Students examine images of human brains that illustrate that specific regions of the brain regulate specific functions. They extend that knowledge to learn that drugs of abuse activate a brain circuit known as the reward system. This same circuit is stimulated in response to basic survival needs, which produces feelings of pleasure.
Specific brain regions control specific brain functions.
By the end of these activities, students will
The brain controls virtually everything humans experience, including movement, sensing our environment, and regulating our involuntary body processes such as breathing, as well as controlling our emotions. Ongoing scientific research into the organization and function of the brain has led, and will continue to lead, to new treatments of diseases such as Parkinson’s disease, epilepsy, stroke, and mental illnesses (including depression and schizophrenia).
The brain is the organ of behavior. It is also the organ of our minds. Both overt behavior and consciousness are manifestations of the work of our brains. Other people can see an individual’s overt behaviors, whereas consciousness is apparent only in our individual minds. The field of neuroscience studies how people control their behaviors, thoughts, and feelings, and how these actions sometimes get out of control.
The brain processes a huge amount of information in a remarkably efficient manner. Think about driving a car. It is something most of us do without much difficulty. But to do it properly, we must perform a remarkable number of tasks. First we have to make sure that our body is in working order: heart rate and breathing have to be properly regulated and body temperature held steady, and we certainly have to be sure we don’t fall asleep. Despite the complexity of these tasks, we carry them out with no conscious involvement on our part. Then, there are the things we are aware of. We have to see the road and hear the traffic (or the radio), use information from our feet, legs, hands, and arms to know where the gas pedal and steering wheel are, and then generate the body movements to control the direction and speed of the car. All of this often takes place while we are talking to someone else in the car, or even while talking on the phone (although this is not a good idea). The magnitude and speed of data processing needed to do this are stunning, but most of us consider driving to be an easy task.
How does the brain carry out multiple tasks at one time? The answer is that the brain splits the larger task—driving, in our example—into smaller ones: seeing, hearing, moving, and so forth. Even those tasks are split into their component parts. One part of the human brain analyzes the movement of objects that we see, while another part is responsible for actually recognizing them. In short, specific parts of the brain carry out specific tasks. Not only that, but each part of the brain specializes in a specific kind of task. This means that whenever that task needs to be done, the appropriate information is processed by that part of the brain.
The flip side of this organizational scheme is that if a part of the brain is damaged, then the job it used to undertake cannot be done. For example, damage to the occipital lobe at the back of the brain can cause blindness, but it has no effect on a person’s ability to hear or move. Because the job of seeing is highly compartmentalized, individuals who have lost one aspect of sight, such as the ability to see colors or to recognize faces, may still be able to do other visual tasks. Imagine being able to recognize someone by hearing his or her voice, but not being able to recognize his or her face when you see it.
The advantage of this localization of function is when larger jobs are parceled out throughout the brain, they all can be done at once. This “division of labor” adds great speed to our ability to perceive what is happening in the world around us, to analyze it, and then to generate appropriate responses. Dealing with information in this way is called parallel processing.1 (Superscript numbers refer to references listed by section on the References page.) Computer scientists have used this concept in the development of computers.
The human brain consists of several large regions, each of which is responsible for some of the activities necessary for life. These include the brainstem, cerebellum, limbic system, diencephalon, and cerebral cortex.2,3
The brainstem is the part of the brain that connects the brain and spinal cord (Figure 1.2). This part of the brain is involved in coordinating many basic functions such as heart rate, breathing, eating, and sleeping.
The cerebellum coordinates the brain’s instructions for skilled repetitive movements and for maintaining balance and posture.
The limbic system, as discussed in the next section, is involved in regulating emotions, motivations, and movement. It includes the amygdala and hippocampus, which is important for memory formation.
The diencephalon contains the thalamus and hypothalamus. The thalamus is involved in sensory perception and regulating movement. The hypothalamus is an important regulator of the pituitary gland, which directs the release of hormones throughout the body.
The cerebral cortex makes up the largest part of the brain mass and lies over and around most of the other brain structures. It is the part of the brain responsible for thinking, perceiving, and producing and understanding language. The cortex can be divided into areas that are involved in vision, hearing, touch, movement, smell, and thinking and reasoning (Figure 1.3).
Just as specific areas of the brain control seeing and hearing, specific brain areas also regulate emotions, motivations, and movement. These functions are carried out by a part of the brain called the limbic system. The limbic system influences how we respond to the world around us. Imagine a cool sunny day. You finish your work early and head to your favorite park for a leisurely walk with your dog. You are feeling so mellow that when the dog slobbers on your clean shirt, you merely scratch him behind the ears.
You might have a very different reaction on another day when you have to work late, traffic is backed up, and the dog runs away instead of coming to welcome you home. This time when the dog slobbers on you (after he finds his way home again), you shove him away and scold him.
The feelings you have in those two different situations are a result of your limbic system at work. The limbic system uses memories, information about how your body is working, and current sensory input to generate your emotional responses to current situations.
The limbic system is involved in many of our emotions and motivations, particularly those related to survival, such as fear and anger. The system is also involved in pleasurable activities necessary for survival, such as eating and sex. If something is pleasurable, or rewarding, you want to do it again. Pleasurable activities engage the reward circuit (or system), so the brain notes that something important is happening that needs to be remembered and repeated.1,2 The reward system includes several interconnected structures—the ventral tegmental area (VTA), located at the top of the brain stem; the nucleus accumbens; and the prefrontal cortex (Figure 1.4). Neurons from the VTA relay messages to the nucleus accumbens and the prefrontal cortex. Information is also relayed back from the cortex to the nucleus accumbens and the VTA.
Most drugs of abuse activate these same VTA and nucleus accumbens neurons; that is why drugs produce pleasurable feelings to the drug user. And, because the feelings are pleasurable, the user wants to continue to experience the pleasure that he or she felt during previous drug use.
Figure 1.4: This drawing of a brain cut in half illustrates the brain areas and systems involved in the reward system, or pleasure circuit. Neurons in the ventral tegmental area (VTA) extend axons to the nucleus accumbens and part of the prefrontal cortex. Source: National Institute on Drug Abuse (1996) The Brain & the Actions of Cocaine, Opiates, and Marijuana. Slide Teaching Packet for Scientists.
One of the reasons that drugs of abuse can exert such powerful control over our behavior is that they act directly on the more evolutionarily primitive brainstem and limbic structures, which can override the cortex in controlling our behavior.
Different drugs of abuse affect the neurons of the reward system in different ways. The activities in Lesson 3 in this module will elucidate the mechanisms by which drugs of abuse exert their effects.
Scientists increasingly use newer technologies to learn more about how the brain works and how drugs of abuse change how the brain works. Historically, scientists could examine brains only after death, but new imaging procedures enable scientists to study the brain in living animals, including humans.
One of the most extensively used techniques to study brain activity and the effects of drugs on the brain is positron emission tomography (PET). PET measures the spatial distribution and movement of radioisotopes in tissues of living subjects. Because the patient is awake, the technique can be used to investigate the relationship between behavioral and physiological effects and changes in brain activity. PET scans can detect nanomolar concentrations of tracer molecules and achieve spatial resolution of about 4 millimeters. In addition, computers can reconstruct images obtained from a PET scan in two or three dimensions.
PET requires the use of compounds labeled with positron-emitting isotopes.4,5 A cyclotron accelerates protons into the nucleus of nitrogen, carbon, oxygen, or fluorine to generate these isotopes. The additional proton makes the isotope unstable. To become stable again, the proton must break down into a neutron and a positron. The unstable positron travels away from the site of generation and dissipates energy along the way. Eventually, the positron collides with an electron, leading to the emission of two gamma rays at 180° from one another. The gamma rays reach a pair of detectors that record the event. Because the detectors respond only to simultaneous emissions, scientists can precisely map the location where the gamma rays were generated. The labeled radioisotopes are very short-lived. The half-life (the time for half of the radioactive label to disintegrate) of the commonly used radioisotopes ranges from approximately two minutes to less than two hours, depending on the specific compound. Because a PET scan requires only small amounts (a few micrograms) of short-lived radioisotopes, pharmacological and radiological effects are negligible or even nonexistent.
PET scans can answer a variety of questions about brain function, including questions about the activity of neurons. Scientists use different radiolabeled compounds to investigate different biological questions. For example, radiolabeled glucose can identify parts of the brain that become more active in response to a specific stimulus. Active neurons metabolize more glucose than inactive neurons. Active neurons will emit more positrons. This will show as red or yellow on PET scans compared with blue or purple in areas where the neurons are not highly active. PET also helps scientists investigate how drugs affect the brain by enabling them to
Although in the context of drug abuse, PET is currently used only as a research tool, it is a powerful diagnostic and monitoring tool for other diseases. For example, PET scans may be used to locate tumors in cancer patients, monitor the spread of cancer, and evaluate the effectiveness of cancer treatment. PET scans are able to reveal the presence of tumors because of the rapid metabolism characteristic of cancerous cells. PET images reveal this increased glucose utilization by cells that have high metabolic rates. PET is an accurate test for coronary heart disease because it can detect areas of diminished blood flow to the heart. Doctors also employ PET to reveal changes in the brain that occur with Alzheimer’s disease, Parkinson’s disease, or seizure disorders. PET is a valuable tool because it
Different Neuroimaging Techniques Provide Different Information about the Brain
PET scanning is a major neuroimaging technique used in drug abuse research. However, researchers also use other techniques when they are better for answering a specific question. Similar to PET, single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), and electroencephalography (EEG) are noninvasive procedures that can measure biological activity through the skull and reveal the living brain at work.4,6 Each technique has its own advantages, and each provides different information about brain structure and function. Scientists often use more than one technique when conducting their research studies.
Similar to PET, SPECT imaging uses radioactive tracers and a scanner to record data that a computer constructs into two- or three-dimensional images of active brain regions. Because the tracers used in SPECT take longer to decay than those for PET, longer periods of time between tests are required for SPECT so a patient does not receive or accumulate too high a “load” of radioactivity. While PET is more versatile than SPECT and produces more detailed images with a higher degree of resolution, SPECT is much less expensive than PET and can address many of the same drug abuse research questions.
MRI uses magnetic fields and radio waves to produce high-quality two- or three-dimensional images of brain structures without injecting radioactive tracers. In this procedure, a large cylindrical magnet creates a magnetic field around the research volunteer’s head, and radio waves are sent through the magnetic field. Sensors read the signals, and a computer uses the information to construct an image. Using MRI, scientists can image both surface and deep brain structures with a high degree of anatomical detail, and they can detect minute changes in these structures over time. A modification of this technique, called functional MRI (fMRI), enables scientists to see images of blood flow in the brain as it occurs. fMRI provides superior image clarity along with the ability to assess blood flow and brain functions in just a few seconds. However, PET retains the advantage of being able to identify which brain receptors are being bound by neurotransmitters, abused drugs, and potential treatment compounds.
EEG uses electrodes placed on the scalp to detect and measure patterns of electrical activity in the brain. The greatest advantage of EEG is speed: it can record complex patterns of neural activity occurring within fractions of a second after a stimulus has been administered. The drawback to EEG is that it does not provide the spatial resolution of fMRI or PET. Researchers often combine EEG images of brain electrical activity with MRI scans to localize brain activity more precisely.
|For the class||For each group of 3 students||For each student|
|1 transparency of Master 1.3, PET Image Tasks
1 transparency of Master 1.4, Major Regions of the Brain
1 transparency of Master 1.5, Areas of the Cerebral Cortex and Their Functions
1 transparency of Master 1.7, The Reward System
|1 copy of Master 1.1, Positron Emission Tomography (PET) Images*
1 copy of Master 1.2, Interpreting PET Images
|1 copy of Master 1.6, What Happened to Phineas Gage?|
|*The online version of Activity 2 is the preferred approach. Copies of Master 1.1 are needed only if the Internet is unavailable for classroom use. If needed, make one set of color photocopies for each team of three students. Field-test teachers recommend laminating the color copies to help preserve them.|
|1||6 to 8 index cards (3" x 5" or 4" x 6")|
|2||overhead projector, computers (optional)|
|3||computers or overhead projector|
Prepare task cards for Activity 1, Step 1. Decide which tasks you wish students to do. Write the instructions for each task on an index card.
Arrange for the class to have access to the Internet for Activities 2 and 3, if possible.
The specific tasks can and should be very diverse. The following list suggests some appropriate tasks:
The goal for this question is for students to acknowledge that the brain is involved in regulating all human physiological, behavioral, and emotional functions. For example, point out that all students are breathing. When most people think about breathing, they think about the lungs, but not the involvement of the brain. Also, point out that each student’s heart is beating. Although the heart is actually pumping the blood, the brain fulfills an important role in regulating the heartbeat. The involvement of the brain will be more obvious for some of the tasks than for others.
Students will provide a variety of answers, including watching a person’s behavior, using various imaging techniques (such as PET scans, CT scans, or MRI), using animals (either living or dead) for research, and so forth.
Next: Lesson 1 (Page 2 of 2)