[Functional heterogeneity of the piramidal system: corticobulbar and corticospinal tracts]

The pyramidal system is constituted by the cortical fibers reaching the bulbar pyramids at the level of the inferior Olive. Close to 75% of these fibers decussate terminating at bulbar level about half of them while the other half form the crossed corticospinal tract. The majority of the uncrossed fibers descend to the cord forming the direct corticospinal tract. This revision presents evidence demonstrating the functional heterogeneity of the pyramidal system.

The corticobulbar fibers are slower conducting and branch more than the corticospinal fibers. The conduction speed of the corticospinal fibers increases as they reach more caudal segments, thus assuring that the different signals modulating the inter segmental activity reach their targets simultaneously. Those from the somatosensory cortex end in the dorsal horn while those from the primary motor cortex end primarily on interneurons of the reflex pathways to distal limb muscles although in rats, primates and humans about 10% of them terminate directly on motoneurons.

The corticobulbar component of the pyramidal system selects the adequate motor synergies and modulates other descending systems to coordinate the activity of the distal musculature and the associated postural adjustments. The corticospinal component is functionally related to motor activities requiring accuracy, and to motoneuronal recruitment to adjust the contractile force. Both pyramidal subsystems modulate the processing and integration of the ascending somatosensory information generated by the movement itself.