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. COUNT 5
QSN1 What is the center of the nervous system in all creatures? ANS1 The brain QSN2 What is the most complex organ in an animal's body? ANS2 The brain QSN3 In people, how many neurons make up the cerebral cortex? ANS3 15–33 billion QSN4 Neurons in the brain interact with each other by fibers called what? ANS4 axons QSN5 A few animals without spines that do not have a brain are what? ANS5 sponges, jellyfish, adult sea squirts