Spinal cord and spinal nerves: gross anatomy

The gross anatomy of the structures that lie within the vertebral canal and their extensions through the intervertebral foramina, the spinal nerve or radicular (‘root’) canals, is described in this chapter. The internal organization of the spinal cord is described in Chapter 27 .

The spinal cord and its blood vessels and nerve roots lie within a meningeal sheath, the theca, which occupies the central zone of the vertebral canal and extends from the foramen magnum, where it is in continuity with the meningeal coverings of the brain, to the level of the second sacral vertebra in the adult. Distal to this level, the dura extends as a fine cord, the filum terminale externum, which fuses with the posterior periosteum of the first coccygeal segment. Tubular prolongations of the dural sheath extend around the spinal roots and nerves into the lateral zones of the vertebral canal and out into the root canals, eventually fusing with the epineurium of the spinal nerves. Between the theca and the walls of the vertebral canal is the epidural (spinal extradural) space, which is loosely filled with fat, connective tissue containing small arteries and lymphatics, and an important venous plexus. Three-dimensional appreciation of the anatomy of the spinal theca and its surroundings is essential for the efficient management of spinal pain and of spinal injuries, tumours and infections. Equally significant clinically is the anatomy of the often-precarious blood supply of the spinal cord and its associated structures. The increasing application and refinement of diagnostic imaging and endoscopic procedures lend a new importance to topographical detail here.

Spinal Cord

The spinal cord occupies the superior two-thirds of the vertebral canal ( Figs 47.1–47.2 ). It is continuous cranially with the medulla oblongata, just below the level of the foramen magnum, at the upper border of the atlas. It terminates caudally as the conus medullaris. During development, the vertebral column elongates more rapidly than the spinal cord, so that there is an increasing discrepancy between the anatomical level of spinal cord segments and their corresponding vertebrae. At stage 23, the vertebral column and spinal cord are the same length and the cord ends at the last coccygeal vertebra. In later fetal life, the conus medullaris lies between the third lumbar and fifth sacral vertebrae. In premature and term neonates, it lies between the first and third lumbar vertebrae, and in children between the ages of 1 and 7 years, it lies between the twelfth thoracic and third lumbar vertebrae ( , , , , ). In the adult, the spinal cord terminates on average at the level of the middle third of the body of the first lumbar vertebra ( ) (see Fig. 46.81 ), which corresponds approximately to the transpyloric plane. However, it may end as high as the middle third of the body of the eleventh thoracic vertebra or as low as the middle third of the body of the third lumbar vertebra ( ). Its position rises slightly in vertebral flexion, and there is some correlation with the length of the trunk, especially in females. (For dimensional data, consult .)

Fig. 47.1, G , Axial T2-weighted MRI of the cervical spine at the level of C4 showing the ventral and dorsal nerve rootlets. The grey and white matter of the spinal cord can also be differentiated: white matter is lower signal. (A, Dissection by MCE Hutchinson, photograph by Kevin Fitzpatrick, London. B–D, Sobotta Atlas of Human Anatomy © Elsevier GmbH, Urban & Fischer, Munich.)

The filum terminale, a filament of connective tissue approximately 20 cm long, descends from the apex of the conus medullaris. Its upper 15 cm, the filum terminale internum, is continued within extensions of the dural and arachnoid meninges and reaches the caudal border of the second sacral vertebra. Its final 5 cm, the filum terminale externum, fuses with the investing dura mater, and then descends to the dorsum of the first coccygeal vertebral segment. The filum is continuous above with the spinal pia mater. A few strands of nerve fibres, which probably represent the roots of rudimentary second and third coccygeal spinal nerves, adhere to its upper part. The central canal is continued into the filum for 5–6 mm. A capacious part of the subarachnoid space surrounds the filum terminale internum, and is the site of election for access to the cerebrospinal fluid (CSF) via a lumbar puncture (see below).

The spinal cord varies in transverse width, gradually tapering craniocaudally, except at the levels of the enlargements. It is not cylindrical, being wider transversely at all levels, especially in the cervical segments. The cervical enlargement, the source of the large spinal nerves that supply the upper limbs, extends from the third cervical to the second thoracic segments. Its maximum circumference (approximately 38 mm) is in the sixth cervical segment (a spinal cord segment provides the attachment of the rootlets of a pair of spinal nerves). The lumbar enlargement, the source of the large spinal nerves that supply the lower limbs, extends from the first lumbar to the third sacral segments; the equivalent vertebral levels are the ninth to twelfth thoracic vertebrae. Its greatest circumference (approximately 35 mm) is near the lower part of the body of the twelfth thoracic vertebra, below which it rapidly dwindles into the conus medullaris.

Fissures and sulci extend along most of the external surface. A ventral (anterior) median fissure (sulcus) and a posterior median sulcus and septum almost completely separate the cord into right and left halves; they are joined by a commissural band of nervous tissue that contains a central canal (see Fig. 27.1 ).

The ventral median fissure extends along the whole ventral surface with an average depth of 3 mm, although it is deeper at caudal levels. It contains a reticulum of pia mater. Dorsal to it is the anterior white commissure. Perforating branches of the spinal vessels pass from the fissure to the commissure to supply the central spinal region. The posterior median sulcus is shallower, and from it a posterior median septum penetrates more than halfway into the cord, almost to the central canal. The septum varies in anteroposterior extent from 4 to 6 mm, and diminishes caudally as the canal becomes more dorsally placed and the cord contracts. The patency of the central canal diminishes with age: a postmortem study found the canal to be patent along the length of the cord in infants under 1 year of age, but occluded in most segments with increasing age after the second decade ( ).

Dorsal rootlets of spinal nerves enter the cord along a posterolateral sulcus that lies from 1.5 to 2.5 mm lateral to each side of the posterior median sulcus. The white matter between the posteromedian and posterolateral sulci on each side is the dorsal (posterior) funiculus. In cervical and upper thoracic segments, a longitudinal posterointermediate sulcus marks a septum dividing each posterior funiculus into two large tracts on either side, a medial fasciculus gracilis and a lateral fasciculus cuneatus. A ventrolateral (anterolateral) funiculus lies between the posterolateral sulcus and ventral median fissure, and is subdivided into ventral (anterior) and lateral funiculi by ventral spinal rootlets that pass through its substance to issue from the surface of the cord. The ventral funiculus is medial to, and includes, the emerging ventral rootlets, whilst the lateral funiculus lies between the roots and the posterolateral sulcus. In upper cervical segments, nerve rootlets emerge through each lateral funiculus to form the accessory nerve, which ascends in the vertebral canal lateral to the spinal cord and enters the posterior cranial fossa via the foramen magnum (see Fig. 35.11 ).

Dorsal and ventral roots

The paired dorsal and ventral roots of the spinal nerves are continuous with the spinal cord ( Fig. 47.1F ). They cross the subarachnoid space and traverse the dura mater separately, uniting in or close to their intervertebral foramina to form the (mixed) spinal nerves. Since the spinal cord is shorter than the vertebral column, the more caudal spinal roots descend for varying distances around and beyond the cord to reach their corresponding foramina. In so doing, they form a divergent sheaf of spinal nerve roots, the cauda equina, which is gathered round the filum terminale in the spinal theca, mostly distal to the apex of the cord.

Ventral spinal roots contain efferent somatic and, at some levels, preganglionic sympathetic, axons that extend from neuronal cell bodies in the ventral horns and intermediolateral columns, respectively. There are also afferent nerve fibres in these roots. The rootlets comprising each ventral root emerge from the anterolateral sulcus in groups over an elongated vertical elliptical area (see Fig. 47.1F ). Dorsal spinal roots bear ovoid swellings, the spinal ganglia, one on each root proximal to its junction with a corresponding ventral root in an intervertebral foramen. Each root fans out into 6–8 rootlets before entering the cord in a vertical row in the posterolateral sulcus. Dorsal roots are usually said to contain only afferent axons (both somatic and visceral), which are the central processes of unipolar neurones in the spinal root ganglia, but they may also contain a small number (3%) of efferent fibres and autonomic vasodilator fibres.

Each ganglionic neurone has a single short stem that divides into a medial (central) branch that enters the spinal cord via a dorsal root, and a lateral (peripheral) branch that passes peripherally to a sensory end organ. The central branch is an axon while the peripheral one is an elongated dendrite (but when traversing a peripheral nerve is, in general structural terms, indistinguishable from an axon). The region of spinal cord associated with the emergence of a pair of nerves is a spinal segment but there is no actual surface indication of segmentation. Moreover, the deep neural sources or destinations of radicular fibres may lie far beyond the confines of the ‘segment’ so defined.

Meninges

Dura mater

In some areas within the skull, the dura mater can be distinguished from the endosteum, but at the base of the skull around the foramen magnum the two layers are fused and adherent to the bone. Distal to the foramen magnum, within the vertebral column, the dura is distinct from the tissues that line the vertebral canal, and separated from them by the epidural space (see below). The spinal dura mater forms a tube whose upper end is attached to the edge of the foramen magnum and to the posterior surfaces of the second and third cervical vertebral bodies, and also by fibrous bands to the posterior longitudinal ligament, especially towards the caudal end of the vertebral canal. The dural tube narrows at the lower border of the second sacral vertebra. It invests the thin spinal filum terminale, descends to the back of the coccyx, and blends with the periosteum.

Epidural space

The epidural space lies between the spinal dura mater and the tissues that line the vertebral canal (see Fig. 47.2 ). It is closed above by fusion of the spinal dura with the edge of the foramen magnum, and below by the posterior sacrococcygeal ligament that closes the sacral hiatus. It contains loosely packed connective tissue, fat, a venous plexus, small arterial branches, lymphatics and fine fibrous bands that connect the theca with the lining tissue of the vertebral canal. These bands, the meningovertebral ligaments, are best developed anteriorly and laterally. Similar bands tether the nerve root sheaths or ‘sleeves’ within their canals. There is also a midline attachment from the posterior spinal dura to the ligamentum nuchae at atlanto-occipital and atlanto-axial levels ( ). The venous plexus consists of longitudinally arranged chains of vessels, connected by circumdural venous ‘rings’. The anteriorly placed vessels receive the basivertebral veins.

Fig. 47.2, The epidural and subarachnoid spaces.

The shape of the space within each spinal segment is not uniform, though the segmental pattern is metamerically repeated. It is difficult to define the true shape of the ‘space’ because it changes with the introduction of fluid or as a result of preservation techniques. In the lumbar region, the dura mater is apposed to the walls of the vertebral canal anteriorly and attached by connective tissue in a manner that permits displacement of the dural sac during movement and venous engorgement. Adipose tissue is present posteriorly in recesses between the ligamentum flavum and the dura. The connective tissue extends for a short distance through the intervertebral foramina along the sheaths of the spinal nerves. Like the main thecal sac, the root sheaths are partially tethered to the walls of the foramina by fine meningovertebral ligaments.

A variety of pathological processes can occur within the epidural space, compressing the dura and resulting in pain and potential neurological disturbance. Among the more common pathological entities seen are infection, haematoma and tumours ( Figs 47.3–47.4 ).

Fig. 47.3, A T1 post-contrast sagittal lumbar spinal magnetic resonance (MR) image showing a spinal epidural abscess (arrow) adherent to the lumbar dura and spinal nerves in a 28-year-old female with a history of intravenous drug abuse.

Fig. 47.4, A sagittal thoracic MR image showing a large ventral spinal epidural haematoma (arrow) resulting in severe neck and upper thoracic pain and ataxia.

Epidural injections

Contrast media and other fluids injected into the epidural space at the sacral level can spread up to the cranial base. Local anaesthetics injected near the spinal nerves, just outside the intervertebral foramina, may spread up or down the epidural space to affect the adjacent spinal nerves or may pass to the opposite side. The paravertebral spaces of each side communicate via the epidural space, particularly at lumbar levels.

For a review of the morphology of the epidural space and a discussion of the nature of the lining layer of the vertebral canal, see .

Subdural space

The subdural space is a potential space in the normal spine because the arachnoid and dura are closely apposed ( ). It does not connect with the subarachnoid space but continues for a short distance along the cranial and spinal nerves. Accidental subdural catheterization may occur during epidural injections. Injection of fluid into the subdural space may damage the cord either by direct toxic effects or by compression of the vasculature.

Arachnoid mater

The spinal arachnoid mater surrounds the spinal cord and is continuous with the cranial arachnoid mater ( Fig. 47.5 ). It is closely applied to the deep aspect of the dura mater. At sites where vessels and nerves enter or leave the subarachnoid space, the arachnoid mater is reflected on to the surface of these structures and forms a thin coating of leptomeningeal cells over the surface of both vessels and nerves. Thus a subarachnoid angle is formed as nerves pass through the dura into the intervertebral foramina. At this point, the layers of leptomeninges (arachnoid and pia) fuse and become continuous with the perineurium. The epineurium is in continuity with the dura. Such an arrangement seals the subarachnoid space so that particulate matter does not pass directly from the subarachnoid space into nerves. The existence of a pathway of lymphatic drainage from the CSF is controversial.

Fig. 47.5, Part of the spinal cord exposed from the anterior aspect to show the meningeal coverings.

Pia mater

The spinal pia mater (see Fig. 47.5 ) closely invests the surface of the spinal cord and passes into the ventral median fissure. The subpial collagenous layer in the spinal subpial ‘space’ is thicker than it is in the cerebral region and is continuous with the collagenous core of the ligamentum denticulatum (denticulate ligament).

The ligamentum denticulatum is a flat, fibrous sheet on either side of the spinal cord between the ventral and dorsal spinal roots. Its medial border is continuous with the subpial connective tissue of the cord and its lateral border forms a series of triangular processes, the apices of which are fixed at intervals to the dura mater. The first crosses behind the vertebral artery where it is attached to the dura mater, and is separated by the artery from the first cervical ventral root. Its site of attachment to the dura mater is above the rim of the foramen magnum, just behind the hypoglossal nerve; the accessory nerve ascends on its posterior aspect (see Fig. 35.11 ). The last of the dentate ligaments lies between the exiting twelfth thoracic and first lumbar spinal nerves and is a narrow, oblique band that descends laterally from the conus medullaris. Changes in the form and position of the dentate ligaments during spinal movements have been demonstrated by cine-radiography. Beyond the conus medullaris, the pia mater continues as a coating of the filum terminale.

Intermediate layer

In addition to the well-defined coats of arachnoid and pia mater, the cord is also surrounded by an extensive intermediate layer of leptomeninges. This layer is concentrated in the dorsal and ventral regions, and forms a highly perforated, almost lace-like structure that is focally compacted to form the dorsal, dorsolateral and ventral ligaments of the spinal cord. Dorsally, the intermediate layer is adherent to the deep aspect of the arachnoid mater and forms a discontinuous series of dorsal ligaments that attach the spinal cord to the arachnoid. The dorsolateral ligaments are more delicate and fenestrated, and they extend from the dorsal roots to the parietal arachnoid. As the intermediate layer spreads laterally over the dorsal surface of the dorsal roots, it becomes increasingly perforated and eventually disappears. A similar arrangement is seen over the ventral aspect of the spinal cord but the intermediate layer is less substantial.

The intermediate layer is structurally similar to the trabeculae that cross the cranial subarachnoid space, i.e. it has a collagenous core coated by leptomeningeal cells. The intermediate layers of leptomeninges around the spinal cord may act as a baffle within the subarachnoid space to dampen waves of CSF movement in the vertebral canal. Inflammation within the spinal subarachnoid space may result in extensive fibrosis within the intermediate layer and the complications of chronic arachnoiditis ( Fig. 47.6 ).

Fig. 47.6, A , A lumbar spinal nerve and its roots and meningeal coverings. B – D , Extradural anomalies of the lumbar nerve roots.

Coverings and relations of the spinal roots and nerves in the radicular canal

Tubular prolongations of spinal dura mater, closely lined by arachnoid, extend around the spinal roots and nerves as they pass through the lateral zone of the vertebral canal and through the intervertebral foramina (see Fig. 47.5 ; Fig. 47.6A ). These prolongations, the spinal nerve sheaths or root sheaths, gradually lengthen as the spinal roots become increasingly oblique. Each individual dorsal and ventral root runs in the subarachnoid space with its own covering of pia mater. Each root pierces the dura separately, taking a sleeve of arachnoid with it, before joining within the dural prolongation just distal to the spinal ganglion. The dural sheaths of the spinal nerves fuse with the epineurium, within or slightly beyond the intervertebral foramina. The arachnoid prolongations within the sheaths do not extend as far distally as their dural coverings, but the subarachnoid space and the CSF it contains extend sufficiently distally to form a radiologically demonstrable root sleeve for each nerve. Shortening or obstruction of this sleeve seen on magnetic resonance imaging (MRI) indicates compression of the spinal nerve. At the cervical level, where the nerves are short and the vertebral movement is greatest, the dural sheaths are tethered to the periosteum of the adjacent transverse processes. In the lumbosacral region, there is less tethering of the dura to the periosteum, though there may be an attachment posteriorly to the facet joint capsule.

Cerebrospinal Fluid (CSF)

The cerebrospinal fluid is described in detail in Chapter 25 . Although there is free communication between the spinal and cerebral subarachnoid spaces, the mode of circulation of the spinal CSF and the contribution that it makes to the overall circulation of CSF remain uncertain in humans: CSF may be absorbed from the spinal subarachnoid space; spinal arachnoid granulations and villi have been described ( ).

Spinal Nerves

In those body segments that largely retain a metameric (segmental) structure, e.g. the thoracic region, spinal nerves show a common plan ( Fig. 47.7 ). The dorsal, epaxial, ramus passes back lateral to the articular processes of the vertebrae and divides into medial and lateral branches that penetrate the deeper muscles of the back; both branches innervate the adjacent muscles and supply a band of skin from the posterior median line to the lateral border of the scapula ( Fig. 47.8 ). The ventral, hypaxial, ramus is connected to a corresponding sympathetic ganglion by white and grey rami communicantes. It innervates the prevertebral muscles and curves around in the body wall to supply the lateral muscles of the trunk. Near the mid-axillary line, it gives off a lateral branch that pierces the muscles and divides into anterior and posterior cutaneous branches. The main nerve advances in the body wall, where it supplies the ventral muscles and terminates in branches to the skin.

Fig. 47.7, The formation and branching pattern of a typical spinal nerve.

Fig. 47.8, The cutaneous distribution of the dorsal rami of the spinal nerves. The nerves are shown lying on the superficial muscles. The nerves are numbered on the right side; the spines of the seventh cervical, sixth and twelfth thoracic, and first and fifth lumbar vertebrae are labelled on the left side.

Spinal nerves are united ventral and dorsal spinal roots, attached in series to the sides of the spinal cord. The term spinal nerve strictly applies only to the short segment after union of the roots and before branching occurs. This segment, the spinal nerve proper, lies in the intervertebral foramen; it is sometimes mistakenly called the ‘nerve root’. There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. The abbreviations C, T, L, S and Co, with appropriate numerals, are commonly applied to individual nerves as individual ‘root values’. The peripheral nerves emerge through the intervertebral foramina. At thoracic, lumbar, sacral and coccygeal levels, the numbered nerve exits the vertebral canal by passing below the pedicle of the corresponding vertebra, e.g. L4 nerve exits the intervertebral foramen between L4 and L5. However, in the cervical region, nerves C1–C7 pass above their corresponding vertebrae. C1 leaves the vertebral canal between the occipital bone and atlas, and hence is often termed the suboccipital nerve. The last pair of cervical nerves does not have a correspondingly numbered vertebra and C8 passes between the seventh cervical and first thoracic vertebrae. Each nerve is continuous with the spinal cord by ventral and dorsal roots; each dorsal root bears a spinal, sensory ganglion (dorsal root ganglion).

Spinal roots and ganglia

Ventral (anterior) roots

Ventral roots contain axons of neurones in the ventral (anterior) and lateral spinal grey columns. Each emerges as a series of rootlets in two or three irregular rows in an area approximately 3 mm in horizontal width.

Dorsal (posterior) roots

Dorsal roots contain centripetal processes of neurones sited in the spinal ganglia. Each consists of medial and lateral fascicles that both diverge into rootlets that enter the spinal cord along the posterolateral sulcus. The rootlets of adjacent dorsal roots are often connected by oblique filaments, especially in the lower cervical and lumbosacral regions.

Little is known of the detail of the regions of entry and emergence of afferent and efferent rootlets in humans, but these zones of transition between the central and peripheral nervous systems have been extensively described in rodents ( ).

Appearance and orientation of roots at each spinal level

The size and direction of spinal nerve roots vary. The upper four cervical roots are small, while the lower four are large. Cervical dorsal roots have a thickness ratio to the ventral roots of 3 : 1, which is greater than in other regions. The first dorsal root is an exception, being smaller than the ventral, and it is occasionally absent. The conventional view is that the first and second cervical spinal roots are short, running almost horizontally to their exits from the vertebral canal, and that from the third to the eighth cervical levels the roots slope obliquely down. Obliquity and length increase successively, although the distance between spinal attachment and vertebral exit never exceeds the height of one vertebra. An alternative view states that upper cervical roots descend, the fifth is horizontal, the sixth to eighth ascend, the first two thoracic roots are horizontal, the next three ascend, the sixth is horizontal and the rest descend ( ). This view is based on the observation that the cervicothoracic part of the spinal cord grows more in length than other parts.

Thoracic roots, except the first, are small, and the dorsal root only slightly exceeds the ventral in thickness. They increase successively in length. In the lower thoracic region, the roots descend in contact with the spinal cord for at least two vertebrae before emerging from the vertebral canal.

Lower lumbar and upper sacral roots are the largest, and their rootlets are the most numerous. Coccygeal roots are the smallest. confirm that lumbar, sacral and coccygeal roots descend with increasing obliquity to their exits. The spinal cord ends near the lower border of the first lumbar vertebra, and so the lengths of successive roots rapidly increase; the consequent collection of roots is the cauda equina ( Fig. 47.1A ). The largest roots, and hence the largest spinal nerves, are continuous with the spinal cervical and lumbar enlargements and innervate the upper and lower limbs.

Spinal ganglia (dorsal root ganglia)

Spinal ganglia (dorsal root ganglia, DRG) are large groups of neurones on the dorsal spinal roots. Each is oval and reddish: its size is related to that of its root. A ganglion is bifid medially where the two fascicles of the dorsal root emerge to enter the cord. Ganglia are usually sited in the intervertebral foramina, immediately lateral to the perforation of the dura mater by the roots (see Fig. 47.1B ). However, the first cervical ganglion lies on the vertebral arch of the atlas, the second lies behind the lateral atlanto-axial joint, the sacral lie inside the vertebral canal, and the coccygeal ganglion usually lies within the dura mater. The first cervical ganglia may be absent. Small aberrant ganglia sometimes occur on the upper cervical dorsal roots between the spinal ganglia and the cord.

Spinal nerves proper

Immediately distal to the spinal ganglia, ventral and dorsal roots unite to form spinal nerves. These very soon divide into dorsal and ventral rami, both of which receive fibres from both roots (see Fig. 47.6A ). At all levels above the sacral, this division occurs within the intervertebral foramen. Division of the sacral spinal nerves occurs within the sacral vertebral canal, and the dorsal and ventral rami exit separately through posterior and anterior sacral foramina at each level. Spinal nerves trifurcate at some cervical and thoracic levels, in which case the third branch is called a ramus intermedius. At or distal to its origin, each ventral ramus gives off recurrent meningeal (sinuvertebral) branches and receives a grey ramus communicans from the corresponding sympathetic ganglion. The thoracic and first and second lumbar ventral rami each contribute a white ramus communicans to the corresponding sympathetic ganglia. The second, third and fourth sacral nerves also supply visceral branches, unconnected with sympathetic ganglia, which carry a parasympathetic outflow direct to the pelvic plexuses.

Cervical spinal nerves enlarge from the first to the sixth nerve. The seventh and eighth cervical and the first thoracic nerve are similar in size to the sixth cervical nerve. The remaining thoracic nerves are relatively small. Lumbar nerves are large, increasing in size from the first to the fifth. The first sacral is the largest spinal nerve; thereafter the sacral nerves decrease in size. The coccygeal nerves are the smallest spinal nerves. The size of the spinal nerve and its associated structures within the intervertebral foramen is not in direct relation to the size of the foramen. At lumbar levels, though L5 is the largest nerve, its foramen is smaller than those of L1–L4, which renders this nerve particularly liable to compression.

In the radicular (‘root’) canal and intervertebral foramen, the spinal nerve is related to the spinal artery of that level and its radicular branch, and to a small plexus of veins. At the outer end of the foramen, the nerve may lie above or below transforaminal ligaments.

Meningeal nerves

Recurrent meningeal (or sinuvertebral) nerves ( Fig. 47.9 ) occur at all vertebral levels. They are mixed sensory and sympathetic nerves, represented by numerous fine filaments amongst which one, or two to four, larger trunks may be evident. At cervical levels, the autonomic roots arise from the grey rami that form the vertebral nerve. At thoracic and lumbar levels, each nerve is formed by a somatic root from the ventral ramus and by an autonomic root from the grey ramus communicans of that segment. Each nerve pursues a recurrent course through the intervertebral foramen, passing ventral to the spinal nerve, to enter the vertebral canal, where it divides into ascending, descending and transverse branches. These branches communicate with corresponding branches from the segments above and below, and from the opposite side, forming arcades along the floor of the vertebral canal. Meningeal branches of the arcades form a plexus on the ventral surface of the dural sac and nerve root sleeves that attenuates laterally; the posterior paramedian dura is devoid of nerve endings. Skeletal branches are distributed to the posterior longitudinal ligament, the periosteum of the vertebral bodies, and to the posterior and posterolateral aspects of the intervertebral discs ( ). Vascular branches accompany the veins and arteries of the vertebral canal and those of the vertebral bodies. The upper three cervical meningeal nerves ascend through the foramen magnum into the posterior cranial fossa, where they innervate the dura mater that covers the clivus. En route , they innervate the median atlanto-axial joint and its ligaments.

Fig. 47.9, The course and skeletal distribution of the lumbar sinuvertebral nerves. Each nerve supplies the intervertebral disc at its level of entry into the vertebral canal, the disc above, and the intervening posterior longitudinal ligament. In about one-third of cases, the nerve at a particular level may be represented by more than one filament.

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