The intervertebral disc
Between each of the cervical, thoracic and lumbar vertebrae there
is a cushion called "intervertebral disc".
 The
intervertebral disc is composed of two parts: the central portion
with a gelatin-like consistency named "nucleus pulposus"
and a fibrous fold to hold it tightly in place, called "annulus
fibrosis" or " fibrous ring".
The fibrous ring is thicker at the anterior portion of the disc;
the posterior portion is more fragile. This is why, in case of disc
herniation, this occurs mostly on the posterior part of the disc,
causing disc herniation.
As time elapses, the force supported by the disc makes its gelatin
portion wear out and lose height. This process, known as "vertebral
arthritis", is not an illness per se. A majority of the healthy
population above 30 years of age begins to show a certain degree
of deterioration.
 Astronauts,
in weightless conditions, increase their height since the lack of
gravity keeps discs from being compressed; the separation between
vertebrae is therefore increased.
Pressure on the nucleus pulposus is 5- to 15-fold greater than
blood pressure, thus blood cannot reach the nucleus. There are no
nerves in the nucleus; they are located in the most external layers
of the fibrous ring. This explains why disc degeneration may not
be painful while the external layer of the fibrous ring is not affected;
although tissue may wear out there are no nerves to receive and
transmit pain.
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