7.2: Brain Development

Figure \(\PageIndex{1}\): This is generally the diagram of neurons and synapses that is shown. In actual fact, the main neuron pictured here is a motor neuron and neuron shapes and sizes differ greatly across the nervous system, but we use this diagram because the different parts of neurons are most clearly seen and differentiated here.

In the third week of gestation, a section of embryonic cells become differentiated into the neural plate, soon tipping at the edges to form a neural groove, and eventually completing the tipping to form a neural tube. The neural tube is filled with fluid and neurogenesis occurs (at the rate of 250,000 per minute at its fastest rate) in the center of the neural tube. These neurons migrate along nervous system supportive cells called radial glia to the genetically programmed area where they will form structures by adhering to other neurons. The cells also begin to form both dendrites and axons prenatally, such that different structures connect to each other. Dendrite formation continues throughout the lifespan lending to brain plasticity - the changeability of the brain even in adulthood.

While we end up with a total of about 86 billion neurons in average adults, each neuron makes connections with thousands of other neurons such that by some counts there are 100,000 trillion (!!) synapses. Clearly each of these connections cannot be genetically preprogrammed but are affected by a number of genetic, environmental (epigenetic) and experiential conditions that the fetus and child are exposed to.

Axons grow before birth for the most part. Dendrites and dendritic branches continue to branch and deteriorate throughout life. Nowhere else is the adage “use it or lose it” more true than in brain development. If neurons do not connect to other neurons, they die. These synapses and synaptic changes are genetically influenced but the prenatal environment clearly also plays an important part in this process. A number of factors ranging from psychoactive drugs, diet, parental and peer relationships and intestinal flora influence the plasticity of the nervous system throughout life. However, this plasticity is also dependent on the age of the fetus and the child.

Figure \(\PageIndex{2}\): The brain continues developing into early adulthood- seen here is the outer wrinkled surface of the brain with the four lobes (frontal, temporal, parietal and occipital), and lateral fissure and main sulci marked.[1]

The prefrontal cortex that is located behind our forehead continues to grow and mature throughout childhood and experiences an additional growth spurt during adolescence. It is the last part of the brain to mature and will eventually comprise 85 percent of the brain’s weight. Experience will shape which of these connections are maintained and which of these are lost. Ultimately, about 40 percent of these connections will be lost (Webb, Monk, and Nelson, 2001). As the prefrontal cortex matures, the child is increasingly able to regulate or control emotions, to plan activity, strategize, and have better judgment. Of course, this is not fully accomplished in infancy and toddlerhood, but continues throughout childhood and adolescence. This changeability of the brain is called plasticity.

Figure \(\PageIndex{3}\): MRI scans of the human brain at different ages - 1 week, 3 months, 1 year, 2 years, and 10 years..[3]

Brain development in Early Childhood

Brain weight

The brain is about 75 percent its adult weight by two years of age. By age 6, it is approximately 95 percent its adult weight. Myelination and the development of dendrites continues to occur in the cortex and as it does, we see a corresponding change in the child’s abilities. Significant development in the prefrontal cortex (the area of the brain behind the forehead that helps us to think, strategize, and control emotion) makes it increasingly possible to control emotional outbursts and to understand how to play games. Consider 4- or 5-year-old children and how they might approach a game of soccer. Chances are, every move would be a response to the commands of a coach standing nearby calling out, “Run this way! Now, stop. Look at the ball. Kick the ball!” And when the child is not being told what to do, he or she is likely to be looking at the clover on the ground or a dog on the other side of the fence! Understanding the game, thinking ahead, coordinating movement, and handling losing improve with practice and myelination.[4]

Growth in the Hemispheres and Corpus Callosum

Between ages 3 and 6, the left hemisphere of the brain grows dramatically. This side of the brain or hemisphere is typically involved in language skills. The right hemisphere continues to grow throughout early childhood and is involved in tasks that require spatial skills such as recognizing shapes and patterns. The corpus callosum which connects the two hemispheres of the brain undergoes a growth spurt between ages 3 and 6 and results in improved coordination between right and left hemisphere tasks.

Brain development in Middle childhood

The brain reaches its adult size at about age 7. Two major brain growth spurts occur during middle/late childhood (Spreen et al, 1995). Between ages 6 and 8, significant improvements in fine motor skills and eye-hand coordination are noted. Then between 10 and 12 years of age, the frontal lobes become more developed and improvements in logic, planning, and memory are evident (van der Molen & Molenaar, 1994). Children in middle to late childhood are also better able to plan, coordinate activity using both left and right hemispheres of the brain, and to control emotional outbursts. Paying attention is also improved as the prefrontal cortex matures (Markant & Thomas, 2013) [5]

Myelin also continues to develop and the child's reaction time improves as well. Myelination improvement is one factor responsible for these growths. From age 6 to 12, there is rapid myelination all over the brain in areas that are responsible for coordinating different sensory and motor activities. This myelination contributes to increases in information processing speed and the child’s reaction time. The hippocampus, which is responsible for transferring information from the short-term to long-term memory, also shows increases in myelination resulting in improvements in memory functioning (Rolls, 2000).

Changes in the brain during this age enable not only physical development, but also allow children to understand what others think of them and dealing socially with the positive and negative consequences of that. Within this development period, children may struggle with mental health disorders or other health problems. As children are growing and becoming more capable, adults need to remember that children don’t grow physically in isolation. The development of their bodies isn't separate from the changes that are occurring socially, emotionally, and cognitively. Awareness and understanding of their other developmental domains and needs will support the child during these changes.[6]

Adolescence brain growth

Brain growth continues into the early 20s. The development of the frontal lobe, in particular, is important during this stage. Adolescents often engage in increased risk-taking behaviors and experience heightened emotions during puberty; this may be due to the fact that the frontal lobes of their brains—which are responsible for judgment, impulse control, and planning—are still maturing until early adulthood (Casey, Tottenham, Liston, & Durston, 2005).

The brain undergoes dramatic changes during adolescence. Although it does not get larger, it matures by becoming more interconnected and specialized (Giedd, 2015). The myelination and development of connections between neurons continues. This results in an increase in the white matter of the brain, and allows the adolescent to make significant improvements in their thinking and processing skills. Different brain areas become myelinated at different times. For example, the brain’s language areas undergo myelination during the first 13 years. Completed insulation of the axons consolidates these language skills but makes it more difficult to learn a second language. However myelination is itself also an experience-dependent plastic process (Monje, 2018).

[

Figure \(\PageIndex{4}\): During adolescence the brain becomes more interconnected and specialized. Different areas like Broca’s area, auditory cortex, and particularly the prefrontal area grow more complex.[8]

The limbic system, which regulates emotion and reward, is linked to the hormonal changes that occur at puberty. The limbic system is also related to novelty seeking and a shift toward interacting with peers. In contrast, the prefrontal cortex, which is involved in the control of impulses, organization, planning, and making good decisions, does not fully develop until the mid-20s. According to Giedd (2015) the significant aspect of the later developing prefrontal cortex and early development of the limbic system is the “mismatch” in timing between the two. The approximately ten years that separates the development of these two brain areas can result in risky behavior, poor decision-making, and weak emotional control for the adolescent. When puberty begins earlier, this mismatch extends even further.

Teens often take more risks than adults and, according to research, it is because they weigh risks and rewards differently than adults do (Dobbs, 2012). For adolescents, the brain’s sensitivity to the neurotransmitter dopamine peaks, and dopamine is involved in reward circuits so the possible rewards outweigh the risks. Adolescents respond especially strongly to social rewards during activities, and they prefer the company of others their same age. In addition to dopamine, the adolescent brain is affected by oxytocin, which facilitates bonding and makes social connections more rewarding. With both dopamine and oxytocin engaged, it is no wonder that adolescents seek peers and excitement in their lives that could end up actually harming them.

Because of all the changes that occur in the adolescent brain, the chances for abnormal development can occur, including mental illness. In fact, 50% of mental illness occurs by the age 14 and 75% occurs by age 24 (Giedd, 2015). Additionally, during this period of development the adolescent brain is especially vulnerable to damage from drug exposure. For example, repeated exposure to marijuana can affect cellular activity in the endocannabinoid system. Consequently, adolescents are more sensitive to the effects of repeated marijuana exposure (Weir, 2015).

=

Figure \(\PageIndex{5}\): Adolescents prefer the company of others their same age.[10]

The physical growth and the changes of puberty mark the onset of adolescence (Lerner & Steinberg, 2009). For both boys and girls, these changes include a growth spurt in height, growth of pubic and underarm hair, and skin changes (e.g., pimples). Hormones drive these pubescent changes, particularly the increase in testosterone for boys and estrogen for girls.[11]

References:

Markant, C., & Thomas, K. M. (2013). Postnatal brain development. In P. D. Zelazo (Ed.), Oxford handbook of developmental psychology. New York: Oxford University Press.

Monje, M. (2018). Myelin plasticity and and nervous system function. Annu Rev Neurosci., 41, 61-76. doi: 10.1146/annurev-neuro-080317-061853.

Spreen, , Rissser, A., & Edgell, D. (1995). Developmental neuropsychology. New York: Oxford University Press. Sternberg, R. J. (1985). Beyond IQ: A triarchic theory of human intelligence. New York, NY: Cambridge University Press.

van der Molen, , & Molenaar, P. (1994). Cognitive psychophysiology: A window to cognitive development and brain maturation. In G. Dawson & K. Fischer (Eds.), Human behavior and the developing brain. New York: Guilford.

Attributions:

Child Growth and Development by Jennifer Paris, Antoinette Ricardo, and Dawn Rymond, 2019, is licensed under CC BY 4.0

[1] Image by Sebastian023 is licensed under CC BY-SA 3.0

[2] Lifespan Development - Module 4: Infancy by Lumen Learning references Psyc 200 Lifespan Psychology by Laura Overstreet, licensed under CC BY 4.0

[3] Image is in the public domain

[4] Lifespan Development - Module 5: Early Childhood by Lumen Learning references Psyc 200 Lifespan Psychology by Laura Overstreet, licensed under CC BY 4.0

[5] Parenting and Family Diversity Issues by Diana Lang, 2020, published by Iowa State University is licensed under CC BY-NC-SA 4.0

[6] Children’s Development by Ana R. Leon is licensed under CC BY 4.0 (modified by Dawn Rymond); Lifespan Development: A Psychological Perspective by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0 (modified by Dawn Rymond)

[7] Lifespan Development: A Psychological Perspective by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[8] Image by BruceBlaus is licensed under CC BY 3.0

[9] An Introduction to Nutrition- Nutrition through the Life Cycle: From Pregnancy to the Toddler Years by Maureen Zimmerman and Beth Snow is licensed under CC BY-NC-SA 3.0

[10] Image by Bailey Torres on Unsplash

[11] Adolescent Development by Jennifer Lansford is licensed under CC BY-NC-SA 4.0