FoundationalConceptsNeuroscience

Brains are made up of two general classes of cells: neurons and glia
(Fig. 2.1). It is currently estimated that there are around a hundred
billion (1011) nerve cells (neurons) in the human brain and at least
that number of glial cells (glia). The cellular units of signal transmis-
sion are generally considered to be the neurons, although it is now
appreciated that many glia, especially the astrocyte glia, are also di-
rectly involved in signaling.
Animal nervous systems have been undergoing evolutionary re-
finement for hundreds of millions of years, and millions of years were
required for the complexity of the human brain to develop among the
primates. How far back in animal evolution do nervous systems go?
Sponges are ancient animals, believed to have come on the scene more
than half a billion years ago. As far as is presently known, sponges do
not contain nerve cells or any sort of nervous system. Hydra are tiny
(usually smaller than an inch) aquatic animals that have a very simple
nervous system—a loosely connected network of a small number of
cells, allowing for simple signal communication throughout their
body. The jellyfish, distant cousins of hydra, also possess relatively
simple neural networks (Fig. 2.2).