Abstract
The corticospinal and corticobulbar tracts (CST, CBT) are immature at birth, in neuroanatomical
terms of myelination and terminal axonal sprouting for multiple synaptic contact;
these developmental features are not mature until 2 years of age. Physiologically,
the CST is mainly inhibitory. Nevertheless, these pathways have an important role
to play in normal neurological function at this age, though different from their functions
in the older child and adult. They are important in the neonate by 1) inhibition of
the monosynaptic stretch reflexes at spinal cord levels, beginning at 25 weeks gestation
or earlier; 2) influence upon muscle tone, hence posture, by mediating proximal flexion
in axial and limb girdle muscles and distal extension of the fingers and toes and
abduction of the thumbs; 3) antagonism of the proximal extension and distal flexion
and adduction from the medial subcorticospinal pathways of Lawrence and Kuypers; 4)
reinforcement of tactile reflexes, the most important of which are suck and swallow;
5)early individualization of finger movements; 6) transmission of epileptic activity
from the cerebral cortex. Understanding the unique roles of the CST at birth provide
rational, physiological explanations of such neonatal phenomena as clonus, opisthotonus,
strong distal flexion and adduction ("cortical thumb") and fisting), and poor suck
and swallow in affected neonates, regardless of the cause of cerebral cortical impairment
or its reversibility or irreversibility. In summary, the CST and CBT are indeed important
functional pathways in the neonate, but many of their functions are very different
than in the adult. (J Pediatr Neurol 2003; 1(1): 3-8).
Keywords
corticospinal tract - muscle tone - neonate - posture - stretch reflexes - suck -
swallow - tactile reflexes
