Neural are interconnected pathways of nerve cells that integrate sensory input and motor output.

Introduction

"I think, therefore I am."

—Rene Descartes, 17th-century philosopher

Few of us question the crucial importance of the brain. It is vital to our existence. Our brains enable us to think, as René Descartes so skillfully pointed out nearly 400 years ago. Yet the human brain is responsible for so much more. It directs almost everything we do. It controls our voluntary movements, and it regulates involuntary activities such as breathing and heartbeat. The brain serves as the seat of human consciousness: it stores our memories, allows us to feel emotions, and gives us our personalities.

The brain makes up only 2 percent of our body weight, but it consumes 20 percent of the oxygen we breathe and 20 percent of the energy we consume. This enormous consumption of oxygen and energy fuels many thousands of chemical reactions in the brain every second. These chemical reactions underlie the actions and behaviors we use to respond to our environment. In short, the brain dictates the behaviors that allow us to survive.

Scientists have worked for many years to unravel the complex workings of the brain. Their research efforts have greatly improved our understanding of brain function. During the past decade alone, scientific and technical progress in all fields of brain research has been astonishing. Using new imaging techniques, scientists can visualize the human brain in action. Images produced by these techniques have defined brain regions responsible for attention, memory, and emotion. A series of discoveries (in multiple fields of study) has displaced the long-standing assumption that brain cells are stable and unchanging. Amazingly, new findings show that some adult brain cells can divide and grow! In addition, advances in research are allowing scientists to analyze and make progress toward understanding the causes of inherited brain disorders such as Alzheimer's disease and Parkinson's disease. Taken together, these discoveries provide hope for the recovery of nervous system function lost through injury or disease.

Despite these and other significant advances in the field of brain research, most of the processes responsible for the integrated functioning of billions of brain cells remain a mystery. Research on the brain in the new millennium is crucial to our effort to come to a complete understanding of this fascinating organ. In turn, improved understanding makes the development of new treatment options possible. Research continues to bring new insights into how the brain is put together, how it works, and whether damage to the brain can be reversed.

An essential aspect of any scientific research is communicating results to the public in a way that is easily understood. The American public has the opportunity to learn of new research findings about the brain regularly through media reports of scientific breakthroughs and discoveries. However, not all the information we receive is accurate. Commercial products promoted through television advertisements claim to improve memory, enhance concentration, or relieve depression, among other things. The media may oversimplify research findings into a "sound clip" open to misinterpretation. In addition, movies, television shows, and the World Wide Web often contain inaccuracies in their portrayal of research findings about brain structure or function.

To correctly interpret the information transmitted through these venues, we need a better understanding of basic concepts related to the brain. By providing students with a conceptual framework about the brain, we significantly increase our chances of producing an informed public that has the tools to interpret brain research findings. Accordingly, it is the goal of this supplement to provide teachers and students with correct information regarding the brain, its role in the nervous system, and how it provides us with our sense of self.

Myths and Realities about the Brain

As a result of the misinformation presented by various media, many people maintain misconceptions about the brain and brain function. The problem may be compounded by textbooks for middle school students that present little, if any, scientific information on the brain as the organ that controls human behavior. As a result, students may build their understanding of the brain on "fictions" rather than facts. The constructivist learning model holds that when students dispel their own misconceptions, they are more open to constructing a correct understanding of the subject. The following list of commonly held misconceptions about the brain, followed by correct information about each concept, should help teachers address these issues in their classrooms.

As a result of the misinformation presented by various media, many people maintain misconceptions about the brain and brain function.

Myth: The brain is separate from the nervous system.

Reality: Students often assume that the brain and the nervous system are separate, unrelated entities. Furthermore, surveys of middle school students have shown that they often believe organs such as the heart and lungs are part of the nervous system. In reality, the nervous system is composed of only the brain, the spinal cord, neurons, and neural support cells. Section 3, The Nervous System: Composition and Organization, contains more detail about the components of the nervous system.

Myth: The brain is a uniform mass of tissue.

Reality: The only exposure that middle school students often have to brain anatomy is a photo or drawing of a gray, bulbous, wrinkled mass of tissue. This may lead students to believe that the brain is uniform throughout. Although thebrain may appear uniform at a gross anatomic level, it is actually composed of billions of specialized cells. These cells, called neurons and glia, are further organized into specialized functional regions within the brain. This type of variation within the brain is what allows it to function as "command central" of the human body. The cellular nature of the brain is discussed in Section 4, Cells of the Nervous System.

Myth: Control of voluntary activity is the sole purpose of the brain.

Reality: When do we use our brains? Students may have the misconception that they use their brains only when they are doing something, such as thinking or performing a physical action. Most students do not recognize that we use our brains constantly for a variety of activities that, while crucial to our survival, require no conscious thought. For instance, the human brain is responsible for involuntary activities, such as regulating heartbeat, breathing, and blinking. Although the brain controls both voluntary and involuntary activities, different regions of the brain are devoted to each type of task. Activities controlled by the brain are explored in Section 5, The Brain.

Myth: The vertebral column and the spinal cord are the same thing.

Reality: The only exposure most students have to the human spine is as a component in models of skeletons. Thus students may assume that the spine consists solely of the skeletal structure of the vertebral column, or backbone. They can feel their own backbone, and they know that it is a structural component of their body. Students may not realize that the backbone encases the spinal cord, a vital part of our nervous system. The components and functions of the spinal cord are described in Section 6, The Spinal Cord.

Myth: The brain does not change.

Reality: The idea that the brain does not change after growth ceases may be the greatest misconception that students have. In actuality, the brain changes throughout life. During embryonic development and early life, the brain changes dramatically. Neurons form many new connections, and some neurons die. However, scientists have discovered that changes in the brain are not restricted to early life. 8, 12 Even in the adult brain, neurons continue to form new connections, strengthen existing connections, or eliminate connections as we continue to learn. Recent studies have shown that some neurons in the adult brain retain the ability to divide. 11 Finally, damaged neurons have some capability to regenerate if the conditions are right. The changing nature of the brain is discussed further in Section 7, Plasticity and Learning.

The idea that the brain does not change after growth ceases may be the greatest misconception that students have.

Myth: Learning disabilities are the only manifestation of a problem with brain function.

Reality: Many students will encounter someone who has a learning disability during their school years. For many students, this is their only experience with a brain disorder. Because many types of neurological diseases (such as Alzheimer's disease and Parkinson's disease) affect older people, students may not have experience with them. They may not realize that emotional and behavioral conditions such as depression and hyperactivity are also brain disorders. Students should be aware that diseases of, and injuries to, the brain and nervous system afflict millions of Americans of all ages each year. Although some injuries and diseases are of short duration, others are permanent and disabling. Brain disorders are discussed in Section 8, Nervous System Injury.

The Nervous System: Composition and Organization

While our brains control nearly everything we do, the brain does not work alone. The brain is the central part of a complex body system known as the nervous system. The nervous system allows us to respond to the world around us. Both our involuntary actions, such as our blink reflex to bright light, as well as our voluntary actions, such as choosing to put on sunglasses, can be attributed to our nervous system. Such a system must necessarily be both complex and extraordinarily well organized to produce the coordinated functions that define human life. How does our nervous system manage to perform its various functions?

The nervous system is organized into two major subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS; Figure 1). 3, 15 A brief examination of the nervous system's components helps create a broader context in which to understand the brain and brain function.

Figure 1

The human nervous system can be subdivided into the central nervous system (CNS) and the peripheral nervous system (PNS).

The central nervous system (CNS)

The central nervous system consists of the brain and spinal cord. It is the major information-processing center of the body. The spinal cord conducts sensory information (information from the body) from the peripheral nervous system to the brain. After processing its many sensory inputs,the brain initiates motor outputs (coordinated mechanical responses) that are appropriate to the sensory input it receives. The spinal cord then carries this motor information from the brain through the PNS to various locations in the body (such as muscles and glands).

Not all of the body's motor responses travel through the brain for processing. The spinal cord alone is able to direct simple reflex actions, such as the knee jerk reflex, that require a quick response from the body. More complex motor actions, such as some involuntary and all voluntary actions of the body, require brain involvement. The brain is both the integrator and director of information through our bodies. Our brain devotes most of its considerable volume, energy, and computational power to processing various sensory inputs from the body in order to determine and initiate appropriate, coordinated motor output to the body.

The peripheral nervous system (PNS)

The peripheral nervous system is composed of all nerve tissue outside the brain and spinal cord. The PNS delivers information between the body and the central nervous system. It is divided into two subsections: the sensory/somatic nervous system and the autonomic nervous system. The somatic nervous system carries messages between the CNS and the body's sensory organs and voluntary muscles. It allows us to detect changes in the world around us, and it delivers information related to actions that we decide to perform. In contrast, the autonomic nervous system carries messages between the CNS and our internal organs. It delivers information related to automatic tasks such as the regulation of breathing and digestive functions.

Cells of the Nervous System

All components of the nervous system, including the brain, are composed of billions of specialized cells: neurons and glia. Though the two cell types work together to provide the coordinated functioning of the nervous system, the unique structure of each type of cell allows it to perform its specific function.

Neurons

The neuron (Figure 2) is the basic functional unit of the nervous system. Its primary function is communication. Neurons receive information from cells, and then transmit this information to other cells. The transmission of information between cells of the body and neurons enables us to react to changes in our internal and external environments. Neurons have a cell body, which contains a nucleus that directs the cell's activities. Specialized extensions called dendrites bring information into the cell body. Other extensions at the opposite end of the neuron are called axons. These carry information away from the cell body. Information leaves a neuron through axon terminals, the endpoints of the axon. Bundles of axons are called nerves.

Figure 2

A neuron is made up of a cell body, dendrites, and an axon. Dendrites bring information into the cell body; information travels through the axon and exits the cell through axon terminals.

The nervous system includes three general types of neurons: sensory neurons, interneurons, and motor neurons. Sensory neurons are specialized to detect stimuli from the environment, such as light, sound, taste, or pressure. Detection of a stimulus triggers the sensory neuron to transmit a message to the central nervous system. There, the message is relayed to interneurons that integrate the information and generate instructions about how to respond. Instructions are sent back to the peripheral nervous system as messages along motor neurons. The motor neurons then stimulate muscles to contract or relax to make the appropriate responses. They also stimulate glands to release hormones.

Our nervous system is able to pass a message from a sensory neuron, through several interneurons, to a motor neuron within several milliseconds. Though this seems very fast, some sensory inputs (such as pain) requires an even more rapid response. If we touch a hot stove, for instance, it is beneficial for us to pull back as quickly as possible. How does the nervous system handle this reflex response? When responding to input that requires a very fast response, our nervous system allows sensory neurons to relay information through only one interneuron, or to connect directly to motor neurons. By reducing the number of interneurons required for signal processing, reflex responses are able to occur more quickly than other responses. Reflex responses are discussed further in Section 6, The Spinal Cord.

Glia

Glial cells, collectively called glia, greatly outnumber neurons. Why do we need so many glia? The functions of glia, though not as well known as for neurons, are generally to serve as the support structure for our immense neural network. 14 For instance, some glia form myelin, the insulating sheath that surrounds certain axons (Figure 3). Myelin keeps electrical signals contained within axons and enhances the conduction of electrical signals. Other glia are scavengers that remove debris after injury or neuronal death. Some glia guide the migration of neurons and direct the outgrowth of axons during development, while others facilitate communication between neurons. 2 Some glia may even serve to "feed" neurons, providing them with essential nutrients.

Figure 3

Specialized glial cells form myelin sheaths around the axons of neurons.

Transmission of nerve impulses: electrical transmission

Neurons send and receive messages to and from each other and the body. They do this through a two-part process called neural signaling. Neural signaling begins with the generation of an electrical impulse that is passed down the length of one neuron.

How does this work? An electrical impulse is generated when a stimulus (such as sensory input) causes a rapid change in electrical charge in one part of a neuron's membrane. This electrical impulse is one unit of neural information. An electrical impulse flowing along the length of a neuron is called a nerve impulse.

Nerve impulses proceed in just one direction within a neuron—from the dendrites, through the cell body and axon, to the axon terminals (Figure 4). In addition, neurons produce nerve impulses in an all-or-nothing way. For example, if the stimulus that a neuron receives is too weak to trigger a nerve impulse, nothing happens—the neuron does not initiate an impulse. If the stimulus is strong enough or much stronger than the minimum required to trigger a nerve impulse, the neuron does initiate an impulse. However, the neuron does not initiate a stronger impulse in response to a more powerful stimulus. All that is required to initiate a nerve impulse is a minimum, or threshold, amount of stimulation. The frequency of nerve impulses, or the rate at which nerve impulses are initiated in a neuron, determines the intensity of the signal that travels through the neuron.

Figure 4

A nerve impulse is information (in the form of an electrical impulse) flowing through the dendrites, cell body, and axon of a neuron.

Transmission of nerve impulses: chemical transmission

What happens when an impulse reaches the end of one neuron and must move to another neuron? The junction between two neurons or between a neuron and a muscle is called a synapse (Figure 5). The two cells involved in a synapse do not physically touch each other. Instead, they are separated by a very small space. The cell that carries the impulse to the synapse is the presynaptic cell, and the cell that receives the impulse is the postsynaptic cell. Information flows from the axon of the presynaptic cell, across the synapse, to the dendrites of the postsynaptic cell. But how does the information cross the synapse?

Figure 5

A synapse is a junction point between neurons.

When an impulse that is traveling along the presynaptic cell reaches the end of the axon, it causes that cell to release molecules known as neurotransmitters. These molecules are released into the synapse and diffuse approximately 20 millionths of a millimeter to where they bind with receptors on the dendrites of the post-synaptic cell (Figure 6). When neurotransmitters bind to the receptors, the charge across the postsynaptic membrane changes, and if the change is great enough, it triggers a nerve impulse. The new nerve impulse then travels along the postsynaptic cell.

Figure 6

An electrical impulse is converted to a chemical signal to cross a synapse.

Scientists have discovered a large number of neurotransmitters. Some are excitatory—they cause the postsynaptic neuron to become more likely to initiate a nerve impulse. Others are inhibitory—they cause the postsynaptic neuron to become less likely to generate a nerve impulse. How important are neurotransmitters to our nervous system? Ultimately, excitatory and inhibitory neurotransmitters are the very molecules responsible for producing a specific motor response to a sensory input.

The Brain

The brain is the root of what makes us human. We begin our discussion of CNS components with an examination of brain structure and function.

Imaging the brain

Until recently, scientists could gather information about the human brain only by removing it from cadavers, slicing it, staining it, and taking pictures of the sliced sections. This method has many limitations, including the inability to study the brain in action. Technological advances, however, have enabled scientists to use advanced imaging methods to learn more about the function and anatomy of the brain. 20, 29, 30

Static imaging

The first methods used to image the living brain employed static imaging, which provides an image of the brain's anatomy at one point in time. One static imaging method is computed tomography (CT), a computer-assisted technique for assembling a "cross-sectional" X-ray image of the brain. Another imaging method, magnetic resonance imaging (MRI), has mostly replaced CT as the brain-imaging method of choice. MRI reveals the anatomy of the brain in greater detail than does CT, does not require X-rays, and is much more flexible with regard to the plane of sectioning. To create an MRI image of the brain, a strong magnetic field is rotated about the head. Exposure to this energy field induces the brain's many hydrogen atoms to resonate, or jump to a higher energy state. As the field passes through, some of the hydrogens return to a lower energy state. Sensors detect these up and down jumps in energy, and computers coordinate the data gathered. MRI scans can produce strikingly detailed images of the brain. The image produced by an MRI scan depicts a virtual "section" of the brain along the plane of the scan (Figure 7).

Figure 7

An MRI scan allows a scientist to view a detailed black-and-white image of a virtual "section" of the brain.

Functional imaging

While static imaging allows scientists to view the living brain and detect structural changes, it does not allow detection of electrical or chemical brain activity. Incredibly, imaging technologies now allow scientists to view the active brain. One of the most extensively used techniques is positron emission tomography (PET). PET works by measuring the distribution and movement of radioactively labeled molecules in the tissues of living subjects. Because a person is awake during a PET scan, the technique can be used to investigate changes in brain activity while the subject performs assigned tasks. Computers reconstruct PET scan data to produce two-dimensional or three-dimensional images. While MRI scans are used for research and in clinical settings for patient diagnosis, PET scans are used exclusively for research.

Another common method is functional magnetic resonance imaging (fMRI). The technology behind fMRI imaging is similar to that of MRI imaging. However, fMRI imaging takes advantage of a special property of hemoglobin, a blood protein that brings oxygen to body tissues. Hemoglobin that is carrying oxygen has different properties from hemoglobin that is not carrying oxygen. By detecting oxygen-containing hemoglobin, scientists use fMRI to assess changes in blood flow to areas of the brain. Since active regions of the brain receive more blood and more oxygen, fMRI images provide scientists with both functional and anatomical information (Figure 8).

Figure 8

An fMRI scan provides information about activity in the brain. The active areas appear to be lighter than the rest of the brain.

Specialized regions of the brain

How does the human brain process efficiently the billions of neural signals passing through its immensely complex network of cells? Part of the answer lies in its organization. The human brain can be divided into three major sections: the cerebrum (forebrain), the cerebellum (hindbrain), and the brainstem (Figure 9).

Figure 9

The brain can be subdivided into the cerebrum, cerebellum, and brainstem.

Functional imaging techniques have allowed scientists to discover that different areas of the brain regulate different functions. The cerebrum itself can be divided into many functionally specialized regions. One region toward the front of the cerebrum is devoted to decision making, problem solving, and planning. Other regions are devoted to processing specific categories of sensory information. For instance, the cerebrum uses different regions to interpret smell, taste, and hearing information. By devoting sections to specific tasks, the brain is able to process multiple inputs more efficiently. 13 Though groups of neurons in the brain are devoted to specific tasks, multiple sections of the brain must generally work together to process information. For instance, regions of both the cerebrum and the cerebellum work together to regulate body functions such as breathing and heartbeat.

To make the most efficient use of its neurons, the brain processes information by splitting a single behavior into component parts. For instance, when we take a bite of food, there is sensory information (this is an apple), voluntary motor information (lift piece to mouth, chew), and involuntary motor information (salivate) for the brain to process. The different components are split, sent to the appropriate regions of the brain, then processed accordingly. This distributed processing of information adds great speed to our ability to take in information and respond.

The brain and sensory input

Most of us take for granted the subtleties of the world around us. We may not always be aware of the sights, sounds, or smells in our environment. However, our brain is continuously processing signals sent through our eyes, ears, nose, mouth, and skin. In addition to the traditional five senses, scientists now recognize other kinds of sensations, including pain, pressure, temperature, joint position, and movement. Specialized sensory neurons respond to input from the environment. This input is then transmitted to the brain as electrochemical signals. In the brain, signals are received in categories. Thus the processing of sensory input begins with specific regions in the brain separately deciphering each message. Subsequently, multiple types of sensory input are integrated, thus allowing the mass of information to be interpreted into an appropriate (motor) response.

The brain and motor output

If the brain were able to process only sensory input, we would be silent, motionless observers of the world around us. Fortunately, our brains perform another important function. After interpreting sensory input, the brain generates neural impulses that flow through the nervous system to other parts of the body. These impulses, carried by motor neurons, allow us to respond to input from the environment. Some responses are voluntary. We see the door to our house, choose to open that door, and enter. Other responses are involuntary. We hear the sound of a window breaking, interpret this as an unusual (and perhaps frightening) event, and our heart begins to race. Both types of motor responses require interpretation of sensory input and regulation of motor output; however, the parts of the brain involved are different. The cerebrum initiates voluntary movement, while the cerebellum coordinates and smoothes out our movements. Regions of both the cerebrum and the cerebellum work together to regulate involuntary responses. In addition, while the CNS generates information to regulate both voluntary and involuntary responses, the PNS delivers that information to the appropriate parts of the body. In the window-breaking example, a sound (the tinkling of broken glass) sets off a CNS-regulated, PNS-mediated increase in heartbeat. It is interesting to note that thought alone can initiate involuntary reactions. In our example, merely the thought of someone breaking a window to burglarize your home can result in increased heart rate.

The Spinal Cord

The second component of the central nervous system is the spinal cord. The spinal cord is responsible for connecting the peripheral nervous system to the vast signal-processing power of the brain. In addition, neurons of the spinal cord are themselves able to process certain signals from the body. We will first examine the structure of this unique organ, then explore its multiple functions.

The spinal cord is responsible for connecting the peripheral nervous system to the vast signal-processing power of the brain.

Structure of the spinal cord

The nerves of the spinal cord are encased in a protective skeletal structure called the vertebral column. The vertebral column is made up of bones called vertebrae. The rigidity of the vertebral column allows us to stand upright, and it protects our spinal cord from harm. However, the vertebral column also remains somewhat flexible, allowing us a wide range of motion. The vertebrae protecting the spinal cord are composed of much thicker bone than the skull, the bony plates that protect the brain. This suggests how important the spinal cord is to the human body; any system under such heavy protective armor must be important to our existence.

The structure of the spinal cord is directly related to its function as a conveyor of information. Information, in the form of nerve impulses, reaches the spinal cord through sensory neurons and exits the spinal cord through motor neurons. Information enters and departs from the spinal cord through spinal nerves.

Spinal nerves are known as "mixed" nerves because they contain the axons of both sensory and motor neurons. Shortly before reaching the spinal cord, the sensory and motor axons are segregated from one another (Figure 10). 3

Figure 10

The organization of neurons in the spinal cord plays an important role in the effective functioning of our nervous system.

Information is delivered into the spinal cord through the axon terminals of sensory neurons. Once in the spinal cord, the information may flow to motor neurons, to interneurons that pass it directly to motor neurons, or to interneurons that transmit the information to the brain.

The dendrites of spinal motor neurons can receive information from sensory neurons, from interneurons that connect with sensory neurons, or from interneurons that connect to neurons in the brain. The axons of spinal motor neurons are able to deliver information through the spinal nerves and out to all parts of the body (Figure 10). The axon terminals of motor neurons connect to various muscles and glands, delivering information for both voluntary and involuntary actions.

Spinal cord functions

The spinal cord carries out two major activities: generating simple behaviors and transferring information. The response generated by the spinal cord is a reflex: an automatic, involuntary response to a sensory input, such as withdrawing from a hot stove. Some reflex pathways (a set of nerves that relay a particular message) are relatively simple. In the simplest reflex pathways, a motor neuron connects directly to a sensory neuron. The patellar, or knee-jerk, reflex is an example of this type of pathway (Figure 11). When we are hit lightly at a certain point below the kneecap (patella), the lower half of the leg jerks upward.

Figure 11

The patellar reflex is mediated by the spinal cord.

The spinal cord relays information in more complex pathways as well. Some reflex actions require the involvement of multiple spinal interneurons before a motor output can be generated.

Another role of the spinal cord is to relay information between the brain and the peripheral nervous system. Information, in the form of nerve impulses, reaches the spinal cord through sensory neurons of the PNS. These impulses are transmitted to the brain through the interneurons of the spinal cord. Finally, response signals generated in the motor areas of the brain travel down the spinal cord through other interneurons and move to the body in the axons of spinal motor neurons. The spinal cord is thus responsible for mediating all information flow between the body and the brain. Injuries to the spinal cord result in a loss of responsiveness below the injury. This loss is not due to an inability of the muscle to function, but to the inability to relay messages between the body and the brain. Spinal cord injuries are discussed in Section 8, Nervous System Injury.

Plasticity and Learning

The changing brain

For many years, scientists have known that neurons form new connections during the first few years of a human's life. Until recently, however, scientists believed that after this initial phase of neural development, the nervous system was complete and relatively fixed. Scientists have now found that nerve fibers continue to grow and innervate areas of the cerebral cortex in children between age three and puberty. 27 Scientific investigations have demonstrated that even the adult brain generates new neurons within a region important for learning and memory. 8 The brain's ability to change and reorganize in response to some input is known as plasticity. Plasticity is defined by a change in the anatomy of the neuron. New synapses may form, existing synapses may strengthen, some synapses may be eliminated, or more dendrites may form. 16, 17, 19

Learning and memory

Learning is a form of plasticity, since it leads to structural changes in the brain. Brain plasticity functions in both positive and negative directions. 9, 21 Neurons that are stimulated to form additional synaptic connections may grow and strengthen. However, neurons or synapses that are neglected may weaken over time. Factors have been identified that can function as positive or negative regulators of neural plasticity. Several are discussed below.

Positive regulators of neural growth

People first used the saying "use it or lose it" in reference to physical fitness. Now the saying also seems valid for learning and brain function. Practicing a task appears to improve the brain's efficiency. 10 For instance, when a person first learns to play the piano, he or she uses a large amount of the motor section of their brain. However, professional piano players who have been playing for many years devote a much smaller region of their motor cortex to finger dexterity. 16 How is this possible? By repeatedly stimulating the same region of their body (fingers) for the same action (piano playing), their brains have strengthened the related synapses. Thus, fewer neurons are needed to perform the same task.

Additional studies have revealed that physical, social, and mental activity may protect memory and alertness. For example, mice raised in an "enriched" environment, which contains other mice and a variety of stimulating toys, displayed dendrite growth and performed better on learning tasks than inactive, isolated mice did (Figure 12). 16, 28 These results were observed in mice of all ages. Mice raised with toys but no social interaction, as well as mice raised with other mice but no toys, performed less well on learning tasks and showed less neural growth than those raised with both toys and social interaction did. Thus, mental, social, and physical stimulation are all positive regulators of neural growth and seem to have an additive effect on learning and memory. In addition, mice subjected to learning tasks tend to retain more of the dendrite growth they experience during enrichment than do mice that are left alone after enrichment.

Figure 12

Mice raised in an impoverished environment (a) show less dendrite growth (c) than do mice raised in an enriched environment (b, d).

Negative regulators of neural growth

One negative regulator of neural growth is deprivation, the opposite of the positive factor of enrichment. Mice, rats, nonhuman primates, and humans all show lower ability to learn tasks and less dendrite branching when raised in environments deprived of stimulation. Poor nutrition, lack of social contact, and absence of mental engagement can all contribute to deprivation. Another major factor known to have a negative effect on neural growth is stress. Scientists have shown consistently that animals and humans living under constant stress conditions show less neural growth and/or learning than their less-stressed counterparts do. 7

Pain and learning

Intuitively, we can understand the link between pain and learning. It is obviously to our advantage to avoid painful input. By doing so, we can live longer, healthier lives. But how does our body determine what is painful? Recent studies indicate that our bodies have special pain receptors known as nociceptors (for "noxious stimuli receptors"). These receptors only respond to potentially damaging stimuli. 26 For instance, when we hold a mug of hot cocoa, our fingers feel the pleasant warmth, and we tend to hold on to the mug. However, if we splash scalding cocoa on our fingers, we tend to pull our hand back abruptly. The burning sensation in our fingers provided a painful damaging stimulus.

Once a nociceptor response has been activated, the neuron responsible becomes inflamed, leading to hypersensitivity. This means that a lesser stimulus in the same spot will activate the pain response. For instance, once skin has been damaged by sunburn, further stimulation (touching the sunburned skin) is painful. This seemingly counterintuitive step occurs because nociceptors respond as other neurons do: their synapses strengthen with use. Once we injure ourselves and stimulate a pain pathway, we are more likely to feel pain if we stimulate the same region again before it has a chance to heal.

Nociceptor response is useful in everyday learning ("don't touch a hot stove" is a lesson most of us don't have to learn twice). However, the synaptic strengthening of pain pathways can become a problem for those who experience chronic pain. In these individuals, pain appears to sensitize the nociceptor pathway almost indefinitely. Thus, additional stimulation is that much more painful, until it becomes unbearable. Researchers are developing therapies that disrupt synaptic strengthening in a nociceptor-specific manner.

Nervous System Injury

Neurological disorders

An estimated 50 million Americans suffer from disorders of the brain or nervous system. Some brain disorders are influenced by genetics, such as Huntington's disease; some are environmental, resulting from spinal cord or brain injury; and some result from a combination of genetic and environmental factors, such as stroke. Ongoing neuroscience research continues to elucidate the causes for the disorders as well as reveal new therapies to minimize or eliminate the disabilities that result from them.

During the past few years, events have occurred that have raised public awareness of neurological disorders. Several well-known people have experienced either neurological disease or traumatic injuries that have been publicized extensively. For example, President Ronald Reagan was diagnosed with Alzheimer's disease. Alzheimer's disease is a progressive, degenerative brain disease that leads to loss of cognitive function and memory, behavioral changes, personality changes, and impaired judgment. 1 The brains of Alzheimer's patients contain tangled masses of abnormal protein in the cerebrum. 22 Scientists continue to investigate the causes of this disease.

Another human brain disorder is Parkinson's disease, a motor system disorder affecting more than 500,000 Americans. 23 Public knowledge of Parkinson's has been increased by actor Michael J. Fox and former U.S. Attorney General Janet Reno, both public figures who have been forthcoming about their conditions. The disease is characterized by tremor, rigidity, slowness of movement, and impaired balance and coordination. It occurs when neurons in certain sections of the midbrain die or become impaired. The neuronal loss causes a decrease in the level of an excitatory neurotransmitter, which causes the neurons in another part of the brain to initiate aberrant neural impulses. Genetic factors may play a stronger role in some forms of the disease, while environmental factors play a prime role in other forms. 23, 24 The drug L-dopa, sometimes in combination with other drugs, is the standard pharmacological treatment for Parkinson's disease. However, for some individuals, surgical intervention can relieve tremors. Scientists continue to study new ways of treating this debilitating disorder.

Injuries to the nervous system

Our nervous system is especially sensitive to damage by injury. Both brain and spinal cord injuries have the potential to cause severe and life-changing disabilities. However, the type of disability sustained depends greatly on the region of trauma.

The spinal cord is responsible for information transfer between the brain and the body. It follows that injuries to the spinal cord disrupt information transfer. The position of trauma to the spine largely determines the effect of a spinal cord injury on the body. For instance, injuries to the lower half of the spine can lead to paraplegia (paralysis of the lower half of the body with involvement of both legs), while injuries closer to the skull may lead to quadriplegia (paralysis of both arms and both legs). Leading causes of spinal cord injuries are motor vehicle accidents, violence, and falls. 4 Exciting new research provides hope of someday restoring function to paralyzed individuals and improving their quality of life.

Traumatic brain injury (TBI) refers to damage resulting from trauma to the brain. TBI, like spinal cord injuries, may result in impaired physical function. However, the brain not only controls our sensory and motor functions, it is also our center of conscious thought. Therefore, injuries to the brain can affect our cognitive abilities or disturb behavioral and emotional functioning. In addition, brain trauma has the potential to alter personality and, thus, that elusive part of ourselves we often consider unchangeable: our sense of self.

The story of Phineas Gage illustrates a dramatic instance of personality change. 19 Gage was the foreman of a railway construction team in the mid-19th century. On Sept. 13, 1848, an accidental explosion blew a 20-pound metal rod all the way through Gage's head, from below his left cheekbone to just behind his right temple (Figure 13). Amazingly, Gage never lost consciousness. However, the injuries he sustained resulted in a complete reversal of his personality. Before the accident, his calm, collected demeanor and level-headedness made him one of the best foremen on his team. After the accident, his demeanor was characterized by rage, impatience, and gross profanity. Though physically capable of work after a few months of recovery, he was not the same man mentally. He never worked as a foreman again. He spent his remaining days as a farmhand until he died while having a seizure in 1921. Gage's case was the first to draw attention to the effect of brain injuries on personality, and it remains one of the most dramatic cases of personality change due to TBI. 5

Figure 13

Phineas Gage's skull and frontal lobe were pierced by a metal rod, changing his personality forever.

Most TBIs result from closed-head injuries (as opposed to penetrating injuries like that of Phineas Gage). As with spinal cord injury, the leading causes of TBI are motor vehicle accidents, violence (such as can occur with firearms), and falls. Sports injuries, especially to young athletes, are another major cause of brain injury. TBI is a leading cause of death and disability in the United States. The Centers for Disease Control and Prevention's National Center for Injury Prevention and Control estimates that 5.3 million Americans (2 percent of the population) live with a disability resulting from TBI.

Regional specificity and effects of trauma

Trauma to the brain results in different types of disabilities. Some injuries have the capacity to alter a person's sense of self, while others affect abilities, such as speech or vision, but do not affect a person's sense of who they are. Before the advent of functional imaging, scientists used case studies of brain injury to determine functional regions of the brain. Scientists continue to depend upon information from case studies to define brain regions. However, functional imaging has greatly aided these research efforts. Now, doctors and scientists work together to investigate the functional specialization of regions of the living brain.

Sometimes it is difficult to decipher the exact story of brain functionality using case studies and functional imaging of human brains. For instance, while a physician may diagnose Alzheimer's disease, it is not possible to use physical examination of a patient to determine the possible genetic influences on the disease. Similarly, when scientists investigate a brain injury case study, they can determine that some areas of the brain are damaged. However, they may not be sure whether they have determined all areas that have experienced trauma, or whether any of the patient's disability is a direct result of the injured region. In such cases, scientists find it helpful to use animal models to further understand how the brain works. Genetic and trauma studies in animal models such as the mouse and rat have been pivotal in producing the knowledge we have about the brain.

Protecting our nervous system

Nervous system injuries in prominent public figures have drawn attention to the importance of protecting our nervous system. The late actor Christopher Reeve, well known for his role in the movies as Superman, was paralyzed as a result of a fall from a horse. Several football players, including Steve Young and Troy Aikman, suffered concussions that jeopardized their careers and their long-term health. In some cases, neurological disorders cannot be predicted or prevented. Continuing research should be able to provide treatments for some diseases. In other instances, the behaviors that contribute to the development of a neurological problem can be avoided. 6 For example, chronic use of certain drugs, such as methamphetamine and alcohol, can cause neurons in certain parts of the brain to die. Individuals can make choices to avoid these substances. Certain sports injuries can be minimized by using appropriate protective gear (Figure 14).

Figure 14

A helmet can greatly reduce one's chances of sports-related head trauma.

These situations illustrate only a few of the health problems that result from functional disorders of the brain and nervous system. Statistics show that neurological diseases and injuries are surprisingly common. In addition to the toll they take on affected individuals, such diseases and injuries exert a dramatic economic influence on all citizens. Continuing research is key to alleviate this impact. It is unlikely that a single discovery will lead to reversing damage from traumatic injury. Undoubtedly, environmental and genetic factors influence the onset, severity, and progression of many neurological diseases. Scientific researchers need to examine these disorders thoroughly in order to develop effective treatments and cures. 25

By learning the fundamental principles of brain structure and function, students will better understand how they respond to and interact with their environment and how scientific research contributes to better health. By learning how individual behaviors can alter the function of the brain either positively or negatively, students will be better prepared to prevent neurological trauma.

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Glossary

Alzheimer's disease

A progressive, neurodegenerative disease characterized by loss of function and death of nerve cells in several areas of the brain. This leads to loss of cognitive functions such as memory and language.

animal model

A laboratory animal used in research that simulates processes comparable to those that occur in humans.

axon

A long, branching outgrowth of a neuron that carries information, in the form of a nerve impulse, away from the cell body of the neuron. Each neuron has one axon, which can be over a foot long. A neuron delivers information to other cells through the axon terminals at the end of its axon.

brain

The center of thought and emotion in the central nervous system. The brain is responsible for the coordination and control of body activities and the interpretation of information from inside and outside the body.

cell body

In neurons, the main part of the cell around the nucleus excluding long processes such as axons and dendrites.

central nervous system (CNS)

One of the two major divisions of the nervous system. The CNS consists of the brain and spinal cord.

cognitive function

The mental process of knowing, thinking, learning, and judging.

dendrite

A branching outgrowth of a neuron that carries information, in the form of a nerve impulse, into the cell body of the neuron. Each nerve usually has many dendrites.

glial cells (glia)

Specialized cells that surround neurons. They provide mechanical and physical support to neurons and electrical insulation between neurons.

interneuron

A type of neuron that connects only with other neurons and thus delivers information only between neurons.

involuntary response

A nervous system response, such as a reflex, that is not under the control of the brain, and thus does not involve choice.

learning

The acquisition of knowledge or skill. It occurs in, and may lead to changes in, the brain.

motor neuron

A neuron that delivers information from a sensory neuron or interneuron to muscles or glands in the body in order to produce a response.

motor output

The body’s response to motor information delivered through motor neurons.

myelin

The insulating sheath that surrounds axons.

nerve

A bundle of neurons bound together by a protective sheath of connective tissue.

nerve impulse

Information in the form of an electrochemical signal that travels through a neuron in response to a stimulus.

nervous system

The entire, integrated system of nerve tissue in the body. It is composed of the brain, spinal cord, nerves, and glia. The human nervous system can be subdivided into the central nervous system (CNS) and the peripheral nervous system (PNS).

neural pathway

An interconnected set of neurons that delivers information (in the form of a nerve impulse) related to a body function.

neural signaling

The delivery of information through a neural pathway.

neuron

A specialized cell that delivers information within the body.

paralysis

Loss or impairment of motor function due to damage of the nervous system by injury or disease. Paralysis below the waist only is referred to as paraplegia; complete paralysis below the neck is referred to as quadriplegia.

peripheral nervous system (PNS)

One of the two major divisions of the nervous system. Nerves in the PNS connect the central nervous system (CNS) with sensory organs, other organs, muscles, blood vessels, and glands.

plasticity

The ability of the brain to change through the formation or strengthening of connections between neurons in the brain.

positron emission tomography (PET)

A technique for imaging the brain in action. PET images show active regions in the brain.

postsynaptic cell

A neuron that receives a nerve impulse across a synapse.

presynaptic cell

A neuron that sends a nerve impulse across a synapse.

reflex

A type of involuntary response.

sensory input

Information received by the body through sensory organs, such as the eyes, ears, nose, and skin.

sensory neuron

A neuron that delivers sensory information from the sensory organs to inter-neurons or motor neurons.

spinal cord

The part of the central nervous system that is located inside the vertebral column. Neurons in the spinal cord connect neurons in the brain to neurons in the body.

stimulus

Any information coming into the body that is capable of generating a nerve impulse.

synapse

The space separating the axon terminals of a neuron from the dendrites of the next neuron in a neural pathway.

traumatic brain injury (TBI)

Injuries to the brain caused by physical trauma to the head.

vertebral column

The series of bones that extend from the skull to the tailbone. It provides support and forms a flexible, protective case for the spinal cord. The bones of the vertebral column are called vertebrae.

voluntary response

A nervous system response that is under control of the brain, and thus involves choice.

What are interconnected groups of nerve cells that integrate sensory input and motor output?

What are interconnected pathways of nerve cells?

What are the three interconnected functions of the nervous system?

Are interconnected clusters of neurons in the central nervous system?