sometimes cause cell growth in the direction of contact or in other
circumstances cause growth away from the direction of contact. The
mechanisms of attraction and repulsion are mediated by coupling
the activation of membrane receptors—such as ephrin to the ephrin
receptor, or Slit to Robo—to the dynamics of the cytoskeleton. Growth
of microfilaments and microtubules in specific directions can pro-
mote extension of an axon in one direction (attraction) or another
(repulsion)—all quite elegant and elaborate, and only beginning to be
understood in detail.
While much of neurogenesis and gliogenesis takes place in the womb,
a great deal of the wiring up of neurons, especially over large regions
of the cerebral cortex, continues to occur after birth. In the year after
birth, vast numbers of new synaptic connections form between neu-
rons throughout the cerebral cortex. In fact, many more connections
between neurons are formed during the first year of life than will
eventually be retained. As fast as connections are forming, they are
also being assessed for utility. Synapses that are used become stabi-
lized and strengthened. Synapses that are not used are eliminated, a
process called synaptic pruning.
These processes are aspects of neuroplasticity. Strengthening and
weakening of synapses may happen as a result of molecular processes
taking place either presynaptically, postsynaptically, or both. An
example of a presynaptic mechanism to strengthen a synapse is pro-
longing the state of depolarization in the axon terminal, so that volt-
age-gated Ca** channels remain open for a longer time, resulting in
the fusion of more vesicles and increased release of neurotransmitter.
More neurotransmitter released means more signal passed to the next
cell—thus, a stronger synapse.
Presynaptic effects may be mediated by receptors on the axon