Brain.......... is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfishes and starfishes have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, balance, taste and smell. Human brain contains roughly 100 billion neurons linked with up to 10,000 synaptic connections each. 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 and target them to specific recipient cells.

Brain is protected by the skull and meninges, surrounded by cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the delicate nature of the brain makes it vulnerable to numerous diseases and several types of damage. Because these problems generally manifest themselves differently in humans than in other species, an overview of brain pathology and how it can be treated is deferred to the human brain, brain damage and neurology articles.