Final answer:
Substances from the blood can access the CNS through the blood-brain barrier, which allows selective passage of molecules like glucose and amino acids.
Step-by-step explanation:
The central nervous system (CNS) in vertebrates is protected by a barrier called the blood-brain barrier (BBB). The BBB allows only specific types of molecules to enter the CNS from the blood. Glucose, amino acids, water, gases, and ions can cross the BBB, but most other substances, including white blood cells, cannot pass through. This barrier is important for protecting the CNS from toxic or pathogenic substances, but it also poses a challenge for drug delivery to the nervous system.
In vertebrates, substances from the blood can access the central nervous system (CNS) primarily through an active transport process via the blood-brain barrier (BBB). This barrier selectively allows certain substances to enter the CNS, such as glucose, amino acids, water, gases, and ions, while restricting others including white blood cells. The BBB, while protecting the CNS from harmful substances, poses challenges in drug delivery for pharmaceutical companies, as therapeutic agents must be able to traverse this barrier to affect the nervous system.