Final answer:
Medications often cannot affect the central nervous system (CNS) because of the blood-brain barrier, a selective membrane that allows only specific molecules to pass from the bloodstream into the CNS, hindering many drugs from reaching their targets within the brain and spinal cord.
Step-by-step explanation:
The Blood-Brain Barrier and Medication Delivery
Many medications introduced into the bloodstream cannot directly affect neurons of the CNS primarily due to the blood-brain barrier (BBB). This barrier is constituted by the endothelial cells of brain capillaries, which form tight junctions, effectively restricting the passage of most substances. Unlike other tissues in the body, the CNS capillaries have fewer fenestra and pinocytotic vesicles, reducing permeability. Only select molecules, such as glucose, amino acids, water, some gases, and ions, can enter the CNS through active transport mechanisms or because they are small enough to pass through the barrier. This unique feature serves to protect the brain from pathogens and toxins, but it also presents considerable challenges for delivering drugs to the CNS, as most pharmaceutical compounds cannot cross the BBB.
Astrocytes are a type of glial cell that contribute to the formation of the BBB by extending their end feet around the capillaries, further limiting the access of substances from the bloodstream into the brain tissue. The BBB plays a pivotal role in maintaining the CNS environment but also complicates the treatment of CNS disorders and infections, as many therapeutic agents are unable to reach their target within the brain.
The final answer is that the blood-brain barrier created by the lining of the capillaries in the CNS forms a selective barrier, which is the main reason many medications cannot directly affect the CNS.