Mental Health: A Report by the Surgeon General

Physical development of the nervous system provides the architecture for mental function (cognition, mood, and intentional behavior). As can be inferred from the discussion of brain complexity in the introductory section, nervous system development is arguably one of the most monumentally complicated developmental achievements. One hundred billion neurons must form elaborate and precise arrays of interconnections. Neurons begin the developmental process as undifferentiated cells, cells so seemingly anonymous that they are almost indistinguishable from other cells in an embryo. On the basis of genetic and epigenetic10 influences, the cells must first specialize, or differentiate, into neurons, migrate to their final position, and then send their growing axons (the branch of a neuron that transmits impulses) to project over long distances in order to form synapses with distant target cells (Kandel et al., 1995).

Most neurobiologists are astounded at the level of precision that neurons achieve in their interconnections. The process of nervous system development has been studied at increasingly complex levels—molecular, cellular, tissue, and behavioral levels. Yet, while researchers have charted many of the behavioral milestones of development because they are so amenable to observation and analysis, far less is known about molecular, cellular, and tissue interactions that underlie them.

Four overarching findings or organizing principles have been gleaned from decades of neuroscience research. The first finding is that the formation of connections between neurons and their target cells depends on axons growing along anatomical pathways that are studded with signaling molecules, much like landing lights illuminate the runway for a descending plane. The second finding is that an axon’s reaching the vicinity of, and locating, its correct target cell depends on diffusable chemical signals being transmitted from the target cell. The third finding is that if an axon does not reach its correct target, it is likely to die. This phenomenon, known as cell death, or apoptosis, is so common that it affects up to half of all developing neurons. The brain overproduces the number of cells it needs, from which it pares down to only the correct connections (Kandel et al., 1995). Finally, neuron activity is essential to strengthening the connections that are formed. In other words, stimulation from the environment—which is translated into neuron activity—is vital for the forging of normal neural development (Shatz, 1993; Kandel, 1995). This is a fundamental principle that is revisited later in this section. This principle helps to explain why, for example, babies who are deprived of a stimulating environment during their first year sometimes suffer irreparable developmental effects.

Behavior at birth consists of a repertoire of simple reflexes, that is, inborn neurological reactions that are involuntary in nature. Two examples are the sucking reflex and the rooting reflex,11 both of which are designed to ensure food intake. Over time, the infant displays an expanded repertoire of fine and gross motor skills (e.g., crawling, walking) that begin to unfold in the first few months and year of life. These include the cherished ability to smile, which helps to solidify a social bond with parents and caregivers. What begins as a child’s biological survival need for food—evidenced by such behaviors as rooting and sucking—can turn into a social, interpersonal experience with the caregiver, as in the smile of an infant at the sight of a nurturing parent. These burgeoning motor capabilities are the forerunners of more complex behavioral and mental functions, but the actual relationships between early and later abilities, and their molecular and cellular basis, are understood only in the most rudimentary terms.

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