Chapter 8 Spinal Cord and Nerve Roots

The spinal cord provides innervation for the trunk and limbs through the paired spinal nerves and their peripheral ramifications. Through them it receives primary afferent fibres from peripheral receptors located in widespread somatic and visceral structures. It also sends motor axons to skeletal muscle and provides autonomic innervation of cardiac and smooth muscle and secretory glands. Many functions are regulated by intraspinal reflex connections. Profuse ascending and descending pathways link the spinal cord with the brain. They allow higher centres to monitor and perceive external and internal stimuli and modulate and control spinal efferent activity.

The spinal cord is essentially a segmental structure. It gives rise to 31 pairs of segmentally arranged spinal nerves, which are attached to the cord by a linear series of dorsal and ventral rootlets. Dorsal rootlets contain afferent nerve fibres, and ventral rootlets contain efferent fibres (see Fig. 1.5). Groups of adjacent rootlets coalesce to form dorsal or ventral nerve roots. These cross the subarachnoid space and unite to form functionally mixed spinal nerves as they pass through the intervertebral foramina. The dorsal roots bear dorsal root ganglia, which contain the cell bodies of primary afferent neurones.

Topographical Anatomy of the Spinal Cord

Macroscopic Anatomy of the Spinal Cord and Spinal Nerves

The gross anatomy of the structures that lie within the vertebral canal and its extensions through the intervertebral foramina, the spinal nerve or radicular (‘root’) canals is the subject of this chapter. The spinal cord, 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 (Ch. 4), 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, 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 new importance to topographical detail here.

Spinal Cord (Medulla)

The spinal cord is an elongated, approximately cylindrical part of the central nervous system, occupying the superior two-thirds of the vertebral canal (Figs 8.1–8.6). Its average length in European males is 45 cm; its weight is approximately 30 g (for dimensional data, consult Barson and Sands 1977). It extends from the upper border of the atlas to the junction between the first and second lumbar vertebrae; this lower level varies, and there is some correlation with the length of the trunk, especially in females. The termination may be as high as the caudal third of the twelfth thoracic vertebra or as low as the disc between the second and third lumbar vertebrae, and its position rises slightly in vertebral flexion. The spinal cord is enclosed in the dura, arachnoid and pia mater, separated from each other by the subdural and subarachnoid spaces, respectively. The former is a potential space, whereas the latter contains cerebrospinal fluid (CSF). The cord is continuous cranially with the medulla oblongata and narrows caudally to the conus medullaris, from whose apex a connective tissue filament, the filum terminale, descends to the dorsum of the first coccygeal vertebral segment. The spinal cord varies in transverse width, gradually tapering craniocaudally, except at the levels of the cervical and lumbosacral enlargements. It is not cylindrical, being wider transversely at all levels, especially in the cervical segments.

Fig. 8.1 Median sagittal section of the lumbosacral part of the vertebral column showing the conus medullaris and filum terminale. The section has opened up the subarachnoid space as far as the first sacral vertebra. Note the difference in levels between the inferior limits of the spinal cord and meninges. Note that there are two inaccuracies in this figure: the epidural space is not shown, and the fibres of interspinous ligaments should slope dorsocranially.

Fig. 8.2 Main features of the spinal cord.

Fig. 8.3 Brain and spinal cord with attached spinal nerve roots and dorsal root ganglia, photographed from the dorsal aspect. Note the fusiform cervical and lumbar enlargements of the cord and the changing obliquity of the spinal nerve roots as the cord is descended. The cauda equina is undisturbed on the right but has been spread out on the left to show its individual components.

(Dissection by M. C. E. Hutchinson, GKT School of Medicine; photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)

Fig. 8.4 Lower end of the spinal cord, filum terminale and cauda equina exposed from behind. The dura mater and the arachnoid have been opened and spread out.

Fig. 8.5 Transverse section of the spinal cord at a midthoracic level.

Fig. 8.6 Diagram of a spinal cord segment showing the formation of a typical spinal nerve and the gross relationships of the grey and white matter. Note the dorsal nerve rootlets in a single linear row, and the ventral rootlets in three or more rows.

The cervical enlargement is the source of the spinal nerves that supply the upper limbs. It extends from the third cervical to the second thoracic segments; its maximal 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 corresponds to the innervation of the lower limbs and extends from the first lumbar to the third sacral segments, the equivalent vertebral levels being the ninth to twelfth thoracic vertebrae. The greatest circumference (apprpoximately 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 of the cord. An anterior median fissure and a posterior median sulcus and septum almost completely separate the cord into right and left halves, but they are joined by a commissural band of nervous tissue that contains a central canal.

The anterior 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 of neuroglia penetrates more than halfway into the cord, almost to the central canal. The septum varies in anteroposterior extent from 4 to 6 mm, and it diminishes caudally as the canal becomes more dorsally placed and the cord contracts.

A posterolateral sulcus exists from 1.5 to 2.5 mm lateral to each side of the posterior median sulcus. Dorsal roots (strictly rootlets) of spinal nerves enter the cord along this sulcus. The white substance between the posterior median and posterolateral sulcus on each side is the posterior funiculus. In cervical and upper thoracic segments, a longitudinal posterointermediate sulcus marks a septum dividing each posterior funiculus into two large tracts: the fasciculus gracilis (medial) and the fasciculus cuneatus (lateral). Between the posterolateral sulcus and anterior median fissure is the anterolateral funiculus. This is subdivided into anterior and lateral funiculi by ventral spinal roots that pass through its substance to issue from the surface of the cord. The anterior funiculus is medial to, and includes, the emerging ventral roots; 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 spinal accessory nerve (cranial nerve XI), which ascends in the vertebral canal lateral to the spinal cord and enters the posterior cranial fossa via the foramen magnum (Fig. 8.7).

Fig. 8.7 Dissection showing the brain stem and upper five cervical spinal segments after the removal of large portions of the occipital and parietal bones, cerebellum and roof of the fourth ventricle. On the left, the foramina transversaria of the atlas and of the third, fourth and fifth cervical vertebrae have been opened to expose the vertebral artery. On the right, the posterior arch of the atlas and the laminae of the succeeding cervical vertebrae have been removed.

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 roots of rudimentary second and third coccygeal spinal nerves, adhere to its upper part. The central canal is continued into the filum for 5 to 6 mm. A capacious part of the subarachnoid space surrounds the filum terminale internum and is the usual access site for lumbar puncture.

Dorsal and Ventral Roots

The paired dorsal and ventral roots of the spinal nerves are continuous with the spinal cord (see Figs 8.6 and 8.10). 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. Because 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, mostly distal to the apex of the cord, a divergent sheaf of spinal nerve roots, the cauda equina, which is gathered around the filum terminale in the spinal theca.

Ventral spinal roots contain efferent somatic and, at some levels, efferent sympathetic nerve fibres that emerge from their spinal sources. There are also afferent nerve fibres in these roots. The rootlets constituting each ventral root emerge from the anterolateral sulcus over an elongated vertical elliptical area. Dorsal spinal roots bear ovoid swellings, the spinal ganglia, one on each root proximal to its junction with the corresponding ventral root in an intervertebral foramen. Each root fans out into six to eight 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) from unipolar neurones in 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 branch, which enters the spinal cord via a dorsal root, and a lateral branch, which passes peripherally to a sensory end-organ. The central branch is an axon, whereas the peripheral one is an elongated dendrite (but when traversing a peripheral nerve it 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

The single layer of dura that lines the cranial cavity divides into two layers as it passes downward through the foramen magnum, although it is still a single layer as it forms the anterior and posterior atlanto-occipital membranes. Within the vertebral column, it has been suggested that the outer endosteal layer becomes the periosteum of the vertebral canal, which is separated from the spinal dura mater by an extradural (epidural) space (see later). This interpretation, which would make the epidural space ‘intradural,’ is not generally agreed on (Newell 1999). 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, as well as to the posterior longitudinal ligament by fibrous bands, especially toward 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. The meningeal coverings of the spinal roots and nerves are described later in this chapter.

Epidural Space

The epidural space lies between the spinal dura mater and the tissues that line the vertebral canal (Fig. 8.8). It is closed above by fusion of the spinal dura with the edge of the foramen magnum, and below by the posterior sacrococcygeal ligament, which 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 the atlanto-occipital and atlanto-axial levels (Dean and Mitchell 2002). The venous plexus consists of longitudinally arranged chains of vessels connected by circumdural venous ‘rings.’ The vertebral venous plexus is commonly known as Batson’s plexus. The anteriorly placed vessels receive the basivertebral veins.

Fig. 8.8 Epidural space.

(Adapted with permission from Rosse, C., Gaddum-Rosse, P., 1997. Hollinshead’s Textbook of Anatomy, 5th ed. Lippincott-Raven, Philadelphia, Fig. 13-3.)

The shape of the space within each spinal segment is not uniform, although 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. 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 the lumbar levels.

Subdural Space

The subdural space is a potential space in the normal spine because the arachnoid and dura are closely apposed (Haines, Harkey and Al-Mefty 1993). 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 extradural 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 arachnoid mater that surrounds the spinal cord is continuous with the cranial arachnoid mater (Figs 8.9, 8.10). 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 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. 8.9 Part of the spinal cord exposed from the anterior aspect to show meningeal coverings.

Fig. 8.10 Transverse section through the spinal cord and meninges showing the relationships of the meninges and ligaments with the spinal cord and roots: dura mater (yellow), outer layer of arachnoid mater (pale blue), intermediate layer of arachnoid mater (dark blue), pia mater (pink), and subpial connective tissue (green).

Pia Mater

The spinal pia mater closely invests the surface of the spinal cord and passes into the anterior median fissure (see Figs 8.9, 8.10). As in the cranial region, there is a subpial ‘space’; however, over the surface of the spinal cord the subpial collagenous layer is thicker than in the cerebral region, and it is continuous with the collagenous core of the ligamentum denticulatum.

The ligamentum denticulatum is a flat, fibrous sheet that lies on each 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. There are usually 21 processes on each side. 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 spinal accessory nerve ascends on its posterior aspect (see Fig. 8.7). 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 cineradiography.

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 surrounded by an extensive intermediate layer of leptomeninges (see Fig. 8.10). 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, in that a collagenous core is 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 in the spinal column. Inflammation within the spinal subarachnoid space may result in extensive fibrosis within the intermediate layer and the complications of chronic arachnoiditis.

Spinal Nerves

In those body segments that largely retain a metameric (segmental) structure (e.g. the thoracic region), spinal nerves exhibit a common plan (Fig. 8.11). The dorsal, epaxial ramus passes back, lateral to the articular processes, 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 scapular line. 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. 8.11 Formation and branching pattern of a typical spinal nerve.

Spinal nerves are united ventral and dorsal spinal roots, attached in series to the sides of the spinal cord. Strictly speaking, the term ‘spinal nerve’ 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; in clinical practice it is often loosely termed 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 corresponding numerals, are commonly applied to individual nerves. The nerves emerge through intervertebral foramina. At the thoracic, lumbar, sacral and coccygeal levels, the numbered nerve exits the vertebral canal by passing below the pedicle of the corresponding vertebra: for example, the L4 nerve exits the intervertebral foramen between L4 and L5. However, in the cervical region, nerves C1 to 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 of the latter bears a spinal ganglion (dorsal root ganglion).

Spinal Roots and Ganglia

Lower lumbar and upper sacral roots are the largest, and their rootlets are most numerous. Coccygeal roots are the smallest. Kubik and Müntener (1969) 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 the lengths of successive roots rapidly increase; the consequent collection of roots is the cauda equina (see Fig. 8.3). The largest roots, and hence the largest spinal nerves, are continuous with the spinal cervical and lumbar swellings and innervate the upper and lower limbs.

Spinal Ganglia (Dorsal Root Ganglia)

Spinal ganglia 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 located in the intervertebral foramina, immediately lateral to the perforation of the dura mater by the roots (see Fig. 8.4). However, the first and second cervical ganglia lie on the vertebral arches of the atlas and axis, the sacral lies inside the vertebral canal, and the coccygeal ganglion usually lies within the dura mater. The first cervical ganglion 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 (Figs 8.11, 8.12). These soon divide into dorsal and ventral rami, both of which receive fibres from both roots. 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, and 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 contributes a white ramus communicans to the corresponding sympathetic ganglia. The second, third and fourth sacral nerves also supply visceral branches, unconnected with sympathetic ganglia, that carry a parasympathetic outflow directly to the pelvic plexuses.

Fig. 8.12 Constitution of a typical spinal nerve. The upper part of the diagram shows the somatic components of the spinal nerve roots; the lower part shows the visceral components: somatic efferent and preganglionic sympathetic fibres (red), somatic and visceral afferent fibres (blue) and postganglionic sympathetic fibres (black).

Cervical spinal nerves enlarge from the first to the sixth nerve. The seventh and eighth cervical and the first thoracic nerves 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.

Meningeal Branches

The recurrent meningeal or sinuvertebral nerves number two to four filaments on each side and occur at all vertebral levels. Each receives one or more rami from a nearby grey ramus communicans or directly from a thoracic sympathetic ganglion; most then pursue a recurrent (often perivascular) course into the vertebral canal through the intervertebral foramen ventral to the dorsal root ganglion. Here these mixed sensory and sympathetic nerves divide into transverse, ascending and descending branches that are distributed to the dura mater, walls of blood vessels, periosteum, ligaments and intervertebral discs in the anterolateral region of the vertebral canal. Fine meningeal branches occasionally pass dorsal to reach the spinal ganglia to innervate the dorsal dura, periosteum and ligaments; others pass ventrally to the posterior longitudinal ligament. Ascending branches of the upper three cervical meningeal nerves are large and are distributed to the dura mater in the posterior cranial fossa. Meningeal nerves are clinically important in relation to referred pain, which is characteristic of many spinal disorders and occipital headache.

Coverings and Relations of the Spinal Roots and Nerves in the Radicular Canal

Tubular prolongations of the spinal dura mater, closely lined by the arachnoid, extend around the spinal roots and nerves as they pass through the lateral zone of the vertebral canal and through the intervertebral foramina (Figs 8.9, 8.10, 8.13). These prolongations, the spinal nerve sheaths (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 its contained CSF extend sufficiently distally to form a radiologically demonstrable ‘root sleeve’ for each nerve. Shortening or obstruction of this sleeve seen on myelography 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, although there may be an attachment posteriorly to the facet joint capsule.

Fig. 8.13 Lumbar spinal nerve and its roots and meningeal coverings.

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

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, although L5 is the largest nerve, its foramen is smaller than those of L1–4, which renders this nerve particularly vulnerable to compression.

Functional Components of Spinal Nerves

Each typical spinal nerve contains somatic and visceral (autonomic) fibres.

Somatic components

Somatic components are efferent and afferent. Somatic efferent fibres that innervate skeletal muscles are axons of α, β and γ neurones in the spinal anterior grey column. Somatic afferent fibres convey impulses into the central nervous system from receptors in the skin, subcutaneous tissues, muscles, tendons, fasciae and joints; they are peripheral processes of unipolar neurones in the spinal ganglia.

Visceral components

Visceral components are also afferent and efferent and belong to the autonomic nervous system. They include sympathetic or parasympathetic fibres at different spinal levels. Preganglionic visceral efferent sympathetic fibres are axons of neurones in the spinal lateral grey column in the thoracic and upper two or three lumbar segments; they join the sympathetic trunk along corresponding white rami communicantes and synapse with postganglionic neurones distributed to non-striated muscle or glands. The preganglionic visceral efferent parasympathetic fibres are axons of neurones in the spinal lateral grey column of the second to fourth sacral segments; they leave the ventral rami of corresponding sacral nerves and synapse in pelvic ganglia. The postganglionic axons are distributed mainly to muscle or glands in the walls of the pelvic viscera. Visceral afferent fibres have cell bodies in the spinal ganglia. Their peripheral processes pass through white rami communicantes and, without synapsing, through one or more sympathetic ganglia to end in viscera. Some visceral afferent fibres may enter the spinal cord in the ventral roots.

Central processes of ganglionic unipolar neurones enter the spinal cord by posterior roots and synapse on somatic or sympathetic efferent neurones, usually through interneurones, completing reflex paths. Alternatively, they may synapse with other neurones in the spinal or brain stem grey matter, which gives origin to a variety of ascending tracts.

Variations of Spinal Roots and Nerves

The courses of spinal roots and nerves in relation to the thecal sac and vertebral and radicular canals may be aberrant. An individual intervertebral foramen may contain a duplicated sheath, nerve and roots, which are then absent at an adjacent level. Abnormal communications between roots may occur within the vertebral canal. These anomalies have been described and classified for the lumbosacral spine by Neidre and Macnab (1983).

Rami of the Spinal Nerves

Ventral (anterior primary) rami supply the limbs and the anterolateral aspects of the trunk; in general, they are larger than the dorsal rami. Thoracic ventral rami run independently and retain a largely segmental distribution. Cervical, lumbar and sacral ventral rami connect near their origins to form plexuses. Dorsal rami do not join these plexuses.

Dorsal (posterior primary) rami of spinal nerves are usually smaller than the ventral rami and are directed posteriorly. Retaining a segmental distribution, they all (except for the first cervical, fourth and fifth sacral and coccygeal) divide into medial and lateral branches that supply the muscles and skin of the posterior regions of the neck and trunk (Fig. 8.14).

Fig. 8.14 Cutaneous distribution of the dorsal rami of the spinal nerves. The nerves are shown lying on the superficial muscles; on the left side, the limit of the skin area supplied by these nerves is indicated by the dotted line. 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 in bold on the left side.

Cervical dorsal spinal rami

Each cervical spinal dorsal ramus, except the first, divides into medial and lateral branches that innervate muscles. In general, only medial branches of the second to fourth, and usually the fifth, supply the skin. Except for the first and second, each dorsal ramus passes back, medial to a posterior intertransverse muscle, and curves around the articular process into the interval between semispinalis capitis and semispinalis cervicis.

The first cervical dorsal ramus, the suboccipital nerve (see Fig. 8.7), is larger than the ventral. It emerges superior to the posterior arch of the atlas and inferior to the vertebral artery and enters the suboccipital triangle to supply the rectus capitis posterior major and minor, obliquus capitis superior and inferior and semispinalis capitis. A filament from the branch to the inferior oblique joins the second dorsal ramus. The suboccipital nerve occasionally has a cutaneous branch that accompanies the occipital artery to the scalp and connects with the greater and lesser occipital nerves. It may also communicate with the accessory nerve.

The second cervical dorsal ramus is slightly larger than the ventral and all the other cervical dorsal rami (see Figs 8.7, 11.10B). It emerges between the posterior arch of the atlas and the lamina of the axis, below the inferior oblique, which it supplies. It receives a connection from the first cervical dorsal ramus and divides into a large medial and smaller lateral branch. The medial branch, termed the ‘greater occipital nerve’, ascends between the inferior oblique and semispinalis capitis, pierces the latter and trapezius near their occipital attachments and is joined by a filament from the medial branch of the third dorsal ramus. It ascends with the occipital artery, divides into branches that connect with the lesser occipital nerve and supplies the skin of the scalp as far forward as the vertex. It supplies the semispinalis capitis and, occasionally, the back of the auricle. The lateral branch supplies the splenius capitis, longissimus capitis and semispinalis capitis and is often joined by the corresponding third cervical branch.

Greater occipital neuralgia is a syndrome of pain and paraesthesia felt in the distribution of the greater occipital nerve. It is usually due to an entrapment neuropathy as the nerve pierces the attachment of the neck extensors to the occiput. A similar syndrome may be caused by upper facet joint arthritis involving the second cervical root.

The third cervical dorsal ramus is intermediate in size between the second and fourth. It courses back around the articular pillar of the third cervical vertebra, medial to the posterior intertransverse muscle, and divides into medial and lateral branches. Its medial branch runs between spinalis capitis and semispinalis cervicis and pierces splenius and trapezius to end in the skin. Deep to trapezius it gives rise to a branch, the third occipital nerve, that pierces trapezius and ends in the skin of the lower occipital region, medial to the greater occipital nerve and connected to it. The lateral branch often joins a branch of the second cervical dorsal ramus. The dorsal ramus of the suboccipital nerve and medial branches of the dorsal rami of the second and third cervical nerves are sometimes joined by loops to form the posterior cervical plexus.

The dorsal rami of the lower five cervical nerves curve back around the vertebral articular pillars and divide into medial and lateral branches. Medial branches of the fourth and fifth cervical nerves run between semispinalis cervicis and semispinalis capitis, reach the vertebral spines and pierce splenius and trapezius to end in the skin. The fifth medial branch may not reach the skin. The medial branches of the lowest three cervical nerves are small and end in semispinalis cervicis, semispinalis capitis, multifidus and interspinales. The lateral branches supply iliocostalis cervicis, longissimus cervicis and longissimus capitis.

Thoracic dorsal spinal rami

Thoracic dorsal rami pass backward close to the vertebral facet joints to divide into medial and lateral branches. Each medial branch emerges between a joint and the medial edges of the superior costotransverse ligament and intertransverse muscle. Each lateral branch runs in the interval between the ligament and the muscle before inclining posteriorly on the medial side of levator costae.

Medial branches of the upper six thoracic dorsal rami pass between and supply semispinalis thoracis and multifidus, then pierce rhomboids and trapezius and reach the skin near the vertebral spines.

Medial branches of the lower six thoracic dorsal rami mainly supply multifidus and longissimus thoracis and occasionally the skin in the median region. Lateral branches increase inferiorly in size and run through, or deep to, longissimus thoracis to the interval between it and iliocostalis cervicis, supplying these muscles and levatores costarum. The lower five or six also have cutaneous branches and pierce serratus posterior inferior and latissimus dorsi in line with the costal angles. Some upper thoracic lateral branches supply the skin. The twelfth thoracic lateral branch sends a filament medially along the iliac crest, then passes down to the anterior gluteal skin. Medial cutaneous branches of the thoracic dorsal rami descend close to the vertebral spines before reaching the skin; lateral branches descend across as many as four ribs before becoming superficial. The branch of the twelfth thoracic reaches the skin a little above the iliac crest.

Lumbar dorsal spinal rami

Lumbar dorsal rami pass back, medial to the medial intertransverse muscles, and divide into medial and lateral branches. Medial branches run near the vertebral articular processes to end in multifidus. They are related to the bone between the accessory and mammillary processes and may groove it, crossing a distinct notch or even a foramen. Lateral branches supply erector spinae. In addition, the upper three rami give rise to cutaneous nerves that pierce the aponeurosis of latissimus dorsi at the lateral border of erector spinae and cross the iliac crest posteriorly to reach the gluteal skin, some extending as far as the level of the greater trochanter.

Sacral dorsal spinal rami

Sacral dorsal rami are small, diminishing downward. Other than the fifth, they all emerge though the dorsal sacral foramina. The upper three are covered at their exit by multifidus and divide into medial and lateral branches. Medial branches are small and end in multifidus. Lateral branches join together and with lateral branches of the last lumbar and fourth sacral dorsal rami, forming loops dorsal to the sacrum. Branches from these loops run dorsal to the sacrotuberous ligament and form a second series of loops under gluteus maximus. From these, two or three gluteal branches pierce the gluteus maximus (along a line from the posterior superior iliac spine to the coccygeal apex) to supply the posterior gluteal skin.

The dorsal rami of the fourth and fifth sacral nerves are small and lie below multifidus. They unite with each other and with the coccygeal dorsal ramus to form loops dorsal to the sacrum; filaments from these supply the skin over the coccyx.

Coccygeal dorsal spinal ramus

The coccygeal dorsal spinal ramus does not divide into medial and lateral branches. Its connections and distribution were noted earlier.

Dermatomes

The cutaneous area supplied by one spinal nerve, through both rami, is a dermatome (Fig. 8.15). Typically, dermatomes extend around the body from the posterior to the anterior median line. The upper half of each zone is supplemented by the nerve above, and the lower half by the nerve below. The area supplied by dorsal rami is limited laterally by the dorsolateral line, which descends laterally from the occiput to the medial end of the acromion, continues to the posterior aspect of the greater trochanter and curves medially to the coccyx. Cutaneous strips supplied by dorsal rami do not correspond exactly to those served by ventral rami and differ in both breadth and position. Dermatomes of adjacent spinal nerves overlap markedly, particularly in the segments least affected by development of the limbs (i.e. second thoracic to first lumbar). In some regions (e.g. upper anterior thoracic wall), cutaneous nerves supplying adjoining areas are not from consecutive spinal nerves, and the overlap is minimal. When the second thoracic spinal ramus is severed, anaesthesia is sharply demarcated, but some overlap for awareness of painful and thermal stimuli may exist. Likewise, after section of a peripheral nerve (e.g. ulnar nerve at the wrist), the area of tactile loss is always greater than that of lost pain and temperature sensation. Hence, the area of total anaesthesia and analgesia following section of peripheral nerves is always less than might be anticipated from their anatomical distribution.

Fig. 8.15 Dermatomes. The small diagram shows the regular arrangement of dermatomes in the upper and lower limbs of the embryo.

(By permission from Moffat, D.B., 1993. Lecture Notes on Anatomy, 2nd ed. Blackwell Scientific, Oxford.)

Internal Organization

In transverse section, the spinal cord is incompletely divided into symmetric halves by a dorsal (posterior) median septum and a ventral (anterior) median sulcus (see Fig. 8.5). It consists of an outer layer of white matter and an inner core of grey matter. The dimensions and relative volumes of white matter and centrally aggregated neurone cell bodies vary according to the level (Fig. 8.16). The amount of grey matter at any level is a function of the amount of muscle, skin and other tissues innervated by neurones at that level. It is therefore largest by proportion in the cervical and lumbar enlargements, because neurones in these segments of the cord innervate the limbs; it is attenuated at thoracic levels. The absolute amount of white matter is greatest at cervical levels and decreases progressively at lower levels; this is the case because descending tracts shed fibres as they descend, and ascending tracts accumulate fibres as they ascend.

Fig. 8.16 Transverse sections through the spinal cord at representative levels. Note the changes in overall profile and the relative changes in grey and white regions, their shapes, sizes and proportions (magnification ×5).

In the centre of the spinal grey matter, the central canal extends the whole length of the spinal cord. Rostrally, the central canal extends into the caudal half of the medulla oblongata, where it opens into the fourth ventricle. Caudally, in the conus medullaris, it expands as a fusiform ‘terminal ventricle’ that is 8 to 10 mm long, contains CSF and is lined by columnar, ciliated epithelium (ependyma). The terminal ventricle is obliterated at approximately 40 years of age.

Spinal Grey Matter

In three dimensions, the spinal grey matter is shaped like a fluted column (see Fig. 8.6). In transverse section (see Fig. 8.16) the column is often described as being ‘butterfly shaped’ or resembling the letter ‘H.’ It consists of four cellular masses, referred to as the dorsal and ventral horns (or columns), which project dorsolaterally and ventrolaterally toward the surface. The grey matter that immediately surrounds the central canal and unites the two sides is termed the dorsal and ventral grey commissure. The dorsal horns are the site of termination of primary afferent fibres, which enter via the dorsal roots of spinal nerves. The tip of the dorsal horn is separated from the surface of the cord by a thin dorsolateral tract (tract of Lissauer) in which primary afferents ascend and descend for a short distance before terminating in the subjacent grey matter (Figs 8.17, 8.18). The dorsal horn may be described in terms of a head, neck and base, the individual constituents of which are described in more detail later. The ventral horns contain efferent neurones whose axons leave the spinal cord in ventral nerve roots. A small intermediate lateral horn is present at the thoracic and upper lumbar levels and contains the cell bodies of preganglionic sympathetic neurones.

Fig. 8.17 Simplified diagram of the main tracts of the spinal cord. The ascending tracts are shown in red on the right side of the figure; the descending tracts are shown in yellow on the left side; the ‘intersegmental’ tracts are shown in orange on both sides. Many tracts are omitted (see Fig. 8.18).

Fig. 8.18 Approximate relative positions of nerve fibre tracts of the human spinal cord at the mid-cervical (A) and lumbar (B) levels.

(Adapted from Crosby et al: Correlative Anatomy of the Nervous System, New York, Macmillan, 1962.)

Spinal grey matter is a complex mixture of neuronal cell bodies (somata), their processes (neurites) and synaptic connections, neuroglia and blood vessels (Fig. 8.19). Neurones in the grey matter are multipolar and vary in size and other features, particularly the length and arrangement of their axons and dendrites. Many are Golgi types I and II neurones. Axons of Golgi type I neurones pass out of the grey matter into ventral spinal roots or spinal tracts. Axons and dendrites of Golgi type II neurones are largely confined to the nearby grey matter. The distribution of neurones may be intrasegmental—deployed within a single segment—or intersegmental—spread through several segments.