FoundationalConceptsNeuroscience

its perception from the rotated eyeball.
Instead, the frog’s nervous system appears to have formed exactly
the same connections between the eye and the brain as were there
before the optic nerve was cut. This was a profound result. It inspired
Sperry to formulate what came to be called the chemoaffinity hypoth-
esis, proposing that nerve cells use specific chemical signals to guide
their wiring during development and during neural regeneration.
In the decades since Sperry’s experiments and proposal of chemi-
cally guided nerve growth, a variety of protein molecules and mecha-
nisms have been discovered that regulate the processes of cell growth,
differentiation, migration, and synaptogenesis. Collectively one refers
to this diverse group of molecules as nerve growth and guidance
factors.
The first nerve growth factor, or neurotrophin, to be discovered was
named simply “nerve growth factor,” or NGF. Others now known are
called things like BDNF (brain-derived neurotrophic factor), GDNF
(glia-derived neurotrophic factor), and NT3 (neurotrophin-3). These
are all proteins that have been found to somehow promote the growth
or survival of neurons. Promoting survival is a particularly important
mechanism related to neuronal growth, because large numbers of
neurons are eliminated by a kind of programmed cell death during the
early development and wiring of the nervous system. In some regions
of the brain, more than 50 percent of the neurons may be eliminated
during early development.
Other proteins, involved in axon and dendrite guidance, as well as
other developmental processes, go by such names as ephrin, netrin,
neuropilin, plexin, semaphorin, Slit, and Robo. Guidance sometimes
involves direct contact between one protein anchored to one cell and
another protein anchored to another. For example, ephrin proteins on
one cell bind to ephrin receptor proteins on another. Such binding can