38 CHAPTER 2
Neurons
Now that you know the essential functions that different parts of the brain perform,
it’s time to discuss how these functions occur. All brain activity depends on neurons,
and malfunctions at the neuronal level often contribute to psychological disorders
(Lambert & Kinsley, 2005). The brain contains numerous types of neurons, which
have different functions, shapes, and sizes. Most neurons interact with other neu-
rons. In some cases, neurons activate, or act to “turn on,” other neurons; in other
cases, neurons inhibit, or act to “turn off,” other neurons. We can classify neurons
into three main types:- sensory neurons receive input from the sense organs (eyes, ears, and so on);
- motor neurons carry output that stimulates muscles and glands; and
- interneurons lie between other neurons—sensory neurons, motor neurons, and/or
other interneurons—and make up most of the neurons in the brain.
Sets of connected neurons that work together to accomplish a basic process,
such as making you recoil when you touch a hot stove, are called brain circuits;
sets of brain circuits are organized into brain systems, which often can involve
most of an entire lobe—or even large portions of several lobes. Many forms of psy-
chopathology arise because specifi c brain circuits are not working properly, either
alone or as part of a larger brain system. To understand brain circuits, consider an
analogy to a row of dominoes: When one domino falls, it causes the next in line
to fall, and so on, down the line. Similarly, when a neuron within a brain circuit is
activated, it in turn activates sequences of other neurons. However, unlike a domino
in a row, the average neuron is connected to about 10,000 other neurons—and
thus a complex pattern of spreading activity occurs when a brain circuit is acti-
vated. For each input, a brain system produces a specifi c output—for instance,
an interpretation of the input, an association to it, or a response based on it.
Ultimately, it is the pattern of activated neurons that is triggered—by a sight, smell,
thought, memory, or other event—that gives rise to our cognitive and emotional
lives. A pattern of neurons fi ring makes us desire that third piece of chocolate cake
or causes us to recoil when a spider saunters out from behind it. Brain systems
allow us to think, feel, and behave.
Psychopathology can arise when neurons fail to communicate appropriately,
leading brain circuits to produce incorrect outputs. For example, people with
schizophrenia appear to have abnormal circuitry in key parts of their frontal lobes
(Pantelis et al., 2003; Vidal et al., 2006). To understand such problems—and pos-
sible treatments for them—you need to know something about the structure and
function of the neuron and its methods of communication.The Cell Body
To see how neurons can fail to communicate appropriately, we must take a closer
look at their anatomy. Figure 2.4 shows that a neuron has three parts: a receiving
end, a sending end, and a middle part, called the cell body. Neurons, like all hu-
man cells, have a nucleus, which regulates all of the functions of the neuron, and
acell membrane, which forms an outer covering. When a neuron receives a suf-
fi cient amount of excitatory input, very small holes in the cell membrane—called
channels—open, and the neuron’s internal balance of chemicals will change to the
point where the neuron fi res. It is this fi ring that sends information to other neurons
(Lambert & Kinsley, 2005).
Each neuron registers the sum total of inputs, both those that try to stimulate
it to fi re and those that try to inhibit it from fi ring. The neuron, then, balances the
two sorts of inputs against each other and only fi res if the stimulating infl uences
substantially outweigh the inhibiting ones (Kandel, Schwartz, & Jessell, 2007). To
understand how fi ring occurs, we need to look at the two other major parts of the
neuron: the axon and the dendrites.Brain circuits
Sets of connected neurons that work together
to accomplish a basic process.
Brain systems
Sets of brain circuits that work together to
accomplish a complex function.