The spine is a column of roughly 33 bones, called vertebrae, that runs from the base of your skull to your tailbone. It serves three essential purposes: holding your body upright, protecting the spinal cord, and allowing your torso to bend, twist, and move. Those 33 bones are stacked with cushioning discs between them, held together by ligaments, and surrounded by layers of muscle that keep everything stable and mobile at the same time.
The Five Regions of the Spine
The vertebral column is divided into five distinct sections, each with a different shape and job.
- Cervical (neck): 7 vertebrae. This is the most mobile part of your spine. It supports the weight of your head and lets you nod, tilt, and turn. Healthy young adults can rotate their neck about 70 degrees in each direction and flex it forward roughly 58 degrees.
- Thoracic (mid-back): 12 vertebrae. Each one connects to a pair of ribs, forming the bony cage that protects your heart and lungs. Because the ribs limit movement, this region is the stiffest part of the spine.
- Lumbar (lower back): 5 vertebrae. These are the largest and strongest, bearing most of your body’s weight. The lumbar spine flexes forward about 70 degrees and rotates roughly 38 degrees to each side.
- Sacrum: 5 vertebrae fused into a single triangular bone. It connects your spine to your pelvis.
- Coccyx (tailbone): 4 fused vertebrae at the very bottom, where pelvic floor muscles and ligaments attach.
The Natural Curves
A healthy spine is not straight when viewed from the side. It has an S-shaped profile created by alternating curves. The neck and lower back curve gently inward (lordosis), while the mid-back and sacrum curve outward (kyphosis). In adults, thoracic kyphosis averages about 44 degrees and lumbar lordosis about 32 degrees. These curves work like a spring, distributing mechanical stress so no single section absorbs all the force when you walk, run, or jump.
How Spinal Discs Work
Between each pair of movable vertebrae sits an intervertebral disc. These discs act as shock absorbers and spacers, preventing bone from grinding against bone. Each disc has three parts: a gel-like center, a tough outer ring, and thin cartilage plates that anchor it to the vertebrae above and below.
The center is about 70 to 80 percent water. When you bear weight, that water content lets the center pressurize evenly, converting downward force into outward pressure. The tough outer ring then contains that outward push, spreading the load across its layered fibers. This system reduces the stress on any single vertebra and lowers the risk of fractures over time.
Discs also give the spine much of its flexibility. Every time you bend forward to tie your shoes or twist to look behind you, discs compress on one side and expand on the other, allowing controlled movement between vertebrae.
Protecting the Spinal Cord and Nerves
Running through a channel in the center of the stacked vertebrae is the spinal cord, a bundle of nerve tissue that relays signals between your brain and the rest of your body. The bony rings of each vertebra form a protective tunnel, shielding this tissue from impact.
Branching off the spinal cord are 31 pairs of spinal nerves that exit the spine through small openings between adjacent vertebrae. These are organized by region: 8 cervical pairs, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each pair serves a specific zone of the body, carrying motor commands out to muscles and sensory information back to the brain. That’s why a problem at a specific spinal level, like a herniated disc in the lower back, can cause pain, numbness, or weakness in a very predictable area of the leg or foot.
Ligaments and Muscles That Hold It Together
Bones and discs alone wouldn’t keep the spine stable. Ligaments run along the front and back of the vertebral column, connecting one vertebra to the next and preventing excessive movement in any direction. They act like reinforced tape holding the stack in alignment.
Muscles provide the active support. Your core muscles, including the deep muscles along the spine, the abdominals wrapping around the front, and the muscles along your sides, work together to control posture and movement. When these muscles are strong, they absorb a significant share of the mechanical load, reducing the demand on discs and ligaments. Exercises that challenge core stability, like planks, directly support spinal health for this reason.
How the Spine Changes With Age
Over time, intervertebral discs lose water content and become thinner. This process, called disc dehydration, is a normal part of aging and one reason people lose height as they get older. Research shows disc height decreases progressively with age, and the lowest spinal levels, closest to the sacrum, tend to thin fastest. This shrinkage happens independently of body weight, muscle mass, or bone density; it’s primarily a consequence of time.
As discs thin, the space around spinal nerves narrows. This can increase sensitivity to pain and reduce comfortable range of motion. The vertebrae themselves may also develop small bone spurs as the body tries to stabilize segments that have lost disc cushioning.
Bulging Discs vs. Herniated Discs
Two of the most common disc problems are bulging and herniation, and they’re often confused. A bulging disc happens when the outer layer of the disc swells outward, like a hamburger patty that’s wider than its bun. Usually a quarter to half of the disc’s circumference is affected, and only the outer layer is involved.
A herniated disc is different. A crack forms in the tough outer ring, and some of the softer inner material pushes through. Only the small area of the crack is affected, not the whole disc, but the protruding material is more likely to irritate nearby nerve roots. That irritation usually comes from inflammation rather than direct pressure, which is why herniated discs are more likely to cause pain, tingling, or weakness than bulging discs.
Both conditions can exist without any symptoms at all. Many people discover a bulging or herniated disc incidentally, during an MRI ordered for something else entirely.