A grown-up's brain weighs about 3 pounds; your brain is only about 2 percent of your body weight. Your brain uses about 20 percent of your body's energy.

The brain and the spinal cord together are called the central nervous system. The spinal cord is a long bundle of nerves about 18 inches (45 cm) long. Nerves that run through the whole body make up the peripheral nervous system.

Neurons have branch-like parts called axons and dendrites; a neuron's branches, called dendrites, receive signals from other neurons. Neurons pass messages along using tiny electrical signals. At a synapse, one neuron releases chemicals that cross a tiny gap to the next neuron. The gap at a synapse is only 20 to 40 nanometers wide, far thinner than anything you can see. Motor neurons carry messages from the brain to the body, telling muscles when to move.

Camillo Golgi developed a stain that made nerve cells, with their axons and dendrites, clearly visible. Golgi thought the nervous system was one connected net, but Cajal showed it is made of separate nerve cells. Cajal developed the neuron doctrine: the idea that the individual neuron is the basic unit of the nervous system. Cajal made superb pen-and-ink drawings of nerve cells, and people still call them works of art. Camillo Golgi and Santiago Ramon y Cajal shared the 1906 Nobel Prize in physiology or medicine for their work on the nervous system.

Scientists counted brain cells by turning brain tissue into a liquid suspension and counting the cell nuclei in small samples. As of November 2026, direct counts found about 86 billion neurons in the human brain, fewer than the 100 billion that scientists repeated for years. As of November 2026, the human brain has about as many glial cells as neurons.

Myelin is a fatty coating wrapped around the axons of many neurons. Myelin helps signals travel faster along a nerve fiber; myelin is broken by small gaps called nodes of Ranvier, and signals seem to jump from node to node. Signals in nerve fibers wrapped in myelin can travel as fast as about 120 meters per second, around 250 miles per hour.

A thick band of nerve fibers, the corpus callosum, carries messages between the brain's two hemispheres; language is mostly handled on the left side of the brain, and attention more on the right. The idea that some people are left-brained and others right-brained is a myth. A study of resting brain scans from 1,011 people aged 7 to 29 found no sign that people are left-brain or right-brain dominant.

The hippocampus helps the brain form long-term memories; scientists studied London taxi drivers, who are experts in the layout of the city, to see whether the hippocampus changes with experience. Experiences shape the brain, and they can keep shaping it even in adulthood. Skills you do not use can weaken over time, and the brain can prune connections it uses little. Parts of the brain keep maturing until people are in their 20s; the brain can change at any age, though change is a bit easier when you are younger.

On September 13, 1848, a railroad crew member named Phineas Gage was hurt when blasting rock sent an iron rod through his head. Phineas Gage survived the accident, but he lost the sight in his left eye. Gage's doctor said his personality changed after the injury. Phineas Gage lived about twelve years after his accident.

Neurosurgeon William Beecher Scoville removed parts of Henry Molaison's brain, including the hippocampus, in 1953; afterward Molaison could not form new long-term memories. Henry Molaison's short-term memory was preserved, while his long-term memory was impaired. Studying Henry Molaison showed that removing the hippocampus and nearby structures harms memory in this way, so the hippocampus is needed for normal memory. After Henry Molaison died, his brain was donated to science and studied, giving the first look at the damage inside it.

Words to know

• myelin: fatty coating
• hippocampus: a memory part
• synapse: gap between neurons
• nanometers: tiny lengths