Chapter 41: The spinal cord and meninges

Spinal cord

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).

External features.

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).

Nerve roots.

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).

Internal structure.

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.

Blood Supply (see figs. 41-2 and 41-3).

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)

Dura mater.

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.

Arachnoid mater.

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).

Pia mater.

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.

Questions

41-1 At which level does the spinal cord end?

41-1 The spinal cord ends at the level of the L1 or 2 vertebra. The range is from one vertebra higher to one vertebra lower: T12 to L3 (A.F. Reimann and B. J. Anson, Anat. Rec.,88:127, 1944). In the newborn, the spinal cord extends to L3 vertebra.

41-2 What is a myelomere?

41-2 A myelomere is the segment of spinal cord to which a given pair of dorsal and ventral roots is attached.

41-3 How are myelomeres related to vertebrae in level?

41-3 Lower myelomeres are at higher levels than their correspondingly numbered vertebrae. Thus, myelomere S1 is opposite T12 or L1 vertebra (see fig. 41-1). For details see R. Louis (Anat. Clin., 1:3,1978).

41-4 Where are spinal ganglia found?

41-4 Spinal ganglia are found typically in the intervertebral foramina. Foramina is the plural of foramen: there is no such word as foraminae.

41-5 How many spinal nerves are present in the body?

41-5 There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

41-6 Where do spinal nerves emerge in relation to their correspondingly numbered vertebrae?

41-6 The emergence of spinal nerves can be appreciated by remembering that (a) C1 nerve emerges between the skull and atlas and (b) C8 nerve emerges between C7 and T1 vertebra. Hence all spinal nerves from T1 downward emerge below their correspondingly numbered vertebrae.

41-7 What is the cauda equina?

41-7 The cauda equina is the bundle of nerve roots in the vertebral canal below those of L1. The term, which means "horse's tail" in Latin, is a translation of a Hebrew term found in the Talmud. Compression of the cauda equina (e.g., from a massive nuclear herniation) may cause pain and numbness (buttocks, back of thighs and legs, and soles), weakness in the lower limbs, and paralysis of the bladder and intestine.

41-8 At which levels is subarachnoid space found below the spinal cord?

41-8 The spinal cord ends at L1 or 2; the subarachnoid space continues to the second sacral level. Between these levels, frequently above or below L.V.5, the subarachnoid space is tapped in lumbar puncture. The needle traverses the interspinous ligament and pierces the ligamentum flavum between the laminae. It traverses the epidural space before entering the subarachnoid space. Valuable practical information and illustrations can be found in R. Macintosh, Lumbar Puncture and Spinal Analgesia, 2nd ed., Livingstone, Edinburgh, 1957. See also figure 41-1.

41-9 Which Latin and Greek roots are used with reference to the spinal cord?

41-9 The Latin word spina is used with reference to the spinal cord (e.g., corticospinal tracts) as well as to the vertebral column. The Latin word medulla means "marrow" but is used in English chiefly for the medulla oblongata, the lowest part of the brain stem (and formerly called the bulb). The medulla is continued downward as the spinal cord (medulla spinalis in Latin). The corresponding Greek word is myelos (marrow), as used in myelomeningocele, a type of spina bifida cystica in which the spinal cord and meninges protrude through the vertebral defect. Other examples are myelography (contrast radiography of the spinal cord) and poliomyelitis (literally gray-matter spinal cord inflammation). The root myelo-, however, is used also with reference to the bone marrow (e.g., in the cell termed the myeloblast).

Figure legends

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.)

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