Final answer:
A herniated nucleus pulposus occurs when the gel-like nucleus pulposus in an intervertebral disc protrudes through the weakened outer layer called the anulus fibrosus, causing compression of a spinal nerve and resulting in pain and muscle weakness.
Step-by-step explanation:
A herniated nucleus pulposus occurs when the gel-like nucleus pulposus in an intervertebral disc protrudes through the weakened outer layer called the anulus fibrosus. This can happen due to injury or increasing age, where the posterior anulus fibrosus is weakened and unable to withstand the pressure when bending forward or lifting heavy objects. The herniation of the nucleus pulposus can result in compression of a spinal nerve, causing pain and muscle weakness in the affected areas.