What Are The Main Parts Of The Human Nervous System?

The principle organ in the human nervous system is the brain, which forms part of the central nervous system. The main nerves that lead out of the brain form the spinal cord, which extends from the base of the brain to the bottom of the back. Nerves branch out from this a reach into every part of the body. Once the nerves leave the spinal column, they are said to belong to the peripheral nervous system.

The peripheral nervous system is divided into two parts, the somatic and autonomic nervous systems. The somatic system is responsible for controlling voluntary actions - those that you consciously decide to do - and the autonomic system keeps control of the involuntary body functions such as breathing and heart rate. Some of these can be overridden by the somatic system - you can hold your breath for example, but some cannot. Most people cannot will their heart rate to slow down but it can be changed by behaviour (if you want it to beat faster, you run about!).

Cerebrum, Cerebellum, Medulla

Brain spinal cord sensory and motor nerves

Basically, the brain, the cerebellum, and the spinal cord are the main parts.  There are other parts, but those are the main parts.