the central gray, the course of the trochlear nerve fibers is directed caudad and dorsad. The caudal course of the nerve is attributed in part to the shifting forward of the cells of origin during development (Ariens Kappers, Huber, and Crosby, ’36). The fibers of the trochlear nerve runalmost parallel to the mesencephalic root of the trigeminal nerve for a short distance. As the fibers of the trochlear nerve approach the anterior medullary velum, they begin to turn toward the midline. Here they lie almost perpendicular to the fibers of the mesencephalic root of the trigeminal nerve. As the midline is approached, the fibers of the trochlear nerve can then be traced into the ventral part of the decussation of that nerve. After they pass to the other side, the fibers of the trochlear nerve emerge from the decussation and make their exit from the dorsolateral surface of the brainstem. No trochlear fibers were observed which failed to decussa.te as van Gehuchten (’03) noted in the rat and rabbit. ' The course of the trochlear nerve within the brain lies deep, but close to the mesencephalic root of the trigeminal nerve and the nucleus of that root. It has been suggested by several authors that there are connectionsxbetween the mesencephalic root of the trigeminal nerve and the trochlear nerve (Freeman, ’27, Weinberg, ’28 and others). Such a connection has been difficult to demonstrate or rule out. There are cells of the mesencephalic V type, however, which are thought to be definitely related to the trochlear nerve. Cells of this type occasionally occur -in the trochlear nucleus (fig. 2), as already mentioned. There are cells of the mesencephalic V type in the region lateral to the anterior medullary velum. Some of these cells appear to be connected with the trochlcar nerve. An occasional cell may be found directly in the course of this nerve (fig. 7). These cells may appear singly or in clusters. Occasionally, a process of one of these cells may be followed directly among the fibers of the trochlear nerve (fig. 8). A few scattered cells of the mesencephalic V type may be found in the anterior medullary Velum. An example of one of these cells is shown in figure 9. A bipolar cell is shown in the anterior medullary velum lying; ventral to the fibers of the trochlear nerve. A process from either end of the cell extends out among the fibers of the trochlear nerve. A similar cell was described by Golgi (1893). V-*'indle (’32 a) described a cell which sent one branch into the trochlear nerve and another branch into the mesencephalic tract of a cat.
  
Fig. 6 A diagram made from several sagittal sections of the brainstem of 2:
 
25-mm. human embryo (no. B. 1194). Pyridine silver preparation (13-20).
 
  
 +
Fig. 6 A diagram made from several sagittal sections of the brainstem of 2: 25-mm. human embryo (no. B. 1194). Pyridine silver preparation (13-20).
  
  
 
  
Fig. 7 A section through the trochlear nerve within the rnesencephalon. Note
+
Fig. 7 A section through the trochlear nerve within the rnesencephalon. Note the nerve cell in the path of the nerve. From a 5»month—old human fetus (no. 64). Pyridine silver preparation (19—3—4).
the nerve cell in the path of the nerve. From a 5»month—old human fetus (no. 64).
 
Pyridine silver preparation (19—3—4).
 
  
 
 
  
 
 
  
Fig. 8 A transverse section through the mesencephalon of a 5-m.onth~old fetus
+
Fig. 8 A transverse section through the mesencephalon of a 5-m.onth~old fetus (no. 64) at the level of tl1e anterior medullary velum. Note the close relation of the IV nerve to the meseneephalie root of the V nerve and its nucleus. Pyridine silver preparation (section 21-3-3). X 94.
(no. 64) at the level of tl1e anterior medullary velum. Note the close relation of the
 
IV nerve to the meseneephalie root of the V nerve and its nucleus. Pyridine silver
 
preparation (section 21-3-3). X 94.
 
  
 
 
 
  
Fig. 9 A photomicrograph of a nerve cell in the anterior medullary velum
 
of a. 5—month—old human fetus (no. 63). The processes of the nerve cell extend
 
dorsally among the fibers of the troehlear nerve. Pyridine silver preparation (193—4). X 450.
 
  
 +
Fig. 9 A photomicrograph of a nerve cell in the anterior medullary velum of a. 5—month—old human fetus (no. 63). The processes of the nerve cell extend dorsally among the fibers of the troehlear nerve. Pyridine silver preparation (193—4). X 450.
  
Peripheral to the decussation in the anterior medullary
 
veluin, the fibers of the trochlear nerve leave the dorsolateral
 
surface of the brainstem grouped in one or two small bundles.
 
After leaving the brain, these fibers collect into a single small
 
trunk, which can be traced ventrad close along the surface of
 
the brainstem. Before reaching the orbit the fibers of the
 
trochlear nerve pierce the dura mater and pass forward between the layers of the tentorium cerebelli.
 
  
As the trochlear nerve passes forward to enter the orbit it is
+
Peripheral to the decussation in the anterior medullary veluin, the fibers of the trochlear nerve leave the dorsolateral surface of the brainstem grouped in one or two small bundles. After leaving the brain, these fibers collect into a single small trunk, which can be traced ventrad close along the surface of the brainstem. Before reaching the orbit the fibers of the trochlear nerve pierce the dura mater and pass forward between the layers of the tentorium cerebelli.
joined by a small bundle of fibers from the ophthalmic division
 
of the trigeminal nerve. This branch of the trigeminal nerve
 
leaves the ophthalmic division just peripheral to the semilunar ganglion. It joins the trochlear nerve (figr. 10) and may
 
be followed back toward the brain. Part of these fibers are
 
thought to be distributed to the dura mater along the course
 
of the trochlear nerve. A few of them can be followed between
 
the layers of the dura mater into the 1'0/gion of the transverse
 
sinus.
 
  
Just peripheral to the place where the above anastomotic
+
As the trochlear nerve passes forward to enter the orbit it is joined by a small bundle of fibers from the ophthalmic division of the trigeminal nerve. This branch of the trigeminal nerve leaves the ophthalmic division just peripheral to the semilunar ganglion. It joins the trochlear nerve (figr. 10) and may be followed back toward the brain. Part of these fibers are thought to be distributed to the dura mater along the course of the trochlear nerve. A few of them can be followed between the layers of the dura mater into the 1'0/gion of the transverse sinus.
branch joins the trochlear nerve, a short part of the course of
 
the trochlear nerve lies in intimate relationship with the
 
ophthalmic nerve. The fibers of the two nerves lie so close
 
together that it is extremely difficult to determine in serial
 
sections whether there is an exchange of fibers between them.
 
After crossing the ophthalmic nerve, the trochlear nerve may
 
be traced forward into the superior oblique muscle. The part
 
of the trochlear nerve peripheral to the place where it crosses
 
the ophthalmic nerve is larger than the part of the nerve at
 
its exit from the brain. This difference in size is sufficient to
 
lead one to conclude that the trigeminal nerve contributes
 
fibers to the distal part of the trochlear nerve.
 
  
 +
Just peripheral to the place where the above anastomotic branch joins the trochlear nerve, a short part of the course of the trochlear nerve lies in intimate relationship with the ophthalmic nerve. The fibers of the two nerves lie so close together that it is extremely difficult to determine in serial sections whether there is an exchange of fibers between them. After crossing the ophthalmic nerve, the trochlear nerve may be traced forward into the superior oblique muscle. The part of the trochlear nerve peripheral to the place where it crosses the ophthalmic nerve is larger than the part of the nerve at its exit from the brain. This difference in size is sufficient to lead one to conclude that the trigeminal nerve contributes fibers to the distal part of the trochlear nerve.
  
The lacrimal nerve occasionally receives ‘a small branch
 
from the trochlear nerve. This contribution to the lacrimal
 
nerve is probably composed of fibers which are derived from
 
the ophthalmic nerve and which simply accompany the
 
trochlear nerve for a short distance. It has been reported
 
that the trochlear nerve occasionally sends a branch to the
 
  
Fig. 10 A diagram drawn from the sagittal sections of the brainstem of a
+
The lacrimal nerve occasionally receives ‘a small branch from the trochlear nerve. This contribution to the lacrimal nerve is probably composed of fibers which are derived from the ophthalmic nerve and which simply accompany the trochlear nerve for a short distance. It has been reported that the trochlear nerve occasionally sends a branch to the orbicularis oculi muscle (Piersol, ’30).‘ This branch, however, was not observed in any of the material studied by the author. In addition to the other communicating branches already mentioned, there are small filaments from the cavernous plexus of nerves Which join the trochlear nerve.
36-mm. human fetus (no. 34) showing a portion of the course of the trochlear
 
  
nerve. Copper protargol preparation.
+
Fig. 10 A diagram drawn from the sagittal sections of the brainstem of a 36-mm. human fetus (no. 34) showing a portion of the course of the trochlear nerve. Copper protargol preparation.
  
orbicularis oculi muscle (Piersol, ’30).‘ This branch, however, was not observed in any of the material studied by the
+
==Discussion==
author. In addition to the other communicating branches
 
already mentioned, there are small filaments from the
 
cavernous plexus of nerves Which join the trochlear nerve.
 
  
DISCUSSION
+
The investigations of several authors (Huber, ’00; Tozer and Sherrington, ’10, and others) have shown that there are sensory endings in the extrinsic muscles of the eye and that these endings degenerate after cutting the third and fourth cranial nerves (Tozer and Sherrington, ’10, and Tarkhan, ’34) . It has been more difficult to demonstrate clearly the location of the cell bodies of the sensory neurons involved.
  
The investigations of several authors (Huber, ’00; Tozer
 
and Sherrington, ’10, and others) have shown that there are sensory endings in the extrinsic muscles of the eye and that
 
these endings degenerate after cutting the third and fourth
 
cranial nerves (Tozer and Sherrington, ’10, and Tarkhan, ’34) .
 
It has been more difficult to demonstrate clearly the location
 
of the cell bodies of the sensory neurons involved.
 
  
The anatomical evidence submitted here would indicate that
+
The anatomical evidence submitted here would indicate that the proprioceptive neurons for the superior oblique muscle are of the mesencephalic V type. They are thought to be located in the brainstem close to the level of the trochlear nerve. The details of the connections of these neurons have been discussed, and they are in general agreement with the position taken by Weiriberg (’28) and Tar-khan (’34) on this question. The experimental work of Freeman (’27), Tarkhan_ (’34), and others strongly suggests that there are proprioceptive fibers in the fourth cranial nerve. The existence of proprioceptive neurons is indicated, after the removal of the orbital contents, by changes in the cells of the mesencephalic nucleus at the level of the trochlear nerve (Freeman, ’27). Corbin (’40) states that such fibers from the mesencephalic nucleus do not join the trochlear nerve extracranially from branches of the trigeminal nerve. Corbin and Harrison (’40 and ’42) found that no action potentials were elicited from the mesencephalic root of V as a result of stretching the extrinsic muscles of the eye. It was possible, however, to obtain action potentials from the oculomotor nucleus and the intramedullary portion of the oculomotor nerve. Corbin (’40) pointed out that if cells of the mesencephalic nucleus do contribute fibers to the trochlear nerve, this contribution must come from cells located between the levels of the trochlear nucleus and the decussation of that nerve. It seems quite likely that the majority of the cells in question may occupy this position. Tozer (’12), however, found no change in the meseneephalic nucleus after cutting the third and fourth cranial nerves. VVe thus have experimental evidence for and against the theory that eertainlcells of the mesencephalic nucleus contribute fibers to the trochlear nerve. It should be remembered that the results gained from the chromatolysis of cells and the Marchi method are not always reliable and that conclusions should be made with care. The more recent Work of Corbin and Oliver (’42) further indicates that the extrinsic ocular muscles are not innervated by the mesencephalic root of the fifth nerve. These authors are of the opinion that the cell bodies of the sensory neurons in question are located within the brainstem and are intermingled With the motor cells. Leinfelder and Black (’42) suggest that proprioceptive impulses may originate in other tissues than the extraocular muscles and their tendons. Tenon’s capsule is listed as a possibility for such nerve endings.
the proprioceptive neurons for the superior oblique muscle are
 
of the mesencephalic V type. They are thought to be located
 
in the brainstem close to the level of the trochlear nerve. The
 
details of the connections of these neurons have been discussed,
 
and they are in general agreement with the position taken by
 
Weiriberg (’28) and Tar-khan (’34) on this question. The
 
experimental work of Freeman (’27), Tarkhan_ (’34), and
 
others strongly suggests that there are proprioceptive fibers
 
in the fourth cranial nerve. The existence of proprioceptive
 
neurons is indicated, after the removal of the orbital contents,
 
by changes in the cells of the mesencephalic nucleus at the
 
level of the trochlear nerve (Freeman, ’27). Corbin (’40)
 
states that such fibers from the mesencephalic nucleus do not
 
join the trochlear nerve extracranially from branches of the
 
trigeminal nerve. Corbin and Harrison (’40 and ’42) found
 
that no action potentials were elicited from the mesencephalic
 
root of V as a result of stretching the extrinsic muscles of the
 
eye. It was possible, however, to obtain action potentials from
 
the oculomotor nucleus and the intramedullary portion of the
 
oculomotor nerve. Corbin (’40) pointed out that if cells of the
 
mesencephalic nucleus do contribute fibers to the trochlear nerve, this contribution must come from cells located between
 
  
the levels of the trochlear nucleus and the decussation of that
 
nerve. It seems quite likely that the majority of the cells in
 
question may occupy this position. Tozer (’12), however,
 
found no change in the meseneephalic nucleus after cutting
 
the third and fourth cranial nerves. VVe thus have experimental evidence for and against the theory that eertainlcells
 
of the mesencephalic nucleus contribute fibers to the trochlear
 
nerve. It should be remembered that the results gained from
 
the chromatolysis of cells and the Marchi method are not always reliable and that conclusions should be made with care.
 
The more recent Work of Corbin and Oliver (’42) further
 
indicates that the extrinsic ocular muscles are not innervated
 
by the mesencephalic root of the fifth nerve. These authors
 
are of the opinion that the cell bodies of the sensory neurons
 
in question are located within the brainstem and are intermingled With the motor cells. Leinfelder and Black (’42) suggest that proprioceptive impulses may originate in other
 
tissues than the extraocular muscles and their tendons.
 
Tenon’s capsule is listed as a possibility for such nerve
 
endings.
 
  
Proprioceptive fibers for the oculomotor nerve may have an
+
Proprioceptive fibers for the oculomotor nerve may have an origin similar to those of the trochlear nerve. The origin for those of the abducens nerve is a little more difficult to explain. While it seems to the author that cells of the mesencephalic V type offer the most likely source of proprioceptive fibers for the extrinsic ocular muscles, other possibilities should be examined. Perhaps there are patterns of proprioceptive conduction entirely different from those we now understand. Is it possible that motor nerves, as we know them, may play some part in this function“?
origin similar to those of the trochlear nerve. The origin for
 
those of the abducens nerve is a little more difficult to explain.
 
While it seems to the author that cells of the mesencephalic V
 
type offer the most likely source of proprioceptive fibers for
 
the extrinsic ocular muscles, other possibilities should be examined. Perhaps there are patterns of proprioceptive conduction entirely different from those we now understand. Is
 
it possible that motor nerves, as we know them, may play
 
some part in this function“?
 
  
SUMMARY
+
==Summary==
  
The nucleus of the trochlear nerve in human embryos and
+
The nucleus of the trochlear nerve in human embryos and fetuses is formed by a main mass of cells and often by an additional smaller mass of cells which lies farther caudad. This caudal group of cells may be bilateral or unilateral. When it is unilateral, it may occur on either the right or the left side. The size of the main nuclear group is often in inverse proportion to the size of the caudal group. Small cells are scattered along the course of the trochlear nerve Within the brain during certain stages of development. Sometimes these cells form a distinct cluster.
fetuses is formed by a main mass of cells and often by an
 
additional smaller mass of cells which lies farther caudad.
 
This caudal group of cells may be bilateral or unilateral.
 
When it is unilateral, it may occur on either the right or the
 
left side. The size of the main nuclear group is often in inverse proportion to the size of the caudal group. Small cells
 
are scattered along the course of the trochlear nerve Within
 
the brain during certain stages of development. Sometimes
 
these cells form a distinct cluster.
 
  
Occasional cells of the mesencephalic fifth type occur in the
+
Occasional cells of the mesencephalic fifth type occur in the main nucleus of the trochlear nerve. These cells are easily recognized in older fetuses. They also occur in relation to the course of the trochlear nerve within the brain. The processes of some of these cells extend among the fibers of the trochlear nerve.
main nucleus of the trochlear nerve. These cells are easily
 
recognized in older fetuses. They also occur in relation to the
 
course of the trochlear nerve within the brain. The processes of some of these cells extend among the fibers of the trochlear
 
nerve.
 
  
The cou1'se of the trochlear nerve at one point lies in close
+
The cou1'se of the trochlear nerve at one point lies in close relation to the mesencephalic root of the fifth nerve. It is difficult to determine to what extent there is an exchange of fibers. The trigeminal nerve contributes fibers to the trochlear nerve along its peripheral course. Some of these fibers
relation to the mesencephalic root of the fifth nerve. It is
 
difficult to determine to what extent there is an exchange of
 
fibers. The trigeminal nerve contributes fibers to the trochlear
 
nerve along its peripheral course. Some of these fibers
 
  
accompany the trochlear nerve toward the brain and are lost
+
accompany the trochlear nerve toward the brain and are lost in the meninges.
in the meninges.
 
  
LITERATURE CITED
+
==Literature Cited==
  
AR1i:'Ns KAPPERS, C. U., G. C. HUBER AND E. C. CROSBY 1936 The comparative
+
AR1i:'Ns KAPPERS, C. U., G. C. HUBER AND E. C. CROSBY 1936 The comparative anatomy of the nervous system of verteberates, including man. The Macmillan Co., New York.
anatomy of the nervous system of verteberates, including man. The
 
Macmillan Co., New York.
 
  
CORBIN, K. B. 1940 Observations of the peripheral distribution of fibers arising
+
CORBIN, K. B. 1940 Observations of the peripheral distribution of fibers arising in the mesencephalic nucleus of the fifth cranial nerve. J. Comp. Neur., vol. 73, pp. 153-177.
in the mesencephalic nucleus of the fifth cranial nerve. J. Comp. Neur.,
 
vol. 73, pp. 153-177.
 
  
Comanv, K. B., AND F. HARRISON 1940 Function of mesencephalic root of fifth
+
Comanv, K. B., AND F. HARRISON 1940 Function of mesencephalic root of fifth cranial nerve. J. Nenrophysio1., vol. 3, pp. 4234135.
cranial nerve. J. Nenrophysio1., vol. 3, pp. 4234135.
 
  
~——~—~~——- 1942 Further attempts to trace the origin of afferent nerves to the
+
~——~—~~——- 1942 Further attempts to trace the origin of afferent nerves to the extrinsic eye muscles. J. Comp. Neur., vol. 77, pp. 187~190.
extrinsic eye muscles. J. Comp. Neur., vol. 77, pp. 187~190.
 
  
CORBIN, K. B., AND R. K. OLIVER 1942 The origin of fibers to the grape-like
+
CORBIN, K. B., AND R. K. OLIVER 1942 The origin of fibers to the grape-like endings in the insertion third of the extra-ocular muscles. J. Comp. Neur., vol. 77, pp. 171—186.
endings in the insertion third of the extra-ocular muscles. J. Comp.
 
Neur., vol. 77, pp. 171—186.
 
  
FREEMAN W. 1927 The columnar arrangement of the primary afferent centers
+
FREEMAN W. 1927 The columnar arrangement of the primary afferent centers in the brain-stem of man. J. Nerv. and Ment. Dis., vol. 65, pp. 378——397.
in the brain-stem of man. J. Nerv. and Ment. Dis., vol. 65, pp. 378——397.
 
  
GEIICCHTEN, A. VAN 1903 Recherches sur 1’origine réelle et le trajet intracérebral des nerfs moteurs par la méthode de la dégénérescence wallérienne
+
GEIICCHTEN, A. VAN 1903 Recherches sur 1’origine réelle et le trajet intracérebral des nerfs moteurs par la méthode de la dégénérescence wallérienne indirecte. Le Névraxe, vol. 5, pp. 265-337.
indirecte. Le Névraxe, vol. 5, pp. 265-337.
 
  
GOLGI, C. 1893 Interim a1l’origine del quarto nervo cerebrale. Arch. Ital. de
+
GOLGI, C. 1893 Interim a1l’origine del quarto nervo cerebrale. Arch. Ital. de Biologie, vol., 19. (Quoted from Freeman.)
Biologie, vol., 19. (Quoted from Freeman.)
 
  
1117131112, G. C. 1900 Sensory nerve terminations in the tendons of the extrinsic
+
1117131112, G. C. 1900 Sensory nerve terminations in the tendons of the extrinsic eye~muscles of the cat. J. Comp. Neur., vol. 10, pp. 152-158.
eye~muscles of the cat. J. Comp. Neur., vol. 10, pp. 152-158.
 
  
LEINFELDER, P. J ., AND N. M. BLACK, JR. 1942 Experimental transposition of
+
LEINFELDER, P. J ., AND N. M. BLACK, JR. 1942 Experimental transposition of extraocular muscles in monkeys. Amer. J. 0phth._. vol. 25, pp. 974-976.
extraocular muscles in monkeys. Amer. J. 0phth._. vol. 25, pp. 974-976.
 
  
PEARSON, A. A. 19412» The development of the nervus terminalis in man. J.
+
PEARSON, A. A. 19412» The development of the nervus terminalis in man. J. Comp. Neux-., vol. 75, pp. 39-66. 1941b The development of the olfactory nerve in man. J. Comp. Neur., vol. 75, pp. 199-217. PIn1>.soL, G. A. 1930 Human Anatomy. Ninth edition (Huber). J. B. Lippincott 00., Philadelphia.
Comp. Neux-., vol. 75, pp. 39-66.
 
1941b The development of the olfactory nerve in man. J. Comp.
 
Neur., vol. 75, pp. 199-217.
 
PIn1>.soL, G. A. 1930 Human Anatomy. Ninth edition (Huber). J. B. Lippincott 00., Philadelphia.
 
  
SABIN, F‘. R. 1901 An atlas of the medulla and midbrain. The Friedenwald Co.,
+
SABIN, F‘. R. 1901 An atlas of the medulla and midbrain. The Friedenwald Co., Baltimore, Md.
Baltimore, Md.
 
  
 
SANO, F. 1941 fiber die zwei«order mehrfache Teilung des Trochleariskerns
 
SANO, F. 1941 fiber die zwei«order mehrfache Teilung des Trochleariskerns
  
beim Menschen und einigen Saugetieren. Tokyo Igakkwai Zassi, vol. 55,
+
beim Menschen und einigen Saugetieren. Tokyo Igakkwai Zassi, vol. 55, pp. 490-497.
pp. 490-497.
 
  
TARKHAN, A. A. 1934 The innervation of the extrinsic ocular muscles. J. Anat.,
+
TARKHAN, A. A. 1934 The innervation of the extrinsic ocular muscles. J. Anat., vol. 68, pp. 293—313.
vol. 68, pp. 293—313.
 
  
TOZER, F. M. 1912 On the presence of ganglion cells in the roots of the III, IV,
+
TOZER, F. M. 1912 On the presence of ganglion cells in the roots of the III, IV, and VI cranial nerves. J. Physiol., vol. 45. Proc. Physiol. Soc., p. XV.
and VI cranial nerves. J. Physiol., vol. 45. Proc. Physiol. Soc., p. XV.
 
  
 
Toznn, F. M., AND C. S. SHERRJNGTON 1910 Receptors and afferents of the
 
Toznn, F. M., AND C. S. SHERRJNGTON 1910 Receptors and afferents of the
Line 468: Line 194:
 
vol. 82, pp. 450-457.
 
vol. 82, pp. 450-457.
  
TSUCHIDA, U. 1906 Ueber die Ursprungskerne der Augenbewegungsnerven und
+
TSUCHIDA, U. 1906 Ueber die Ursprungskerne der Augenbewegungsnerven und fiber die mit diesen in Beziehung stehenden Bahnen im Mittel« und Zwischenhirn. Arb. a.. d. hirnanat. Inst. in Ziirich, Bd. 2, S. 1. (Quoted from Ariéns Kappers, Huber and Crosby.)
fiber die mit diesen in Beziehung stehenden Bahnen im Mittel« und
+
 
Zwischenhirn. Arb. a.. d. hirnanat. Inst. in Ziirich, Bd. 2, S. 1. (Quoted
+
VAN VALKENBURG, C. T. 1912 On the splitting of the nucleus trochlearis. Kon. Akad. v. Wetensch. te Amsterdam, Proc. sect. sc., vol. 14, pt. 2, p. 1023.
from Ariéns Kappers, Huber and Crosby.)
+
 
 +
WEINBERG, E. 1928 The mesencephalic root of the fifth nerve. J. Comp. New-., vol. 46, pp. 249-405.
  
VAN VALKENBURG, C. T. 1912 On the splitting of the nucleus trochlearis. Kon.
+
WINDLE, W. F. 1932a The neurofibrillar structure of the 7-mm. cat embryo. J. Comp. Neur., vol. 55, pp. 99-138. 1932b The neurofibrillar structures of the five-and-one-half-millr meter cat embryo. J. Comp. Neur., vol. 55, pp. 315—331.
Akad. v. Wetensch. te Amsterdam, Proc. sect. sc., vol. 14, pt. 2, p. 1023.
 
  
WEINBERG, E. 1928 The mesencephalic root of the fifth nerve. J. Comp. New-.,
+
ZAHAJSZKY, E. VON 1940 Beitréige zur Kenntnis der Entvvickelung des N. ocula motorius, des N. trochlearis und des motorischen N. trigeminus. Anat. Anz., Bd. 89, S. 316-332.
vol. 46, pp. 249-405.
 
  
WINDLE, W. F. 1932a The neurofibrillar structure of the 7-mm. cat embryo.
+
{{Footer}}
J. Comp. Neur., vol. 55, pp. 99-138.
 
1932b The neurofibrillar structures of the five-and-one-half-millr
 
meter cat embryo. J. Comp. Neur., vol. 55, pp. 315—331.
 
  
ZAHAJSZKY, E. VON 1940 Beitréige zur Kenntnis der Entvvickelung des N. ocula
+
[[Category:Draft]]
motorius, des N. trochlearis und des motorischen N. trigeminus. Anat.
 
Anz., Bd. 89, S. 316-332.
 

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Pearson AA. The trochlear nerve in human fetuses. (1943) J Comp. Neurol. : 29-43.

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This historic 1943 paper by Pearson described development of the fetal human trochlear nerve.


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The Trochlear Nerve in Human Fetuses

Anthony A. Pearson


Loyola University School of Medicine, Chicago, Illinois

Ten Figures (1943)

Introduction

The early development of the trochlear nerve in mammalian embryos has been described by Windle (’32 a, ’32 b), Zahajszky ( ’40), and others. The discussion will be limited to the later development of the trochlear nerve and its relation to the trigeminal nerve.‘ This study is based on serial sections of human embryos and fetuses stained with various silver methods. The whole heads of the smaller fetuses and em— bryos were sectioned and studied; however, only the brains of the larger fetuses were prepared. Most of the material used in this study has been listed in previous papers (Pearson, ’41 a, ’41 b) and only necessary data will be given here.

Nucleus Of Origin

The nucleus of the trochlear nerve is easily recognized in older human embryos and fetuses as a large mass of multipolar neurons located just caudal to the nucleus of the oculo— motor nerve. There is no wide interval between the nuclei of the third and fourth cranial nerves. A distinct thinning out of the cells of the motor column, however, indicates a boundary between the nuclei of these two nerves. The trochlear nucleus is in line with the lateral groups of cells of the oculomotor nucleus. It is located i11 its characteristic position, dorsal to and partly imbedded in the medial longitudinal fasciculus.


The nucleus of the trochlear nerve is formed by one main mass of cells and sometimes by an additional smaller mass of cells which lies farther caudad (figs. 1, 2, 3). The caudal group of cells may be bilateral or unilateral. VVhen it is unilateral, it may occur on either the right _or the left side.


This subdivision of the trochlear nucleus into a larger cephalic portion and a smaller caudal portion has been observed by several investigators (Sabin, ’O1; Tsuchida, ’06; van Valkenburg, ’12; and Sano, ’41). Tsuchida reported a subdivision in the trochlear nucleus in some 20 to 30% of the brainstems studied, this differentiation occurring in most of the specimens on only one side. In the material studied‘ by the author the percentages were somewhat higher. The caudal division of the trochlear nucleus occurred nearly as often on both sides as on only one side. When the caudal division was present on one side only, it was found to occur about as frequently on the right side as on the left side.


ABBREVIATIONS

aq.cer., or cer.aq., aquaeductus cerebri

cer., cerebellum

c.mes.V., cells of the mesencephalic root /of V

dcc.N.IV, decussation of the trochlear nerve

f.1.m., fasciculus longitudinalis medialis

4th vent., fourth ventricle

gang.N.V, ganglion semilunare

med.ob1., medulla oblongata

me.root V, mesencephalic root of the trigeminal nerve

mes.V et nu., mesencephalic root of V and nucleus

N.III, nervus oculomotorius

N .IV, ncrvus trochlearis

N.V, nervus trigeminus

nu.IV, nucleus nervi trochlearis

nu.IV p.caud., nucleus trochlearis (pars caudali)

V., or vent., ventriculus

X, 3. group of cells in the path of the trochlear nerve


Fig. 1 A agittal section through the brainstem of a 29-mm. human fetus (no. 29). Note the rostral and caudal divisions of the trochlear nucleus. Copper protargol preparation (section 16-1-2). X 12.


Fig. 2 A transverse section through the mesencephalon of a 5-month-old fetus (no. 64) at the level of the main nucleus of the trochlear nerve. In the left nucleus of the IV nerve there are two cells of the mesencephalic V type. Pyridine silver preparation (section 29-3-4). X 24.


In one human fetus of about 5 months of age (no. 63) the trochlear nucleus on each side was a single mass of cells. In another fetus (no. 64) of about the same age, there was a caudal division of the trochlear nucleus on each side (fig. 3). This caudal division was a small group of darkly staining Inultipolar neurons. It was located about 1 mm. caudal to the


Fig. 3 A transverse section through the mesencephalon of a 5-month-old fetus (no. 64) at the level of the caudal division of the trochlear nucleus. Pyridine silver preparation (section 24-3-3). X 24.

main nucleus and was approximately 0.2 mm. in length. This caudal nucleus of the trochlear nerve was slightly smaller on the right side than on the left. There are about sixty cells in the caudal nucleus on the right side and about eighty-three cells on the left side. This is only an approximation, as the cells were closely packed and it was difficult to make an exact count. In the fetus which possessed only the main nucleus of the trochlear nerve it is interesting to note that this main nucleus measured slightly more than 1 mm. in length and was quite symmetrical. In the other fetus of this same age, tl1e main nucleus measured less than 1 mm. in length. On the right side, the main nucleus was about 0.9 mm. in length and the caudal division contained about sixty cells. The main nucleus of the trochlear nerve on the left side measured about 0.6 mm. and the caudal division contained about eighty—three cells. The cross sectional area of the main nuclei of the trochlear nerve in both fetuses appeared to be approximately equal. These two fetuses are thought to be fair examples of the cellular arrangement of the trochlear nerve nuclei. This suggests that the size of the main nucleus is in inverse proportion to the size of the caudal division. There is no indication that the fiber connections of the caudal division are any different from those of the main nucleus of the trochlear nerve nor is there any apparent physiological significance to the separation of the cells of the trochlear nerve into groups.

A few cells of the mesencephalic V type were observed in the main nucleus of the trochlear nerve (fig. 2, left). These cells are easily distinguished from the usual multipolar neurons of the trochlear nucleus by their shape and size. Weinbe1'g' (’28) has made a similar observation.

In certain embryos, scattered cells have been observed along the course of the trochlear nerve central to its" decussation (fig. 4). These cells are thought to resemble neuroblasts, although their exact character is diflicult to determine. Occasionally, such cells form a distinct cluster. In one 3-month-old human fetus, such a group was found in the central course of the trochlear nerve on each side (fig. 5). This group is located dorsolateral to the medial longitudinal fasciculus and at a plane slightly caudal to the level of the trochlear nuclei. Some of the cells appear to be bipolar, although their exact shapes are not distinct. Sabin (’01) has also reported the presence of groups of cells along the root fibers of the trochlear nerve.


Fig. 4 A sagittal section through the brainstem of a 17-mm. human embryo (no. 16). Scattered cells are shown along the course of the trochlear nerve. Pyridine silver preparation (8—3~2). X 75.

cm. car,


Fig. 5 A transverse section through the mesencephalon of 9. 63-mm. human fetus (no. 57). A small cluster of cells is hown in the path of the IV nerve. Copper protargol preparation (section 48—2—3). X 14.


The Course of the Nerve

Fibers of the trochlear nerve are collected into several small bundles along the dorsolateral border of the trochlear nucleus toward its caudal end. When a caudal division of the trochlear nucleus is present an additional bundle of fibers arises here. This bundle soon joins the larger bundles of the trochlear nerve. The fibers of the trochlear nerve follow an oblique course through the periphery of the central gray which surrounds the cerebral aqueduct. At first the fibers pass dorsolaterad. On reaching the lateral part of the central gray, the course of the trochlear nerve fibers is directed caudad and dorsad. The caudal course of the nerve is attributed in part to the shifting forward of the cells of origin during development (Ariens Kappers, Huber, and Crosby, ’36). The fibers of the trochlear nerve runalmost parallel to the mesencephalic root of the trigeminal nerve for a short distance. As the fibers of the trochlear nerve approach the anterior medullary velum, they begin to turn toward the midline. Here they lie almost perpendicular to the fibers of the mesencephalic root of the trigeminal nerve. As the midline is approached, the fibers of the trochlear nerve can then be traced into the ventral part of the decussation of that nerve. After they pass to the other side, the fibers of the trochlear nerve emerge from the decussation and make their exit from the dorsolateral surface of the brainstem. No trochlear fibers were observed which failed to decussa.te as van Gehuchten (’03) noted in the rat and rabbit. ' The course of the trochlear nerve within the brain lies deep, but close to the mesencephalic root of the trigeminal nerve and the nucleus of that root. It has been suggested by several authors that there are connectionsxbetween the mesencephalic root of the trigeminal nerve and the trochlear nerve (Freeman, ’27, Weinberg, ’28 and others). Such a connection has been difficult to demonstrate or rule out. There are cells of the mesencephalic V type, however, which are thought to be definitely related to the trochlear nerve. Cells of this type occasionally occur -in the trochlear nucleus (fig. 2), as already mentioned. There are cells of the mesencephalic V type in the region lateral to the anterior medullary velum. Some of these cells appear to be connected with the trochlcar nerve. An occasional cell may be found directly in the course of this nerve (fig. 7). These cells may appear singly or in clusters. Occasionally, a process of one of these cells may be followed directly among the fibers of the trochlear nerve (fig. 8). A few scattered cells of the mesencephalic V type may be found in the anterior medullary Velum. An example of one of these cells is shown in figure 9. A bipolar cell is shown in the anterior medullary velum lying; ventral to the fibers of the trochlear nerve. A process from either end of the cell extends out among the fibers of the trochlear nerve. A similar cell was described by Golgi (1893). V-*'indle (’32 a) described a cell which sent one branch into the trochlear nerve and another branch into the mesencephalic tract of a cat.


Fig. 6 A diagram made from several sagittal sections of the brainstem of 2: 25-mm. human embryo (no. B. 1194). Pyridine silver preparation (13-20).


Fig. 7 A section through the trochlear nerve within the rnesencephalon. Note the nerve cell in the path of the nerve. From a 5»month—old human fetus (no. 64). Pyridine silver preparation (19—3—4).


Fig. 8 A transverse section through the mesencephalon of a 5-m.onth~old fetus (no. 64) at the level of tl1e anterior medullary velum. Note the close relation of the IV nerve to the meseneephalie root of the V nerve and its nucleus. Pyridine silver preparation (section 21-3-3). X 94.


Fig. 9 A photomicrograph of a nerve cell in the anterior medullary velum of a. 5—month—old human fetus (no. 63). The processes of the nerve cell extend dorsally among the fibers of the troehlear nerve. Pyridine silver preparation (193—4). X 450.


Peripheral to the decussation in the anterior medullary veluin, the fibers of the trochlear nerve leave the dorsolateral surface of the brainstem grouped in one or two small bundles. After leaving the brain, these fibers collect into a single small trunk, which can be traced ventrad close along the surface of the brainstem. Before reaching the orbit the fibers of the trochlear nerve pierce the dura mater and pass forward between the layers of the tentorium cerebelli.

As the trochlear nerve passes forward to enter the orbit it is joined by a small bundle of fibers from the ophthalmic division of the trigeminal nerve. This branch of the trigeminal nerve leaves the ophthalmic division just peripheral to the semilunar ganglion. It joins the trochlear nerve (figr. 10) and may be followed back toward the brain. Part of these fibers are thought to be distributed to the dura mater along the course of the trochlear nerve. A few of them can be followed between the layers of the dura mater into the 1'0/gion of the transverse sinus.

Just peripheral to the place where the above anastomotic branch joins the trochlear nerve, a short part of the course of the trochlear nerve lies in intimate relationship with the ophthalmic nerve. The fibers of the two nerves lie so close together that it is extremely difficult to determine in serial sections whether there is an exchange of fibers between them. After crossing the ophthalmic nerve, the trochlear nerve may be traced forward into the superior oblique muscle. The part of the trochlear nerve peripheral to the place where it crosses the ophthalmic nerve is larger than the part of the nerve at its exit from the brain. This difference in size is sufficient to lead one to conclude that the trigeminal nerve contributes fibers to the distal part of the trochlear nerve.


The lacrimal nerve occasionally receives ‘a small branch from the trochlear nerve. This contribution to the lacrimal nerve is probably composed of fibers which are derived from the ophthalmic nerve and which simply accompany the trochlear nerve for a short distance. It has been reported that the trochlear nerve occasionally sends a branch to the orbicularis oculi muscle (Piersol, ’30).‘ This branch, however, was not observed in any of the material studied by the author. In addition to the other communicating branches already mentioned, there are small filaments from the cavernous plexus of nerves Which join the trochlear nerve.

Fig. 10 A diagram drawn from the sagittal sections of the brainstem of a 36-mm. human fetus (no. 34) showing a portion of the course of the trochlear nerve. Copper protargol preparation.

Discussion

The investigations of several authors (Huber, ’00; Tozer and Sherrington, ’10, and others) have shown that there are sensory endings in the extrinsic muscles of the eye and that these endings degenerate after cutting the third and fourth cranial nerves (Tozer and Sherrington, ’10, and Tarkhan, ’34) . It has been more difficult to demonstrate clearly the location of the cell bodies of the sensory neurons involved.


The anatomical evidence submitted here would indicate that the proprioceptive neurons for the superior oblique muscle are of the mesencephalic V type. They are thought to be located in the brainstem close to the level of the trochlear nerve. The details of the connections of these neurons have been discussed, and they are in general agreement with the position taken by Weiriberg (’28) and Tar-khan (’34) on this question. The experimental work of Freeman (’27), Tarkhan_ (’34), and others strongly suggests that there are proprioceptive fibers in the fourth cranial nerve. The existence of proprioceptive neurons is indicated, after the removal of the orbital contents, by changes in the cells of the mesencephalic nucleus at the level of the trochlear nerve (Freeman, ’27). Corbin (’40) states that such fibers from the mesencephalic nucleus do not join the trochlear nerve extracranially from branches of the trigeminal nerve. Corbin and Harrison (’40 and ’42) found that no action potentials were elicited from the mesencephalic root of V as a result of stretching the extrinsic muscles of the eye. It was possible, however, to obtain action potentials from the oculomotor nucleus and the intramedullary portion of the oculomotor nerve. Corbin (’40) pointed out that if cells of the mesencephalic nucleus do contribute fibers to the trochlear nerve, this contribution must come from cells located between the levels of the trochlear nucleus and the decussation of that nerve. It seems quite likely that the majority of the cells in question may occupy this position. Tozer (’12), however, found no change in the meseneephalic nucleus after cutting the third and fourth cranial nerves. VVe thus have experimental evidence for and against the theory that eertainlcells of the mesencephalic nucleus contribute fibers to the trochlear nerve. It should be remembered that the results gained from the chromatolysis of cells and the Marchi method are not always reliable and that conclusions should be made with care. The more recent Work of Corbin and Oliver (’42) further indicates that the extrinsic ocular muscles are not innervated by the mesencephalic root of the fifth nerve. These authors are of the opinion that the cell bodies of the sensory neurons in question are located within the brainstem and are intermingled With the motor cells. Leinfelder and Black (’42) suggest that proprioceptive impulses may originate in other tissues than the extraocular muscles and their tendons. Tenon’s capsule is listed as a possibility for such nerve endings.


Proprioceptive fibers for the oculomotor nerve may have an origin similar to those of the trochlear nerve. The origin for those of the abducens nerve is a little more difficult to explain. While it seems to the author that cells of the mesencephalic V type offer the most likely source of proprioceptive fibers for the extrinsic ocular muscles, other possibilities should be examined. Perhaps there are patterns of proprioceptive conduction entirely different from those we now understand. Is it possible that motor nerves, as we know them, may play some part in this function“?

Summary

The nucleus of the trochlear nerve in human embryos and fetuses is formed by a main mass of cells and often by an additional smaller mass of cells which lies farther caudad. This caudal group of cells may be bilateral or unilateral. When it is unilateral, it may occur on either the right or the left side. The size of the main nuclear group is often in inverse proportion to the size of the caudal group. Small cells are scattered along the course of the trochlear nerve Within the brain during certain stages of development. Sometimes these cells form a distinct cluster.

Occasional cells of the mesencephalic fifth type occur in the main nucleus of the trochlear nerve. These cells are easily recognized in older fetuses. They also occur in relation to the course of the trochlear nerve within the brain. The processes of some of these cells extend among the fibers of the trochlear nerve.

The cou1'se of the trochlear nerve at one point lies in close relation to the mesencephalic root of the fifth nerve. It is difficult to determine to what extent there is an exchange of fibers. The trigeminal nerve contributes fibers to the trochlear nerve along its peripheral course. Some of these fibers

accompany the trochlear nerve toward the brain and are lost in the meninges.

Literature Cited

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Cite this page: Hill, M.A. (2020, August 11) Embryology Paper - The trochlear nerve in human fetuses. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_trochlear_nerve_in_human_fetuses

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