Читать книгу Introduction to Abnormal Child and Adolescent Psychology - Robert Weis - Страница 91

Scientific advances have given us increasingly more detailed pictures of the brain and nervous system from infancy through adolescence. Studies examining children over time have yielded several principles of brain development (Roberts, 2020).

1. The brain consists of 100 billion neurons.

A neuron is a nerve cell that is typically very narrow and very long. Most neurons are small. You could place 50 neurons side by side within the period that ends this sentence. Neurons vary from 1 millimeter to more than 1 meter in length. Neurons are also very numerous; if you counted each neuron in your brain, one neuron per second, it would take you more than 3,000 years to finish.

The structure of a neuron can tell us something about its function. The center of most neurons contains the cell body. Its main purpose is to perform metabolic functions for the cell, that is, to keep the cell alive. The neuron also has dendrites, fingerlike appendages that receive information from either outside stimuli (e.g., light, pressure) or other neurons. Finally, the neuron has a longer axon, which relays information from the dendrites and cell body to the terminal endings of the neuron. Neurons relay information electrically, by controlling the positively and negatively charged particles that are allowed to enter the cell. Information is conducted down the axon in a manner analogous to electricity flowing down a wire. Mammalian axons are wrapped in a fatty substance called myelin (produced by Schwann cells), which increases conduction and speeds the electrical impulse (Image 2.5).

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2. Neurons communicate using chemical messengers.

Each neuron typically forms many connections with other neurons, forming a complex neural network. Although information travels within neurons electrically, it travels between neurons chemically. When an impulse reaches the end of an axon, it triggers the release of a chemical messenger called a neurotransmitter. The neurotransmitter is released into the synapse, a small cleft between neurons. In the synapse, neurotransmitters can be detected by other neurons, causing them to change their electrical charge. Sufficient stimulation by neurotransmitters can cause other nerve cells to become active, thus sending the impulse to the next neuron.

Neurotransmitters have different functions. Some are excitatory—that is, they increase the positive charge of neurons, making them more likely to become active. For example, dopamine is an excitatory neurotransmitter that is important for attention and concentration. Insufficient dopamine in certain brain regions is associated with ADHD. Other neurotransmitters are inhibitory—that is, they increase the negative charge of neurons, making them less likely to become active. For example, gamma aminobutyric acid (GABA) is an inhibitory neurotransmitter. Alcohol causes an increase in GABA, slowing reaction time, judgment, and decision-making. Most psychotropic medications and drugs affect behavior by enhancing or attenuating the effects of neurotransmitters.

3. The brain is organized from the bottom up.

Evolutionarily older areas of the brain develop first, followed by more complex, higher-order brain regions. For example, the brain stem consists of the medulla, pons, and midbrain and is largely responsible for basic metabolic functions such as heart rate, respiration, and arousal. It is developed at birth and is necessary to keep us alive (Ganzel & Morris, 2016).

Similarly, the cerebellum is a brain region located near the back of the brain; it is chiefly responsible for balance and coordinated motor activity. It develops rapidly during the first year of life. Interestingly, the cerebellum undergoes a second round of maturation during early adolescence. Researchers believe the cerebellum plays a role in mental gracefulness and efficiency in addition to adroitness in physical movement. Maturation of the cerebellum during adolescence might explain the increased physical gracefulness exhibited by older adolescents as well as a general increase in mental efficiency across development.

Just above the brainstem, in the center of the brain, are two important regions that also mature relatively early. The basal ganglia are located between the brainstem and the higher-level cortical regions. The basal ganglia perform many important functions. One of their primary roles is to help control movement. Another function is to filter incoming information from the senses and relay this information to other brain regions where it can be processed. The basal ganglia have also been implicated in the regulation of attention and emotions. Researchers believe that structural changes in the basal ganglia during childhood and adolescence might account for children’s increased motor functioning, attention, and emotional processing during the school-age years.

Finally, the limbic system is located deep inside the brain, behind the cortex. Two important components of the limbic system are the amygdala and hippocampus. The amygdala aids in our understanding and expression of emotions, especially negative feelings, such as fear and rage. The hippocampus also plays a role in emotional processing, especially the formation of emotion-laden memories (Image 2.6).

4. Higher-order regions may not mature until adulthood.

The cerebral cortex is the outermost shell of the brain. It is divided into four lobes. The occipital lobe, located near the back of the brain, is primarily responsible for visual processing. This brain region appears to undergo the most change from birth through age 2 years. In contrast, the volume of the parietal lobe (located on the sides and top of the brain) peaks around age 6. The parietal lobe is primarily responsible for integrating visual, auditory, and tactile information. The temporal lobe (located on the sides and bottom of the brain) also shows peak growth during the first 6 years of life. The temporal lobe has multiple functions, including hearing, language, and the expression and regulation of emotions.

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The volume of the frontal lobe peaks in late childhood or early adolescence. The frontal cortex plays an important role in language production, problem-solving, and memory—skills that develop rapidly during childhood. A particular region of the frontal lobe, the prefrontal cortex, shows peak growth in early adolescence and reorganization into early adulthood. This brain region is responsible for planning, organizing, and prioritizing activity to meet long-term goals. Development of the prefrontal cortex is believed to underlie young adults’ increased capacity for attention, inhibition, and overall self-regulation (de Haan & Johnson, 2016).

5. Experience can affect the brain.

Although it may seem that brain maturation determines development, the relationship between maturation and behavior is bidirectional. The brain can change in response to experience. Biological maturation and environmental experiences interact in three ways to shape the developing brain (Cicchetti, 2019).

First, certain aspects of brain development are gene driven. These aspects are largely impervious to the effects of experience and almost entirely determined by genetics. For example, the development of the brain stem and migration of neurons from the center of the brain to the cortex is believed to be genetically preprogrammed. Developmental psychologists sometimes refer to this importance of genes over experience in embryonic development as canalization (Blair, Raver, & Finegood, 2016).

Second, some aspects of brain development are experience expectant—that is, the formation of the brain region is partially dependent on information received from the environment. Infants have an overabundance of neural connections, many of which they do not need. Connections that are used are maintained and strengthened while connections that are not used atrophy and die. Whether a connection is maintained or pruned depends on experience. For example, an infant exposed to the Japanese language during the first few years of life may strengthen neural connections responsible for processing the sounds used in Japanese. However, infants not exposed to Japanese during this early period of development may lose neural connections that play a role in processing this language. Consequently, children who are not exposed to Japanese in infancy or early childhood may find it difficult to speak the language without an accent. Developmental psychologists often refer to periods of development in which experience can greatly shape neural structure and functioning as developmentally sensitive periods.

Third, brain development can be experience dependent—that is, environmental experiences in later life can lead to the formation of new neural connections or to changes in the brain’s organization or structure. Neural plasticity refers to the brain’s malleability, that is, its capacity to change its structure and/or functioning in response to environmental experiences. These experiences can be either internal or external. Internal experiences alter the immediate environment of the brain and nervous system. For example, exposure to too much testosterone or stress hormone can lead to structural changes in various brain regions. In contrast, external experiences come from outside the organism. For example, an infant exposed to environmental toxins can experience brain damage (Cicchetti, 2015).

Neuroscientists have discovered that the brain is remarkably adaptive to environmental stressors, especially when these stressors occur early in life. Perhaps the most striking example of brain plasticity is seen following a surgical procedure called a functional hemispherectomy. This surgery is performed on some children who have medically intractable epilepsy that arises in one hemisphere of the brain. These seizures cause severe impairment, occur very frequently, and are not responsive to medication. The surgeon removes the entire parietal lobe of the nonfunctional hemisphere (the origin of the seizures) and severs the corpus callosum, a bundle of neurons that allow the seizure to travel from one hemisphere to the other.

Despite removal or disconnection of several brain regions, children usually show remarkable recovery from the surgery. Children often experience weakness or mild paralysis on the opposite side of the body. Furthermore, if the left hemisphere is removed, most children experience problems with language. However, children usually recover much of this lost functioning within 6 to 12 months after surgery, as the remaining hemisphere gradually assumes many of these lost functions. In fact, most children who undergo this surgery are able to return to school 6 to 8 weeks later (van Schooneveld, Braun, van Rijen, van Nieuwenhuizen, & Jennekens-Schinkel, 2016).

Positive environmental experiences can also lead to the formation of new neural connections. Long ago, the neuropsychologist Donald Hebb (1949) proposed that the simultaneous activation of neurons can cause the neurons to form new connections. Hebb suggested “neurons that fire together, wire together.” Recently, neuroscientists have been able to show synaptogenesis, that is, the formation of new neural connections due to experience. For example, rats reared in enriched living environments (e.g., given extra space and access to toys and mazes) show differences in brain structure and functioning compared to rats reared in typical cages. Humans who receive extensive training in Braille show growth in brain regions responsible for processing the sense of touch. Even skilled musicians show a reorganization of brain regions responsible for controlling the finger positions of their instruments (Cicchetti, 2019).

Review

 The brain consists of 100 billion neurons that form trillions of synaptic connections. Neurons relay information within themselves electrically; they communicate between one another using chemical messengers called neurotransmitters.

 Brain development is characterized by rapid growth followed by periods of neuronal pruning. Development begins in evolutionarily older brain regions (e.g., brainstem, basal ganglia, limbic system) and ends in regions responsible for higher-order functions (e.g., the cerebral cortex).

 Development can be gene driven, experience expectant, or experience dependent. Environmental experiences can lead to synaptogenesis and the reorganization of neuronal connections (i.e., plasticity).