Nervous System

The nervous system undergoes rapid maturation and growth during the infancy period. In con­trast to other systems that grow rapidly after birth, the ner­vous system grows proportionately more rapidly before birth. The most rapid period of fetal brain growth is be­tween 15 and 20 weeks of gestation, at which time there is a significant increase in neurons. A second increase occurs between 30 weeks' gestation and 1 year of age. At birth, the average brain weighs approximately 325 g. By 1 year of age, the weight has tripled, and the brain weighs approxi­mately 1000 g.⁷ Head circumference, one of the best indi-

cators of brain growth, increases six times as much during the first year as it does during the second year of life.

At birth, the nervous system is incompletely inte­grated but sufficiently developed to sustain extrauterine life. Most of the neurologic reflexes are primitive reflexes. Normal newborn reflexes, which include the Moro (star­tle), rooting (sucking), and stepping (placing) reflexes, can be used to evaluate the newborn and infant's developing central nervous system.

The maturation of the nervous system includes an in­crease in the size of neurons, size and number of glial cells, and number of interneuron connections and branching of axons and dendrites. As this maturation progresses, the level of infant functioning increases from simple to com­plex and from primitive reflexes to purposeful movement. Cortical control of motor functions is closely associated with myelination of nerve fibers. Myelination of the vari­ous nerve tracts progresses rapidly after birth and follows a cephalocaudal and proximodistal direction sequence, beginning with myelination of the spinal cord and cranial nerves and followed by the brain stem and corticospinal tracts.⁷ In general, sensory pathways become myelinated before motor pathways. The acquisition of fine and gross motor skills depends on this myelination and maturation.

The first year of life also is filled with psychosocial de­velopmental milestones for the infant. Basic needs must be met before the infant can accomplish these develop­mental tasks. Erikson described the development of a sense of trust as the task of the first stage.¹⁷ If trust is not acquired, the infant becomes mistrustful of others and frustrated with his or her inability to control the surround­ing environment.