Facial Nerve (CN VII): Anatomical Course, Branches & Bell's Palsy

**1. Functional Components & Nuclei** The Facial Nerve (Cranial Nerve VII) contains four functional modalities: 1. *Special Visceral Efferent (Branchiomotor)*: Supplies muscles of facial expression, stapedius, stylohyoid, and posterior belly of digastric (from motor nucleus of CN VII). 2. *General Visceral Efferent (Parasympathetic)*: From superior salivatory nucleus, supplies lacrimal, submandibular, and sublingual glands. 3. *Special Sensory (Taste)*: Carries taste from anterior 2/3 of the tongue via chorda tympani to the nucleus tractus solitarius. 4. *General Somatic Afferent*: Sensation from concha of auricle and external auditory meatus to spinal nucleus of CN V. **2. Intracranial & Intrapetrous Course** * Exits the brainstem at the cerebellopontine angle (CPA) with the nervus intermedius and vestibulocochlear nerve (CN VIII). * Traverses the internal acoustic meatus into the petrous temporal bone, where it enters the facial canal. * Turns sharply backward at the **Geniculate Ganglion** (first genu), then descends in the medial wall of the middle ear cavity and posterior wall of tympanic cavity to exit the skull via the **Stylomastoid Foramen**. * Intrapetrous branches include: - *Greater Petrosal Nerve*: Arises at geniculate ganglion; relays in pterygopalatine ganglion to lacrimal gland. - *Nerve to Stapedius*: Dampens excessive vibration of the stapes; lesion causes painful hyperacusis. - *Chorda Tympani*: Arises 6 mm above stylomastoid foramen; crosses middle ear cavity over tympanic membrane; joins lingual nerve to supply taste to anterior 2/3 of tongue and preganglionic fibers to submandibular ganglion. **3. Extracranial Course & Terminal Branches** After exiting the stylomastoid foramen, the trunk enters the posteromedial surface of the parotid gland. It runs superficial to the retromandibular vein and external carotid artery, dividing into two primary divisions (temporofacial and cervicofacial) which yield 5 terminal motor branches (mnemonic: **Two Zebras Bit My Cheek**): 1. *Temporal*: Frontalis, orbicularis oculi (upper), corrugator supercilii. 2. *Zygomatic*: Orbicularis oculi (lower part). 3. *Buccal*: Buccinator, orbicularis oris, zygomaticus major. 4. *Marginal Mandibular*: Depressor labii inferioris, depressor anguli oris, mentalis. 5. *Cervical*: Platysma muscle. **4. 🔬 Upper vs Lower Motor Neuron Facial Palsy** * **UMN Facial Palsy (e.g., Stroke, Capsular Infarct)**: - The forehead muscles (frontalis) receive **bilateral corticobulbar innervation**. - Therefore, an UMN lesion causes paralysis of only the contralateral *lower* facial muscles, sparing the forehead (patient CAN wrinkle forehead). * **LMN Facial Palsy (e.g., Bell's Palsy, CPA Tumor)**: - Affects the ipsilateral facial nerve trunk or nucleus. - Causes paralysis of **all** ipsilateral facial muscles, *including* the forehead (patient CANNOT wrinkle forehead or close eye). * **Bell's Palsy**: Acute idiopathic LMN facial paralysis, often post-viral (HSV-1 reactivation) edema causing entrapment in the fallopian canal. Features include loss of nasolabial fold, mouth angle drooping, inability to close eye (Bell's phenomenon: eyeball rolls upward on attempted closure), and hyperacusis. Management: Oral Prednisolone within 72 hours, eye lubrication. **5. 📖 Standard Textbook Reference** * *BD Chaurasia's Human Anatomy*, 8th Ed., Vol. 3: Head and Neck, Ch. 7 & 15, pp. 95–112, 275–282. * *Gray's Anatomy*, 42nd Ed., Section 4: Neuroanatomy, Cranial Nerves, pp. 3312–3316.