Corticonuclear fibers

Corticonuclear fibres (a.k.a. corticobulbar fibres), are descending fibres that connect the motor areas of the cerebral cortex with motor systems associated with the cranial nerve nuclei in the brainstem. They arise predominantly from the face and head region of the primary motor cortex, with contributions from premotor areas, and descend through the corona radiata, mainly the genu of the internal capsule, and the cerebral peduncle.

The principal cranial motor nuclei receiving corticonuclear fibres include:

• Oculomotor nucleus (CN III) – controls most extraocular muscles and levator palpebrae superioris.

• Trochlear nucleus (CN IV) – supplies the superior oblique muscle.

• Motor nucleus of the trigeminal nerve (CN V) – supplies the muscles of mastication and other branchial motor derivatives.

• Abducens nucleus (CN VI) – controls the lateral rectus.

• Facial motor nucleus (CN VII) – supplies the muscles of facial expression and other branchial motor derivatives.

• Nucleus ambiguus – provides branchial motor neurons whose axons travel principally with CN IX and CN X to muscles of the pharynx, larynx and related structures; its cranial accessory component is traditionally associated with CN XI.

• Hypoglossal nucleus (CN XII) – supplies the intrinsic and most extrinsic muscles of the tongue.

The Edinger–Westphal nucleus (CN III), superior salivatory nucleus (CN VII), inferior salivatory nucleus (CN IX) and dorsal motor nucleus of the vagus (CN X) are also subject to descending cortical influence. However, these are preganglionic parasympathetic nuclei, whose cortical regulation occurs through broader descending autonomic pathways.

Clinical correlate

Most cranial motor nuclei receive corticonuclear fibres from both cerebral hemispheres (bilateral innervation). Therefore, damage to the corticonuclear fibres on one side is often partly compensated for by input from the opposite hemisphere.

Two important exceptions have predominantly contralateral cortical innervation; the part of the facial motor nucleus that controls the lower face and the part of the hypoglossal nucleus that controls the genioglossus muscle. Therefore, a unilateral corticonuclear (upper motor neuron) lesion typically causes weakness of the lower face on the opposite side, while forehead movement is relatively preserved because the muscles of the upper face receive bilateral cortical input. Also, the upper motor neuron lesion may also cause weakness of the opposite side of the tongue.