Due to the strains to which they are exposed, the intervertebral discs of the lumbar spine are particularly susceptible to rupture. The soft gelatinous material of the nucleus pulposus leaks through the previously restraining layers of the annulus fibrosis.
Herniation, which may or may not be accompanied by an audible “pop,” allows the soft, jelly-like inner material to ooze out into the space normally reserved for the nerve roots. This either presses directly on the nerve root, or initiates an inflammatory reaction that sometimes has no effect, but often brings immediate and considerable pain. There may be numbness or other sensory changes along the exact anatomical route of the fibres that are affected. With a good physical examination, MRI and electrodiagnostic techniques, clinicians can determine just which nerves are affected, and how seriously. This is critical for treatment. In the pictured case the the herniated disc impinges on the reddened nerve root. This is extremely important for treatment.