The brain makes up just two percent of our total body weight, but crammed inside are approximately 86 billion neurons, surrounded by 180,000 kilometers of insulated fibers connected at 100 trillion synapses. It’s a vast biological supercomputer.
The cells in the brain communicate using electrical signals. When a message is dent, thousands of microscopic channels open, allowing positively charged ions to flood across the membranes. Afterwards, more than 1 million miniature pumps in each cell move the ions back again ready for the next impulse.
The cell bodies of the neurons, and their connections, are contained within the grey matter, which consumes 94 per cent of the oxygen delivered to the brain. Different areas are responsible for different functions, and wiring them together is fatty network of fibers called white matter.
When a signal reaches the end of a nerve cell, tiny packets of chemicals signals spill out onto the surrounding neurons. These connections, called synapses, allow messages to be passed from one cell to the next. Each neuron can receive thousands of inputs, coordinating them in time and space, and by type of chemical, to decide what to do next.
Scientists have been electrical the brain to see how it responds to to different signals, recording electrical activity to map thoughts and using imaging like function MRI to track the blood flow increases that reveal when nerve cells are firing. The cells of the brain can also be studied inside the lab. Thanks to these investigations we know more about this incredible structure than ever before, but our understanding is only just beginning. There is so much more to learn.
From a single cell to an incredibly intricate network in just nine months.
Within weeks of fertilization, neuron progenitors starts to form; these stem cells will go on to become all of the cells of the central nervous system. They organize into a neural tube when the embryo is barely the size of a pen tip, and then patterning begins, laying out the structural organization of the brain and spinal cord. At its peak growth rate, the developing brain can generate 250,000 new neurons every minute. By the time a baby is born, the process still isn’t complete. But, by the age of two, the brain is 80 per cent of its adult size.
This astonishing structure is formed and refined as pregnancy progress.
Brain development starts just three after fertilization. The first structure is the neural tube, which divides into regions that later become the forebrain, midbrain, hindbrain and spinal cord.
The pattern of the brain and spinal cord is now laid out and is gradually refined, controlled by gradients of signaling molecules that assign different areas for different functions.
As the embryo becomes larger, the brain continues to increase in size and neurons migrate and organize. The surface of the brain gradually begins to fold. At this point, a fetus only measures about five centimeters in length.
Before a baby is born, around half of the nerve cells in the brain are lost and connections are pruned, leaving only the most useful. This process continues after birth.
The brain folds in on itself to cram in more processing power. The folds and pockets of our brains are a biological rarity that we only share with a few other species, including dolphins, some primate and elephants. It’s a clever evolutionary adaptation that allows intelligent species to a squash huge amount of cortical tissue into a small space, allowing enormous brainpower to be crammed into our relatively small skulls.
Folding start during the second trimester of pregnancy, creating ridges (gyri) and fissures (sulci), but the biology behind the distinctive wrinkles is stranger than you might think. The organization of the brain is determined by complex cascades of chemical signals, but the overall shape seems to be the result of simple physics. Grey matter sits on the outside of the brain, and during development its growth rapidly outpaces the growth rapidly outpaces the growth of white matter underneath. This puts mechanical stress on the structure, forcing the outside to buckle and curl.
More wrinkled brains are associated with higher intelligence (brain sizes not to scale).