Even when children receive the full schedule of vaccinations, many do not receive them on the timetable recommended by the National Vaccine Advisory Committee. Vaccine timeliness is important because the efficacy of early and late vaccination is not always known and may vary by disease. When a child receives a vaccination may be just as important as whether the child receives it in promoting disease resistance.
What Do You Think?
1 What do you think about the use of vaccines?
2 In your view, what is the most important reason in favor of vaccinations?
3 Why do you think some parents object to their use? How might you respond to their objections?
We have seen that growth proceeds rapidly over the first 2 years of life and follows consistent patterns. Body growth is largely maturationally driven but also influenced by contextual factors. Whereas breastfeeding and introducing healthy solid foods can aid infants’ growth, malnutrition and a poor diet can hinder infants’ development with effects that can persist throughout childhood and even into adulthood.
Thinking in Context 4.1
1 Why are marasmus and kwashiorkor uncommon in the United States? What contextual factors place children in developing nations at risk for these impairments?
2 How might you increase parents’ knowledge about health topics such as breastfeeding or nutrition? If you were assigned to design such a program, what would you need to know to begin your design? How would you go about gathering information and ideas (see Chapter 1 for information about research design)?
Brain Development During Infancy and Toddlerhood
All of the developments in infants’ physical and mental capacities are influenced by the dramatic changes that occur in the brain. At birth, the brain is about 25% of its adult weight, and it grows rapidly throughout infancy, reaching about 70% of its adult weight by 2 years of age (Lyall et al., 2015). As the brain develops, it becomes larger and more complex.
Processes of Neural Development
The brain is made up of billions of cells called neurons. Neurons are specialized to communicate with one another to make it possible for people to sense the world, think, move their body, and carry out their lives. Brain development begins well before birth. Neurogenesis, the creation of new neurons, begins in the embryo’s neural tube. 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, 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 tends to outnumber 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 4.4, 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, & Tesky, 2016).
At birth, the neural networks of axons and dendrites 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. As the dendrites grow and branch out, neurons form synapses and thereby increase connections with others, 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).
Figure 4.4 Glial Cell-Neuron Relationship
Neurons migrate along thin strands of glial cells.
Source: Gasser and Hatten (1990).
Figure 4.5 The Human Brain
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 (Monk, Webb, & Nelson, 2001). By age 3, children have more synapses than at any other point in life, with at least 50% more synapses than in the adult brain. 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). 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 contributes to advances in neural communication because axons coated with myelin transmit neural impulses more quickly than unmyelinated axons (Markant & Thomas, 2013). With increases in myelination, infants and children process information more quickly. Their thought and behavior becomes 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).
The Cerebral Cortex
The brain comprises different structures with differing functions, located across four lobes. The various parts of the brain work together, but as shown in Figure 4.5, each lobe is specialized to a certain extent. In addition, the brain comprises two hemispheres that are joined by a thick band of neural fibers known as the corpus collosum. Although all four lobes appear on both hemispheres, the hemispheres are not identical. The right and left hemispheres are specialized for different functions, known as lateralization. For most people, language is governed by the left hemisphere. Each hemisphere of the brain (and the parts of the brain that comprise each hemisphere) is specialized for particular functions and becomes more specialized with experience. This process of the hemispheres becoming specialized to carry out different functions is called lateralization (Duboc, Dufourcq,