The vertebral column supports the body's weight, protects the spinal cord, and allows for a wide range of trunk and neck movements. Understanding its regional differences, curvatures, and structural components is essential for the rest of musculoskeletal anatomy.
Vertebral Column Anatomy
Regions of the Vertebral Column
The vertebral column has five regions, each with a distinct number of vertebrae and a specific role.
- Cervical region contains 7 vertebrae (C1–C7). These form the neck and support the skull.
- Thoracic region includes 12 vertebrae (T1–T12). They connect to the ribs and form the upper back.
- Lumbar region has 5 vertebrae (L1–L5). This region bears the weight of the upper body and allows significant flexion and extension.
- Sacral region consists of 1 sacrum, formed by the fusion of 5 sacral vertebrae (S1–S5). It connects the spine to the hip bones at the sacroiliac joints.
- Coccygeal region includes 1 coccyx, made up of 3–5 fused coccygeal vertebrae. It serves as an attachment point for pelvic floor muscles and ligaments.
A quick count: 7 + 12 + 5 = 24 individual vertebrae, plus the sacrum and coccyx, giving a total of 26 bones in the adult vertebral column.
Spinal Curvatures from Birth to Adulthood
The spine is not straight. It has four curves that develop at different stages of life, and knowing when each appears is a common exam topic.
Primary curves are present at birth. They are concave anteriorly (curve toward the front of the body):
- Thoracic curve accommodates the thoracic organs (heart and lungs).
- Sacral curve provides a stable base for upright posture.
Secondary curves develop after birth. They are convex anteriorly (curve away from the front):
- Cervical curve develops around 3 months of age, when an infant begins to hold their head up.
- Lumbar curve develops around 12–18 months, when a child starts to walk.
Together, these alternating curves increase flexibility, absorb shock during walking and running, distribute mechanical stress across the column, and help maintain balance in an upright posture.
Vertebral Structure and Function
Vertebral Components and Associated Structures
Before looking at regional differences, make sure you can identify the basic parts of a typical vertebra:
- Vertebral body is the thick, anterior portion and the main weight-bearing structure. Bodies get progressively larger from cervical to lumbar because each lower region supports more weight.
- Vertebral arch is the posterior portion, formed by two pedicles and two laminae. Together with the body, it encloses the vertebral foramen. The stacked foramina of all vertebrae form the vertebral canal, which houses the spinal cord.
- Processes project from the arch. The spinous process extends posteriorly, the two transverse processes extend laterally, and the superior and inferior articular processes bear facets for articulation with adjacent vertebrae.
- Facet joints (zygapophyseal joints) are formed where the articular processes of adjacent vertebrae meet. They guide and limit the direction of spinal motion in each region.
- Intervertebral foramina are openings between adjacent vertebrae through which spinal nerves exit to reach the rest of the body.
Vertebral Features Across Spinal Regions
Each region has distinguishing features. Focus on the differences that let you identify an isolated vertebra on a practical exam.
Cervical vertebrae (C1–C7):
- Smallest and lightest vertebral bodies
- Transverse foramina in each transverse process, through which the vertebral arteries pass (this feature is unique to cervical vertebrae)
- Bifid (forked) spinous processes on C2–C6; C7 has a long, non-bifid spinous process (the vertebra prominens, which you can feel at the base of the neck)
- C1 (atlas) is unique: it lacks both a body and a spinous process. It articulates with the occipital condyles of the skull, allowing the "yes" nodding motion.
- C2 (axis) has the dens (odontoid process), a tooth-like projection that extends superiorly into C1. The atlas pivots around the dens, producing the "no" head-rotation motion.
Thoracic vertebrae (T1–T12):
- Intermediate in size
- Long, slender spinous processes that angle sharply inferiorly (they overlap like shingles)
- Costal facets on the bodies and transverse processes for rib articulation (this feature is unique to thoracic vertebrae)
Lumbar vertebrae (L1–L5):
- Largest and most massive bodies, reflecting the heavy load they carry
- Short, thick, rectangular spinous processes that project straight posteriorly
- Lack both transverse foramina and costal facets
Sacrum:
- Triangular bone formed by the fusion of 5 sacral vertebrae
- Sacral foramina (anterior and posterior) allow passage of sacral spinal nerves
- Articulates laterally with the hip bones
Coccyx:
- Small, triangular bone formed by the fusion of 3–5 coccygeal vertebrae
- Attachment site for pelvic floor muscles and the gluteus maximus
Composition of Intervertebral Discs
Intervertebral discs sit between adjacent vertebral bodies from C2 down to the sacrum. They have two distinct parts:
- Annulus fibrosus is the tough outer ring. It consists of concentric layers of fibrocartilage with collagen fibers arranged in alternating directions. This crisscross pattern gives the ring its strength and resists tearing during twisting motions.
- Nucleus pulposus is the gel-like core with a high water content. It absorbs and redistributes compressive forces placed on the spine.
When the annulus fibrosus weakens or tears, the nucleus pulposus can bulge outward. This is a herniated (slipped) disc, which most commonly occurs in the lumbar region because of the heavy loads and range of motion there. The bulging material can compress nearby spinal nerves, causing pain, numbness, or weakness.
Overall, intervertebral discs provide flexibility between vertebrae, act as shock absorbers, maintain separation between vertebral bodies, and help prevent nerve compression under normal conditions.
Ligaments Supporting the Vertebral Column
Several ligaments reinforce the vertebral column and limit excessive movement in specific directions.
- Anterior longitudinal ligament (ALL) runs along the anterior surface of the vertebral bodies from the skull to the sacrum. It limits hyperextension (bending too far backward).
- Posterior longitudinal ligament (PLL) runs along the posterior surface of the vertebral bodies, inside the vertebral canal. It limits hyperflexion (bending too far forward). It is narrower than the ALL, which is one reason disc herniations tend to occur posterolaterally.
- Ligamentum flavum connects the laminae of adjacent vertebrae. It is highly elastic, which helps it spring the spine back to an upright position after flexion.
- Interspinous ligaments connect the spinous processes of adjacent vertebrae and limit flexion.
- Supraspinous ligament runs along the tips of the spinous processes from C7 to the sacrum and also limits flexion. (In the cervical region above C7, this ligament is called the nuchal ligament.)
- Intertransverse ligaments connect the transverse processes of adjacent vertebrae and limit lateral flexion.