As of November 2026, a human brain has about 86 billion neurons. Nerve cells called neurons send messages all over your body. 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. Motor neurons carry messages from the brain to the body, telling muscles when to move.
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. Myelin is a fatty coating wrapped around the axons of many neurons. Signals in nerve fibers wrapped in myelin can travel as fast as about 120 meters per second, around 250 miles per hour.
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. Camillo Golgi and Santiago Ramon y Cajal shared the 1906 Nobel Prize in physiology or medicine for their work on the nervous system.
The biggest part of the brain, the cerebrum, has a left half and a right half. The wrinkly outside of the cerebrum is called the cortex. Each side of the cerebrum has four lobes. 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. The frontal lobe is where planning, reasoning and problem solving happen.
On September 13, 1848, a railroad crew member named Phineas Gage was hurt when blasting rock sent an iron rod through his head. The iron rod that hit Phineas Gage was about 3 and a half feet (43 inches) long and weighed 13 pounds. 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.
In 1953 a surgeon removed parts of Henry Molaison's brain, including the hippocampus, and afterward he could not form new long-term memories. The hippocampus helps the brain form long-term memories.
When you learn, your neurons form new connections. The brain's ability to change with experience and learning is called neuroplasticity. The brain can change at any age, though change is a bit easier when you are younger. Parts of the brain keep maturing until people are in their 20s.
Words to know
- synapse: gap between neurons
- myelin: fatty coating
- hippocampus: a memory part
- dendrites: branches that receive
