The blood-brain barrier is a highly specific, partially permeable membrane that consists of endothelial cells. It establishes communication between the central and peripheral nervous systems. Since the brain is a vital organ of the body, it needs to be protected against harmful agents from seeping in from the blood. The blood-brain barrier acts as a checkpoint and screens substances before they enter into the complex central nervous system.
This barrier restricts the flow of numerous substances, such as blood solutes, hydrophilic molecules, immune factors, immune cells, and antibodies. It only allows the passage of certain substances into the cerebrospinal fluid, such as glucose, water, hydrophobic molecules (oxygen and carbon dioxide), specific hormones, and amino acids that are required for the healthy functioning of the brain.
Components of the Blood-Brain Barrier
1. Endothelial Cells
These cells initiate tight contacts to maintain strict passage control. This characteristic feature is what separates the central nervous system endothelial cells from the ones found in the peripheries. Endothelial cells are also involved in vasoregulation and insulin signaling processes.
Neurons are excitatory cells that communicate with each other via synaptic connections in the brain. They are responsible for receiving sensory information so that a motor response can be generated in return.
Neurons actively engage themselves in a plethora of brain activities, such as dopamine signaling, enhancing cognition, strengthening synaptic connections, promoting peripheral metabolism, and initiating mitochondrial function.
These glial cells are found in a much greater proportion than the neurons. Astrocytes are responsible for glucose uptake from the blood-brain barrier. They not only initiate mitochondrial function, but also cause the release of adenosine triphosphate (ATP), which is primarily responsible for the transferring and storing of energy in cells.
Pericytes were previously known as Rouget cells. These cells are found wrapped around the endothelial cells within capillaries. Pericytes promote homeostasis in the brain, and also sensitize the neurons to insulin. Their role in protecting endothelial cells is widely studied.