[ Science vol. 331 p. 708 ’11 ] “The human brain contains about 80 billion neurons that communicate with each other via specialized connections or synapses. A typical adult brain has about 150 trillion synapses.” Well back in the day, when I was starting out, the brain was thought to have about a billion neurons and supporting elements (called glia) combined. Should you believe this any more than you should believe the latest government guidelines about diet? I’m not sure. Consider the following: The elements of the brain are packed so closely together that 100 years ago some serious, intelligent neuroscientists did not think the brain was made of cells like the rest of the body. We now know why — neurons and their processes are so mushed up against one another that you can’t see the space between them at the resolution of the light microscope (which is all they had back then) — only about 2000 Angstroms. Aside from the seeing problem there is the counting problem. 24 x 60 x 60 x 365 = 31,536,000. So given a plucky neuroanatomist, one hundred years time and a lot of stamina he’s only up to 3,143,600,000 neurons at the end. Even counting the number of synapses on a given neuron is tricky — given the fact that you need electron microscopy to even see them, and serial sections at that.
This is typical of a fair amount that gets said about the brain. At least the above statement gives 2 references so you can at least set how they got these figures
The genre goes all the way back to Aristotle, who thought that the function of the brain was to cool the blood, a case of mistaking the CPU (central processing unit) for the fan.
Descartes thought the pineal gland (which along with the pituitary is one of the few unpaired midline structures of the brain) was the seat of the soul. For why and a nice discussion see http://plato.stanford.edu/entries/pineal-gland/. Don’t dismiss him, the dichotomy between thought and physical action continues to plague philosophy (and neuroscience).
In these cases, at least you can reach the source and make up your own mind. Not so with urban myth, the classic being the young male child castrated in the men’s room of a shopping center (always unnamed).
Example #1: You lose 10,000 neurons a day. This presumes that you can count them, which I doubt. I’ve never been able to track this one down and it’s been around for decades. It’s not a big deal if we really have 80 billion of the things. 365 x 10,000 x 100 is 365,0000,000 = 3,650,000,000. If we have 1 billion (as we were thought to when I was starting out) you’re in serious trouble if not dead before you get past 40.
Example #2: You only use 5 – 10% of your brain at any one time. I’ve never been about to track this one down. However, whatever your brain is doing, it’s consuming a lot of oxygen and glucose while you’re sitting there reading this. The brain has a mass of 1200 – 1500 grams (around 3 pounds in the 70 kiloGram man beloved of introductory physiology courses everywhere). So it’s about 2% of body weight. However right now, your brain is consuming 20% of the oxygen you breathe and 20% of the glucose in your blood, and 20% of the blood your heart is pumping. Not only that your brain lives from heartbeat to heartbeat as it has essentially energy reserves of its own and depends on what the blood brings it. This is why you lose consciousness with a minute or so of choking. Even worse, once an area of the brain is deprived of blood (or oxygen and glucose in the blood) it begins eating itself. 90 years ago Haldane said that Anoxemia (lack of oxygen in the blood) not only stops the machine but wrecks the machinery. This, along with the very limited regenerative capacity of the brain is why strokes are so devastating.
There are plenty more, but enough cynicism. The next post will take the first reference seriously e.g. — assume that it’s true. Until then just consider what it would take for you to accept an explanation of how the brain does what it does, given all those neurons and their connections.
Chemiotics II 
Comments
I believe the “we only use 10 % of our brain” came from the not so distant past when we thought glia only had a support function.
Possibly the source, but I remember hearing it quoted to the effect that we could accomplish a lot more if we only tried harder. As a medical student I remember a case presentation of a brain tumor case which began as a personality change which was blamed on the tumor (just about all intrinsic brain tumors are not made of neurons, but glia of various sorts). One of the attendings asked ‘can glia think?’.
I was told in neurobiology class that the notion we only use 5% or 10% or some absurd amount of our brains came from the 19th century neuroanatomists. They could ablate parts of the brains of animals or stimulate them with electricity and assign them to sensory or motor cortex, but with humans they could only do this with the aforementioned much smaller amount. I was not given a source for this attribution. In the same class, I was also told the story about the frog (or was it a rabbit) that would sit in a pot of water and not jump out if you only raised the temperature by 1 degree a minute, so it’d cook to death.
Nonetheless, a lot of this enters the public consciousness, and even the consciousness of very intelligent people — e.g. Neal Stephenson who appears to have built his latest novel “Fall” around the proposition that if you knew the complete wiring diagram of someone’s brain you could reconstruct their conscious experience — for details see — https://luysii.wordpress.com/2019/09/03/prolegomena-to-reading-fall-by-neal-stephenson/