Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

The Animal Self: Neurobehavioral Correlates 145

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

Although most adult neurons do not divide, the synaptic connec-
tions are constantly in a state of flux. High-resolution microscopy in the
living, adult mouse cerebral cortex reveals the dendritic spines in con-
stant morphologic changes and movements, observable in a span of less
than ten minutes.^7
Plasticity of the developing brain is evident in its remarkable ability
to recover from injury. Newborn rats with almost half a cerebral hemi-
sphere removed may grow up with no apparent motor deficit. Humans
born with extensive injury to the language centers in the left hemisphere
grow up with little speech impairment (confirmed by functional MRI). In
these instances, the opposite hemisphere takes over the lost function as
a result of circuit rewiring. However, the regenerative capacity decreases
rapidly after a critical period. For instance, human infants with congeni-
tal cataract, if uncorrected within the first three years, remain blind even
after surgery in later life.
Molecular recognition in synaptic connection is most dramatically
demonstrated in lower vertebrates. Roger Sperry, in the 1940s, showed
that when the optic nerve of a frog is cut and allowed to regenerate, the
individual axons at the cut end from the eye seek out their respective
partners in the midbrain in a topographically precise manner. The match-
ing of the two parts by chemical recognition is known as chemo-affinity.^8
In recent years, molecular markers on neuronal surface carrying
repulsive or adhesive messages have been confirmed. In the fruit fly,
a gene called DSCAM encodes a large family of axon guidance recep-
tors. Alternative splicing of the gene gives rise to 38 thousand different
cell surface recognition proteins.^9 Other DSCAM proteins are found in
mouse,^10 chick retina,^11 and the invertebrate sea slug.^12

7.5 Parallel Evolution of Brain and Behavior

Figure 7.12 shows changes in adaptive behavior with respect to phylog-
eny. All behaviors are outcomes of interplay between internal (genetic
and epigenetic programs) and external factors (environment) shaped by
natural selection. When the “brain” is nothing but a head ganglion at the