36 PSYCHOLOGY
sages from the feet, the hands, and other parts of the body are sent to the brain.
Motor messages from the brain are used to move the legs, the arms, and other
parts of the body. If the spinal cord is damaged, this can greatly impair the indi-
vidual’s ability to both experience and move the body.
The brain resides at the top of the spinal cord and, as indicated, is the chief
executive officer (CEO) of the nervous system. It is traditional in studying the
brain to describe its structures and their functions. Each structure is given a name
and a location in the brain. Then the purpose of the structure is identified.
Let’s start toward the bottom of the brain and work our way up. Immediately
above the spinal cord is the brain stem.It has the obvious function of connecting
the brain to the spinal cord, and may even be thought of as an extension of the
spinal cord. Located within the brain stem is a structure called the reticular acti-
vating system (RAS).It sends forth nerves to the higher levels of the brain. The
RAS functions like both a light switch and a rheostat. When you wake up sud-
denly, the RAS has stimulated you to do so. When you fall asleep quickly, the
RAS has also induced this behavior. On the other hand, when you are drowsy, the
RAS is acting like a rheostat. It is reducing the flow of the kind of information
that makes you attentive and alert. Also, you can be overly alert—what people call
“hyper.” In this case, the flow of information to the higher levels of the brain is
excessive.(a) The spinal cord acts as.(b) What structure acts like both a light switch and a rheostat?
Answers: (a) a two-way communication highway; (b) The reticular activating system.The medullais located toward the front of the brain stem. One of its princi-
pal functions is to regulate the respiration rate. The pons,associated with the
medulla, is a bulging structure also located toward the front of the brain stem; it
resides above the medulla. It too is involved in the regulation of breathing. In
addition, it plays a role in the regulation of sleep and attention.
The cerebellum(i.e., “little brain”) is located toward the back of the brain
stem. Maintaining your sense of balance and coordinating your muscle move-
ments are functions of the cerebellum.
The hypothalamusis located just under the thalamus and above the brain
stem. (The prefix hypomeans “under” or “beneath.” A few paragraphs down you
will find a reference to the thalamus.) The hypothalamus has a number of func-
tions. Of particular interest is the regulation of biological drives. For example,
both excitatory and inhibitory impulses for hunger arise from the hypothalamus.
One starts eating and then stops eating depending, to some extent, on signals from
the hypothalamus. Freud spoke of a psychological agent called the id,the con-
stellation of our primal, pleasure-oriented impulses. The hypothalamus is notthe
id, because the id is a psychological concept and the hypothalamus is a physiolog-