Understanding Psychological Disorders: The Neuropsychosocial Approach 39
The Axon
Theaxon is the part of the neu-
ron that sends signals when a
neuron fi res. The axon is a long,
threadlike structure covered by a
layer of fatty material, known as
the myelin sheath, that insulates
it electrically. Though a neuron
has only a single axon, it often
branches extensively, allowing
signals to be sent simultane-
ously to many other neurons
(Shepherd, 1999). When a neu-
ron has been stimulated to the
point that it fires, a wave of
chemical activity moves from
the cell body down the axon
very quickly. This wave is called
anaction potential. When the
action potential reaches the end of
the axon, it typically causes chemi-
cals to be released. These chemicals are stored in structures called terminal buttons,
and they affect other neurons, muscles, or glands.
If stimulation does not cause a neuron to fi re when it should, the circuit ofwhich it is a part will not function correctly—and psychopathology may result. Let’s
consider why a neuron might not fi re when stimulated appropriately.
The Dendrites
Neurons fi re when they are appropriately stimulated. But how are they stimulated?
Two ways: First, through dendrites,which receive signals from other neurons. These
dendrites are highly branched, so a single neuron can receive many different signals
at the same time. Received signals move along the dendrites to the cell body (Kandel,
Schwartz, & Jessell, 2007). Second, in some cases, neurons receive inputs directly on
their cell bodies. Such inputs are produced not only by other neurons, but also by
glial cells. Glial cells are involved in the “care and feeding” of neurons, and act as
a kind of support system (in fact, glialmeans “glue” in Greek; Lambert & Kinsley,
2005). The brain has about ten times as many glial cells as neurons, which implies
that glial cells are important. In fact, researchers have learned that glial cells do much
more than provide support services; they can directly stimulate neurons, and play a
role in modulating input from other neurons (Parpura & Haydon, 2000).
Given the roles of neurons and glial cells in brain function, it is not surprisingthat researchers have found that at least some patients with psychological disorders
(specifi cally, the sorts of mood disorders we consider in Chapter 6) have lost both
types of cells. One possible reason for such defi cits may be that stress early in child-
hood (and even to the mother, prior to a child’s birth) can disrupt the development
of both neurons and glial cells (Zorumski, 2005).
Chemical Signals
The way neurons communicate is crucial for understanding psychopathology. In
many cases, psychological disorders involve faulty signaling among neurons, and ef-
fective medications operate by altering the ways in which signals are produced or
processed (Kelsey, Newport, & Nemeroff, 2006). Subsequent chapters of this book
will describe how particular signaling problems contribute to some psychological dis-
orders and how certain medications compensate for such problems. To understand
these problems with chemical signaling, we now need to consider the following: what
Action potential
The wave of chemical activity that moves
from the cell body down the axon when a
neuron fi res.2.4 • The Neuron
Figure 2.4f 24 h
g4NucleusDendritesMyelin
sheathTerminal buttonsCell
bodyAxonNeuralim
puls
e