The facial nerve (cranial nerve VII) is a complex nerve responsible for facial expression, taste sensation from the anterior two-thirds of the tongue, and several autonomic functions. Its neuroanatomy is intricate, involving a long intracranial course and multiple branches.
Origin and Course
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Brainstem Nuclei: The facial nerve originates from several nuclei located in the pons region of the brainstem. These include:
- Motor Nucleus: Controls the muscles of facial expression.
- Superior Salivatory Nucleus: Controls lacrimal, submandibular, and sublingual glands.
- Nucleus of the Solitary Tract: Receives taste sensation from the anterior two-thirds of the tongue via the chorda tympani nerve.
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Intracranial Course: The facial nerve exits the brainstem and travels through the internal auditory meatus alongside the vestibulocochlear nerve (CN VIII).
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Facial Canal (Petrous Temporal Bone): Within the petrous portion of the temporal bone, the facial nerve traverses a bony canal known as the facial canal. This is where the nerve's most common pathologies occur due to the narrow confines of the canal.
Branches within the Facial Canal
Within the facial canal, the facial nerve gives off several important branches:
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Greater Petrosal Nerve: Carries preganglionic parasympathetic fibers to the pterygopalatine ganglion, which then innervates the lacrimal gland (tear production) and nasal mucosa.
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Nerve to Stapedius: Innervates the stapedius muscle in the middle ear, which dampens loud sounds.
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Chorda Tympani: Carries taste sensation from the anterior two-thirds of the tongue and preganglionic parasympathetic fibers to the submandibular ganglion, which then innervates the submandibular and sublingual salivary glands.
Exit from the Skull and Extracranial Branches
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Stylomastoid Foramen: The facial nerve exits the skull through the stylomastoid foramen, located posterior to the styloid process and inferior to the mastoid process.
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Parotid Gland and Terminal Branches: After exiting the stylomastoid foramen, the facial nerve enters the parotid gland, where it divides into its five main terminal branches. It's important to note that the facial nerve passes through the parotid gland but does not innervate it.
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Terminal Branches: These branches innervate the muscles of facial expression:
- Temporal Branch: Frontalis (forehead), orbicularis oculi (eye).
- Zygomatic Branch: Orbicularis oculi, zygomaticus major and minor (cheek).
- Buccal Branch: Buccinator (cheek), orbicularis oris (mouth).
- Marginal Mandibular Branch: Depressor anguli oris, depressor labii inferioris, mentalis (lower lip and chin).
- Cervical Branch: Platysma (neck).
Mnemonics to remember the branches:
- To Zanzibar By Motor Car (Temporal, Zygomatic, Buccal, Mandibular, Cervical)
- Ten Zebras bit My Carrot (Temporal, Zygomatic, Buccal, Marginal Mandibular, Cervical)
Summary
The facial nerve's neuroanatomy is characterized by its origin in the brainstem, a complex course through the temporal bone, and division into multiple branches that control facial expression, taste, and autonomic functions. Its long and winding path makes it susceptible to injury and disease, leading to conditions such as Bell's palsy.
