CN VII and VIII

Learning Objectives

• Describe the anatomical pathways of CN VII and CN VIII

• Describe the nerve components of CN VII and CN VIII

• Describe the functions associated with each of these nerves

• Describe select clinical correlations associated with each nerve

CN VII and CN VIII Pathway (commonality)

• Emerge from the brainstem between the pons and medulla

• Internal auditory meatus → Petrous portion of temporal

  • CN VII usually found superficial to CN VIII

• Paths diverge from here

CN VIII Vestibulococlear - Hearing and Balance (Special Sensory)

CN VIII Pathway (continuted)

• Cochlear and vestibular

• Conveys SSA info on sound and equilibrium

Divisions of the Ear

• External, middle, inner ear

• CN VII innervates inner ear

• CN VII innervates external and middle ear

Ear Structure: Inner Ear

• Composed of semicircular canals, vestibule, and cochlea

• Innervation by vestibulocochlear n.

  • Has distinct vestibular and cochlear portions

    • Vestibular portion → Semicircular canals and vestibule

    • Cochlear portion → Cochlea

• Semicircular canals oriented in 3 orthogonal planes

  • Horizontal canal: turning head left and right

  • Superior canal: nodding hear

  • Posterior canal: touching head to shoulder

  • If damaged → Vertigo

• Structure - Tube within a tube

  • Components from outside → inside

    • Bony/osseous labyrinth - bone

    • Perilymph - fluid

    • Membranous labyrinth

    • Sensory epithelium - lining

    • Endolymph - fluid

• Cochlea compartments

  • Scala vestibuli (vestibular duct) - contains perilymph

  • Tympanic duct (scala tympani) - contains perilymph

  • Cochlear duct (scala media) - contains endolymph

  • Organ of Corti sits on top of the basilar membrane in the cochlear duct

    • Transmits fluid displacement to the brain via hair cells

Path of sound

• Sound stimulate stapes to tap the oval window → Pressure waves travel through cochlea → As it passes, displaces basilar membrane and stimulates cochlear portion of n. → Remaining pressure waves shake into the middle ear cavity via the round window (dissipate pressure)

  • Place theory: Stimulation of hair cells in different parts of the basilar membrane results in differences in sound frequencies

  • High frequencies: close to middle ear

  • Low frequencies: close to cochlear apex

• Sound processing

  • Anterior cochlear nucleus: Process low frequency sounds

  • Posterior cochlear nucleus: Process high frequency and loud sounds

Clinical considerations: Presbycusis

• Presbycusis: Very gradual loss of hearing sensitivity related to aging beginning in young adults

  • Begins with frequencies over 15-16,000

• Affects high frequencies more and greater in men

  • Loss of sensitivity to high frequencies → hard to distinguish between certain consonants especially in noisy environments

• Use hearing aids to amplify low frequencies

Clinical considerations: Cochlear implants

• Implant takes place of hairs on basilar membrane and directly stimulate cochlear n.

• Typically implanted inn younger individuals - speech development

CN VII Facial - SS, SM Para/Pre, Taste

Nerve Components

• SS: Skin of the concha of ear and posterior to ear

• SM: Muscles of facial expression, stapedius, posterior belly of digastric, and stylohyoid

• Para/Pre: For lacrimal (tears), submandibular (salivary), sublingual (salivary), + mucous membranes of nasopharynx and the hard and soft palate

• Special sensory: Taste to anterior 2/3 of tongue and hard and soft palate

CN VII Pathway (continued)

• Once enters internal auditory meatus → Travels anteriorly within petrous portion of temporal bone → Right angle laterally at geniculate ganglion → exits stylomastoid foramen

  • Geniculate ganglion: Collection of sensory neurons

  • Branches of CN VII before exiting stylomastoid foramen: Greater (and lesser) petrosal n., n. to stapedius, chorda tympani

CN VII Somatomotor

• SM to muscles of facial expression, stapedius, posterior belly of digastric, and stylohyoid

• Branches of CN VII (once exited stylomastoid foramen) - Superior → Inferior + posterior

  • Temporal: Innervate frontalis + orbicularis oculi (superior portion)

  • Zygomatic: Innervate orbicularis oculi (inferior portion)

  • Buccal: Innervate buccinator; orbicularis oris

  • Marginal mandibular

  • Cervical: Innervate platysma

  • Posterior auricular: SS behind ear

Clinical considerations: facial nerve paralysis

• Mild weakness to total paralysis → cause significant facial distortion or total loss of muscle tone on affected side

• Upper motor neuron lesions (UMNL) vs. Lower motor neuron lesions (LMNL)

  • UMNL: Forehead will be spared

    • Caused by stroke, subdural hemorrhage

  • LMNL: Entire face affected (lesion after exit stylomastoid foramen)

    • Caused by Bell palsy, Ramsay Hunt syndrome

• Forehead vs. Lower face innervation

  • Forehead innervation: innervated by facial nucleus in the pons from L and R motor cortex

    • Therefore, UMNL has an unaffected forehead because forehead still connected to one side of motor cortex

  • Lower face innervation: innervated from lower part of facial nucleus in the pons ONLY contralateral side of motor cortex

Clinical considerations: Bell’s Palsy (LMNL)

• Symptoms

  • Facial asymmetry

  • Drooping of eyelid; inability to close eye; cornea drying (orbicularis oculi - no SM)

  • Drooling of mouth and drooling (orbicularis oris - no SM)

  • Accumulation of food between cheek and teeth when chewing (buccinator - no SM)

• Causes

  • Irritated facial nerve → swollen due to inflammation

  • Viral infection (BSV1; Lyme disease; Extreme cold)

  • Peripheral n. problem after stylomastoid foramen

• Treatment: minimal; eye-patch → usually spontaneously recover after several days

CN VII Somatosensory

• SS to skin of the concha of ear and posterior to the ear

• Posterior auricular branch of CN VII innervates occipitalis and most auricular muscles

  • Most posterior branch of facial runs on top of the mastoid process

• Part of tympanic membrane receies SS from CN VII

  • N. that innervate the tympanic membrane: CN V3, VII, IX, and X

CN VII Para/Pre

• Para/pre to lacrimal (tears), submandibular (saliva), and sublingual glands (saliva) + mucous membranes of nasopharynx and hard and soft palate

Greater (and lesser) petrosal n.

• First branch off geniculate ganglion → Hiatus for the greater petrosal n. → Travel under CN V ganglion → Cross into foramen lacerum → Pterygopalatine ganglion (in fossa)

• Path of para/pre from CN VII that travel on the greater petrosal n.

  • Para/pre from CN VII travel on greater petrosal n. → Joins with deep petrosal (from the tympanic plexus) → Forms the nerve of the pterygoid canal → Synapse at the pterygopalatine ganglion → Exit to V2

Parasympathetic innervation of lacrimal gland (increase tear production)

• Superior salvatory nucleus (in the brain) → Facial n. → Greater petrosal n. → N. of the pterygoid canal VII synapse at pterygopalatine ganglion → Zygomatic branch of V2 → Lacrimal n. of V1 → Lacrimal gland (increase tear production)

Sympathetic innervation of lacrimal gland (decrease tear production)

• Lateral horn T1-4 → Ventral root → Spinal n. → Ventral ramus → White communicating ramus → Sympathetic trunk (ascend) → Synapse at superior cervical chain ganglion → ICA plexus → Deep petrosal n. → N. to pterygoid canal → Pass through pterygoid ganglion → CN V2 → Lacrimal n.

Parasympathetic innervation of the nasal mucosa, hard/soft palate (increase mucous secretion)

• Superior salivatory nucleus → Facial n. → Greater petrosal n → N. of the pterygoid canal → Synapse at pterygopalatine ganglion → Greater and lesser palatine n. → Nasal & palatine mucosal glands

Sympathetic innervation of the nasal mucosa, hard/soft palate (decrease mucous secretion)

• Lateral horn T1-4 → Ventral root → Spinal n. → Ventral ramus → White communicating ramus → Sympathetic trunk (ascend) → Synapse at superior cervical chain ganglion → ICA plexus → Maxillary a.

  • To nasal cavity: Nasopalatine/sphenopalatine a.

  • To palate: Descending palatine a.

• Can also get to destinations via ICA plexus → Deep petrosal n. → N. of pterygoid canal → Pass through pterygoid ganglion → Nasal and palatine mucosal glands

Parasympathetics to submandibular and sublingual glands (increase saliva secretion)

• Superior salivatory nucleus → Facial n. → Chorda tympani → Lingual n. → Synapse at submandibular ganglion → Submandibular gland OR lingual nerve, sublingual gland

Sympathetics to submandibular and sublingual glands (decrease saliva secretion)

• Lateral horn T1-4 → Ventral root → Spinal n. → Ventral ramus → White communicating ramus → Sympathetic trunk (ascend) → Synapse at superior cervical chain ganglion → ECA plexus → Facial or lingual artery plexus → Submandibular/sublingual gland

  • Facial a. plexus → superior side

  • Lingual a. plexus → inferior side

CN VII Special sensory

• Taste to anterior 2.3 of tongue and hard and soft palate

• Chorda tympani → Petrotympanic fissure → Hitchhike on lingual n.