system is a network within the body that functions to manipulate
external and internal information. It is specialized for rapid commu-
nication of signals throughout the body, and the brain is considered
to be the locus of central control in the nervous system. Animals
presumably have nervous systems to facilitate movement through
the world. Collection and analysis of sensory information and co-
ordination with the mechanisms of movement are needed to safely
accomplish the task of moving around in an environment that is often
challenging and sometimes unpredictable. Plants and fungi, the other
kingdoms of multicellular organisms, do not move around in the fash-
ion of animals and have evolved other, nonneural ways to flourish.
The human brain (together with the brains of a few other mam-
mals) is perhaps the most complex structure known. This complexity
is embodied in its makeup, consisting of several hundreds of billions
of cells interconnected by hundreds of trillions of connections. Each
of these connections is a locus of signal transfer between cells, so at
all times there are vast networks of cells intercommunicating in some
manner. The complexity of these networks of intercellular communi-
cation is staggering.
Figure 2.1. Nerve cell (left) and astrocyte glial cell (right).