Brain

The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate species. Only a few species do not have a brain; diffuse or localized nerve nets are present instead. The brain is usually located in the head, close to the primary sensory organs for such senses as vision, hearing, balance, taste, and smell. The brain is the most complex organ in a vertebrate's body. In a typical Terran human, the cerebral cortex (the largest part) is estimated to contain 15–33 billion neurons, each connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.

Physiologically, the function of the brain is to exert centralized control over the other organs of the body. The brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment. Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain.

The operations of individual brain cells are now understood in considerable detail and the way they cooperate in ensembles of millions is solved. Modern neurosciences treat the brain as a biological computer, very different in mechanism from an AI, but similar in the sense that it acquires information from the surrounding world, stores it, and processes it in a variety of ways, analogous to the central processing unit (CPU) in a computer.