Lecture on the Facial Nerve (7th Cranial Nerve)

Overview

• Facial nerve is the 7th cranial nerve
• Responsible for motor innervation of facial muscles, taste for the anterior 2/3 of the tongue, and production of saliva and tears
• Consists of different fibers and nuclei

Facial Nerve Pathway

• Within the Pons and Upper Medulla
  • Motor nucleus: sends out motor fibers
  • Superior salivatory nucleus: sends out fibers to salivary and lacrimal glands
  • Sensory fibers synapse with the spinal nucleus of the trigeminal nerve and nuclei of the solitary tract
  • Pontomedullary junction: where fibers form the motor root and intermediate nerve
• Motor Root
  • Travels through internal acoustic meatus and enters the facial canal
  • Forms a bend at the geniculum, where the geniculate ganglion is located
  • Gives off the nerve to the Stapedius muscle and continues to the stylomastoid foramen
• Superior Salivatory Nucleus Fibers
  • Pass through geniculate ganglion without synapsing
  • Form the greater petrosal nerve, which merges with the deep petrosal nerve to form the nerve of the pterygoid canal
  • Innervates lacrimal gland and mucous glands of nasal cavity, maxillary sinus, and palate
  • Branch to form chorda tympani, which terminates in the submandibular ganglion

Functional Components

• Special Visceral Efferent Fibers (SVE)
  • Originate from the motor nucleus in the lower pons
  • Supply muscles of facial expression
• General Visceral Efferent Fibers (GVE)
  • Preganglionic fibers from the superior salivatory nucleus
  • Innervate the submandibular, sublingual salivary glands, and lacrimal gland
• Special Visceral Afferent Fibers (SVA)
  • Originate from taste buds of the anterior 2/3 of the tongue
  • Synapse in the geniculate ganglion, then to the nucleus of the solitary tract
• General Somatic Afferent Fibers (GSA)
  • Originate from the skin of the external ear
  • Synapse in the geniculate ganglion, then to the spinal nucleus of the trigeminal nerve

Nuclei of the Facial Nerve

• Motor nucleus
• Superior salivatory nucleus
• Synapse with the spinal nucleus of trigeminal nerve
• Synapse with the nucleus of the solitary tract

Intracranial Course

• Facial nerve arises from two divisions: motor root and intermediate nerve
• Both penetrate the dura mater and travel through the internal acoustic meatus
• Fuse to form the facial nerve at the geniculate ganglion
• Greater petrosal nerve emerges from geniculate ganglion
• Chorda tympani emerges to carry taste and parasympathetic fibers

Extracranial Course

• Exits the skull via the stylomastoid foramen
• Gives off branches: stylohyoid, digastric, and posterior auricular nerves
• Continues into the parotid gland, forming the parotid plexus
• Five terminal branches: temporal, zygomatic, buccal, marginal mandibular, cervical
  • Provide motor innervation to facial muscles

Clinical Relevance

• Upper Motor Neuron Lesion
  • Affects the contralateral middle and lower face, sparing forehead and eyes
• Lower Motor Neuron Lesion (e.g., Bell's palsy)
  • Causes complete paralysis of ipsilateral facial muscles
  • Rapid onset, often recovers within days to weeks with steroid treatment

Summary

• Facial nerve is complex with multiple functional components
• Important for motor and sensory innervation in the face
• Damage can lead to various forms of facial paralysis
• Next topic will be the vestibulocochlear nerve (8th cranial nerve).