Neuroanatomy of cranial nerve VII
| Understanding the neuroanatomy of cranial nerve VII (CN VII) is key in differentiating upper motor neuron from lower motor neuron facial palsy, as it provides a road map of innervation from its origin, which will help localise the lesion during clinical examination. The facial nuclei originate in the pons and receive corticobulbar motor fibres from the primary motor cortex, travelling through the internal capsule.5,7 Disruption of these supranuclear pathways, above the level of the facial nucleus, cause a centrally originating contralateral facial palsy of which acute stroke is the most common aetiology. |
| Intracranially, the motor and sensory roots of CN VII arise in the pons and travel through the internal acoustic meatus along an opening in the petrous temporal bone. They then enter the facial canal where the roots join together forming the facial nerve, which then exits the cranium at the stylomastoid foramen.7 Intracranial damage to the facial nerve roots prior to their exit, from traumatic damage to the temporal bone, neoplastic disease of the middle ear or mastoid, or infective and inflammatory causes, often leads to compression of the CN VII nerve roots causing loss of lacrimation, taste, salivation and stapedial reflex, along with facial palsy. |
| The facial nerve then branches into the parotid gland where it splits into its temporal, zygomatic, buccal, marginal mandibular and cervical branches, which are responsible for innervating the muscles of facial expression.5,8 Compression or damage here often by trauma or neoplasm within the parotid gland or mandible, leads to isolated lower motor neuron facial palsy. |