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Detailed Notes on the Cranial Nerves

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Overview

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Cranial Nerves on the Base of the Brain

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Fiber Component Classification

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Memory Mnemonics

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Individual Cranial Nerves

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CN I - Olfactory Nerve

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CN II - Optic Nerve

Note: The optic nerve is technically a CNS tract (myelinated by oligodendrocytes, not Schwann cells).

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CN III - Oculomotor Nerve

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CN IV - Trochlear Nerve

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CN V - Trigeminal Nerve

• Purely sensory (GSA)
• Sensation from forehead, anterior scalp, upper eyelid, cornea, dorsum of nose, nasal mucosa (upper), paranasal sinuses, dura of anterior cranial fossa
• Passes through cavernous sinus lateral wall → superior orbital fissure

• Purely sensory (GSA)
• Sensation from lower eyelid, cheek, upper lip, upper teeth and gums, hard palate, nasal mucosa (lower), maxillary sinus
• Exits via foramen rotundum → pterygopalatine fossa

• Mixed: sensory (GSA) + motor (BE)
• Sensation from lower lip, chin, lower teeth and gums, anterior two-thirds tongue (general sensation, NOT taste), ear, temporal scalp
• Motor to: muscles of mastication (masseter, temporalis, medial and lateral pterygoids), tensor tympani, tensor veli palatini, mylohyoid, anterior belly of digastric
• Exits via foramen ovale

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CN VI - Abducens Nerve

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CN VII - Facial Nerve

• Greater petrosal nerve (from geniculate ganglion): preganglionic parasympathetics → pterygopalatine ganglion → lacrimal gland, nasal/palatal glands
• Nerve to stapedius: motor to stapedius muscle (dampens loud sounds)
• Chorda tympani: taste (anterior 2/3 tongue) + parasympathetics → submandibular ganglion → submandibular and sublingual glands

• BE: All muscles of facial expression, scalp muscles, posterior belly of digastric, stylohyoid, stapedius
• SA: Taste from anterior 2/3 of tongue (via chorda tympani)
• GVE: Lacrimal, submandibular, sublingual glands + nasal/palatal mucous glands
• GSA: Skin of external acoustic meatus and auricle

• LMN (Bell's palsy): Complete ipsilateral facial paralysis (forehead involved - forehead has bilateral cortical representation)
• UMN (stroke): Contralateral lower face weakness only (forehead spared)
• Additional features: loss of taste anterior 2/3 tongue, hyperacusis (stapedius paralysis), dry eye/mouth

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CN VIII - Vestibulocochlear Nerve

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CN IX - Glossopharyngeal Nerve

• SA: Taste from posterior 1/3 of tongue
• GSA: Sensation from posterior 1/3 tongue, palatine tonsils, oropharynx, middle ear mucosa, pharyngotympanic tube, mastoid air cells
• GVA: Sensory from carotid body (chemoreceptors) and carotid sinus (baroreceptors)
• GVE: Preganglionic parasympathetics → otic ganglion → parotid gland (via lesser petrosal nerve)
• BE: Stylopharyngeus muscle (3rd pharyngeal arch)

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CN X - Vagus Nerve

• GSA: Larynx, laryngopharynx, external acoustic meatus, auricle, posterior cranial fossa dura
• GVA: Aortic body/arch baroreceptors, esophagus, bronchi, lungs, heart, abdominal viscera (foregut + midgut)
• SA: Taste around the epiglottis
• GVE: Parasympathetics to pharynx, larynx, thoracic viscera (heart, lungs), abdominal viscera (foregut + midgut up to splenic flexure)
• BE: Palatoglossus (tongue), soft palate muscles (except tensor veli palatini), pharyngeal muscles (except stylopharyngeus), laryngeal muscles

• Pharyngeal branch: pharyngeal plexus (with CN IX)
• Superior laryngeal nerve: internal (sensory above vocal folds) + external (motor to cricothyroid)
• Recurrent laryngeal nerve: all intrinsic laryngeal muscles (except cricothyroid) + sensory below vocal folds

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CN XI - Accessory Nerve

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CN XII - Hypoglossal Nerve

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Parasympathetic Ganglia of the Head

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Foramina and Cranial Nerve Exit Points

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Brainstem Levels of Cranial Nerve Nuclei

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Clinical Lesion Summary

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High-Yield Clinical Points

1. CN III palsy with dilated pupil = compressive lesion (posterior communicating artery aneurysm, uncal herniation). Pupil-sparing CN III palsy = diabetic microvascular disease (vasa nervorum ischemia - pupillomotor fibers on the periphery are spared).
2. Bell's palsy vs. UMN facial palsy: Bell's palsy involves the forehead (complete ipsilateral paralysis). UMN lesion (stroke) spares the forehead because the frontalis muscle has bilateral cortical representation.
3. Jugular foramen syndrome (CN IX, X, XI) - caused by jugular foramen tumours (e.g. glomus jugulare, metastases).
4. CN VI palsy and raised ICP: CN VI has the longest intracranial course; it can be compressed by raised ICP independent of the site of lesion - hence a "false localizing sign."
5. The four parasympathetic cranial nerves: III, VII, IX, X (mnemonic: 3, 7, 9, 10). Remember: parasympathetic fibers in CN III, VII, and IX are distributed with branches of the trigeminal nerve to reach their target organs.
6. Chorda tympani (branch of CN VII): crosses the middle ear, exits via the petrotympanic fissure, joins the lingual nerve (CN V3). Carries taste (anterior 2/3 tongue) and presympathetic fibers to submandibular ganglion.
7. Tongue innervation summary:
   • General sensation: CN V3 (lingual nerve)
   • Taste anterior 2/3: CN VII (chorda tympani)
   • Taste posterior 1/3: CN IX
   • Motor (all except palatoglossus): CN XII
   • Motor palatoglossus: CN X
8. Laryngeal innervation (CN X):
   • All intrinsic laryngeal muscles innervated by the recurrent laryngeal nerve EXCEPT the cricothyroid (external branch of superior laryngeal nerve)
   • Sensory above vocal folds: internal branch of superior laryngeal nerve
   • Sensory below vocal folds: recurrent laryngeal nerve

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