Introducing the brain: floating cells (Introduction)

by David Turell , Tuesday, May 03, 2022, 16:08 (933 days ago) @ David Turell

Brain cells float in fluid:

https://physicstoday.scitation.org/doi/full/10.1063/PT.3.4999

"It’s now known that every cell in the brain is separated from its neighbor by a fluid-filled extracellular space (ECS), which forms sheets and tunnels, as shown on page 26 in a computer reconstruction of the ECS in a rat’s brain. That interstitial fluid is predominantly an aqueous solution of sodium chloride with small concentrations of many other essential substances, such as potassium, calcium, and several amino acids and peptides. The ECS also hosts a sparse extracellular matrix of larger molecules. The space between cells is exceedingly narrow—much of it only tens of nanometers wide—and thus one of the most difficult domains of the brain to study in the living state. But without the ECS, electrical signals wouldn’t pass between neurons, metabolic substances and chemical signals wouldn’t disseminate, and drugs wouldn’t reach their targets. The long journey to finally observe and understand the ECS was made possible by innovations in the analysis of molecular diffusion.

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"The possibility of flow in the ECS has since received further support. Recent work suggested that the flow originates in the perivascular spaces, which form a sheath around the many blood vessels that penetrate the brain. Cerebrospinal fluid enters the sheaths surrounding arteries near the brain surface and then moves into the brain. The proposed idea is that some of that fluid leaves the perivascular spaces and moves through the ECS to exit at the perivascular space around veins. Flow in the perivascular space has been established, but flow in the ECS is not yet confirmed.

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"Why does brain tissue have so much ECS? One answer is that, in order to exchange electrical signals, nerve cells maintain a potential difference between their insides and outsides. That potential arises from a difference in ionic concentrations across the ion-selective cell membrane that creates a battery. So there must be a reservoir of ions external to the membrane, which is maintained by active transport of ions across cell membranes.
Another reason for the ECS is that substances need to diffuse between cells. Some of those substances, such as glucose, are involved in cellular metabolism, and some are waste products of metabolism. Those substances move to and from the vast network of blood vessels that permeate the brain. Other substances are signaling molecules that pass between cells. That chemical communication channel has long been discussed14 and today is widely accepted and commonly called volume transmission.

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"Finally, dramatic changes in ECS volume have been shown to occur in pathological conditions, such as ischemia or stroke, when the local blood flow is cut off to part or all of the brain. The resulting lack of oxygen and glucose causes cells to rapidly swell and thus shrink the ECS volume fraction as some of the interstitial fluid’s salt enters cells and water follows suit to maintain osmotic balance. In that process, the ECS volume fraction drops as low as 5%, and the tortuosity increases2 to around 2.0. Similar changes occur in what’s known as spreading depression or spreading depolarization—a condition thought to underly some types of migraine headache—when diffusion appears to briefly stop altogether.

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"In addition to being 3D, the ECS is a dynamic structure that varies on multiple timescales. The change in volume fraction between sleeping and waking states shows variance sustained over hours,13 while an analysis of spreading depression saw dramatic changes in volume fraction over tens of seconds or minutes.6 Recently, rapid extracellular volume pulsations lasting a second or so have been detected during epileptiform activity."

Comment: the brain turns out to have an extremely complex design at the cellular level which adds to our knowledge of how it works. No other organ has each cell floating in a special fluid.