Your spinal discs sit between each pair of vertebrae (the individual bones of your spine), running from just below the base of your skull down to your tailbone. You have 23 of these discs in total, and each one acts as a cushion that keeps the bones above and below it from grinding together. They’re the primary shock absorbers of your spine and carry most of your body weight when you stand, sit, or move.
Exact Placement Between Vertebrae
Each disc is sandwiched directly between the flat surfaces of two neighboring vertebrae. The bottom of one vertebra and the top of the one below it form a joint, with the disc filling the space in between. Your first disc sits between the second and third vertebrae in your neck (called C2 and C3), and your last disc sits between the lowest lumbar vertebra and the top of the sacrum (L5 and S1), right at the base of your lower back. There is no disc between the top two vertebrae in your neck, which is why those two bones allow your head to rotate and nod so freely.
Discs are anchored to the vertebrae above and below by thin layers of cartilage called endplates. These endplates are the only route for nutrients to reach the disc, since discs have no blood supply of their own. Instead, nutrients from tiny blood vessels in the vertebral bone slowly seep through the endplate and diffuse into the disc tissue.
Discs in Different Regions of the Spine
Your spine has three main movable regions, and discs are present in all of them. Their size and thickness change depending on where they are.
- Cervical (neck): Seven vertebrae with six discs between them. These are the smallest discs in your spine, roughly 3 to 5 millimeters thick at the lower cervical levels.
- Thoracic (mid-back): Twelve vertebrae with twelve discs. These are relatively thin as well, dipping to about 3 millimeters in the upper thoracic region and increasing to around 7 millimeters near the bottom of the ribcage.
- Lumbar (lower back): Five vertebrae with five discs, including the one at the very bottom where the spine meets the sacrum. Lumbar discs are the largest and thickest in the body because they bear the most weight.
The lumbar and cervical regions are the most flexible parts of your spine, which is why they’re also the most common sites of disc problems.
What a Disc Looks Like Inside
A spinal disc has two distinct parts. The outer layer is a tough ring of crisscrossing collagen fibers, organized into concentric layers much like the rings of a tree trunk. This outer ring holds the disc together and resists the forces of bending and twisting. The inner core is a soft, gel-like substance that is mostly water mixed with proteins. It looks translucent and jelly-like.
When you put weight on your spine, the gel-like center compresses slightly and pushes outward against the tough outer ring. This transfers the load evenly in all directions, similar to how a water balloon spreads pressure when you squeeze it. The gel has a special property called viscoelasticity, meaning it absorbs energy during sudden impacts (like jumping or running) and slowly dissipates that force into the surrounding tissue. This is what makes your discs such effective shock absorbers during everyday activities.
How Close Discs Are to Your Nerves
This is the detail that matters most when a disc causes pain. Your spinal cord runs through a channel directly behind the vertebrae and discs, and nerve roots branch off from the cord at each spinal level, exiting through small openings between adjacent vertebrae. The back edge of each disc sits just in front of this nerve pathway.
In a healthy disc, nerve fibers only penetrate the outermost two or three layers of the tough outer ring. The interior of the disc has no nerves at all. But when a disc degenerates or is injured, nerve fibers can grow deeper into the disc, sometimes reaching all the way into the gel-like center. This ingrowth of nerves into tissue that was previously nerve-free is one reason damaged discs become painful even without pressing on a nearby spinal nerve.
Where Disc Problems Happen Most Often
Disc herniations, where the gel-like center pushes through a tear in the outer ring, follow a very predictable pattern. About 95% of lumbar disc herniations in adults between ages 25 and 55 occur at just two levels: between the fourth and fifth lumbar vertebrae (L4-L5), or between the fifth lumbar vertebra and the sacrum (L5-S1). These two discs sit at the very bottom of your movable spine and bear the greatest combination of weight and bending force.
In the neck, the most common site for a herniated disc is between C6 and C7, which is roughly at the base of your neck. Thoracic disc herniations are rare by comparison, but when they do occur, about 75% happen below the T8 level, with T11-T12 being the most common thoracic site.
Herniations tend to happen toward the back and slightly to one side of the disc. This is because the outer ring is structurally weaker in that area and lacks reinforcement from the ligaments that support the front and center of the disc. When disc material pushes out in this direction, it enters the space where nerve roots exit the spine, which is why a herniated disc so often causes pain, numbness, or weakness radiating into an arm or leg.