The brain is the primary organ of the central nervous system in almost all animals, including humans. It collects information from sensory organs and from the body’s nerve cells, which in vertebrates are linked to it through the spinal cord. Brain cells process this information in order to decide and execute actions, functions they can perform millions of times in one second. In humans, the brain also initiates language, reasoning, and creativity. This organ is the subject of continued study, and some of its functions are still not fully understood.
Like the rest of the nervous system, the brain is composed of nerve cells, also called neurons. Unique among cells, neurons are designed to transmit information among themselves and other cells with electrical and chemical impulses. Neurons have great length for this purpose; a single cell can stretch more than 3 feet (1 meter) in human beings. These cells form a dense network in the brain to transmit and process information rapidly. In primitive creatures, the brain may be no more than a wide spot on the spinal cord. In advanced creatures such as humans, 100 billion neurons combine to create a complex organ weighing more than 3 pounds (1.36 kg).
In humans, the brain has several sections, each of which controls different physical or mental functions. At the top of the spinal column, the brain stem regulates involuntary functions such as breathing and circulation and connects to the spinal cord’s nerves. Nearby structures called the thalamus and hypothalamus deal with pain, the sex drive, and sleep. Motor functions and balance are maintained by the cerebellum, also located at the base of the brain.
The main structure of the human brain is the cerebrum, divided into right and left hemispheres. Much of this structure is composed of millions of interlaced axons, the parts of nerve cells responsible for transferring information. This area, collectively known as white matter, coordinates sensory input, hormone function, and some simple emotions. The outermost layer, the cerebral cortex, deals with complex tasks such as language, memory, and advanced thought or emotion.
The cerebral cortex is composed of neurons. Their gray color in preserved brains inspired the phrase gray matter. Axons from these cells make up the underlying white matter. Any cell or axon can communicate with any other through connections called synapses, electrical signals in the neuron that set off chemical signals, which in turn create other electrical signals in the receiving neuron. This process, repeated millions of times a second over trillions of possible synapses, accounts for the brain’s functioning. The precise details of this functioning are the subject of continuing scientific study.