Interesting questions are whether neurons begin to function only when their axons have acquired a myelin sheath; whether myelination is under the control of the cell body, the axon, or both; and whether the usual myelin stains yield sufficient information as to the time of onset and degree of the myelination process. At best these correlations can be only approximate. It seems likely that systems of neurons begin to function before the first appearance of myelin, at least insofar as shown in conventional myelin stains. These correlations will undoubtedly be restudied using more delicate measures of function and finer staining techniques, as well as the techniques of quantitative biochemistry, phase and electron microscopy, and the emerging techniques of connectivity and network imaging.
Neural Development in the Fetus
The human fetus is capable of a complex series of reflex activities, some of which appear as early as 5 weeks of postconceptional age. Cutaneous and proprioceptive stimuli evoke slow, generalized, patterned movements of the head, trunk, and extremities. More discrete movements appear to differentiate from these generalized activities. Reflexes subserving blinking, sucking, grasping, and visceral functions, as well as tendon and plantar reflexes, are all elicitable in late fetal life. They seem to develop along with the myelination of peripheral nerves, spinal roots, spinal cord, and brainstem. By the 24th week of gestation, the neural apparatus is functioning sufficiently well to give the fetus some chance of survival should birth occur at this time. However, most infants fail to survive birth at this age, usually owing to an inadequacy of pulmonary function. Thereafter, the basic neural equipment matures so rapidly that, by the 30th week, postnatal viability is relatively common. It seems that nature prepares the fetus for the contingency of premature birth by hastening the establishment of vital functions necessary for extrauterine existence.
It is in the last trimester of pregnancy that a complete timetable of fetal movements, posture, and reflexes would be of the greatest value, for mainly during this period does the need for a full clinical evaluation arise. That there are recognizable differences between infants born in the sixth, seventh, eighth, and ninth months of fetal life has been documented by Saint-Anne Dargassies, who applied the neurologic tests earlier devised by André-Thomas and herself. Her observations document prevailing postures; control and attitude of head, neck, and limbs; muscle tonus; and grasp and sucking reflexes. These findings are of interest and may well be a means of determining exact age, but many more observations are needed with follow-up data on later development before they can be fully accepted. Part of the difficulty here is the variability of the premature infant’s neurologic functions, which may change literally from hour to hour. Even at term there may be variability in neurologic functions from one day to the next. This variability reflects in part the effects of parturition and the effects of medications given to the mother, as well as the inaccurate dating of conception and rapid developmental changes in the brain.
Development During the Neonatal Period, Infancy, and Early Childhood
At term, effective sucking, rooting, and grasping reactions are present. The infant is able to swallow and cry, and the startle reaction (e.g., Moro reflex, as described in the following text) can be evoked by loud sound and sudden extension of the neck. Support and steppage movements can be demonstrated by placing the infant on its feet, and incurvation of the trunk by stroking one side of the back. Also present at birth is the placing reaction, wherein the foot or hand, brought passively into contact with the edge of a table, is lifted automatically and placed on the flat surface. These neonatal automatisms depend essentially on the functioning of the spinal cord, brainstem, and possibly diencephalon and pallidum. The Apgar score, a universally used but somewhat imprecise index of the well-being of the newly born infant, is in reality a numerical rating of the adequacy of brainstem-spinal mechanisms (breathing, pulse, color of skin, tone, and responsivity) (Table 27-3).