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Neurons, especially in the CNS (brain/spinal cord), are quite sensitive, and are usually kept in a sheltered environment separated from the blood by the blood-brain barrier. Astrocytes and other glial cells provide a controlled environment for the neurons. They do take nutrients from the blood, but they act as a filter to only let through the right things and in the concentrations that neurons prefer.

Even in other tissues, blood isn't usually supposed to leave the blood vessels, and can cause trouble if it does. Perhaps the easiest to understand is coagulation - if everything's clotted up, that will disrupt whatever else is supposed to be happening in that tissue. There are also immune molecules which tend to get activated and cause inflammation when outside blood vessels. Inflammation generally interferes with regular tissue function, and CNS neurons are particularly sensitive to it.

Blood also contains higher concentrations of stuff like iron that would damage neurons.

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Yes, blood is toxic to neurons. We have a blood-brain barrier that only lets some things through (either passively if they're small enough like oxygen or alcohol, or actively for bigger molecules).

The brain sits in cerebrospinal fluid (CSF) rather than blood.

As for the actual mechanisms that blood will damage the brain -- first, if you've got a brain bleed, it'll increase intracranial pressure and make it harder for new oxygenated blood to get in, which will kill your neurons from simply lack of oxygen. Second, even a small bleed will kill neurons, because things like the sodium concentration in your CSF will get messed up, and then when the blood dies in your brain it'll leave behind a bunch of iron from hemoglobin which will also kill your neurons.

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Think of it like a neuron is your house and the blood is a Highway. We need the things traveling on the highway (like people, Amazon orders, etc) to make the house function. But there are a lot of other things on the road that if they directly contacted the house (like a 16-wheeler moving at 60 mph) would cause damage. Our neurons need the oxygen and nutrients, but sometimes the things that bring the oxygen (like hemoglobin which contains iron) can be toxic if they are released onto bare neurons. That’s why neurons are separated from direct contact with blood by other cell types like astrocytes and pericytes which more carefully regulate how neurons get what they need from blood.

Thanks for your information!

There's several reasons for this. There are many things in the blood stream that would be toxic to neurons if they passed the blood-brain barrier. Also, blood spilling out of a hemorrhagic stroke doesn't oxygenate the brain in the normal way though capillaries, where the blood ells progress to veins and then eventually are re-oxygenated. Blood spilling out onto the brain causes most neurons downstream to be deprived of oxygen and nutrients.

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I would like a neurologist/immunologist to correct me if I'm wrong but. Isn't the biggest reason is that while differenciating leukocytes reacting with brain's molecules aren't destroyed the same way ones reacting with other irgans are? So basically they see a foreign material and attack it?

Also, pooled non-circulation blood is an infection risk. "Bacteria love blood."

I know this isn't a perfect analogy, but I'm imagining like how fuel and air power the combustion in an engine, but you wouldn't want it in the oil system.

Yup! Your brain sits in a bath not of blood, but a different substance called cerebrospinal fluid (CSF). It is on the "internal' side of the blood brain barrier. Nutrients and water diffuse across the blood brain barrier through very fine capillaries. Blood is too thick and "sticky" for the brain to be able to use while still performing its role. CSF itself is almost entirely water - about 99%. CSF also serves to cushion your brain and act as an immunological barrier between your brain and the rest of your body.

In addition, most organs are adversely affected by direct contact with blood. You don't want blood in your stomach or kidneys or liver either! Blood is meant to stay in your blood vessels and diffuse nutrients across various tissue barriers while the glymphatic / lymphatic systems draw metabolic and immunologic waste away to be excreted.

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Well, what they are saying is that blood, and its contents, have some direct detrimental effects, but the most immediate, significant damage is due to secondary effects of all the leaking blood. When blood escapes the vessels, it builds up, and causes pressure to increase within the skull, harming the brain. Also, with the controlled flow of the blood interrupted, the brain does not get the oxygen it needs. These are factors secondary to the blood flow no longer being controlled by vessels.

 

The direct effects of disruption of the blood-brain-barrier (neurons exposed to inappropriate stuff/concentrations of stuff, and products of decomposing RBCs) is indeed damaging, but it is the deprivation of oxygen that has the most, lasting effect. The neurons recover from the damaging, but not so lethal, effects created by the presence of blood, but the secondary lack of oxygen, is what kills the neurons.

Great explain, thank you very much!

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Can blood/brain barrier be breached by bacterial infection thus damaging the brain? I am asking this because someone close to me was recently treated for sepsis and infection on spine, left the hospital but is now investigated for dementia due to saviour symptoms that started during the infection? Just wondering 🤔

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Yes it can. During any infection of the CNS the blood brain barrier becomes more porous for everything, and people experience CNS symptoms. If the infection destroyed some neurons, those don't grow back. CNS neuronal damage is permanent.

What is the blood-brain barrier physically made of? If I opened a head, what would it look like?

There are thin cells called pericytes wrapped around the small blood vessels, and around that you have extensions of astrocyte cells. The barrier is too small to see with the naked eye.

In fact, blood is the only source of nutrients and oxygen needed for neurons to survive and operate.However, there are certain substances in the blood that can be toxic to neurons if they are present in excessive amounts

>If the infection destroyed some neurons, those don't grow back. CNS neuronal damage is permanent.

I still wonder why I had (and have) worse motor distance latency than sensor distance latency at the test my neurologist did before EMG, which is called "Nerve Conduction Study". Abnormal values. The EMG results were ok.

He asked me if I was exposed to toxins as a I child.

Initially I thought: maybe rapid development during puberty contributed to the stretching of nerves affecting those just like stretch marks? But then why would it only affect motor speed and not sensor?

Probably it has something to do with what you wrote. Irreversible damage/death of neurons due to childhood illnesses or something. Why motor and not sensor is an open question, could be different parts of the CNS affected.

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Dr liberman is my surgeon. He is works renowned and is very knowledgeable. He did my first done surgery, and my brain surgery. One thing I respect about him is that he has great bedside manner as well, and doesn't mind explaining things like this to people. I'm sure if you reach out to him, he would explain happily. But there are some good answers here already to sum it up pretty well. Blood is basically suffocating to the brain because of the clotting agents, and blood breaks down over time unlike cerebral spinal fluid. The breaking down of blood cells causes processes to happen that are not safe for the brain or spinal fluid.

Blood doesn’t normally contact neurons. The brain is surrounded by cerebrospinal fluid, not blood. There is actually a blood-brain barrier. If blood touches neurons it can damage them but if blood is touching your neurons there is something badly damaged anyway.

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