The brain is a complex organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It contains billions of nerve cells that are connected by synapses to other cells and can send a train of signal pulses throughout the body.he brain is a complex organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It contains billions of nerve cells that are connected by synapses to other cells and can send a train of signal pulses throughout the body.