Do the Corticospinal and Corticobulbar Tracts Mediate Functions in the Human Newborn?

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Abstract
Unlike the numerous dispersed bulbospinal pathways that are already well myelinated at term, the more compact corticospinal and corticobulbar tracts are only beginning their myelination cycle in late gestation and do not complete it until two years of age. During this same period, these pathways also develop extensive ramification of terminal axonal segments, growth of collateral axons, and proliferation of synapses. Despite their immaturity in the fullterm human newborn, several proposed functions may be attributed to the descending pathways from the neonatal cerebral cortex: a) a contribution to the differential development of passive muscle tone and resting postures; in general they function as an antagonist to the “subcorticospinal pathways” in mediating proximal flexion and distal extension, except for the rubrospinal tract which is probably synergistic with the corticospinal tract; b) enhancement of tactile reflexes originating in the brainstem and spinal cord, including suck and swallow; c) relay of epileptic activity of cortical origin; d) inhibition of complex stereotyped motor reflexes including many phenomena formerly termed “subtle seizures”; e) a possible influence on muscle maturation, particularly in relaying cerebellar impulses that modify the histochemical differentiation of myofibres. However, the bulbospinal tracts are probably more influential on muscle development. The corticospinal and corticobulbar tracts subserve different needs in the newborn than at older ages, but are functionally important pathways even at birth.