The
brain is the center of the
nervous system in all
vertebrate and most
invertebrate animals—only a few invertebrates such as
sponges,
jellyfish, adult
sea squirts and
starfishes do not have one, even if diffuse neural tissue is present. It is located in the head, usually close to the primary sensory organs for such senses as vision, hearing, balance, taste, and smell. The brain of a vertebrate is the most complex organ of its body. In a typical human the
cerebral cortex 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.