Neurologic Exam-Pediatric | Developmental Anatomy | Myelination and Development

Developmental Anatomy 04 - Myelination and Development: The next developmental process that greatly contributes to brain growth and maturation is myelination. Myelination begins early in the 3rd trimester with the most rapid period of myelination occurring in the first two years of life. The myelination process follows a specific time course and pattern. The acquisition of neurodevelopmental milestones parallels and reflects this pattern. Myelination occurs early for motor-sensory roots, special senses and the brainstem; those structures necessary for reflex behavior and survival. The corticospinal tract starts to myelinate at 36 weeks gestation and myelination is completed by the end of the 2nd year of life. Myelination of the corticospinal tract begins at the proximal portion of the axon and the shortest axons are the first to myelinate. The axons for the upper extremities and the trunk myelinate next. The axons for the lower extremities, which are the longest axons, are the last to myelinate with the process being completed by 24 months of age. This myelination pattern correlates with the progressive head-to-toe acquisition of developmental milestones. The motor patterns of the immature brain are predominately under brainstem control and lack cortical modulation until there is the necessary maturation of these higher brain center connections.