An Inventory of
All the Brain Cells
That Let You Run,
Jump and Roll
A project to map the motor cortex
used the widest range of tools for
probing brain cells ever deployed
in a single, coordinated effort
Pioneering neuroscientist Santiago
Ramón y Cajal jump-started the
search for a “components catalog”
of the human brain toward the end
of the 19th century. His intricate
drawings of brain cells, complete
with their weblike connections, still
appear in many textbooks. Looking
for brain parts is driven by more
than curiosity. Before the genera-
tions-long endeavor of deciphering
the brain can proceed, neuroscien-
tists need to first identify its multi-
tude of component parts and then
figure out what each one does.
The task is complicated by the
many ways cells can differ. Ramón
y Cajal provided glimpses of the
shapes that distinguish some cell
types but also left a virtually infinite
amount of work for future genera-
tions of neuroanatomists. Cells can
differ by location, biochemistry and
other properties. These different
descriptors often do not correspond
to one another in any simple way, a
fact that has fueled debates about
how to define cell types. As tools to
record the signals that neurons use
to communicate became available,
researchers have tried to categorizecells by comparing their different
firing patterns, the specialty of the
discipline known as electrophysiolo-
gy. This effort comes closer to
classifying what cells do, but is still
descriptive in that it describes
behavior rather than morphology.
The journey toward a definition
that describes cells according to
their function comes to an end atthe genome, the blueprint that
underlies all other biological proper-
ties. That these efforts are now
bearing fruit is demonstrated by
a large, international consortium,
funded by the National Institute of
Health’s BRAIN Initiative. It has
produced a genomics-based census
of the cell types in one region, the
primary motor cortex, which is Andriy Onufriyenko/Getty ImagesNEWS