Your brain is a three-pound organ made up of roughly 86 billion nerve cells, and it runs on about 20% of your body’s energy supply despite accounting for only 2% of your body weight. It’s the most energy-hungry organ you have, and nearly everything you experience, from a passing thought to a reflexive flinch, is the result of electrical and chemical signals racing between those billions of cells. Here’s how it all fits together.
What the Brain Is Made Of
By composition, your brain is about 60% fat, with the remaining 40% being a mix of water, protein, carbohydrates, and salts. That high fat content isn’t filler. Fat forms the insulating layers around nerve fibers that allow signals to travel quickly. The average adult brain weighs about three pounds.
For decades, textbooks claimed the brain contained 100 billion neurons and ten times as many support cells. Neither number held up to direct counting. Work by neuroscientist Suzana Herculano-Houzel established that the human brain contains approximately 86 billion neurons and a roughly equal number of non-neuronal cells, a 1:1 ratio, not the 10:1 figure still repeated in many sources. Those non-neuronal cells (called glia) insulate nerve fibers, supply nutrients, and clean up debris.
The Three Major Regions
The brain has three broad divisions, each responsible for different categories of work.
The cerebrum is the large, wrinkled mass that sits on top and makes up most of what you see. It handles conscious thought, memory, planning, imagination, and language. When you recognize a friend’s face, read a sentence, or weigh a decision, the cerebrum is doing the heavy lifting.
The cerebellum is a fist-sized structure tucked beneath the back of the cerebrum. It coordinates movement, especially learned physical skills. Playing piano, catching a ball, or maintaining your balance while walking all depend on cerebellar circuits that refine and time your muscle actions.
The brainstem connects the brain to the spinal cord and manages the functions you never have to think about: heart rate, breathing, blood pressure, swallowing, and sleep-wake cycles. Parts of the brainstem also control reflexive eye movements and route sensory information between the body and higher brain regions.
How Neurons Send Signals
Everything your brain does comes down to neurons communicating with each other through a combination of electrical pulses and chemical messengers. The electrical part is called an action potential, and it works like a chain of falling dominoes along the length of a nerve cell.
At rest, a neuron has a slight negative charge inside relative to outside. When the cell receives enough stimulation to cross a threshold, channels in the membrane snap open and let positively charged sodium ions flood in. This flips the voltage positive in that spot, which triggers the next set of channels down the line to open, and so on, sending a rapid wave of electrical activity along the cell. The whole depolarization event at any one point lasts about one millisecond.
Right behind that wave, a second set of channels opens to let potassium ions flow out, restoring the negative resting charge. The cell actually dips slightly more negative than its baseline for a brief moment before resettling. This quick reset allows the neuron to fire again almost immediately.
In nerve fibers wrapped in fatty insulation (myelin), signals can travel up to 150 meters per second, roughly 335 miles per hour. Uninsulated fibers are far slower, topping out around 10 meters per second. This is why diseases that damage myelin, like multiple sclerosis, cause such dramatic problems with movement and sensation.
What Happens at the Gap Between Neurons
Neurons don’t physically touch each other. There’s a tiny gap, called a synapse, between the sending cell and the receiving cell. When an electrical signal reaches the end of a neuron, it triggers the release of chemical messengers called neurotransmitters. These molecules drift across the gap and lock onto receptors on the next cell, either encouraging it to fire its own electrical signal or discouraging it from firing.
Four neurotransmitters do most of the brain’s signaling work. Glutamate is the main excitatory messenger, responsible for pushing neurons to fire. GABA is the primary inhibitory messenger, accounting for roughly 40% of the brain’s inhibitory signaling and acting as a brake to prevent runaway activity. Dopamine plays a central role in learning, motivation, reward, and motor control. Serotonin modulates mood, sleep, and a wide range of psychological processes, which is why so many psychiatric medications target it.
The balance between excitation and inhibition is critical. Too much excitation can cause seizures. Too much inhibition can cause sedation or loss of function. Your brain is constantly adjusting these signals in real time.
How the Brain Learns and Adapts
Your brain physically rewires itself in response to experience. This capacity, called neuroplasticity, operates through two complementary processes.
The first is strengthening. When two neurons repeatedly fire together, the connection between them becomes more efficient. This principle, often summarized as “neurons that fire together wire together,” was first proposed by psychologist Donald Hebb in 1949 and has since been confirmed at the molecular level. The strengthening process depends on the size of calcium signals at the synapse: larger calcium influxes trigger the molecular cascade that makes the connection stronger.
The second is pruning. Connections that are rarely used get weakened and eventually eliminated. This uses some of the same molecular machinery as strengthening (both require activation of the same type of receptor and calcium signaling) but diverges at a key step, leading to the removal of receptors from the synapse rather than the addition of them. Pruning is not a sign of damage. It’s how your brain becomes more efficient, clearing away unused wiring so the circuits you actually rely on can work faster.
Both processes are especially active during childhood and adolescence. The prefrontal cortex, the region behind your forehead responsible for planning, impulse control, and complex decision-making, is one of the last brain areas to fully mature. It doesn’t reach structural completion until around age 25, which helps explain why teenagers can be brilliant in many ways yet still struggle with long-term planning and risk assessment.
Why the Brain Needs So Much Fuel
Your brain consumes about 20% of all the glucose-derived energy your body produces, a staggering share for an organ that makes up just 2% of your body weight. Nearly all of that glucose is fully burned with oxygen to produce energy, consuming roughly 5.5 to 5.8 molecules of oxygen for every molecule of glucose. This is why you feel mentally foggy when you haven’t eaten or when your blood sugar drops: your brain is one of the first organs to feel the shortage.
This enormous energy demand also means the brain generates a lot of metabolic waste. Clearing that waste turns out to be one of the most important things that happens while you sleep.
How Your Brain Cleans Itself During Sleep
The brain has its own waste-clearance system, sometimes called the glymphatic system, that ramps up during deep sleep. The mechanism, detailed in a 2024 study published in Cell, works like a pump driven by rhythmic pulses in your blood vessels.
During the deep stages of non-REM sleep, a brain region called the locus coeruleus releases slow, rhythmic waves of norepinephrine. These waves cause blood vessels in the brain to contract and relax in a steady rhythm. Each contraction pushes blood volume down slightly, and cerebrospinal fluid rushes in to fill the space. This pulsing flow of fluid washes through brain tissue, carrying away waste products, including proteins linked to neurodegenerative diseases.
The frequency of these norepinephrine oscillations during deep sleep directly predicts how much waste gets cleared. Interestingly, the common sleep medication zolpidem was found to suppress these oscillations and reduce glymphatic flow, highlighting that not all sleep is equally effective at brain cleaning. The micro-architecture of your sleep, the specific pattern of deep-sleep stages, matters as much as total hours.
The Brain’s Speed and Scale
To put the brain’s complexity in perspective: 86 billion neurons, each forming thousands of connections, create an estimated 100 trillion synapses. Signals cross those synapses in fractions of a millisecond, and the fastest nerve fibers carry information at highway speeds. All of this runs on roughly 20 watts of power, about the same as a dim light bulb. No computer comes close to matching that efficiency for the range of tasks your brain handles every second, from keeping your heart beating to composing a sentence to recognizing the smell of coffee from across a room.