Point 14











Corticobulbars to Motor VII - Stick DiagramYou will recall that the cortex sends axons to cranial nerve motor nuclei. These are called CORTICOBULBAR fibers (remember those to the hypoglossal and nucleus ambiguus?). A unilateral lesion of the corticobulbar fibers to motor VII, for example in the motor cortex, results in weakness of the muscles of expression of the face BELOW THE EYE ON THE SIDE CONTRALATERAL TO THE LESION. The frontalis muscle (wrinkles forehead) and the orbicularis oculi muscle (closes eyelid) are unaffected. The accepted explanation states that BILATERAL (crossed and uncrossed) corticobulbar projections from the cerebral cortex influence the lower motor neurons (within Motor VII) innervating the frontalis muscle and orbicularis oculi, while only CROSSED corticobulbar projections influence the lower motor neurons innervating the muscles of the LOWER face.

A lesion in the face representation of area 4 (motor cortex) will mean that those motor neurons in the contralateral region (ventral) of motor VII that innervate the lower facial muscles are completely deprived of cortical input. In contrast, the lower motor neurons in that part of motor VII that innervate the upper facial muscles still have cortical input from the ipsilateral motor cortex. Such muscles, therefore, contract when the patient wants to voluntarily contract them. REMEMBER, THERE IS NO MUSCLE ATROPHY FOLLOWING A LESION OF THE CORTICOBULBAR FIBERS.

Corticobulbars to Motor VIIYou should now think about the resulting neurological deficits following a lesion of the LEFT motor cortex that interrupts all CORTICOSPINAL fibers and CORTICOBULBAR fibers to motor VII, nucleus ambiguus and the hypoglossal nucleus. There will be a RIGHT hemiplegia, the tongue will deviate to the RIGHT upon protrusion, and the lower facial muscles on the RIGHT will be weak. Any problems with swallowing? Will the uvula deviate when you say ahhh? THINK! THIS IS VERY IMPORTANT.