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 is a structure about 18 inches long and one inch wide that runs through the spinal column. The brain sits on top of the spinal cord within the skull, weighs about three pounds, and is made up of about 100 billion cells. Together, the brain and spinal cord have the following functions:

• CNS activity underlies the complex processing of sensory information.
• CNS activity underlies judgement- and decision-making, which direct our responses to sensory information.

The rest of the nervous system consists of the peripheral nervous system (PNS). The PNS has two major functions:

• Sensory stimuli from the external world activates sensory receptors in the PNS, which generate activity that is sent through nerves in the PNS and into the CNS.
• CNS activity associated with mental processes is sent through nerves in the PNS to the muscles, glands, and organs of the body.

In short, information about events in the outside world is sent through the PNS to the CNS, the CNS analyzes this information and makes decisions based on it, and these decisions activate parts of the PNS linked to bodily responses to the events. When something touches the skin on your arm, for example, tactile (touch) receptors in the skin are activated and this information is sent through nerves in your arm into the spinal cord and then the brain. The CNS activity is associated with processing this sensory information and, perhaps, forming a conscious perception that the skin is being touched. Based on this information processing, a decision about what to do and how to do it are made. This decision is constructed into a motor message[∂] that exits the CNS. The motor message travels down your arm to the appropriate muscles, perhaps causing them to contract. In this way, the PNS and CNS work together in adjusting (adapting) our behavior to internal and external demands. Martin (1986) 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)

Any disruption of communication between the PNS and the CNS will lead to disturbances in our ability to sense or 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. Because of this, parts of the body innervated[∂] by the motor pathways below the cut no longer are able to respond (for example, a person will show paralysis of the muscles below the cut). In addition, any sensory information that enters the spinal cord below the cut cannot move past it and, therefore, cannot be sent to the brain, 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. These spinal-cord-induced movements make up several basic reflexes[∂]. An example of a spinal reflex is the withdrawal of the hand from a hot object. If your finger touches a hot stove, the sensory information travels up your arm and into the spinal cord where it is processed in a rapid but relatively superficial manner. If the temperature is extreme enough, your spinal cord will send out a motor message to the muscles of your hand and arm, which will cause them to quickly pull away from the stove. This happens before you feel any pain, and sometimes even when the temperature is very cold instead of hot. You may remember, for example, putting your finger under very cold running water and then pulling it away before realizing that the water wasn’t hot. This is because your spinal cord does not process sensory information in complex ways that allow you to consciously feel heat and pain. More complex processing of sensory information occurs in your brain and, because it takes a little longer for information to get to the brain and be processed, you may already have withdrawn your hand before you consciously perceived that the water was not hot.

Normal walking also 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): your spinal cord controls the walking movements. Cats that have had their spinal cords lesioned[∂] above the part that controls the hind legs — a technique that eliminates the brain’s influence— still are able to 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 any decision made in the brain to start walking no longer can be sent to the spinal cord. The scratch reflex in dogs also is controlled by the spinal cord. When a dog's skin is irritated by a flea, the dog will move a hind leg up to the source of the irritation and begin a rhythmic scratching pattern. This is true even in dogs whose CNS has been experimentally lesioned above the part of the spinal cord that controls the scratch reflex.

Since the 1600s, a great deal of evidence has accumulated that confirms that the normal functioning of the brain is essential for complex mental processing of information and the production of complex behavioral patterns (Zimmer, 2005).

We will distinguish three main parts of the brain (see Figure 1). The first part is the brain stem, which is composed of a set of neural[∂] structures making up the extension of the spinal cord into the skull. Activity within the brain stem is associated with attention, daily patterns of sleeping and waking, and a variety of reflexes essential to survival. Second, the limbic system is composed of a set of neural structures surrounding the upper part of the brain stem. Activity within the limbic system is associated with the regulation and expression of emotions, memory formation, and biological drives. Third, the cerebral cortex, which is the largest subdivision of the human brain, is composed of the wrinkly tissue that covers the outermost parts of the brain. Activity in the cerebral cortex is associated with many of the so-called “higher functions” of humans, such as language ability, reasoning ability, the planning of actions, and perception.

Figure 1. The three subdivisions of the brain and some of their major structures (adapted from Kassin, 2001, p. 55)

The ultimate goals of brain research are:

• to analyze the brain into its component parts;
• to determine how these components are organized;
• and to determine how their activity is coordinated when perceiving, reasoning, planning, responding, and so on.

Until about 1980, the human brain could be studied primarily as a sort of "black box" — a sealed container with "machinery" that, other than during neurosurgery, could be observed directly only by X-ray imaging, the recording of electrical activity in the outer layers of the cerebral cortex, or the dissection of brains during autopsies. All these methods for observing the brain, however, suffer from severe limitations: X-ray images show little detail; the EEG gives only a crude measurement of the summed activity of millions of neurons; and, after death, the brain changes very quickly. Researchers' inability to observe living brains in action meant that it was very difficult for them to find links between the activity of particular brain structures, on the one hand, and mental and behavioral functioning[∂], on the other.

The nature of brain research changed dramatically with the development of new technologies that provided detailed images of brain structures and their activity. We now are able to look at which areas of the brain are most active while performing tasks such as reading, evaluating, or deciding. Since 1980, the pace of discoveries about the brain and its functions has exploded.

Study Questions for Section 3-2

1. What are the main functions of the CNS?
2. What are the main functions of the PNS?
3. How would you define the concept of a "motor message" in your own words?
4. What is an example of a motor message not mentioned in the text?
5. If the spinal cord were cut just below the point where it enters the skull, what problems would result?
6. What are some examples of spinal-cord reflexes?
7. What are the three major subdivisions of the brain?
8. What are the main functions of the brain stem?
9. What are the main functions of the limbic system?
10. What are the main functions of the cerebral cortex?
11. What are the ultimate goals of brain research?

Go to Quiz 3-2 questions

Go to Readings Section 3-3