Spinal discs are located in between the vertebrae. They play an important role in absorbing shock, supporting and accentuating many of the spinal movements.
The inner, jelly like, portion of a disc is called the nucleus pulposus whilst the tough, outer layer is called the annulus fibrosus.
What causes a disc protrusion/herniation:
Spinal disc herniations occur for many reasons. One of the main causes in incorrect posture or heavy lifting. When the spine moves in to a forward (flexed) position, additional body weight is placed on the front portion of the disc. Due to the additional pressure at the front portion of the disc, the nucleus will move in a posterior (backwards) direction in to the posterior portion of the annulus fibrosis. If this is done with enough force, small tears may occur in the annulus which may bulge out slightly causing a disc bulge. The spinal cord, and nerve roots are located very closely behind the spinal disc. If the bulge is severe enough to push out beyond the boundaries of the vertebral body (and often in to the nearby nerve roots), it is known as a disc protrusion.
Another cause of disc protrusion/herniation is disc degeneration. As one ages, the discs lose some of the fluid that makes them spongy. This leads to a gradual process of disc flattening and hardening. This starts early in life and can even be seen in some imaging scans in early adulthood. This leads to easier disc bulging or cracking when pressure is applied to the discs. The protrusions caused, again are likely to lead to nerve root compression. When a nerve root is compressed, pain is often felt along the path of the nerve. This is why it is common for individuals to suffer with referred pain in to the buttocks or legs.
References:
Spine Health, 2016. Lumbar Herniated Disc: What You Should Know. [Online] Available at: <https://www.spine-health.com/conditions/herniated-disc/lumbar-herniated-disc> [Accessed 5th April 2018].