Brain and Nervous System
All our movement, thoughts, sensations, and bodily functions are controlled by the brain and nervous system, the most highly evolved among all living creatures and the least understood. Neuroscientists are only beginning to unravel the myriad mysteries of the human brain, and many predict we will never fully understand so many of the things we take for granted: memory, language, and creativity.
The largest part of the brain is the cerebrum, which sets us apart from other animal species; not only is it the seat of intelligence and creativity, but it is also the source of such emotions as love, jealousy, forgiveness, anger, and other human qualities.
Taken as a whole, the nervous system is actually a complex branching network of systems with many overlapping parts and functions, all controlled by the brain and its spinal cord extension. Such automatic, or involuntary, functions as breathing, circulation, and digestion are directed largely by the autonomic nervous system, which is divided into the sympathetic and parasympathetic components. In simplified terms, these two systems act as switches to turn organs on and off, thus maintaining a state of balance.
Superficial sensory nerves receive messages from the outside world and transmit them to the brain, where they are interpreted and sent back through the body via the cranial or spinal nerves. All this takes only a split second and often requires little or no thought. However, when something goes awry with the brain or other components of the nervous system, manifestations can be disastrous, ranging from trivial movement disorders to paralysis and dementia.