Anatomy of the back and vertebral column

The vertebral column is the jointed bony support of the back, typically formed from 33 vertebrae arranged in five regions. It supports the head and trunk, permits movement and encloses the spinal cord, the part of the central nervous system extending from the brainstem. Back muscles move and stabilise this framework.

What are the five regions of the vertebral column?

The column belongs to the axial skeleton, the bones forming the central framework of the head and trunk. Its regions are named according to their position, from the neck to the lower end of the column:

• Cervical region: seven vertebrae, C1–C7, form the cervical vertebral column in the neck.
• Thoracic region: twelve vertebrae, T1–T12, form the thoracic vertebral column and articulate, or form joints, with the ribs.
• Lumbar region: five lumbar vertebrae, L1–L5, form the lower back.
• Sacral region: five vertebrae fuse to form the sacrum, which joins the column to the hip bones.
• Coccygeal region: usually four vertebrae form the coccyx, the small terminal bone below the sacrum; their number and degree of fusion vary.

The familiar total of 33 therefore describes the usual developmental arrangement, not 33 separate bones in an adult. The sacrum transfers weight from the column to the pelvis, the bony ring formed by the hip bones and sacrum.

What are the normal curves of the spine?

Viewed from the side, the column has cervical, thoracic, lumbar and sacral curves. The cervical and lumbar curves are convex anteriorly, meaning they bulge towards the front; the thoracic and sacral curves are convex posteriorly, towards the back.

The thoracic and sacral curves are primary curves, retained from the general curvature present before birth. The cervical and lumbar curves are secondary curves, developing as head control and upright posture become established after birth.

These alternating curves help distribute loads while keeping the head and trunk balanced over the pelvis. They describe the normal side-view shape of the column, rather than bends from side to side.

What are the parts of a typical vertebra?

A typical vertebra has a body, its anterior weight-bearing mass, and a vertebral arch behind it. Together they surround the vertebral foramen, the opening through the vertebra. Successive foramina form the vertebral canal, which contains the spinal cord, nerve roots and their coverings.

The arch consists of pedicles, short bridges extending backwards from the body, and laminae, plates completing the arch posteriorly. A spinous process projects backwards, while transverse processes project to either side; these bony projections provide attachment sites for muscles and ligaments.

Superior and inferior articular processes project above and below the arch. Their smooth joint surfaces, called facets, meet neighbouring vertebrae at facet joints. These are synovial joints: their surfaces are enclosed by a capsule containing lubricating synovial fluid.

Notches above and below adjacent pedicles help form intervertebral foramina, openings between vertebrae through which spinal nerves pass. Regional differences reflect different mechanical roles:

• A typical cervical vertebra has a relatively small body and a transverse foramen, an opening in each transverse process.
• A typical thoracic vertebra has articular facets for ribs on its body and transverse processes, with variations near the ends of the region.
• A typical lumbar vertebra has a large body and broad, sturdy processes suited to bearing substantial trunk loads.

How do the atlas and axis differ from other vertebrae?

The atlas is C1, the vertebra immediately beneath the skull. It lacks a body and a conventional spinous process, consisting instead of anterior and posterior arches joined by lateral masses, the thickened sides of its ring.

The upper surfaces of these masses meet the occipital condyles, rounded joint surfaces on the underside of the skull. These atlanto-occipital joints permit mainly flexion and extension: forward and backward bending associated with nodding.

The axis is C2. Its dens, a tooth-like projection extending upwards from its body, acts as a pivot around which the atlas and head rotate at the atlanto-axial joints.

The transverse ligament of the atlas passes behind the dens and holds it against the anterior arch. Alar ligaments extend from the dens to the skull and limit rotation. There is no intervertebral disc between the atlas and axis.

How do intervertebral discs and ligaments connect vertebrae?

An intervertebral symphysis is a cartilaginous joint between neighbouring vertebral bodies, containing an intervertebral disc. Discs occur from the junction between C2 and C3 down to the junction between L5 and the sacrum.

Each disc has an annulus fibrosus, an outer ring of layered fibrocartilage, and a nucleus pulposus, a softer, gel-like centre. Fibrocartilage combines collagen fibres with cartilage tissue to resist tension and compression. Cartilaginous endplates separate most of the disc from the adjacent vertebral bodies.

Discs distribute compressive loads and allow small movements between individual vertebrae. Together with facet joints, they permit flexion, extension, lateral flexion—bending sideways—and rotation, although the available movement differs between regions.

Ligaments are bands of connective tissue joining bones and helping control joint movement. Important vertebral ligaments include:

• Anterior longitudinal ligament: runs along the front of vertebral bodies and discs and limits extension.
• Posterior longitudinal ligament: runs along the backs of the bodies inside the vertebral canal and helps limit flexion.
• Ligamenta flava: elastic ligaments connecting adjacent laminae and assisting return from flexion.
• Interspinous ligaments: connect neighbouring spinous processes.
• Supraspinous ligament: joins spinous process tips and continues superiorly as the nuchal ligament in the neck.
• Intertransverse ligaments: connect adjacent transverse processes and help limit lateral flexion.

Where does the spinal cord end, and what is the cauda equina?

The spinal cord continues from the medulla oblongata, the lowest part of the brainstem, into the vertebral canal. In adults, it usually ends around the L1–L2 vertebral level as the conus medullaris, its tapered lower end.

Spinal cord segments are portions of the cord associated with particular pairs of spinal nerves. They do not all lie opposite the correspondingly numbered vertebrae because the column grows longer than the cord during development.

Below the conus, lumbar, sacral and coccygeal nerve roots descend towards their exit openings. This bundle is the cauda equina, meaning horse’s tail; nerve roots are bundles of nerve fibres connecting spinal nerves to the cord. The filum terminale, a slender connective-tissue strand, extends downwards from the conus and helps anchor the cord.

The cord is enclosed by the meninges, three protective membranes: dura mater, arachnoid mater and pia mater. Cerebrospinal fluid surrounds it in the subarachnoid space, between the arachnoid and pia; this fluid-filled space continues below the cord around the cauda equina.

What are the superficial, intermediate and deep back muscles?

Back muscles are grouped by their position and function. Superficial and intermediate groups are commonly called extrinsic, while deep muscles are intrinsic, forming the principal muscle system that moves and stabilises the vertebral column.

Erector spinae comprises iliocostalis, longissimus and spinalis. The transversospinal group includes semispinalis, multifidus and rotatores, whose fibres generally ascend medially from transverse or other vertebral processes towards spinous processes above; semispinalis capitis inserts on the occipital bone. Deep muscles receive posterior rami, the backward-running branches of spinal nerves.

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