The central nervous system (CNS) consists of the brain and spinal cord. It is the only part of the nervous system protected by bone — a fact that suggests that CNS activity is extremely important for the survival of organisms. The spinal cord of humans is about 18 inches long and one inch wide. The brain sits within the skull and on top of the spinal cord, weighs about three pounds, and is made up of about 100 billion cells. The brain and spinal cord have at least two general functions:
- CNS activity processes sensory information from the environment.
- CNS activity is the fundamental basis for intellectual skills and abilities.
The rest of the nervous system consists of the peripheral nervous system (PNS). The PNS also has at least two general functions:
- Activity of sensory receptors in the PNS (e.g., those in the retina of the eye) provides the sensory information processed by the CNS.
- Activity of the body’s muscles, glands, and organs is caused by activity of the PNS’s nerves, which are responding to signals sent by the CNS.
Information about events in the outside world is sent through the PNS to the CNS. The CNS analyzes this information and makes decisions about how to respond. The CNS then sends messages to the PNS, which transmits these messages to parts of the body involved in making the responses. For example, when an insect crawls across the skin of your arm, tactile (touch) receptors in the skin are activated, and this information is sent through nerves to the spinal cord, and from there to the brain. The spinal cord and brain processes this sensory information (e.g., you may have a conscious perception of something crawling on your skin). Based on this information processing, a decision is made about how to respond. The CNS sends this information to the PNS. The message travels to the appropriate muscles, probably causing you to slap the insect off your arm. This example shows that the PNS and CNS work together in adjusting (adapting) our behavior to internal and external demands. Martin (1987) described the interaction between the PNS and the CNS in this way:
The brain and spinal cord play a role in virtually every physiological activity — from swallowing to sweating, from listening to … music … to making love. Yet without the many millions of … [extremely small] fibers of the peripheral nervous system, fibers that supply every organ, every muscle, every scaly patch of skin, there would be no communication between brain and body. The brain would languish like an unprogrammed computer, and the body would be functionless — some marvelous machine that could never be powered up. (p. 174)
In other words, any disruption of communication between the PNS and CNS will result in disturbances in our ability to sense and respond to the environment. When the spinal cord is severed, any neural activity sent from the brain into the spinal cord cannot move past that point into the PNS. Because of this, parts of the body innervated by the motor pathways below the cut no longer can respond. For example, a person will develop muscle paralysis below the cut. In addition, any sensory information that enters the spinal cord below the cut cannot move past it, which means that the person is unable to sense parts of the body below the cut.
The intact (undamaged) spinal cord, on the other hand, not only transfers information between the brain and all parts of the PNS, it also processes some of the incoming sensory information and sends out its own motor messages (Kalat, 2008). A motor message consists of activity that begins in the CNS and then is sent through the PNS to muscles, glands, and/or organs of the body. Motor messages allow the body to respond to environmental events. These motor messages sent by the spinal cord cause several basic reflexes (involuntary responses to sensory stimuli). An example of a spinal reflex is withdrawing a hand from a hot object. If your finger touches a hot stove, for instance, the sensory information travels up your arm and into the spinal cord where it is processed rapidly. If the temperature is extreme enough, your spinal cord sends a motor message to the muscles of your hand and arm, which causes you to quickly pull them away from the stove — so quickly that the behavior occurs even before you feel pain.
You probably have, at some point, pulled your hand away reflexively from something that was very cold (such as cold water coming out of a faucet) rather than very hot, and then felt stupid, especially if another person saw you do this. But you shouldn’t have felt stupid. The spinal cord processes sensory information superficially, which in this case means that it senses only an extreme temperature, not whether this extreme temperature is hot or cold. The sort of complex processing that allows you to consciously perceive heat or cold occurs in your brain; but, because the information takes a little longer to get to the brain, you already had withdrawn your hand before you consciously perceied that the water was cold.
Normal walking is controlled mostly by spinal reflexes. Once you make a decision to walk and then begin walking, your brain no longer is involved (unless some problem arises). Cats that have had their spinal cords lesioned (surgically cut or removed) above the part that controls the hind legs — a technique that eliminates the brain’s influence— still can move their legs in a normal walking pattern when placed on a treadmill. If the cats are not forced to walk, however, they will simply lie on the floor because the brain cannot transmit the message to begin walking.
The scratch reflex of dogs is controlled by a spinal reflex. When a dog’s skin is irritated by, say, a flea, it will move a hind leg to the source of that irritation and begin a rhythmic scratching pattern. This is true even in dogs whose spinal cord has been lesioned above the part that controls the scratch reflex because motor messages from the brain are unnecessary. All that is needed is sensory information traveling from the PNS to the spinal cord. The spinal cord processes this information rapidly (but superficially) and, even though the dog cannot consciously perceive the irritation (the sensory information can’t get to the brain), its hind leg will reflexively scratch the appropriate part of the body.
Although the PNS and the spinal cord are (of course) essential components of the nervous system, in Chapter 3, you’ll learn primarily about the structures and functions of the brain because this is essential for understanding several topics to be discussed later in this book.
Subdivisions of the Brain
The brain can be divided into three main parts: the brain stem, the limbic system, and the cerebral cortex (see Figure 1). Each part consists of a number of neural structures (neural means something that either is a part of or is related to the nervous system).
- The brain stem is an extension of the spinal cord into the skull. It consists of a set of neural structures involved in attention, sleeping and waking, and a variety of reflexes essential to survival.
- The limbic system forms a ring around the upper part of the brain stem. It consists of a set of neural structures involved in the regulation and expression of emotions, memory formation, and biological drives.
- The cerebral cortex is the wrinkly outer part of the brain — the part of the brain you can most easily see. It consists of a set of neural structures involved in producing the so-called “higher functions” of humans, such as language, reasoning, the planning of actions, and perception.
Since the 1600s, scientific research has demonstrated conclusively that the normal functioning of the brain is essential for complex mental processing of information, as well as for the production of complex behavioral patterns (Zimmer, 2004). Brain research today has three general goals:
- to analyze the brain into its component parts;
- to determine how these components are organized;
- to determine how the activity of these components is coordinated in performing various functions (such as perceiving, reasoning, planning, etc.)
Until about 1980, the human brain was studied primarily as a sort of “black box” — a sealed container with “machinery” that researchers could not observe directly, except during brain surgery or only with crude techniques, such as X-ray imaging, EEG recordings, or dissection during autopsies. These techniques suffer from severe limitations:
- X-ray imaging. Images made using X-rays provide little detail of brain structures.
- EEG. The EEG is able to measure the activity of only those cells in the outer layers of the brain; and, in addition, it measures the summed activity of millions of brain cells (i.e., it can’t look at one or a small number of cells).
- Autopsies. After death, the brain changes very quickly, which means that autopsy results don’t necessarily reflect what happens in living brains
The inability to observe living brains in action made it very difficult for researchers to find links between (a) the activity of particular brain structures and (b) mental and behavioral functioning.
The nature of brain research changed dramatically with the development of new techniques that provided detailed images of brain structures and their activity. We now can determine which areas of the brain are most active when we perform tasks such as reading, making judgments, or deciding. Since 1980, the pace of discoveries about the brain and its functions has exploded.
Study Questions for Section 3-1
- What are the main functions of the CNS?
- What are the main functions of the PNS?
- How would you define the concept of a “motor message” in your own words?
- What is an example of a motor message not mentioned in the text?
- If the spinal cord were cut just below the point where it enters the skull, what problems are likely to develop?
- What are some examples of spinal-cord reflexes?
- What are the three major subdivisions of the brain?
- What are the main functions of the brain stem?
- What are the main functions of the limbic system?
- What are the main functions of the cerebral cortex?
- What are the ultimate goals of brain research?
Martin, R. (1987). Matters of gray and white: A neurologist, his patients, and the mysteries of the brain. New York: Henry Holt & Co.
Groves, P. M., & Rebec, G. V. (1992). Introduction to biological psychology (4th ed.). Dubuque, IA: Brown.
Kalat, J. W. (2008). Biological psychology (10th ed.). Belmont, CA: Wadsworth.
Zimmer, C. (2004). Soul made flesh: The discovery of the brain — and how it changed the world. New York: Free Press.