The human nervous system is possibly the single most complex object in the entire cosmos, or at least the most complex object in our section of the galaxy. This is because it includes the brain, with ten billion neurons and many times more interneural connections. The human brain is a more dense source of complexity than anything we have yet seen, including the bodies of all animals and any variety of inanimate phenomena or objects.
However, it is more than just the brain. All animals have a nervous system, but only vertebrates have a complex nervous system that include the central nervous system (CNS) and peripheral nervous system (PNS) as components. The CNS consists of the nerves and neurons found in the spine and brain, while the PNS is everything else.
In older animals, the nervous system was mainly a sensor network connected directly to the skeletomuscular system, allowing external and internal causes to give rise to organism-centric effects called behaviors. In more complex organisms, it functions as an independent entity, processing inputs extensively before returning carefully chosen outputs. In all animals, including humans, the vast majority of this is unconscious, automatically executed by neural programs that have been hardwired by millions of years of evolutionary design.
The central nervous system is the most complex system and the most difficult to understand or reverse-engineer in any species, although efforts in this direction have had some success. For the purposes of this article, it can be thought of as the brain and spine, which includes the brain stem. Further subdivisions are the province of cognitive science and neuroanatomy.
The peripheral nervous system has several subdivisions. The first level consists of the somatic nervous system and the autonomic nervous system, which refer to the nerves just under the skin and the nerves everywhere else, respectively. Despite their names, both mostly execute automatically, but the autonomic system is so named because it is responsible for the body’s maintenance functions, which have a reputation for being opaque to conscious control. The nerves we use to consciously control our bodies are part of the somatic system, but these function automatically even in the event of a coma.
The autonomic nervous system is further divided into sympathetic and parasympathetic. A third division, the enteric nervous system, is also occasionally referred to. The sympathetic system responds to stress, danger, and the like, and is responsible for the release of adrenaline, among other things. The parasympathetic system is dominant during rest and helps us go to sleep and digest food. These two tend to balance each other, creating a degree of harmony in the body. The enteric system is responsible for some nerves around the intestines, and is known to function properly even when disconnected from the other two systems.