25th June 2023

Decussations and bilateral innervation

A cranial nerve (i.e. the axon to and from its lower motor neuron nuclei) does not decussate, i.e. cross over to the opposite side, with the exceptions of the trochlear nerve (CN 4), and the medial fibres of the optic nerve (CN 2).

CN 2, 4, 7, and 12 cranial nerve nuclei receive bilateral innervation from upper motor neuron cortical inputs; therefore, cortical lesions can affect tracts to the cranial nuclei and subsequently result in an observed disorder of cranial nerve function, despite the lesion not affecting the cranial nerve directly.

Lower motor cranial nerve nuclei disorders will always result in an ipsilateral disorder except for CN 2 and CN 4, while lesions that affect upper motor cranial nuclei will result in bilateral or contralateral manifestations. The common examples of these will be discussed in the following sections.