Nervous System Development

The human brain is an extraordinarily complex organ in terms of structure as well as function. It is one of the earliest systems to begin development in the fetus (just 2 1/2 weeks after fertilization) and yet the last to completely mature, well into childhood. From the midpoint of gestation until birth, the volume of the brain increases almost 17 times, with brain cells being produced at astonishing rates. As the fetal brain develops, however, it doesn't just uniformly expand; it goes through a carefully orchestrated sequence of developmental steps that involve dramatic changes in organization as well as size. As Susan Tucker Blackburn explains, in Maternal, Fetal, & Neonatal Physiology, "The nervous system does not 'turn on' at birth; rather nervous system activity has increased steadily during pregnancy."

There are many different types of brain cells, each with specific functions. Neurons are the brain cells that transmit information via electrochemical signals. In simplified terms, most neurons have a cell body, called the soma, and a long nerve fiber, called the axon, that carries signals to other nerve cells. Neurons are supported by another large class of brain cells called glial cells. The two types of cells also work together to provide the axons with a sheath (myelin) that acts as insulation for the nerve fiber.

The axons connect to other brain cells and can be very long (the longest axons in the body are those that stretch from the spinal column all the way to the big toe). Within the brain, the area where neuron cell bodies are congregated is referred to as gray matter. The area where the nerve fibers, the axons, are located, is referred to as white matter.

By the end of the second trimester, certain white matter tracts have already formed, though many are still rapidly growing at this time. A new magnetic resonance imaging (MRI) technique called diffusion tensor imaging (DTI) allows researchers to map subtle developmental changes in the fetal brain, including the growth of white matter. This enables researchers to track normal development processes as well as spot abnormalities caused by genetic problems or biochemical injuries. White matter, for example, is particularly sensitive to damage in fetal alcohol syndrome.

Measuring Neurodevelopment

Beyond making maps of the changing structure of fetal and infant brains, how do researchers evaluate the function and performance of developing brains?

Measuring infant vision is one way to assess neurodevelopment because the eye is a window into brain development. Infant vision can be measured directly with special electrophysiological instruments. One test is the electroretinogram (ERG) which is used to measure the response of the retina in infants younger than 6 months. Another is the visual evoked potential (VEP), which measures the combination of the response of the retina and the transmission of electrical impulses to the infant's visual cortex as the infant responds to a series of changing black and white patterns.

Measuring cognition in infants is arguably more complex. A number of tests have been developed to evaluate an infant's cognitive development, but they are necessarily subjective and often only suggestive. Most involve nonverbal tests and techniques that require researchers to make inferences from infant behavior such as head and eye movements. Some assess motor and neural development by testing a child's grasping and other coordination skills; others evaluate an infant's interest in new objects or ability to locate hidden objects. The most complex of the tests, used with toddlers, measure the child's problem-solving and verbal abilities.

"It's a fascinating field," explains psychologist John Colombo of the University of Kansas, "but there's no single accepted measure for cognitive development in infants and toddlers. For that age range, we depend on the responses available. From birth to nine months, that means we measure 'looking' plus factors such as heart rate and respiration. When the baby is able to sit up, at eight or nine months, we can entice the baby to respond further. And at 12 to 24 months, we can begin to use language and start to measure simple memory, more complicated memory, and the ability to use symbols."

Attention is one of the most basic cognitive functions. Infants will pay attention to stimuli in their environment until they lose interest or are distracted by other stimuli. Researchers try to determine how easily a baby's attention is held and how easily the baby is distracted. How long a baby looks at something, however, usually depends on age. Focusing on an object might mean a lack of response or curiosity in a younger baby, but in an older baby could mean the ability to concentrate.

Responding to novelty is another measure of a baby's mental development. Babies eventually pay less attention to objects that become familiar. Researchers evaluate how often and how much longer infants look at new objects than at familiar objects as an indication of their developing sense of visual memory. As infants mature, they develop problem-solving skills such as being able to recognize that a toy has been placed behind a box or under a cloth.

In short, while scientists notice the same developmental steps that parents do, researchers try to fit their observations into patterns of age-appropriate development. And in the process, they try to isolate and measure the impact of specific factors, including nutrition. Fatty acids such as DHA are certainly critical nutrients, and because many researchers believe DHA is so important and recognize that levels vary, many international groups have recommended that formulas for both term and preterm babies be supplemented with DHA. There are, however, many other nutritional components that also affect cognitive development, including choline, iron, zinc, and others.

Sleep Patterns

Sensory systems and sleep patterns also offer glimpses into an infant's neurodevelopment. We may not be able to easily apply yardsticks to a baby's developing sensory capacity, but we do know that the infant is contending with a rapidly changing environment. A newborn goes from an environment that is dark to one that can be blindingly light; from one of constant warmth to one of variable coolness; from one of muted, rhythmic sounds to one that can shift from silence to overwhelming din; from one where the tactile experience is wet and smooth to one characterized by dryness, handling, rubbing, and sometimes pain.

"Sleep-wake patterns are an excellent window to the infant's neurologic status," as well, writes Blackburn. "Sleep is required for brain development." And for an infant, explains Blackburn, sleep is not merely rest. "Infant development entails increasing amounts of quiet sleep as well as increasing periods of quiet alertness. Both of these states reflect sophisticated neural control. Sustaining a state consistently or making a transition from one state to another requires tremendous neural organization."

Though it might seem like small comfort to bleary-eyed parents in the middle of the night, the changing sleep-wake patterns of their infants represent baby brains hard at work.

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