A collection of specialized cells (neurons) in the head that regulates behavior as well as sensory and motor functions. Neurons grow long threadlike structures—an axon and a dendritic tree—from their cell bodies, which provide them with a rapid communication network throughout the body. The axon uses pulses to transmit a signal to thousands of other neurons or to muscle or gland cells. The dendritic tree uses waves of electric current to integrate the pulses from thousands of other neurons. Groups of neurons form ganglia in chains along both sides of the body axis from “head to tail.” The largest of these paired groups, the brain, is in the head, where the distance receptors (nose, eyes, and ears) are located. These receptors respond to smells, sights, and sounds coming so far from the collective that the collective has time to receive the inputs, interpret them as signals, plan an action before being overtaken by circumstance, and act while monitoring and correcting its action. These are the minimal functions of a brain. The power of a brain lies not in its size but in the complexity of the connections among its functional parts.