Flexi Says: The nervous system has two main divisions: the central nervous system and the peripheral nervous system. The central nervous system (CNS) includes the brain and spinal cord. The brain is the control center of the nervous system. An average adult human brain weighs between 1 and 1.5 kg (3 lb). An adult brain uses about 20-25% of the total energy used by the body, while the developing brain of an infant consumes around 60% of the total energy used by the body.