Introducing the brain: intense protection (Introduction)

by David Turell , Wednesday, June 21, 2023, 16:22 (531 days ago) @ David Turell

The blood/brain barrier reveiwed:

https://www.quantamagazine.org/how-the-brain-protects-itself-from-blood-borne-threats-2...

"Embedded in the walls of the hundreds of miles of capillaries that wind through the brain, the barrier keeps most molecules in the blood from ever reaching sensitive neurons. Much as the skull protects the brain from external physical threats, the blood-brain barrier protects it from chemical and pathogenic ones.

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"Like the rest of the body, the brain needs circulating blood to deliver essential nutrients and oxygen and to carry away waste. But blood chemistry constantly fluctuates, and brain tissue is extremely sensitive to its chemical environment. Neurons rely on precise releases of ions to communicate — if ions could flow freely out of the blood, that precision would be lost. Other types of biologically active molecules can also twang the delicate neurons, interfering with thoughts, memories and behaviors.

“'It’s really there to control the environment for proper brain function,” said Richard Daneman, an associate professor of pharmacology at the University of California, San Diego.

"So the blood-brain barrier provides protection, but it’s not a discrete structure like the walls around a fortress. Instead, the term refers to the unique properties of the blood vessels in the brain and those of the neighboring brain cells that wrap closely around those vessels.

"Most of the body’s capillaries are “leaky” at a molecular level to allow the free flow of nutrients and other substances. Their permeability is crucial to the function of organs such as the kidney and liver.

"But the brain’s blood vessels are built to a higher, less leaky standard. The endothelial cells that make up the capillary walls are pinned tightly together by structures called tight junctions. Thin parallel protein strands stick the cells together like “wires through the bricks,” said Elisa Konofagou, a professor of biomedical engineering and radiology at Columbia University. A few kinds of molecules can get past, but in small amounts. And they are mostly very small and water-soluble.

"But the brain also needs many other molecules such as glucose and insulin, which can’t squeeze between the tight junctions. The barrier is therefore also lined with pumps and receptors that, like bouncers for an elite club, permit only certain molecules in — and quickly eject most trespassers. Beyond the capillary wall itself are layers of supportive cells including pericytes and astrocytes, which also help to maintain the barrier and adjust its permeability.

"The blood-brain barrier is “there to deliver what the brain needs,” Daneman said. But not every part of the brain needs the same molecules, so the barrier is not the same everywhere. The barrier in the olfactory bulb, for example, acts differently and has a different protein composition than the barrier in the hippocampus, Rhea said.

"In fact, some parts of the brain don’t have a traditional blood-brain barrier at all. In the choroid plexus, a tissue in the large cavities of the brain that produces cerebrospinal fluid (CSF), the walls of blood vessels are much leakier. They have to be because the “blood-CSF” barrier of the choroid plexus needs to secrete half a liter of CSF into the brain every day, and that kind of output requires extensive amounts of water, ions and nutrients from the blood.

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"Even though this protective function isn’t perfect, it is so universally useful that every organism with a complex nervous system has something resembling a blood-brain barrier, Daneman said.

"Even flies and other insects, which don’t have blood vessels, have one. Their equivalent of blood simply sloshes through organs inside their exoskeleton, but their equivalent of a brain is sheathed in protective glial cells.

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"Where once the blood-brain barrier was thought of as a static, unchanging wall, scientists now view it as dynamic and “living,” Lehtinen said. It likely “grows and develops in different ways in different parts of the nervous system.” It temporarily creaks open naturally when we are in deep REM sleep or when we exercise. It changes with exposure to hormones and drugs, closing off old avenues for entry or opening new ones. When some molecules bind to the barrier, its cells can sometimes signal to the brain how to act without ever letting the molecule through, Rhea said.

"So rather than a stone rampart around a medieval fortress, the blood-brain barrier is like a magical wall in which doors appear and disappear, and windows grow bigger and smaller. Some parts crumble, some parts get built back — and it is constantly changing.

"'The blood-brain barrier is “never static,” Rhea said. “It’s never just this wall that needs to be overcome.'”

Comment: the barrier exists all through the levels of evolution. It reeks of purposeful design. It seems reasonable to assume the brain and barrier were designed all at once. Defenseless brains would not have survived. Excerpts are from an article reviewing how to treat a brain.