Medullary corticonuclear fibers

Medullary corticonuclear fibres are descending cortical fibres that terminate in or influence cranial motor systems within the medulla oblongata. They are principally associated with motor control involving:

• Nucleus ambiguus – contains branchial motor neurons whose axons travel principally with CN IX and CN X to muscles of the pharynx, larynx and related structures involved in swallowing and phonation.

• Hypoglossal nucleus (CN XII) – supplies the intrinsic and most extrinsic muscles of the tongue and is important in voluntary movements of the tongue during speech, swallowing and food manipulation.

Clinical correlate

The nucleus ambiguus receives substantial bilateral cortical influence. Consequently, a unilateral lesion of the corticonuclear pathway is often partly compensated for by input from the opposite cerebral hemisphere, helping to preserve many pharyngeal and laryngeal motor functions.

The hypoglossal motor system also receives bilateral cortical influence overall. However, the part controlling the genioglossus muscle receives predominantly contralateral corticonuclear input. Therefore, a unilateral corticonuclear (upper motor neuron) lesion may produce weakness of the contralateral genioglossus. On protrusion, the tongue tends to deviate away from the side of the cortical lesion, towards the weaker side.

Autonomic motor nuclei of medulla oblongata

The inferior salivatory nucleus (CN IX) and dorsal motor nucleus of the vagus (CN X) are also subject to higher descending influence from cerebral cortex. However, these are preganglionic parasympathetic nuclei, whose cortical regulation occurs through broader descending autonomic pathways. Therefore, they should not be regarded as equivalent direct targets of the voluntary corticonuclear motor fibres.