Spinal cord and spinal nerves: gross anatomy

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CHAPTER 43 Spinal cord and spinal nerves: gross anatomy

This chapter deals with the gross anatomy of the structures which lie within the vertebral canal and its extensions through the intervertebral foramina, the spinal nerve or radicular (‘root’) canals. 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, 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 (MEDULLA)

The spinal cord occupies the superior two-thirds of the vertebral canal (Fig. 18.1, Fig. 43.1). It 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 cord extends from the upper border of the atlas to the junction between the first and second lumbar vertebrae: its average length in European males is 45 cm, its weight approximately 30 g. (For dimensional data consult Barson & Sands 1977.)

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: this arrangement continues until the third fetal month. At birth, the spinal cord terminates at the lower border of the second lumbar vertebra, and may sometimes reach the third lumbar vertebra. In the adult, the spinal cord is said to terminate at the level of the disc between the first and second lumbar vertebral bodies, which lies a little above the level of the elbow joint when the arm is by the side, and also lies approximately in the transpyloric plane (p. 1054). However, there is considerable variation in the level at which the spinal cord ends. It may end below this level in as many as 40% of subjects, or opposite the body of either the first or second lumbar vertebra: very occasionally it ends as high as the caudal third of twelfth thoracic or as low as the disc between the second and third lumbar vertebrae. Its position rises slightly in vertebral flexion, and there is some correlation with the length of the trunk, especially in females. 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 is the source of the large spinal nerves that supply the upper limbs. It 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 is the source of the large spinal nerves that supply 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. 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. 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 which 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 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.

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 the 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 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 rootlets which pass through its substance to issue from the surface of the cord. The anterior 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 spinal accessory nerve which ascends in the vertebral canal lateral to the spinal cord and enters the posterior cranial fossa via the foramen magnum (Fig. 28.11).

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 5–6 mm. A capacious part of the subarachnoid space surrounds the filum terminale internum, and is the site of election for access to the CSF (lumbar puncture).

DORSAL AND VENTRAL ROOTS

The paired dorsal and ventral roots of the spinal nerves are continuous with the spinal cord (Fig. 43.1F; see also p. 754). 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 which 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 (Fig. 43.1F). 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 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) 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 which divides into a medial (central) branch which enters the spinal cord via a dorsal root, and a lateral (peripheral) branch which 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 which 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 which line the vertebral canal (Fig. 43.2). It is closed above by fusion of the spinal dura with the edge of the foramen magnum, and below by the posterior sacrococcygeal ligament which closes the sacral hiatus. It contains loosely packed connective tissue, fat, a venous plexus, small arterial branches, lymphatics and fine fibrous bands which 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 (Dean & Mitchell 2002). The venous plexus consists of longitudinally arranged chains of vessels, connected by circumdural venous ‘rings’. The anteriorly placed vessels receive the basivertebral veins.

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.

Subdural space

The subdural space is a potential space in the normal spine because the arachnoid and dura are closely apposed (Haines et al 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 epidural injections. Injection of fluid into the subdural space may either damage the cord by direct toxic effects or by compression of the vasculature.

ARACHNOID MATER

The spinal arachnoid mater, which surrounds the spinal cord, is continuous with the cranial arachnoid mater (Fig. 43.3). 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.

PIA MATER

The spinal pia mater (Fig. 43.3) closely invests the surface of the spinal cord and passes into the anterior median fissure. 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 which 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 (Fig. 28.11). The last of the dentate ligaments lies between the exiting twelfth thoracic and first lumbar spinal nerves and is a narrow, oblique band which 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.

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 (Fig. 43.3, Fig. 43.4A). 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 its contained CSF extend sufficiently distally to form a radiologically demonstrable root sleeve for each nerve. Shortening or obstruction of this sleeve seen on 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 on page 242. 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 remains uncertain in man: CSF may be absorbed from the spinal subarachnoid space, and spinal arachnoid granulations and villi have been described (Kiddo et al 1976).

SPINAL NERVES

In those body segments which largely retain a metameric (segmental) structure, e.g. the thoracic region, spinal nerves show a common plan (Fig. 43.5). The dorsal, epaxial, ramus passes back lateral to the articular processes of the vertebrae and divides into medial and lateral branches which 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. 43.6). The ventral, hypaxial, ramus is connected to a corresponding sympathetic ganglion by white and grey rami communicantes. It innervates the prevertebral muscles and curves round in the body wall to supply the lateral muscles of the trunk. Near the midaxillary line it gives off a lateral branch which 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.

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, 1 coccygeal. The abbreviations C, T, L, S and Co, with appropriate numerals, are commonly applied to individual nerves. 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 C1–7 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; the latter each bears a spinal ganglion (‘dorsal root ganglion’).

SPINAL ROOTS AND GANGLIA

Ventral (anterior) roots

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