Neuroscientists did have a rough idea. Some estimates suggested 10 to 20 billion neurons for the human cerebral cortex, others some 60 to 80 billion in another region called the cerebellum. With the rest of the brain known to be fairly sparse in comparison, the number of neurons in the whole human brain was definitely closer to billion than to just 10 billion far too little or 1 trillion way too many. But there we were, neuroscientists armed with fancy tools to modify genes and light up parts of the brain, still in the dark about what different brains were made of and how the human brain compared to others.
So I devised a way to easily and rapidly count how many cells a brain is made of. I spent 15 years collecting brains and then turning them into soup that I examined under the microscope. As it turned out, there are many ways to put brains together: Primates like us have more neurons in the cerebral cortex than most other mammals, no matter the size of the brain. A brain can be large but made of relatively few neurons if those neurons are huge, like in an elephant; primate neurons are small, and bird neurons are even tinier, so even the smallest bird brains can hide lots of neurons.
But never as many as the largest primate brain: ours. And not just that: I recently found that the more cortical neurons, the longer the species takes to develop into adulthood , just like it takes longer to assemble a truckload of Legos into a mansion than a handful into a little house. And for as yet unknown reasons, along with more cortical neurons comes a longer life.
Getting more cortical neurons thus seems to be a two-for-one bargain: Buy more mental capabilities, and along comes more lifetime to learn to use them. Lots more neurons cost lots more energy , though. The human brain is made up of a complex network of neurons.
These neurons serve as the building blocks of the nervous system, transmitting information to and from the brain and throughout the body. You probably expect that a large number of neurons are required for such a complex process, but just how many neurons are there in the human brain?
According to many estimates, the human brain contains around billion neurons give or take a few billion. This estimate has often been reported for many years in neuroscience and psychology textbooks and for many years was simply accepted as a relatively close approximation. Recently, however, Brazilian researcher Dr. Suzana Herculano-Houzel discovered that these estimates might not be entirely accurate. While the number is widely cited, she found that no one seemed to know where or when this number originated.
Estimating the number of neurons in the brain seems fairly simple on the surface. Simply take a sample of the brain, count the number of neurons in that sample and then extrapolate that information to account for the remaining brain volume. While this seems like a fairly straightforward approach, neuron density differs in different regions of the brain.
Counting neurons in a high-density part of the brain might lead to a high estimate while counting those in a lower density region might lead to an excessively low estimate. To overcome this problem, the researchers utilized a method that involved dissolving the cell membranes in order to create a sort of "brain soup" so that they could then count the number of cell nuclei in a sample.
How many neurons did the researchers find in the brains they analyzed? And not one that we looked at so far has billion. Even though it may sound like a small difference the 14 billion neurons amount to pretty much the number of neurons that a baboon brain has or almost half the number of neurons in the gorilla brain. So that's a pretty large difference actually," explained Herculano-Houzel.
So, according to this new research, the human brain likely has somewhere around 86 billion neurons. According to Herculano-Houzel, human brains are remarkably similar to primate brains with one important distinction: we have far more brain cells that require a tremendous amount of energy to fuel and maintain.
The sheer number of neurons present in the human brain becomes more apparent when compared to other species. So how many neurons are in the brains of other animals? While the human brain might not have the mythic billion neurons as long suspected, 86 billion is still nothing to sneeze at.
The density of neurons may vary by factors of up to 1, across structures. Even within a single structure, different layers can consist of different numbers of neurons. The method involves dissolving cell membranes while preserving nuclear membranes each neuron consists of one nuclear membrane , producing what Herculano-Houzel refers to as brain soup. In the brain soup, there are free-floating nuclei that are relatively easy to count by sampling tiny amounts of the soup.
All the nuclei from the cells are stained blue, collected, and counted. Counting the nuclei is simple and requires no special training. In the book The Human Advantage , Herculano-Houzel provides a description of what went into developing the technique, including detail on the early failed attempts to create brain soup. Other researchers, including Christopher von Bartheld from the University of Reno and Jon Kaas from Vanderbilt University, have shown this method to be fast, reliable, and relatively easy to apply Bahney and von Bartheld Results, from the isotropic fractionator indicate the human brain has an average of 86 billion neurons and 85 billion non-neuronal cells glial cells and endothelial cells.
Fourteen billion is a significant number of neurons. It becomes even more significant when considering each neuron may connect to thousands of other neurons. She has counted brains cells in a range of different species, so her work is also important regarding nonhuman animals. Textbook writers and top brain scientists are acknowledging her groundbreaking work. In a recent conversation with Bryan Kolb, coauthor of Fundamentals of Human Neuropsychology , he told me there would be no referrals to the billion neuron myth in future editions of the text.
I contacted Kolb after reading that the human brain contained about billion neurons in a textbook he coauthored.
0コメント