Question

What is 90% of the brain made of?

Answer 1

90% of the brain is made of glial cells and microglia.

Although it used to be thought that 90% of the brain was made of just glial cells, but current scientific consensus and modern research now indicates that the human brain also contains roughly an equal number of neurons and non neuronal cells including glia, with estimates of around 86 billion neurons and 40 to 50 billion glial cells.

And by weight, the brain is mainly composed of water or around 73 percent to 78 percent of water and lipids of around 10 percent to 12 percent, instead of just a specific cell type.

By mass, the brain is not defined by it's cell count percentages, and instead the brain is defined by it's chemical make up which includes 73 percent to 78 percent water, 10 to 12 percent fats or lipids and 8 percent of proteins.

The brain is the most protected organ in the human body and is enclosed in a solid and thick cranium or skull.

The brain in the human body is also shielded further by 3 protective membranes that are called dura mater, arachnoid mater and pia mater as we well CSF or cerebrospinal fluid for cushioning against shock and the blood brain barrier or BBB.

The skull or cranium is what provides a rigid and armored and protective casing for the human brain.

The CSF or cerebrospinal fluid acts a liquid cushion to absorb physical shocks to the brain and provide nutrients to the brain as well.

The meninges are 3 layers of protective membranes that protect the brain, which include the dura mater, arachnoid mater and pia mater that surround the brain.

And the blood brain barrier or BBB is a highly selective membrane, which keeps harmful substances and toxins out of the brain while also allowing essential nutrients into the brain.

Other organs like the lungs and the heart have some protection as well like the rib cage, but no other organ other than the human brain has as many layers of physical and biological defense as the brain does.

Over millions of years, the human brain has evolved distinct features, particularly within the neocortex, resulting in increased size, complexity, and unique cellular composition due to specialized molecular expressions and intricate connectivity.

The most damaging factors to the brain are often chronic lifestyle issues like poor sleep, chronic stress, a sedentary lifestyle, and unhealthy diets (processed foods, high sugar, bad fats), alongside specific substances like excess alcohol and smoking, all of which can impair function and increase risks for neurodegenerative diseases.

However, acute physical trauma, certain medications, and substance abuse (like opioids) can also cause severe, immediate damage.  

The brain is incredibly complex, weighing about 3 pounds but using 20% of the body's energy, containing billions of neurons with trillions of connections, generating its own electricity, and capable of rewiring itself through neuroplasticity to form new pathways, essentially having a vast, virtually unlimited storage capacity.