Introduction
We can trace the development of a nervous system, and correlate with it the parallel phenomena of sensation and thought. We see with undoubting certainty that they go hand-in-hand. But we try to soar in a vacuum the moment we seek to comprehend the connexion between them... Man the object is separated by an impassable gulf from man the subject.John Tyndall, British physicist, in Fragments of Science for Unscientific People: A Series of Detached Essays, Lectures and Reviews , 1892
The distinction between the brain and the mind, as described by Tyndall, is but one of many questions that have fascinated scientists regarding the human nervous system. Several Nobel Prizes have been awarded to scientists who have helped elucidate the workings of nerves and nervous systems, usually with the aid of studies in non-human organisms.
While you’re reading this, your nervous system is performing several functions simultaneously. The visual system is processing what is seen on the page; the motor system controls the turn of the pages (or click of the mouse); the prefrontal cortex maintains attention. Even fundamental functions, like breathing and regulation of body temperature, are controlled by the nervous system. A nervous system is an organism’s control center: it processes sensory information from outside (and inside) the body and controls all behaviors—from eating to sleeping to studying to finding a mate.
Diversity of nervous systems
Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in [link] . Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves. Flatworms of the phylum Platyhelminthes have both a central nervous system (CNS), made up of a small “brain” and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.