The brain serves as a center for the nervous system in most invertebrate and all vertebrate animals. The brain is located in the head and is typically very close to the primary sense organs, utilized for senses including hearing, balance, taste, vision and smell. The brain is the single most complex organ in a vertebrate's body. Humans have a cerebral cortex that is estimated to contain 15-33 billion neurons in a typical human. Each neuron is connected with synapses to thousands of other neurons. The neurons communicate via long protoplasmic fibers also known as axons. The axons then carry signal pulses known as action potentials to various, distant locations in the brain or body. These pulses target specific cells in order to trigger actions.
Looking at the brain from a biological and evolutionary point of view, the main purpose of the brain is to exert some sort of control over the other organs in the body. The brain acts for the rest of the body by either generating different patterns of muscle activity or by secreting chemicals also known as hormones. Having a centralized, control system like this allows for the most rapid and yet coordinated responses possible with any change in the environment. A few basic responses, like reflexes, are controlled by the peripheral ganglia or the spinal cord, but purposeful and sophisticated control of behaviors is left to the centralized brain's capabilities.
Not only does the brain make the body run, it also forms a structure that allows the mind to work. When psychology first became a studied discipline, it was believed that the mind existed separately from the brain. Today, scientists know this is false and are continuing to explore various cognitive functions and learn a great deal every day about the brain. While science, psychology and technology has come a great distance in understanding how the mind and brain function, there is still a long ways to go.