Chapter 41: The spinal cord and meninges
The spinal cord, about 45 cm in length, extends from the foramen magnum, where it is continuous with the medulla oblongata, to the level of the first or second lumbar vertebra (The range is T12 to L3). Below that level, the vertebral canal is occupied by spinal nerve roots and meninges. A fibrous strand, the filum terminale, continues from the spinal cord down to the coccyx (fig. 41-1).
The spinal cord presents a cervical and a lumbar enlargement at the levels of attachment of the nerves to the limbs. The inferiormost end of the cord is conical and is termed the conus medullaris. The coccygeal nerves are attached to it. The cord presents a posterior median sulcus and an anterior median fissure, lateral to which the dorsal and ventral root filaments are attached (figs. 41-2 and 41-3). The segment of spinal cord to which a given pair of dorsal and ventral roots is attached is a myelomere (fig. 41-2). Because the adult spinal cord does not extend down as far as the vertebral column does, the lower myelomeres are not opposite their correspondingly numbered vertebrae. Thus myelomere S1 is opposite the T12 vertebra (see fig. 41-1).
Each dorsal root presents a swelling, the spinal (dorsal root) ganglion, which lies near or within the intervertebral foramen. Distal to the ganglion, each dorsal root combines with the corresponding ventral root to form a spinal nerve (figs. 41-2 and 41-3). There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. The first pair of spinal nerves emerges between the atlas and the skull; hence C1 to 7 nerve roots leave the vertebral canal above the correspondingly numbered vertebrae. C8 emerges below the C7 vertebra, and all the remaining spinal nerves leave inferior to the corresponding vertebrae. The nerve roots below L1, and those which occupy the vertebral canal inferior to the cord, resemble a horse's tail and hence are collectively called the "cauda equina" (fig. 41-4).
In cross section, the spinal cord is seen to consist of gray matter, which is shaped like the letter "H" surrounded by white matter (see figs. 41-2 and 41-3). Regional differences occur; e.g., the contour of the gray matter varies, and the amount of white matter decreases as one descends the cord. There is a central canal running the length of the spinal cord, which extends from the fourth ventricle of the brain to the upper part of the filum terminale.
Because of the discrepancy between the levels of the myelomeres and their corresponding vertebrae, the lower spinal roots become increasingly oblique (figs. 41-1 and 41-4). Because myelomere S1 is opposite the T12 vertebra, the S1 nerve roots must descend steeply in order that the rami can emerge through the first sacral foramina. The lumbosacral roots are the longest and the thickest. The lumbar nerves increase in size from above downward, whereas the lumbar intervertebral foramina decrease in diameter. Thus the L5 nerve root, the thickest, traverses the narrowest foramen. Therfore, it has an increased chance of compression by pathology compromising the foramen.
The spinal cord contains the descending motor tracts and the ascending sensory tracts. The cervical and lumbar enlargements contain the neurons that supply the limbs. The cervical part of the cord conteins motor neurons giving rise to the spinal part of the accessory nerve and contains the neurons that supply the diaphragm. The thoracic and upper lumbar parts of the cord contain preganglionic sympathetic neurons, and the sacral cord contains parasympathetic preganglionic neurons giving rise to pelvic splanchnic nerves.
The spinal cord is supplied by three longitudinal arterial channels, which are reinforced by segmental (e.g. branches from intercostal and lumbar) arteries. The anterior spinal artery (from the vertebral artery) lies in the anterior median fissure. Two posterior spinal arteries (also from the vertebral artery, directly or indirectly) descend lateral to the posterior median sulcus. Although these arterial channels may run the length of the spinal cord, they are not sufficient to supply the entire cord. They are reinforced at intervals by segmental arteries arising from intercostal or lumbar arteries that follow the nerve roots (radicular arteries) to the spinal cord. The segmental reinforcements are very important in reinforcing the longitudinal channels. There is usually at least one large contribution every 4-6 segments and there is often a large vessel in the lower thoracic region that is critical to the supply of the lumbar enlargement (arteria magna of Adamkiewicz).
Meninges (see fig. 41-3)
The spinal cord, like the brain, is surrounded by the three meninges. The dura mater extends from the foramen magnum to the sacrum and coccyx (see fig. 41-1). The dura is attached to the foramen magnum and the periosteium covering the uppemost cervical vertebrae and their ligaments. Through the remainder of the vertebral canal, the dura is not attached to the vertebrae, being separated by the epidural (or peridural or extradural) space, which contains fat and the internal vertebral venous plexus. In caudal analgesia, an anesthetic solution injected into the sacral hiatus diffuses upward into the epidural space (see fig. 41-1). This may be used in surgical procedures relating to pelvic and perineal regions. Extensions of dura (dural sheaths) surround the nerve roots and spinal ganglia, and continue into the connective tissue coverings (epineurium) of the spinal nerves.
The arachnoid invests the spinal cord loosely. Continuous with the cerebral arachnoid above, it traverses the foramen magnum and descends to about the S2 vertebral level. The subarachnoid space, which contains cerebrospinal fluid (C.S.F.), is a wide interval between the arachnoid and pia. Because the spinal cord ends at about the level of the L2 vertebra, whereas the subarachnoid space continues to S2, access can be gained to the C.S.F. by inserting a needle between the vertebral lamina below the end of the cord, a procedure termed lumbar puncture (see fig. 41-1). By this means, the pressure of C.S.F. can be measured, the fluid can be analyzed, a spinal anesthetic can be introduced, or fluid can be replaced by a contrast medium for radiography (myelography).
The pia mater invests the spinal cord closely, ensheathes the anterior spinal artery (as the linea splendens), and enters the anterior median fissure. Laterally, the pia forms a discontinuous longitudinal septum, the denticulate ligament (see fig. 41-3), which sends about 21 tooth-like processes laterally to fuse with the arachnoid and dura on each side. The ligament is a surgical landmark in that it is attached to the spinal cord about midway between the attachments of dorsal and ventral roots.
Further details concerning the spinal cord should be sought in books on neuroanatomy.
41-1 At which level does the spinal cord end?
41-2 What is a myelomere?
41-3 How are myelomeres related to vertebrae in level?
41-4 Where are spinal ganglia found?
41-5 How many spinal nerves are present in the body?
41-6 Where do spinal nerves emerge in relation to their correspondingly numbered vertebrae?
41-7 What is the cauda equina?
41-8 At which levels is subarachnoid space found below the spinal cord?
41-9 Which Latin and Greek roots are used with reference to the spinal cord?
Figure 41-1 Median section of the vertebral column, showing the different levels of the vertebral bodies, myelomeres, and spinous processes. The spinal cord ends at the Ll/2 vertebral level and the subarachnoid space at S1/2 level. Cisternal, lumbar, and epidural punctures are shown. As an example of a spinal nerve, the S1 nerve can be seen arising from myelomere S1 opposite the T12 vertebra, descending (as part of the cauda equina), and emerging from the first sacral foramen.
Figure 41-2 A myelomere of the spinal cord, and one of its two associated spinal nerves. In A: A, anterior median fissure; P, posterior median sulcus. B shows the arterial supply to the cord.
Figure 41-3 Horizontal section of the spinal cord showing the meninges. The dura is in yellow, the arachnoid in red, and the pia in blue. The anterior and posterior spinal arteries are shown. C.S.F., cerebrospinal fluid in the subarachnoid space.
Figure 41-4 The spinal cord and cauda equina in situ. posterior aspect, made visible by a laminectomy on the right-hand side. The dorsal rami are omitted. The intervertebral discs are shown in blue. It can be seen that prolapse of disc L4/5, for example, would be likely to damage L5 roots. (Based partly on Pernkopf.)