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.