Читать книгу Infants and Children in Context - Tara L. Kuther - Страница 213

The first neurons form early in prenatal development, in the embryo’s neural tube, through a process called neurogenesis. We are born with more than 100 billion neurons, more than we will ever need—and more than we will ever have at any other time in our lives. Some of our neurons die, but neurogenesis continues throughout life and new neurons are formed, although at a much slower pace than during prenatal development (Stiles et al., 2015). As the brain develops, new neurons migrate along a network of glial cells, a second type of brain cell that outnumbers neurons (Gibb & Kovalchuk, 2018). Glial cells nourish neurons and move throughout the brain to provide a physical structure to the brain. As shown in Figure 4.7, neurons travel along glial cells to the location of the brain where they will function, often the outer layer of the brain, known as the cortex, and glial cells instruct neurons to form connections with other neurons (Kolb, Whishaw, & Teskey, 2016).

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Figure 4.7 Glial Cell–Neuron Relationship

Neurons migrate along thin strands of glial cells

Source: Gasser and Hatten (1990).

At birth, the networks of neurons are simple, with few connections, or synapses, between neurons (Kolb et al., 2016). Early in infancy, major growth takes place. Neurons and glial cells enlarge. The dendrites grow and branch out, increasing synapses with other neurons, a process called synaptogenesis. Synaptogenesis peaks in different brain regions at different ages (Remer et al., 2017). The most active areas of synaptogenesis during the first 5 weeks of life are in the sensorimotor cortex and subcortical parts of the brain, which are responsible for respiration and other essential survival processes. The visual cortex develops very rapidly between 3 and 4 months and reaches peak density by 12 months of age. The prefrontal cortex—responsible for planning and higher thinking—develops more slowly and is not complete until early adulthood (Tamnes et al., 2017).

In response to exposure to stimulation from the outside world, the number of synapses initially rises meteorically in the first year of life, and the dendrites increase 500% by age 2 (Schuldiner & Yaron, 2015). Cortical thickness peaks by 2 years of age, but surface area continues to develop throughout childhood (Gilmore, Knickmeyer Santelli, & Gao, 2018). Toddlers have more synapses than at any other point in life (see Figure 4.8). This explosion in connections in the early years of life means that the brain makes more connections than it needs, in preparation to receive any and all conceivable kinds of stimulation (Schuldiner & Yaron, 2015). Those connections that are used become stronger and more efficient, while those unused eventually shrink, atrophy, and disappear. This loss of unused neural connections is a process called synaptic pruning, which can improve the efficiency of neural communication by removing “clutter”—excess unused connections. Little-used synapses are pruned in response to experience, an important part of neurological development that leads to more efficient thought (Lyall et al., 2015).

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Figure 4.8 Synaptogenesis From Birth to Age 2

Source: Gilmore et al. (2018).

Another important process of brain development is myelination, in which glial cells produce and coat the axons of neurons with a fatty substance called myelin. Myelination begins prenatally but accelerates after birth (Gilmore et al., 2018). Myelination contributes to advances in neural communication because axons coated with myelin transmit neural impulses more quickly than unmyelinated axons (Lebel & Deoni, 2018). With increases in myelination, infants and children process information more quickly. Their thought and behaviors become faster more coordinated, and complex (Chevalier et al., 2015). Myelination proceeds most rapidly from birth to age 4, first in the sensory and motor cortex in infancy, and continues through childhood into adolescence and early adulthood (Qiu, Mori, & Miller, 2015).