We now know that neurotransmitters are not the only important part of the machinery. But let's not diminish their importance either. They are deeply involved in how nerve cells communicate with one another. And they are a component of brain function that we can often influence to good ends. Neurotransmitters are chemicals that relay messages from neuron to neuron.
An antidepressant medication tends to increase the concentration of these substances in the spaces between neurons the synapses. In many cases, this shift appears to give the system enough of a nudge so that the brain can do its job better.
How the system works. If you trained a high-powered microscope on a slice of brain tissue, you might be able to see a loosely braided network of neurons that send and receive messages. While every cell in the body has the capacity to send and receive signals, neurons are specially designed for this function.
Each neuron has a cell body containing the structures that any cell needs to thrive. Stretching out from the cell body are short, branchlike fibers called dendrites and one longer, more prominent fiber called the axon. A combination of electrical and chemical signals allows communication within and between neurons. When a neuron becomes activated, it passes an electrical signal from the cell body down the axon to its end known as the axon terminal , where chemical messengers called neurotransmitters are stored.
The signal releases certain neurotransmitters into the space between that neuron and the dendrite of a neighboring neuron. That space is called a synapse. As the concentration of a neurotransmitter rises in the synapse, neurotransmitter molecules begin to bind with receptors embedded in the membranes of the two neurons see Figure 2.
The release of a neurotransmitter from one neuron can activate or inhibit a second neuron. If the signal is activating, or excitatory, the message continues to pass farther along that particular neural pathway.
If it is inhibitory, the signal will be suppressed. The neurotransmitter also affects the neuron that released it. Once the first neuron has released a certain amount of the chemical, a feedback mechanism controlled by that neuron's receptors instructs the neuron to stop pumping out the neurotransmitter and start bringing it back into the cell. This process is called reabsorption or reuptake. Enzymes break down the remaining neurotransmitter molecules into smaller particles.
When the system falters. Brain cells usually produce levels of neurotransmitters that keep senses, learning, movements, and moods perking along.
But in some people who are severely depressed or manic, the complex systems that accomplish this go awry. For example, receptors may be oversensitive or insensitive to a specific neurotransmitter, causing their response to its release to be excessive or inadequate.
Or a message might be weakened if the originating cell pumps out too little of a neurotransmitter or if an overly efficient reuptake mops up too much before the molecules have the chance to bind to the receptors on other neurons. Any of these system faults could significantly affect mood. Kinds of neurotransmitters. Scientists have identified many different neurotransmitters. Here is a description of a few believed to play a role in depression:.
Every part of your body, including your brain, is controlled by genes. Genes make proteins that are involved in biological processes. Throughout life, different genes turn on and off, so that — in the best case — they make the right proteins at the right time. But if the genes get it wrong, they can alter your biology in a way that results in your mood becoming unstable. In a person who is genetically vulnerable to depression, any stress a missed deadline at work or a medical illness, for example can then push this system off balance.
Mood is affected by dozens of genes, and as our genetic endowments differ, so do our depressions. The hope is that as researchers pinpoint the genes involved in mood disorders and better understand their functions, depression treatment can become more individualized and more successful.
Patients would receive the best medication for their type of depression. Another goal of gene research, of course, is to understand how, exactly, biology makes certain people vulnerable to depression. For example, several genes influence the stress response, leaving us more or less likely to become depressed in response to trouble. Perhaps the easiest way to grasp the power of genetics is to look at families.
It is well known that depression and bipolar disorder run in families. The strongest evidence for this comes from the research on bipolar disorder.
Half of those with bipolar disorder have a relative with a similar pattern of mood fluctuations. These numbers don't apply to fraternal twins, who — like other biological siblings — share only about half of their genes. The evidence for other types of depression is more subtle, but it is real. A person who has a first-degree relative who suffered major depression has an increase in risk for the condition of 1. One important goal of genetics research — and this is true throughout medicine — is to learn the specific function of each gene.
This kind of information will help us figure out how the interaction of biology and environment leads to depression in some people but not others. Genetics provides one perspective on how resilient you are in the face of difficult life events. But you don't need to be a geneticist to understand yourself. Perhaps a more intuitive way to look at resilience is by understanding your temperament. Temperament — for example, how excitable you are or whether you tend to withdraw from or engage in social situations — is determined by your genetic inheritance and by the experiences you've had during the course of your life.
Some people are able to make better choices in life once they appreciate their habitual reactions to people and to life events. Cognitive psychologists point out that your view of the world and, in particular, your unacknowledged assumptions about how the world works also influence how you feel. You develop your viewpoint early on and learn to automatically fall back on it when loss, disappointment, or rejection occurs.
For example, you may come to see yourself as unworthy of love, so you avoid getting involved with people rather than risk losing a relationship.
Or you may be so self-critical that you can't bear the slightest criticism from others, which can slow or block your career progress.
Yet while temperament or world view may have a hand in depression, neither is unchangeable. Therapy and medications can shift thoughts and attitudes that have developed over time. At some point, nearly everyone encounters stressful life events: the death of a loved one, the loss of a job, an illness, or a relationship spiraling downward. Some must cope with the early loss of a parent, violence, or sexual abuse. While not everyone who faces these stresses develops a mood disorder — in fact, most do not — stress plays an important role in depression.
As the previous section explained, your genetic makeup influences how sensitive you are to stressful life events. When genetics, biology, and stressful life situations come together, depression can result.
Stress has its own physiological consequences. It triggers a chain of chemical reactions and responses in the body. If the stress is short-lived, the body usually returns to normal.
But when stress is chronic or the system gets stuck in overdrive, changes in the body and brain can be long-lasting. Stress can be defined as an automatic physical response to any stimulus that requires you to adjust to change. Every real or perceived threat to your body triggers a cascade of stress hormones that produces physiological changes. We all know the sensations: your heart pounds, muscles tense, breathing quickens, and beads of sweat appear.
This is known as the stress response. The stress response starts with a signal from the part of your brain known as the hypothalamus. The hypothalamus joins the pituitary gland and the adrenal glands to form a trio known as the hypothalamic-pituitary-adrenal HPA axis, which governs a multitude of hormonal activities in the body and may play a role in depression as well. The more a person interacts with the world, the more a person needs information from the world incorporated into the brain structures.
Synapse overproduction and selection may progress at different rates in different parts of the brain Huttenlocher and Dabholkar, In the primary visual cortex, a peak in synapse density occurs relatively quickly. In the medial frontal cortex, a region clearly associated with higher cognitive functions, the process is more protracted: synapse production starts before birth and synapse density continues to increase until 5 or 6 years of age.
The selection process, which corresponds conceptually to the main organization of patterns, continues during the next 4—5 years and ends around early adolescence. This lack of synchrony among cortical regions may also occur upon individual cortical neurons where different inputs may mature at different rates see Juraska, , on animal studies.
After the cycle of synapse overproduction and selection has run its course, additional changes occur in the brain. They appear to include both the modification of existing synapses and the addition of entirely new synapses to the brain. Research evidence described in the next section suggests that activity in the nervous system associated with learning experiences somehow causes nerve cells to create new synapses.
Unlike the process of synapse overproduction and loss, synapse addition and modification are lifelong processes, driven by experience. This process is probably not the only way that information is stored in the brain, but it is a very important way that provides insight into how people learn.
Alterations in the brain that occur during learning seem to make the nerve cells more efficient or powerful. Animals raised in complex environments have a greater volume of capillaries per nerve cell—and therefore a greater supply of blood to the brain—than the caged animals, regardless of whether the caged animal lived alone or with companions Black et al. Capillaries are the tiny blood vessels that supply oxygen and other nutrients to the brain.
In this way experience increases the overall quality. Using astrocytes cells that support neuron functioning by providing nutrients and removing waste as the index, there are higher amounts of astrocyte per neuron in the complex-environment animals than in the caged groups.
Overall, these studies depict an orchestrated pattern of increased capacity in the brain that depends on experience. Other studies of animals show other changes in the brain through learning; see Box 5. The weight and thickness of the cerebral cortex can be measurably altered in rats that are reared from weaning, or placed as adults, in a large cage enriched by the presence both of a changing set of objects for play and exploration and of other rats to induce play and exploration Rosenzweig and Bennett, These animals also perform better on a variety of problem-solving tasks than rats reared in standard laboratory cages.
Interestingly, both the interactive presence of a social group and direct physical contact with the environment are important factors: animals placed in the enriched environment alone showed relatively little benefit; neither did animals placed in small cages within the larger environment Ferchmin et al.
Thus, the gross structure of the cerebral cortex was altered both by exposure to opportunities for learning and by learning in a social context. Are the changes in the brain due to actual learning or to variations in aggregate levels of neural activity? Animals in a complex environment not only learn from experiences, but they also run, play, and exercise, which activates the brain. The question is whether activation alone can produce brain changes without the subjects actually learning anything, just as activation of muscles by exercise can cause them to grow.
To answer this question, a group of animals that learned challenging motor skills but had relatively little brain activity was compared with groups that had high levels of brain activity but did relatively little learning Black et al.
There were four groups in all. What happened to the volume of blood vessels and number of synapses per neuron in the rats? Both the mandatory exercisers and the voluntary exercisers showed higher densities of blood vessels than either the cage potato rats or the acrobats, who learned skills that did not involve significant. How do rats learn? The objects are changed and rearranged each day, and during the changing time, the animals are put in yet another environment with another set of objects.
These two settings can help determine how experience affects the development of the normal brain and normal cognitive structures, and one can also see what happens when animals are deprived of critical experiences. After living in the complex or impoverished environments for a period from weaning to rat adolescence, the two groups of animals were subjected to a learning experience. The rats that had grown up in the complex environment made fewer errors at the outset than the other rats; they also learned more quickly not to make any errors at all.
In this sense, they were smarter than their more deprived counterparts. And with positive rewards, they performed better on complex tasks than the animals raised in individual cages. It is clear that when animals learn, they add new connections to the wiring of their brains—a phenomenon not limited to early development see, e. But when the number of synapses per nerve cell was measured, the acrobats were the standout group.
Learning adds synapses; exercise does not. Thus, different kinds of experience condition the brain in different ways. Synapse formation and blood vessel formation vascularization are two important forms of brain adaptation, but they are driven by different physiological mechanisms and by different behavioral events. Learning specific tasks brings about localized changes in the areas of the brain appropriate to the task.
For example, when young adult animals were. When they learned the maze with one eye blocked with an opaque contact lens, only the brain regions connected to the open eye were altered Chang and Greenough, When they learned a set of complex motor skills, structural changes occurred in the motor region of the cerebral cortex and in the cerebellum, a hindbrain structure that coordinates motor activity Black et al.
These changes in brain structure underlie changes in the functional organization of the brain. That is, learning imposes new patterns of organization on the brain, and this phenomenon has been confirmed by electro-physiological recordings of the activity of nerve cells Beaulieu and Cynader, Studies of brain development provide a model of the learning process at a cellular level: the changes first observed in rats have also proved to be true in mice, cats, monkeys, and birds, and they almost certainly occur in humans.
Clearly, the brain can store information, but what kinds of information? The neuroscientist does not address these questions. Answering them is the job of cognitive scientists, education researchers, and others who study the effects of experiences on human behavior and human potential. Several examples illustrate how instruction in specific kinds of information can influence natural development processes. This section discusses a case involving language development.
Brain development is often timed to take advantage of particular experiences, such that information from the environment helps to organize the brain. The development of language in humans is an example of a natural process that is guided by a timetable with certain limiting conditions. A phoneme is defined as the smallest meaningful unit of speech sound. Very young children discriminate many more phonemic boundaries than adults, but they lose their discriminatory powers when certain boundaries are not supported by experience with spoken language Kuhl, Native Japa-.
It is not known whether synapse overproduction and elimination underlies this process, but it certainly seems plausible. The process of synapse elimination occurs relatively slowly in the cerebral cortical regions that are involved in aspects of language and other higher cognitive functions Huttenlocher and Dabholkar, Different brain systems appear to develop according to different time frames, driven in part by experience and in part by intrinsic forces. But, as noted above, learning continues to affect the structure of the brain long after synapse overproduction and loss are completed.
There may be other changes in the brain involved in the encoding of learning, but most scientists agree that synapse addition and modification are the ones that are most certain. Detailed knowledge of the brain processes that underlie language has emerged in recent years. For example, there appear to be separate brain areas that specialize in subtasks such as hearing words spoken language of others , seeing words reading , speaking words speech , and generating words thinking with language.
Whether these patterns of brain organization for oral, written, and listening skills require separate exercises to promote the component skills of language and literacy remains to be determined. If these closely related skills have somewhat independent brain representation, then coordinated practice of skills may be a better way to encourage learners to move seamlessly among speaking, writing, and listening.
Language provides a particularly striking example of how instructional processes may contribute to organizing brain functions. The example is interesting because language processes are usually more closely associated with the left side of the brain. As the following discussion points out, specific kinds of experiences can contribute to other areas of the brain taking over some of the language functions.
For example, deaf people who learn a sign language are learning to communicate using the visual system in place of the auditory system. Manual sign languages have grammatical structures, with affixes and morphology, but they are not translations of spoken languages. Each particular sign language such as American Sign Language.
The perception of sign language depends on parallel visual perception of shape, relative spatial location, and movement of the hands—a very different type of perception than the auditory perception of spoken language Bellugi, In the nervous system of a hearing person, auditory system pathways appear to be closely connected to the brain regions that process the features of spoken language, while visual pathways appear to go through several stages of processing before features of written language are extracted Blakemore, ; Friedman and Cocking, When a deaf individual learns to communicate with manual signs, different nervous system processes have replaced the ones normally used for language—a significant achievement.
Neuroscientists have investigated how the visual-spatial and language processing areas each come together in a different hemisphere of the brain, while developing certain new functions as a result of the visual language experiences. In the brains of all deaf people, some cortical areas that normally process auditory information become organized to process visual information. Yet there are also demonstrable differences among the brains of deaf people who use sign language and deaf people who do not use sign language, presumably because they have had different language experiences Neville, , Among other things, major differences exist in the electrical activities of the brains of deaf individuals who use sign language and those who do not know sign language Friedman and Cocking, ; Neville, Also, there are similarities between sign language users with normal hearing and sign language users who are deaf that result from their common experiences of engaging in language activities.
In other words, specific types of instruction can modify the brain, enabling it to use alternative sensory input to accomplish adaptive functions, in this case, communication. Another demonstration that the human brain can be functionally reorganized by instruction comes from research on individuals who have suffered strokes or had portions of the brain removed Bach-y-Rita, , ; Crill and Raichle, It examines societal and individual motivation, sexual motivation, and cumulative effects of optimism and pessimism in human life.
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