[Development and maturation of the pyramidal tract].

The pyramidal tract contains axons that originate from neurons located in layer 5 of the neocortex of the frontal areas 4 and 6 and of the parietal lobe. These neurons are generated during the first half of gestation in humans. The growth of these axons is highly regulated and the mechanisms that control this growth begin to be unravelled. For example, netrins could serve as chemattractants, the adhesion molecule L1 plays a crucial role in the control of axonal decussation at the level of the medulla, the ephrin B3-Eph A4 couple prevents the axons from crossing the midline. During development, the total number of pyramidal axons increases progressively and then decreases by regression of exuberant collaterals. The pyramidal tract is the sole unmyelinated tract in the human spinal cord at birth. This accounts for the protracted central conduction time in newborns. This immaturity of the pyramidal system could explain the existence of specific motor reflexes in newborns (the so-called primary reflexes) that disappear as the pyramidal system matures.