What protective structure surrounds the central nervous system?

Cerebrospinal fluid is the protective structure that surrounds the central nervous system. This clear, colorless fluid is found in the spaces around the brain and spinal cord, effectively acting as a cushion that helps absorb shocks from any impact or movements. Additionally, it provides essential nutrients to the brain and removes waste products, playing a crucial role in maintaining optimal brain function and health. The presence of cerebrospinal fluid also helps to regulate intracranial pressure.

While other options present important elements related to the central nervous system—such as the blood-brain barrier, which protects the brain from harmful substances in the bloodstream, and connective tissues that support the structure of the nervous system—none provide the physical cushioning or the protective fluid environment that cerebrospinal fluid does. Neurons, on the other hand, are the functional cells that carry signals, but they do not have a protective role surrounding the central nervous system. Thus, cerebrospinal fluid is the most accurate choice for the question of what surrounds the central nervous system protectively.