The nervous system is a collection of nerves and cells which transmit signals to different parts of the human body. Humans have four (4) types of nervous system – the cranial nervous system which are the nerves that connect our brain to our eyes, ears, mouth, and other parts of our head, the peripheral nervous system which consists of our sensory neurons and nerves which connect the central nervous system to our arms, legs, hands, and feet, the central nervous system which is the command center of our body (brain, retina, and spinal cord), and the autonomic nervous system which are nerves that connect our central nervous system to the heart, stomach, lungs, bladder, intestines, and sex organs.