Mesencephalic corticonuclear fibers

Mesencephalic corticonuclear fibres are descending cortical fibres that terminate in or influence cranial motor systems within the midbrain (mesencephalon). They are principally associated with the motor control of eye movements involving:

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

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

Importantly, cortical control of eye movements is not simply a direct pathway from the motor cortex to individual oculomotor and trochlear motor neurons. Much of this control occurs through supranuclear gaze centres and interneuronal networks, which coordinate the activity of the ocular motor nuclei to produce conjugate and coordinated eye movements.

The Edinger–Westphal nucleus, associated with CN III, is a parasympathetic nucleus controlling pupillary constriction and accommodation. It is influenced by the cortex through the descending hypothalamosppinal tract and therefore should not be regarded as an equivalent direct target of the voluntary corticonuclear motor fibres.