The maturation and function of the corticospinal and corticobulbar tracts in the human newborn are reviewed from the Departments of Paediatrics, Pathology and Clinical Neurosciences, University of Calgary Faculty of Medicine, Calgary, Alberta, Canada. The myelination of these tracts begins in late gestation but is not complete until two years of age. Functions attributed to these descending pathways in the full term human newborn include the following: 1) Development of passive muscle tone and resting postures; 2) Enhancement of suck and swallow reflexes; 3) Relay of cortical epileptic discharges; and 4) Inhibition of complex stereotyped motor reflexes (“subtle seizures”). The antagonistic balance between flexors and extensors are controlled by the subcortical spinal and corticospinal pathways. If the corticospinal tract is impaired as with perinatal asphyxia the infant assumes distal flexion and proximal extension postures which reflect the influence of subcorticospinal pathways when corticospinal tract antagonism is lacking. The “tonic seizures” of preterm infants with intraventricular hemorrhages are probably not true epileptic phenomena but rather episodes of decerebration. A weak suck, poor feeding, and impaired coordination of swallowing are findings common to cerebral cortical disturbances in the newborn caused by hypoxia, meningitis, hemorrhage or hydrocephalus. Most human neonatal seizures are focal or multifocal and arise in the cerebral cortex and are relayed by the corticospinal tract to spinal motor neurons and to facial and hypoglossal nuclei by the corticobulbar tract. Some neonatal seizures are subcortical in origin. The corticospinal tract probably does not influence muscle maturation because. muscle shows histochemical differentiation at 20-28 weeks gestation, long before myelination of the corticospinal tract begins. Cerebellar influence on muscle tone and coordination is mediated mainly by the corticospinal and corticobulbar tracts. Hypotonia is among the most constant clinical findings in infants with cerebellar hypoplasia. 
COMMENT. Dr. Sarnat’s research concerning the development of the corticospinal tract in the newborn aids the clinician in his understanding of reflexes, seizures, and muscle tone and posture. In another recent study, acridine orange, a fluorochromic stain of nucleic acids, was used to study neural maturation in human brains during development. The increase in cytoplasmic RNA of neurons coincided with the onset of neurotransmitter synthesis, and the presence of orange fluorescence in heterotopic nerve cells served as a marker of the state of maturity and degree of migration.