Cranial Nerves + The Eye PDF

CN VI: Abducens Nerve Function: General somatic motor only → Lateral rectus Course: Emerges from the brainstem at the pontomedullary junction Enters the subarachnoid space and pierces the dura mater to travel in Dorello’s canal and then the cavernous sinus Exits the cranial cavity via the...

CN VI: Abducens Nerve Function: General somatic motor only → Lateral rectus Course: Emerges from the brainstem at the pontomedullary junction Enters the subarachnoid space and pierces the dura mater to travel in Dorello’s canal and then the cavernous sinus Exits the cranial cavity via the superior orbital fissure CN VI Palsy: Damage to the abducens nerve can be caused by: ○ Space occupying lesions pushing down on the brainstem ○ Diabetic neuropathy ○ Thrombophlebitis of the cavernous sinus Clinical features: ○ Affected eye turned inwards → unopposed activity of medial rectus ○ Diplopia CN VII: Facial Nerve Functions: Motor: Muscles of facial expression, posterior belly of digastric, stylohyoid, stapedius Sensory: Concha of the auricle Special sensory: Taste to anterior ⅔ of tongue Parasympathetics: ○ Submandibular and sublingual salivary glands ○ Nasal, palatine and pharyngeal mucous glands ○ Lacrimal glands CN VII: Facial Nerve Intracranial Course: Large motor root + Small sensory root travel through the internal acoustic meatus In the facial canal: the roots fuse → geniculate ganglion → 3 branches: ○ Greater petrosal nerve ○ Nerve to stapedius ○ Chorda tympani Then exits the facial canal and cranium via the stylomastoid foramen Intracranial Course CN VII: Facial Nerve Extracranial Course: After exiting, it gives off 3 branches posteriorly and inferiorly: ○ Posterior auricular nerve ○ Nerve to digastric ○ Nerve to stylohyoid The rest of the nerve (the motor root) continues into the parotid gland, terminating by splitting into five branches innervating the muscles of facial expression: ○ Temporal ○ Zygomatic ○ Buccal ○ Marginal mandibular ○ Cervical CN VII: Facial Nerve - Motor Functions Nerve to stapedius Stapedius Posterior auricular Intrinsic and extrinsic muscles of the outer ear (auricular branch) Occipitalis muscle (occipital branch) Nerve to digastric Posterior belly of digastric Nerve to stylohyoid Stylohyoid Temporal Frontalis, orbicularis oculi, corrugator supercilii Zygomatic Orbicularis oculi Buccal Orbicularis oris, buccinator, zygomaticus Marginal Mentalis mandibular Cervical Platysma CN VII Facial Nerve - Other Functions Chorda Tympani Greater Petrosal Nerve Exits the facial canal and passes over the middle ear Travels anteromedially from the geniculate ganglion Enters the infratemporal fossa and hitchhikes with the Joins with the deep petrosal nerve = nerve of pterygoid lingual nerve canal Some fibres continue on to supply taste to anterior ⅔ Passes through pterygoid canal → pterygopalatine fossa tongue → pterygopalatine ganglion Others form the submandibular ganglion to supply Branches then provide parasympathetics to the lacrimal parasympathetics to the submandibular and sublingual gland and mucous glands of the mouth, nose and salivary glands pharynx Facial Nerve Palsy Symptoms of Facial nerve palsy will depend on where the lesion is and which branches are affected: Extracranial = Motor weakness/paralysis ○ Parotid gland pathology e.g. tumour, mumps ○ Infection of the nerve by herpes virus ○ Idiopathic (Bell’s Palsy) Intracranial: E.g. tumour or infection of middle ear ○ = Motor weakness AND Chorda tympani: Loss of salivation and taste on ipsilateral ⅔ tongue Nerve to stapedius: Ipsilateral hyperacusis (hypersensitivity to sound) Greater petrosal nerve: Ipsilateral loss of lacrimation Differentiating Bell’s palsy from a stroke (supranuclear lesion) The upper portion of the face receives bilateral supply so a lesion to one side of the motor cortex will have forehead sparing Lesions of the facial nerve itself (e.g. Bell’s palsy) occur after the nerves synapse at the pons so there will be forehead paralysis