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.

Camillo Golgi and Santiago Ramon y Cajal shared the 1906 Nobel Prize in physiology or medicine for their work on the nervous system. Cajal made superb pen-and-ink drawings of nerve cells, and people still call them works of art. Cajal's work showed that neurons are separate cells with tiny spaces between them.

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.

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.

The wrinkly outside of the cerebrum is called the cortex. Each side of the cerebrum has four lobes. The thalamus relays messages between the senses and the cerebral cortex. The brain stem is made up of the midbrain, the pons and the medulla.

The biggest part of the brain, the cerebrum, has a left half and a right half. 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.

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. Henry Molaison could still learn new physical skills, even though he could not form new memories of events.

The hippocampus helps the brain form long-term memories. When you learn, your neurons form new connections. Skills you do not use can weaken over time, and the brain can prune connections it uses little. 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.

Words to know

• glial: kind of cell
• myelin: fatty coating
• preserved: kept
• hippocampus: a memory part