Before I get too far into this, let me point you to an excellent summary of the disc's role in low back pain. It was penned by Michael Brown, MD, DC (an excellent physiatrist/pioneer in Bellevue, WA as well as in California) and describes it better than I ever will here: http://michaelnbrownmd.com/michaelnbrownmd/files/Intervertebral_Disc_Derangement.pdf.
Rather than a detailed scientific account, what follows is a simplification for educational purposes for the layperson. There are many other factors including endplate changes and facet changes that cannot be entirely separated from the disc when considering back pain. However, for simplicity I hope you find the following information helpful.
A day in clinic rarely passes where I don't meet someone with pain that I attribute to the aging, so calling "degenerative" disc. The patient (often age 25-65) describes a circular pattern of pain in the spine, worse after sitting/bending, a bit better w/ standing/walking. They may describe "numbness" into the upper thighs or arms, but on further discussion, it is actually a burning, tingling or odd feeling, but not true absence of sensation. The pain my flair in a severe fashion and last days or weeks and then gradually diminish. An MRI may be have been done, and the patient is fearful because they are told their discs are "bulging", "degenerating" and getting shorter. They picture in their mind that means that the remaining vertebra (or the disc) or somehow "pinching" their nerves.
Here is what I tell them: Our "discs" are the shock absorbers between the stacked vertebral bones of our back and neck. These discs wear down and age just like our skin. .Much like our skin, the originally flexible, relatively soft discs, eventually get drier, firmer, less soft and "wrinkly". A young, fifteen year old disc has two main components: (1) a soft, mobile, internal gel-like substance called the "nucleus pulposus" inside (2) the flexible, rings of the "annular" layers (like an onion or the plies of a radial tire). This annulus provides a padded confining home at it's center for the pulposus.
The Young Disc:
This young disc's internal pulposus can push on the padded walls of it's central room and therefore influence the height and outer shape of the entire disc as we bend, turn and extend. This is by design and a major reason that young people are more flexible and heal up back sprains so much faster.
Our Inevitable Disc "Degeneration" : 25-65 years old:
Let me be very clear at the onset, the pain I describe here has NOTHING to do with "pinched nerves" or "lumbar radiculopathy". Nothing described here is particularly dangerous. but MAN, can it hurt at times.
As the disc ages, the pulposus begins to break down the walls of its internal captivity. In it's simplest sense, this allows it to wander into the corridors of it's surrounding annular layer home. This leads to "bulging" of the disc as a whole. Besides the microtrauma caused by this mechanical stress, there is also chemical stress due to enzymes in the pulposus which contribute to the annular "fissures" and "delamination" that ensues. Let me reiterate, this is not always painful.
However, occasionally pulposus can get "stuck" in those layers. Especially if that happens toward the rear of the disc, our body may react by spasming the muscles behind and in in front of that part of the spine, (what we call a "splinting" affect). That rear part of the disc is pretty sensitive, whereas the front part does not appear to be innervated in a way to cause much problem. I suspect that this sensitivity in the back portions is part of the body's early warning system to avoid allowing the pulposus to get too far through the annular layers to a place where it canreally get into trouble. If the pulposus migrates far enough and the outer annular layers are breached to create a "door" to the outside where the nerves live, we have what is called a "herniated disc". Now pain from herniated discs an entirely different kind of pain, (if it hurts)! I'll describe that on a later day. Herniated disc pain is also way less common.
This bulging "degenerative disc" hurts when when sit or bend forward because that allows pulposus to wander further back into the corridors of the annuli. Standing up, or bending backward may initally hurt a bit because some disc is lodged in the layers, but eventually back extension/straightening can help the pulposus get back "to it's roost" in the center. This is a huge reason that I send people to physical therapy to work on this. Injections and surgery, for the most part, do not help this particular problem. But proper education through therapy gives us the mental and physical tools to properly align the disc and use the back in a way that will aggravate it less
Later stages (65+ years old usually):
Allowed to its devices, with time the pulposus has wandered through (and broken down) enough of the annular layers, that it becomes very difficult to discern their separation. It's like a house or maze where someone has removed all the internal walls. Luckily, by this time, the pulposus itself has dried up a lot and is way less mobile. The disc is relatively hard and flattened and way less mobile. At this age, true new disc "herniations" where the pulposus gets out of it's annular confinement seems way less common. As a consequence, "old back" pain is very different than middle-aged back pain. I'll discuss that a later date.