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The vertebral level of origin of spinal nerves in the rat.

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The Vertebral Level of Origin of Spinal Nerves in the Rat
JOHN B. GELDERD A N D SUZZETTE F CHOPIN
Department of A n n t o m y , L S U Medical Center, 1 1 00 Florida Aurnutj,
Building 1 3 7 , N e w Orlenns, Loiirsiunn 701 19 and Department of
A n n t o m y , Untverszty of Sozrth Florida, College of Medicine,
4202 Fowler Avenue, T a m p u , Floridn 33620
Several dissections were performed to determine the level of
spinal cord termination and the vertebral level at which the dorsal and ventral
roots of spinal nerves Cl-S4 emerged from the spinal cord in the rat.
These levels of emergence were then compared to the level of exit from the
vertebral canal. The dissections demonstrated that the effect of differential
growth between spinal cord and vertebral column begins in the lower cervical
region and becomes progressively more pronounced throughout thoracic and
lumbar levels. The disparity between the vertebral level of emergence of spinal
roots from the spinal cord and their level of exit via intervertebral foramina was
found to be considerably larger than was previously reported by Greene (‘68).
It was hrther noted that the spinal cord terminated at the level of the intervertebral disc between the third and fourth lumbar vertebrae, not between the
fourth and fifth lumbar vertebrae as reported by Greene (’68).
ABSTRACT
One of the major reference books for the
anatomy of the rat was published by
Greene i n 1935 and reprinted in 1968. This
text contains a n atlas of gross anatomical
structures of the albino Norway rat. According to the atlas, the dorsal and ventral
roots of spinal nerves were depicted as
arising from the spinal cord a t their corresponding vertebral levels, while the spinal cord itself was shown to terminate a t
approximately the level of the intervertebra1 disc between the fourth and fifth lumbar vertebrae (Greene, ’68, fig. 163).
&man and Innes reported the termination of the spinal cord at “about the third
and fourth lumbar vertebrae” (Zeman and
Innes, ’63: p. 50). These authors stated
that “the segments of spinal cord always
lie higher than the corresponding vertebrae” (Zeman and Innes, ’63: p. 52), a
result of differential growth. A diagram of
a spinal cord was included and demonstrated the emergence of spinal roots from
the spinal cord slightly more rostra1 than
their foramina of exit.
In the present investigation, gross anatomical dissections were performed to esANAT. REC., 188: 4 5 4 8 .
tablish both the vertebral level of spinal
cord termination and the vertebral level of
origin of spinal nerves from the spinal
cord.
MATERIALS AND METHODS
Fifteen mature, female Holtzman rats
and one mature, male Long Evans rat were
anesthetized with pentobarbitol (35 mg/
kg). The animals were sacrificed by intracardial perfusion with a 10% formalin
solution. A complete laminectomy was performed from the first cervical vertebra to
the lower part of the sacrum and the dura
mater removed. The course of the dorsal
and ventral roots of spinal nerves C1-S4
was traced from their spinal cord origin to
their exit via intervertebral foramina. The
dissections were performed with the aid of
a n Olympus MTX operating microscope.
RESULTS
The results of the dissections are summarized in table 1. Throughout most of the
Received Sept. 7 , ’76.Accepted Oct. 1 , ’76.
‘Supported by NIH Training Grant TO1 DE 00241-05,
NIH Grant 25-P-30268 and Edward G. Schleider Foundation Grant 410-11-6121,
45
46
JOHN B. GELDERD AND SUZZETTE F. CHOPIN
TABLE 1
Compnrisoia between w r t e h r n l leuel of orlyin
n n d exit of spincil roots
Spinal root
c1
c2
c3
c4
c5
C6
c7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
T13
L1
L2
L3
LA
L!5
L6
s1
Vertebral level of
origin of spinal
roots from the
s a i n a l cord
Intervertebral
level of exit
Base of skull-C1
c 1-c2
C2-C3
c3-c4
c4-c5
C5-C6
C6-C7
C7-T1
T1-T2
T2-T3
T3-T4
T4-T5
T5-T6
T6-T7
T7-T8
T8-T9
TS-T10
T10-T11
T11-Tl2
T12-Tl3
T13-L1
Ll-L2
L2-L3
L3-L4
IA-L5
L5-L6
L6-S1
s1-s2
s2
s2-s3
54
s3-s4
s4-COC. 1
s3
Spinal cord terminates at L3-L4.
(R),rostral, ( M ) . middle, (B). base
cervical region, the spinal roots arise from
the spinal cord at approximately the same
vertebral level at which the spinal nerves
exit their respective intervertebral foramina. Beginning with the C8 spinal root, a
disparity is observed such that the spinal
roots begin to arise a t significantly more
rostral levels than their corresponding
levels of exit. Spinal roots T1 through T12
emerge from the spinal cord approximately
one vertebra rostral to their level of exit.
As might be expected, the greatest disparity occurs in the lumbar region. Since our
dissections also show that the spinal cord
terminates at the level of the L3-L4 intervertebral disc, the remaining lumbar, sacral
and coccygeal nerve roots must arise from
the spinal cord within the first three lumbar vertebrae. The peripheral effects of
this disparity in the origin and exit of
spinal nerves is perhaps best exemplified
by the spinal nerves comprising the sciatic
nerve. In the rat, the sciatic nerve is composed of spinal segments L4,5,6. The roots
of these three spinal nerves arise from the
spinal cord within the limits of the L1 vertebra (table 1). They must then descend
distances of 2.7 cm (L4) to 4.5 cm (L6)
within the vertebral canal before gaining
access to the periphery (fig. 1). No species
differences were observed between Holtzman and Long Evans hooded rats.
DISCUSSION
These dissections demonstrate two facts:
(1) the spinal cord of the rat terminates a t
the level of the intervertebral disc between
the third and fourth lumbar vertebrae;
and (2) beginning in the lower cervical
region, the dorsal and ventral roots of
spinal nerves arise from the spinal cord a t
more rostral levels than their vertebral
level of exit. This phenomenon becomes
more pronounced i n lower thoracic and
lumbar regions.
Our findings tend to agree with those of
Zeman and Innes (‘63) who also note the
termination of the spinal cord at the level
of the third to fourth lumbar vertebrae.
The present dissections confirm and expand the results of these authors concerning the relationship between the origin of
spinal nerve roots and their corresponding
vertebrae. Spinal nerve roots do indeed
arise from the spinal cord rostral to their
foramina of exit, but our results show that
they originate considerably more rostral
than depicted by Zeman and Innes. In contrast, the results of our dissections are in
complete disagreement with Greene (‘68)
who reported that: (1) the spinal cord of
the rat terminates a t the intervertebral
disc between the fourth and fifth lumbar
vertebrae; and (2) the dorsal and ventral
roots of spinal nerves arise from the spinal
cord at their vertebral level of exit.
Because Greene’s publication is the only
one of which we are aware that deals extensively with the gross anatomical relationships in the rat, it is widely utilized as
a reference and guide by many researchers. Since spinal cord transections are routinely accomplished by performing a laminectomy at a given level then making a
complete cut through the meninges, spinal
nerve roots and spinal cord a t that level,
SPINAL CORD LEVEL VERSUS VERTEBRAL LEVEL
47
Fig. 1 Dorsal view of a gross dissection of the lumbar region i n the rat, showing the origin
of the spinal roots (L4,5,6) of the sciatic nerve (S) a n d their vertebral level of exit (arrows).
The spinal cord caudal to the L1 vertebra (R) h a s been removed so that the location of the
spinal roots may be more clearly seen as they descend within the vertebral canal. X 2.0.
the discrepencies between Greene’s published data and the present data assumes
great significance. If, for example, a spinal
cord transection is performed at the T 1 3
vertebral level, not only will the T 1 3 spinal
nerve roots be severed, but also spinal
nerve roots L1, L2, and L 3 with concomitant damage and cell loss occurring in
spinal segments L 3 and L4 (table 1).Therefore, great care must be exercised when
interpreting anatomical, neurophysiological and behavioral data following lesions
of the spinal cord.
LITERATURE CITED
Greene, E . C. 1968 Anatomy of the rat. Hafner
Publishing Co., Inc., New York.
Zeman, W., a n d J. R. M. Innes 1963 Craigie’s
Neuroanatomy of the Rat. Academic Press, New
York.
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