[Functional development of the brain in the fetus and the infant]

Neuronal multiplication occurs mainly from the 10th to the 20th gestational weeks, after which probably no new nerve cells are formed, though neuronal arborisation and the formation and re-organisation of synapses continues until adulthood. An intriguing question is how the blueprint for the formation of about 100 billion nerve cells and their dendrites and synapses can be contained in the human genome. Environmental factors are probably crucially involved in the development of the brain, particularly from the later stages of gestation onwards. Somatosensory functions are developed at an early stage. The pain threshold is assumed to be lower in the fetus than in the adult. In man, general fetal movements appear from the 8th gestational week, and more complex movements such as sucking, swallowing and breathing during the 10th-12th gestational weeks. These movements are generated by neuronal networks, and seem to occur spontaneously without any sensory stimulation. At birth there is a general excitation of the infant, and the neonate is awake and aroused. Studies in rats have shown a multifold increase in noradrenaline turnover to occur in the brain stem at birth, which is assumed to be related to the arousal of the newborn. Recent studies have shown that there is a switch-on of various excitatory neuropeptide genes at birth, and an increase in noradrenaline in the locus coeruleus which is assumed to be the arousal centre.