A<unk>  is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a vertebrate's<unk> ody. In a human, the cerebral cortex contains approximately 14–16<unk> illion neurons, and the estimated number of neurons in the cerebellum is 55–70<unk> illion. Each neuron is connected<unk> y synapses to several thousand other neurons. These neurons typically communicate with one another<unk> y means of long fibers called axons, which carry<unk> s of signal pulses called action potentials to distant parts of the<unk>  or<unk> ody targeting specific recipient cells. Physiologically,<unk> s exert centralized control over a<unk> ody's other organs. They act on the rest of the<unk> ody<unk> oth<unk> y generating patterns of muscle activity and<unk> y driving the secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment. Some<unk> asic types of responsiveness such as reflexes can<unk> e mediated<unk> y the spinal cord or peripheral ganglia,<unk> ut sophisticated purposeful control of<unk> ehavior<unk> ased on complex sensory input requires the information integrating capabilities of a centralized<unk> . The operations of individual<unk>  cells are now understood in considerable detail<unk> ut the way they cooperate in ensembles of millions is yet to<unk> e solved. Recent models in modern neuroscience treat the<unk>  as a<unk> iological computer, very different in mechanism from an electronic computer,<unk> ut similar in the sense that it acquires information from the surrounding world, stores it, and processes it in a variety of ways. This article compares the properties of<unk> s across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human<unk>  insofar as it shares the properties of other<unk> s. The ways in which the human<unk>  differs from other<unk> s are covered in the human<unk>  article. Several topics that might<unk> e covered here are instead covered there<unk> ecause much more can<unk> e said about them in a human context. The most important that are covered in the human<unk>  article are<unk>  disease and the effects of<unk>  damage.
Brain
Heart
Central nervous system
Peripheral nervous system
Head
Liver
Skin
Kidney