The brain is actually a part of the nervous system. As indicated earlier, the brain and the spinal cord constitute one of the two major divisions of the nervous sys- tem—the central nervous system. Our main concern in this section is with the brain, not the spinal cord. However, let us note that the spinal cord,protected by the bones of the spine, is a two-way communication highway. Sensory mes-
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 hypothalamus is 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 not the id, because the id is a psychological concept and the hypothalamus is a physiolog-
ical structure. Nonetheless, it is correct to say that to a large extent it is the activ- ity of the hypothalamus that gives rise to the psychological processes Freud asso- ciated with the id.
(a) One of the principal functions of the medulla is to regulate .
(b) The regulation of biological drives is associated to a large extent with what structure?
Answers: (a) respiration rate; (b) The hypothalamus.
The pituitary gland—one of the endocrine glands—is located toward the front of the hypothalamus. It is usually called the “master gland” of the body.
This is because it plays a role in regulating the action of the other endocrine glands. This will be discussed in the section on the endocrine system, pages 39–41.
Located just above the hypothalamus is the thalamus.One of the principal functions of the thalamus is to act as a relay center for the sense organs. For exam- ple, the optic nerves transmit visual information to a center in the thalamus. The information is then relayed to a higher area in the cerebral cortex.
The cerebral cortexis the highest part of the brain. The word cortexmeans
“bark” or “covering.” The cortex, a large structure, does in fact sit on top of, or cover, the lower regions of the brain. The cortex has a number of functions. It has dedicated areas for vision, hearing, taste, touch, and smell. It has motor areas
Broca’s area
Cortex
Thalamus
Hypothalamus
Medulla Pons Pituitary gland
Reticular activating system
Cerebellum
Brain stem
Cross section of the brain with selected structures.
allowing for voluntary movements. It has association areas allowing for learning, thinking, and memory. There are two speech areas allowing for language compre- hension and language production. For example, Broca’s areais involved prima- rily in language production. A group of structures in the cortex called the limbic systemplay important roles in our motivational and emotional lives. For exam- ple, damage to the limbic system can be associated with anhedonia(or ahedo- nia), an inability to experience pleasure.
(a) The “master gland” of the body is the .
(b) One of the principal functions of the thalamus is to act as .
(c) What large structure of the brain is associated with our ability to think?
Answers: (a) pituitary gland; (b) a relay center for the sense organs; (c) The cerebral cortex.