The
brain is an
organ that serves as 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
starfish do not have a brain; diffuse or localised
nerve nets are present instead. The brain is located in the head, usually close to the primary sensory organs for such
senses as
vision,
hearing,
balance,
taste, and
smell. The brain is the most complex organ in a vertebrate's body. In a typical human, the
cerebral cortex (the largest part) 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.
