Chapter 4 Neurons, Hormones, and the Brain 125were inhibiting their growth. Findings like this
one hold promise for treatments in humans, but
replication has been difficult; perhaps successful
approaches will need to be tailored to the unique
circumstances of each patient (Noble et al., 2011).
A long road lies ahead, and many daunting
technical hurdles remain to be overcome before
stem-cell research yields practical benefits for hu-
man patients. But exciting developments are on
the horizon, including ongoing clinical attempts to
restore sight in legally blind people and heal dam-
aged heart tissue. Each year brings more incredible
findings about neurons, findings that only a short
time ago would have seemed like science fiction.How Neurons Communicate Lo 4.5
Neurons do not directly touch each other, end to
end. Instead, they are separated by a minuscule
space called the synaptic cleft, where the axon ter-
minal of one neuron nearly touches a dendrite
or the cell body of another. The entire site—the
axon terminal, the cleft, and the covering mem-
brane of the receiving dendrite or cell body—is
called a synapse. Because a neuron’s axon may
have hundreds or even thousands of terminals, a
single neuron may have synaptic connections with
a great many others. As a result, the number of
communication links in the nervous system runs
into the trillions or perhaps even the quadrillions.
Neurons speak to one another, or in some
cases to muscles or glands, in an electrical and
chemical language. The inside and outside of a
neuron contain positively and negatively charged
ions (electrically charged atoms). At rest, the neu-
ron has a negative charge relative to the outside.
But when it is stimulated, special “gates” in the
cell’s membrane open, allowing positively charged
sodium ions to move from the outside to the
inside, making the neuron less negative. If this
change reaches a critical level, it briefly triggers
an action potential, during which gates in the axon
membrane allow even more positively charged so-
dium into the cell, causing it to become positively
charged; as a result, the neuron “fires.” Then posi-
tively charged potassium ions quickly move from
within the axon to the outside, which returns the
cell to its negatively charged resting state.
If an axon is unmyelinated, the action poten-
tial at each point in the axon gives rise to a new ac-
tion potential at the next point; thus, the impulse
travels down the axon somewhat as fire travels
along the fuse of a firecracker. But in myelinated
axons, the process is a little different. Conduction
of a neural impulse beneath the sheath is impos-
sible, in part because sodium and potassium ions
cannot cross the cell’s membrane except at thesynapse The site where
transmission of a nerve
impulse from one nerve
cell to another occurs; it
includes the axon termi-
nal, the synaptic cleft,
and receptor sites in the
membrane of the receiv-
ing cell.action potential A
brief change in electrical
voltage that occurs be-
tween the inside and the
outside of an axon when
a neuron is stimulated;
it serves to produce an
electrical impulse.come from aborted fetuses and from embryos that
are a few days old, consisting of just a few cells.
(Fertility clinics store many such embryos because
several “test tube” fertilizations are created for
every patient who hopes to become pregnant;
eventually, the extra embryos are destroyed.) In
the United States, federal funding for basic stem-
cell research has faced resistance by antiabortion
activists.
An alternative to the use of ES cells is to
reprogram adult cells from certain organs to be-
come stem cells (Takahashi et al., 2007; Yu et al.,
2007). In one study, when stem cells derived from
human nasal cells were transplanted into mice that
had lesions in a part of the brain involved with
memory, the mice performed better on learning
and memory tasks (Nivet et al., 2011). Like ES
cells, induced pluripotent stem (iPS) cells derived
from adult tissues seem to be capable of giving
rise to many kinds of cells. However, they are
harder to keep alive than ES cells are, and it is still
unclear whether they will prove to be as versatile.
Patient-advocacy groups hope that trans-
planted stem cells will eventually help people
recover from serious diseases of the brain and
from damage to the spinal cord and other parts of
the body. Scientists have already had some success
in animals. For example, mice with recent spinal
cord injuries regained much of their ability to walk
normally after being injected with stem cells de-
rived from extracted human wisdom teeth (Sakai
et al., 2012). Microscopic analysis showed that
many of the cells had helped axons regenerate at
the site of the injury, by blocking substances that
induced pluripotent
stem (iPS) cells Stem
cells derived from adult
tissues.In an area associated with learning and memory, im-
mature stem cells give rise to new neurons, and physical
and mental stimulation promotes the production and
survival of these neurons. These mice, who have toys to
play with, tunnels to explore, wheels to run on, and other
mice to share their cage with, will grow more cells than
mice living alone in standard cages.