42 CHAPTER 2
cleft (Meana, Barturen, & Garcia-Sevilla, 1992). If the neurons in an inhibitory
circuit are abnormal in this way, the brain may have diffi culty dampening down re-
petitive thoughts or behaviors—as occurs in certain disorders we will discuss later
in this book.
Second, the sending neurons might produce too much or too little of a neuro-
transmitter substance. Third, the events after a neuron fi res may go awry (Kelsey,
Newport, & Nemeroff, 2006). In particular, when a neuron fi res and sends neuro-
transmitter chemicals to another neuron, not all of these molecules bind to receptors.
Rather, some of the molecules linger in the synaptic cleft and need to be removed.
Special chemical processes operate to reuptake these leftover neurotransmitters,
moving them back into the sending neuron. Sometimes reuptake does not operate
correctly, which may contribute to a psychological disorder. For example, neurons
might not reabsorb enough of the molecules from the synaptic cleft, which results
in chronically high levels of neurotransmitter (Kandel, Schwartz, & Jessell, 2007;
Kelsey, Newport, & Nemeroff, 2006). You might think that this would mean that
the receiving neuron is habitually stimulated too much, but in fact the neuron alters
its sensitivity to the neurotransmitter so that abnormally high amounts must be
produced by the sending neuron to be registered. And if abnormally high amounts
are not in fact produced, the functional results are the same as if not enough neuro-
transmitter were being produced. As we saw earlier, many psychological disorders
are associated with too little of a specifi c neurotransmitter substance.Hormones and the Endocrine System
Not all neurotransmitter substances are released across synapses. Hormones are
neurotransmitter substances that are released directly into the bloodstream and often
function primarily as neuromodulators; their effect is more general and more wide-
spread than that of neurotransmitters released into synapses (Kelsey, Newport, &
Nemeroff, 2006). Like other neurotransmitter substances, hormones can activate
or modulate the activity of neurons. For example, some hormones play a key role
in helping animals respond to stressful situations by altering the functioning of
the ANS (Kandel, Schwartz, & Jessell, 2007). However, traumatic events can dis-
rupt this often–helpful mechanism and contribute to psychological disorders such
as depression (Claes, 2004).
Hormones are produced by glands in the endocrine system, which secretes sub-
stances into the bloodstream. Hormones affect various organs throughout the body.
As we noted when we discussed the HPA axis, cortisol is a particularly important
hormone, which helps the body to cope with challenges by making more resources
available (by breaking down fats and proteins and converting them to sugar); cortisol
is produced by the adrenal glands (which are located right above the kidneys), and
abnormal amounts of cortisol have been linked to anxiety and depression. Some sub-
stances, such as adrenaline, function as neurotransmitters in the brain and also as
hormones in the body (Lambert & Kinsley, 2005). As noted earlier, we cannot link
a psychological disorder solely to the relative amounts of a given transmitter sub-
stance: We must also take into account the nature of the neurons (and the circuits in
which they participate) that have receptors for that substance.The Genetics of Psychopathology
Researchers knew about the inheritance of traits long before the discovery of DNA
(deoxyribonucleic acid, the long molecule that contains many thousands of genes).
Everyone knows that people “take after” their parents in some ways, and genes are
responsible for this phenomenon. Genes affect not only physical traits but also the
brain and, through the brain, thinking, feeling, and behavior; moreover, genes af-
fect how vulnerable people are to particular psychological disorders (Plomin et al.,
1997, 2003).
The study of genetics dates back to Gregor Mendel, an Augustinian monk who
lived and worked in what is now the Czech Republic. In 1866, he published a paperReuptake
The process of moving leftover
neurotransmitter molecules in the synapse
back into the sending neuron.
Hormones
Neurotransmitter substances that are
released directly into the bloodstream and
often function primarily as neuromodulators.