The Corticobulbar Tract

The Corticobulbar Tract
This tract is composed of fibers originating in the precentral gyrus of the lower quarter of the motor cortex. The descending fibers leave the motor cortex and pass through the posterior limb of the internal capsule just anterior and medial to the corticospinal tract fibers. From here they continue on through the cerebral peduncles just medial to the corticospinal tract fibers to terminate in the motor nuclei of cranial nerves III and IV in the midbrain; V, VI. and VII in the pons; and IX, X, XI, and XII in the medulla. The corticobulbar fibers from one side of the brain project to the motor nuclei on both sides of the brainstem (Fig-4).

The corticobulbar fibers (upper motor neurons remember) synapse in cranial nerve motor nuclei of the same (ipsilateral) side, AND cross to the opposite (contralateral) side to synapse on motor nuclei there.

EXECPTIONS: ALL corticobulbar fibers to the motor nucleus of the hypoglossal nerve (XII) cross to the contralateral motor nucleus and, ALL corticobulbar fibers to the lower half of the motor nucleus of the facial nerve (VII) cross to the lower half of the contralateral motor nucleus. bstr431.biostr.washington.edu/syl/lab4/fig402.gif