The Vertebral Column and Spinal Cord

OBJECTIVES: When you have mastered the material in this study guide, you should be able to meet the objectives of Dissection 1.
 

 
ORGANIZATION OF THE VERTEBRAL COLUMN 

The vertebral column is composed of a series of 31 separate bones known as vertebrae. There are seven cervical or neck vertebrae, 12 thoracic vertebrae, and five lumbar vertebrae. The sacrum is composed of five fused vertebrae, and there are two coccygeal vertebrae which are sometimes fused (fig. 1). In the normal adult there are four curvatures in the vertebral column in an anteroposterior plane. These serve to align the head with a vertical line through the pelvis (fig. 2). In the thoracic and sacral regions, these curves are oriented concave anterior and each is known as a kyphosis. In the lumbar and cervical regions the curves are convex anterior and each is known as a lordosis. These latter normal curvatures develop during childhood in association with lifting the head (cervical) and assuming upright sitting (lumbar) and they are thus known as secondary curvatures. The thoracic and sacral curvatures are the same in adult as they are in fetal life and they are known as primary curvatures. Exaggerated kyphosis or lordosis can occur under some normal conditions (e.g. increased lumbar lordosis in pregnancy). A curvature of the vertebral column in a mediolateral plane can occur pathologically and is known as a scoliosis.

 
Structure of a Typical Vertebra

Each vertebra is composed of a body anteriorly and a neural arch posteriorly (fig. 2). The arch encloses an opening, the vertebral foramen, which helps to form a canal in which the spinal cord is housed. Protruding from the posterior extreme of each neural arch is a spinous process and extending from the lateral edges of each arch are transverse processes. These bony elements serve as important sites of attachment of deep back muscles. The neural arch of each vertebrae is divided into component parts by these processes. The parts of the neural arch between the spinous and transverse processes are known as the laminae and the parts of the arch between the transverse processes and the body are the pedicles. At the point where the laminae and pedicles meet, each vertebra contains two superior articular facets and two inferior articular facets. The former pair of facets form articulations, which are synovial joints, with the two inferior articular facets of the vertebra immediately above (or the skull, in the case of the first cervical vertebra) (fig. 2). The pedicle of each vertebra is notched at its superior and inferior edges. Together the notches from two contiguous vertebra form an opening, the intervertebral foramen, through which spinal nerves pass (fig. 3).
 

Regional Differences in Vertebral Structure 
 
Typical cervical vertebrae have large spinal canals, oval shaped vertebral bodies, and articular facets oriented abiquely (fig. 4). Their most characteristic features are their bifid spinous processes and a foramen in their transverse processes. These foramina transversaria contain the vertebral artery and vein. The first and second cervical vertebrae are atypica (fig 5). The first cervical vertebra is known as the atlas, and it is remarkable for having no body. It contains an anterior tubercle instead. Its superior articular facets articulate with the occipital condyles of the skull and are oriented in a roughly parasagittal plane. The head thus moves forward and backwards on this vertebra. The second cervical vertebra contains a prominent odontoid process, or dens, which projects superiorly from its body. It articulates with the anterior tubercle of the atlas, forming a pivotal joint. Side to side movements of the head take place about this joint. The seventh cervical vertebra is sometimes considered atypical since it lacks a bifid spinous process.
 
Thoracic vertebrae form a transition between cervical vertebrae above and lumbar vertebrae below. The upper four thoracic vertebrae are like cervical vertebrae in some respects. They have vertically oriented articular facets and posteriorly directed spinous processes. The lower four thoracic vertebrae contain more lumbar features, like large bodies, robust transverse and spinous processes, and lateral projecting articular facets. The middle four thoracic vertebrae have characteristics between these two regions. These include vertically oriented articular processes and long, slender, and inferiorly inclined spinous processes.

The unique characteristic of thoracic vertebrae are articular facets for the ribs. Each vertebra contains two pairs of these costal demifacets on its body and one on each transverse process (fig. 6). Typical ribs articulate with the inferior demifacet and transverse process of a thoracic vertebra and the superior demifacet of the vertebra below it. The 11th and 12th thoracic vertebrae are sometimes considered atypical because they lack a superior costal demifacet. The 11th and 12th ribs thus articulate only with the 11th and 12th thoracic vertebrae, respectively.
 

 
Lumbar vertebrae are characterized by massive bodies and robust spinous and transverse processes. Their articular facets are oriented somewhat parasagittally, which is thought to contribute the large range of anteroposterior bending possible between lumbar vertebrae. Lumbar vertebrae also contain small mammillary and accessory processes on their bodies. These bony protuberances are sites of attachment of deep back muscles (fig. 7).
 
The sacrum (fig. 8) consists of five fused vertebrae. It articulates with the fifth lumbar vertebra above and the coccyx below, and with the iliac bones on either side. In addition to its characteristic shape, it contains both anterior and posterior foramina through which anterior and posterior rami of spinal nerves pass.
 

INTERVERTEBRAL JOINTS 

 Adjacent vertebrae are connected by three types of intervertebral articulations. Synovial joints are formed between the inferior articular facets of one vertebrae and the superior articular facets of the vertebrae below. These joints are extensively reÐenforced by different ligaments. These ligaments connect the tips of the spinous processes (supraspinous ligaments), the base of the spinous processes (interspinous ligaments), and the transverse processes (intertransverse ligaments). In addition the laminae of adjacent vertebrae are bound together by a ligamentum flavum (fig. 9).
 
The bodies of adjacent vertebrae are connected by specialized cartilaginous joints known as intervertebral discs. Each disc is composed of a central core of gelatinous material, known as the nucleus pulposus, and a surrounding series of fibrous rings known as the annulus fibrosis (fig. 9). Normally body weight is transmitted through the disc by loading the nucleus pulposus, which is then compressed and transfers its loading to the annulus fibrous. In most individuals, the fibers of the annulus fibrosus effectively resist this load, but in some people they do not and the nucleus pulposus is forced out of the disc, or is herniated. A herniated nucleus pulposus can have a profound effect on the adjacent spinal nerves (see below). Two ligaments connect the vertebral bodies anteriorly and posteriorly and thereby reÐenforce the intervertebral disc. The anterior longitudinal ligament (fig. 10) is strong and robust throughout but the posterior longitudinal ligament becomes thin and narrow in the lumbar region. This change in structure of the posterior longitudinal ligament is part of the reason that the overwhelming majority of disc herniations occur posteriorly in the lumbar region.
 

DEEP BACK MUSCLES
 
The intrinsic muscles of the vertebral column lie deep to a number of large muscles which connect the upper limb to the trunk (fig. 11). The most superficial muscles are considered with the upper limb. Overlying the deep back muscles are two thin and relatively insignificant muscles: the serratus posterior superior and the serratus posterior inferior. These connect the vertebral column with the upper and lower ribs, respectively. Although considered accessory muscles of respiration by some, their small size makes their functional significance in this role questionable.

The deep or intrinsic back muscles all are innervated by dorsal rami of spinal nerves and are sometimes called deep dorsal muscles. Muscles that span several spinal segments receive their innervation from dorsal rami of several segmental spinal nerves. Thus, unlike the ventral musculature, the deep back muscles have retained this aspect of their segmental organization.

The deep back muscles are divided into two groups, the erector spinae and the transversospinalis. The erector spinae or sacrospinalis muscle group extend from the pelvis to the back of the skull. In the lumbar region, muscle fibers arise from the robust lumbar aponeurosis and in the lower thoracic regions the erector spinae divides into three longitudinal columns of muscle. The most lateral, the iliocostalis (fig. 12), attaches to the angles of the lower ribs (iliocostalis lumborum), but is continued cranially as a series of long fiber bundles, each spanning about six ribs (iliocostalis thoracic), and into the neck, attaching to transverse processes (iliocostalis cervicis) (fig. 12). The next most medial muscle is the longissimus. It attaches to lumbar and inferior thoracic transverse processes (longissimus dorsi) and in the thoracic region to the adjacent ribs (longissimus thoracis). Upper thoracic bundles arising medial to these fibers attach to cervical transverse processes (longissimus cervicis) or the skull (longissimus capitis). The most medial part of the erector spinae is the spinalis. From the lumbar aponeurosis, fibers of spinalis attach to the lumbar and thoracic spinous processes.

The functions of the erector spinae are not well understood. Electromyographic (EMG) evidence shows that all components are active in flexion as well as extension of the vertebral column. In flexion, these muscles appear to act to control movements rather than produce them, and in full forward bending they are electrically silent. During lateral bending and twisting, EMG studies show that erector spinae muscles on both sides are active, indicating a movement a controlling as well as a movementÐproducing role for these muscles. Since each muscle crosses many segments these muscles probably act to produce or control movements of several vertebrae.

Deep to the erector spinae are a series of muscles connecting the transverse and spinous processes. There are three layers of these transversospinalis muscles (fig. 13). The more superficial span more vertebrae than the deeper muscles. The most superficial is the semispinalis. These cross several segments and are named according to their attachments (thoracic, cervicis, and capitis). Deep to semispinalis and spanning fewer segments (3) is the multifidus. It extends from C2 to L5. The deepest muscles in this group are the rotatores. The rotatores longus crosses two segments and the rotatores brevis only one. The functions of these muscles are like those of the erectors spinae, not wellÐknown. That they cross but a few intervertebral joints suggests a role for the transversospinalis group in the precise control of vertebral position.
 

THE RELATIONSHIP BETWEEN THE VERTEBRAL COLUMN AND SPINAL CORD 

The spinal cord lies within the vertebral canal and is covered by three membranes, known as meninges (fig. 14). The outermost layer is the dura mater, a tough fibrous sheath closely applied to the inner layer of bone surrounding the spinal canal. Between the dura and the bone is a potential space, the epidural space, which normally contains a small amount of fat and vertebral veins. The spinal dura mater is continuous with the dura mater lining the skull and continues to the level of the second sacral vertebra. It covers each of the spinal nerves as they leave the spinal canal and forms a tough sheath about the dorsal root ganglion. Beneath the dura mater is a thin and delicate membrane called the arachnoid mater, because of its resemblance to a spider's web. Normally the arachnoid mater is closely applied to the underside of the dura mater, but a potential space exists, the subdural space, which can fill with blood or pus under pathologic conditions. Beneath the arachnoid mater and intimately applied to the spinal cord is the pia mater. Both the arachnoid and pia mater are continuous with the arachnoid and pia surrounding the brain, but unlike the arachnoid, which follows the dura mater, the pia essentially ends, with the caudal end of the spinal cord, at the level of the second lumbar vertebra. A rope like extension of the pia mater, the filum terminale attaches the end of the spinal cord to the caudal end of the dura mater. In addition, the pia mater contains lateral projections called denticulate ligaments, which connect the spinal cord to the dura mater by projecting between the dorsal and ventral roots. The space between the arachnoid mater and pia mater is the subarachnoid space. It is normally filled with cerebrospinal fluid, which surrounds the entire brain and spinal cord.

 
 

The spinal cord proper begins at the level of the foramen magnum of the skull and ends at the level of the L1ÐL2 intervertebral joint (fig. 15). There it tapers to a coneÐshaped ending known as the conus medullaris. A stalk of pia mater, the filum terminale attaches it to the end of the dura mater at S2. All of the roots of the spinal nerves from L2 to the lowest coccygeal nerve pass caudal to the conus medullaris to exit at their respective intervertebral foramina. This mass of spinal roots within the spinal canal (in the subarachnoid space) is known as the cauda equina. Thus, the area of the subarachnoid space below the L2 vertebra (typically L4) is considered a safe place to insert a needle to sample cerebrospinal fluid, since only nerve roots and not spinal cord are found there. Likewise, the posterior protrusion of a herniated lower lumbar intervertebral disc, while not pressing on the spinal cord, can compress a lumbar nerve root, causing severe pain or physical dysfunction in distinct areas of the lower limb.