A disc in your back is a small, rubbery pad that sits between each pair of vertebrae in your spine. You have 23 of these discs stacked from your neck down to your lower back, and they serve as both shock absorbers and spacers that allow your spine to bend and twist. Each one is roughly the size and shape of a thick coin, though the exact dimensions vary depending on where it sits in the spine.
What a Disc Is Made Of
Every spinal disc has two main parts that work together. The outer layer, called the annulus fibrosus, is a tough ring of cartilage made from densely packed collagen fibers arranged in crisscrossing layers. Think of it like the layers of a radial tire. This outer shell is strong and slightly flexible, designed to contain what’s inside while resisting the twisting and bending forces your spine encounters throughout the day.
The inner core, called the nucleus pulposus, is a soft, gel-like substance that’s roughly 88% water. This high water content is what gives the disc its ability to compress under load and then spring back. The gel is held together by molecules called proteoglycans, which attract and hold onto water the way a sponge does. When you stand, sit, or lift something, the watery core pressurizes inside the tough outer ring, distributing the force evenly across the vertebrae above and below. That pressure then transfers outward to the surrounding structures, preventing any single point on the bone from bearing too much stress.
Between the disc and each vertebra sits a thin layer of cartilage called the endplate. This acts as the gateway for nutrients, since adult discs don’t have their own blood supply. Nutrients from nearby blood vessels seep through these endplates to keep the disc tissue alive, which is one reason discs are slow to heal when they’re injured.
How Discs Work Day to Day
Your discs do three jobs simultaneously: they absorb impact, distribute weight, and allow movement. When you walk or run, the gel core compresses slightly with each step and then rebounds, cushioning the vertebrae from repetitive impact. When you bend forward to pick something up, the disc deforms to accommodate the angle, with the nucleus shifting slightly toward the back of the spine. When you twist, the alternating layers of the outer ring resist the rotational force and keep the disc intact.
Discs also lose and regain water throughout the day. During waking hours, the weight of your body gradually squeezes water out of the nucleus. This is why you’re measurably shorter in the evening than when you wake up. Overnight, while you’re lying down and spinal loads are minimal, the discs rehydrate and expand again.
How Discs Change With Age
Disc degeneration is one of the most universal changes in the human body. A landmark review in the American Journal of Neuroradiology found that 37% of 20-year-olds already show signs of disc degeneration on MRI, even with zero back pain. By age 80, that number rises to 96%. This means disc wear is closer to graying hair than it is to a disease. Most people with degenerating discs never know it.
As discs age, they lose water content. The gel core becomes drier and stiffer, reducing its ability to absorb shock. The outer ring develops small cracks and becomes less resilient. The disc gradually flattens, which narrows the space between vertebrae and can change how the small joints behind the disc line up. None of this necessarily causes pain, but it does make the disc more vulnerable to injury.
Bulging Discs vs. Herniated Discs
When a disc is described as “bulging,” it means the outer layer has expanded outward beyond its normal boundary, a bit like a hamburger patty that’s too wide for its bun. The bulge typically affects a quarter to half of the disc’s circumference, and only the outer cartilage layer is involved. The inner gel stays contained. Many bulging discs cause no symptoms at all and are found incidentally on imaging.
A herniated disc is different. In this case, a crack develops in the outer ring, and some of the soft inner gel pushes through that crack. Only a small area of the disc is affected, but the protruding material extends farther outward than a bulge and is more likely to press against a nearby nerve root. This is the type of disc problem that tends to cause significant pain.
What a Disc Problem Feels Like
Symptoms depend entirely on whether the disc is pressing on a nerve. A damaged disc that isn’t touching any nerve tissue can produce localized back or neck stiffness, or nothing at all. When disc material does compress a nerve, the symptoms typically radiate outward along the path of that nerve, often affecting just one side of the body.
A herniated disc in the lower back commonly sends sharp or burning pain down through the buttock, thigh, calf, and sometimes into the foot. This pattern is often called sciatica. A herniated disc in the neck tends to send pain into the shoulder and arm. The pain frequently intensifies when you cough, sneeze, or shift into certain positions. Beyond pain, you may notice numbness, tingling, or a pins-and-needles sensation in the affected arm or leg. Some people also develop weakness in specific muscles, which can show up as difficulty gripping objects or a tendency to stumble.
How Most Disc Problems Resolve
The natural history of a herniated disc is more favorable than most people expect. The majority of people with leg pain from a herniated disc improve within a few months without surgery. The body gradually breaks down and reabsorbs the protruding disc material over time, which relieves pressure on the nerve.
First-line management focuses on staying active while managing pain. Prolonged bed rest actually slows recovery by allowing joints to stiffen and muscles to weaken. Cold packs during the first few days can help with pain and swelling, followed by gentle heat for comfort. Physical therapy plays a central role, with therapists teaching specific positions and exercises that take pressure off the affected nerve. For persistent pain that doesn’t respond to standard pain relief, a corticosteroid injection near the spinal nerves can reduce inflammation. Massage and acupuncture may also provide modest relief for chronic back and neck pain.
Surgery is reserved for cases where conservative treatment fails after several weeks or months, or where neurological symptoms like significant weakness are progressing.
Symptoms That Need Immediate Attention
In rare cases, a large disc herniation in the lower back can compress a bundle of nerves at the base of the spine, a condition called cauda equina syndrome. This is a surgical emergency. The hallmark warning signs include sudden loss of bladder control or the inability to sense when your bladder is full, loss of bowel control, numbness in the groin or inner thighs (sometimes called “saddle” numbness because it affects the areas that would contact a saddle), and rapidly worsening weakness in one or both legs. Sexual dysfunction can also develop. If you experience any combination of these symptoms, seek emergency care immediately, as delays can lead to permanent nerve damage.