25th June 2023

Rule of Four

The cranial nerves consist of twelve paired nerves that can be characterised as follows:

They can be structurally abstracted as part of the Rule of 4:

• 4 cranial nerves are situated above the pons: CN 1 and 2 in cerebrum, CN 3 and 4 in midbrain
• 4 cranial nerves are situated within the pons: CN 5, 6, 7, and 8
• 4 cranial nerves are situated below the pons: CN 9, 10, 11, and 12
• 4 motor cranial nerve nuclei are in the midline: CN 3, 4, 6 and 12, which are associated with three other important structures – the corticospinal tract (motor pathway), medial lemniscus (vibration and proprioception), and medial longitudinal fasciculus.
• 4 cranial nerve nuclei are in the lateral brainstem: CN 5, 7, 9 and 10, which are associated with three other important structures – the spinocerebellar pathway, spinothalamic pathway, and sympathetic pathway.
• 4 cranial nerves are closely related at the cerebellopontine angle – CN 5, 6, 7, 8, and the cerebellum. Tumours at that area, e.g. a vestibular schwannoma, are likely to cause ipsilateral CN 5,6,7 and 8 palsies with ataxia.