Whereas at a level above the medulla a lesion occurring in one hemisphere would mean that the other hemisphere could still sufficiently innervate the posterior facial motor nucleus, a lesion affecting a lower motor neuron would eliminate innervation altogether because the nerves no longer have a means to receive compensatory contralateral input at a downstream decussation.
32.
Cortical upper motor neurons originate from Brodmann areas 1, 2, 3, 4, and 6 and then descend in the posterior limb of the internal capsule, through the crus cerebri, down through the pons, and to the medullary pyramids, where about 90 % of the axons cross to the contralateral side at the decussation of the pyramids.
33.
Occupying the posterior part of the commissure of the optic chiasma is a strand of fibers, the "'Ventral supraoptic decussation "'( "'commissure of Gudden "', "'Gudden's inferior commissure "'), which is not derived from the optic nerves; it forms a connecting link between the medial geniculate bodies.
34.
Therefore, a lesion that is caudal to the sensory decussation where the remaining 3 / 5 modalities decussate ( at the superior medulla ) will result in contralateral ( opposite sided ) pain and temperature loss ( as this has already crossed over at the anterior white commissure ), whereas loss of crude touch, pressure, and proprioception will be ipsilateral ( same sided ) as these have not yet crossed over.
