Because they are fairly rigid, thoracic vertebrae don't permit much movement and consequently, aren't injured as often as the other vertebrae. The lumbar vertebrae, numbered L1 to L5 are below the thoracic vertebrae and above the sacrum and are most frequently involved in back pain because they carry most of the body's stress. The intervertebral discs are the cushiony material sandwiched between each of the adjacent vertebrae, and are constructed somewhat like a jelly doughnut. The centre of the disc is jelly like and is called the nucleus pulposus. The outer part is made up of slightly elastic woven bands and is referred to as the annulus.
Discs are flat, round structures about a quarter of an inch thick.
They compress when weight is put on them and spring back when weight is removed. The discs keep the vertebrae apart, and permit movement between the vertebrae which allowes our spine to bend. The discs act as shock absorbers, cushioning impact on the spine. Discs do not have a supply of blood vessels to nourish them like other organs in the body. They depend on fluids rich with oxygen and nutrients passing back and forth from the bones above and below them.