Neurology Quiz on Trigeminal Nerve Anatomy

Disclosures

• The presenter has no relationships with companies whose products or services relate to medicine, medical education, or research.

Suggested Reading

• Neuroscience for Dentistry by O'Kane and Barritt, 2022, Chapter 9
• Teach Me Anatomy articles on the Cranial Nerves, including the Summary of the Cranial Nerves by Katherine Sanders

Nervous System Modalities

• Sensory (afferent):
  • General somatic sensory (GSS): general sensation from skin
  • General visceral sensory (GVS): general sensation from internal organs
  • Special somatic sensory (SSS): senses from ectoderm (sight, sound, balance)
  • Special visceral sensory (SVS): senses from endoderm (taste, smell)
• Motor (efferent):
  • General somatic motor (GSM): skeletal muscles
  • General visceral motor (GVM): smooth muscles of the gut and autonomic motor
  • Special visceral motor (SVM): muscles derived from pharyngeal arches

Learning Objectives

• Correlate cranial nerve (CN) numbers with their names.
• Explain the basic sensory and/or motor functions of each CN.
• Identify the origin of each CN in the central nervous system.
• Identify the cranial fossae and foramina each CN travels through.
• Describe the location of ganglia and nuclei for each CN.
• Identify the cortical region for each CN (origin or destination of signal).
• Predict signs and symptoms if a CN is lesioned.

Cranial Nerve I (Olfactory)

• Sensory only, specifically for olfaction (smell).
• Originates from the forebrain.
• Fibers pass through foramina of the cribriform plate (ethmoid bone).
• Located in the anterior cranial fossa.
• Synapse in olfactory glomeruli of the olfactory bulb (continuous with olfactory tract).
• Projects to the olfactory cortex of the temporal lobe.

Clinical Relevance of CN I

• The cribriform plate is a weak spot in the anterior cranial fossa.
• Trauma or fracture to this area can cause anosmia (loss of smell).

Cranial Nerve II (Optic)

• Sensory only, for vision.
• Originates from the forebrain (retinal ganglion cell axons).
• Passes through the optic canal.
• The optic chiasm is located in the middle cranial fossa.
• Axons continue in the optic tract.
• Lateral Geniculate Nucleus (LGN) in the thalamus processes signals.
• Optic radiations project to the occipital lobe.

Clinical Relevance of CN II

• Optic nerve transection (trauma) or other damage (increased intraocular pressure, tumor) results in anopsia (unilateral blindness).

Cranial Nerve III (Oculomotor)

• Motor only, controlling extraocular muscles.
• Innervates superior rectus, medial rectus, inferior rectus, inferior oblique, and levator palpebrae superioris muscles.
• Contains parasympathetic innervation for intraocular muscles (pupillary constrictor and ciliary muscle).
• Originates from the midbrain.
• Passes into the orbit through the superior orbital fissure.
• Located in the middle cranial fossa.

Clinical Presentation of CN III Palsy

• Isolated CN III palsy suggests possible trauma, aneurysm, or brain herniation.
• Symptoms include ptosis (drooping eyelid), mydriasis (pupil dilation), and down and out deviation of the eye.

Cranial Nerve IV (Trochlear)

• Motor only, controlling the superior oblique muscle.
• Actions include intorsion, depression, and abduction.
• Originates from the dorsal midbrain.
• Passes into the orbit through the superior orbital fissure.
• Located in the middle cranial fossa.
• Trochlear nucleus is located within the midbrain.

Cranial Nerve V (Trigeminal)

• Mixed nerve (sensory and motor).
• Three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3).
• Sensory functions include sensation from the face, nasal cavity, and mouth.
• Motor functions control muscles of mastication (chewing).
• Sensory and motor nuclei are located in the pons.
• Foramina include the superior orbital fissure, foramen rotundum, and foramen ovale.

Cranial Nerve VI (Abducens)

• Motor only, controlling the lateral rectus muscle (for abduction).
• Originates from the pontomedullary junction.
• Passes into the orbit through the superior orbital fissure.
• Located in the middle cranial fossa.
• Contains the abducens ganglion in the pons.
• Cortex is located in the frontal lobe.

Cranial Nerve VII (Facial)

• Mixed nerve (sensory and motor).
• Motor functions innervate muscles of facial expression.
• Sensory functions provide taste from the anterior two-thirds of the tongue, and general sensation from skin behind the ear.
• Important parasympathetic functions include salivation and lacrimation.
• Contains various branches for different functions.
• Originates from the pontomedullary junction, passes through the internal acoustic meatus and stylomastoid foramen.

Clinical Considerations for CN VII

• Bell's palsy, cause: idiopathic, Treatment: Corticosteroids,Typically self-resolving within ~6 months, Protect the affected eye, Prevent

Cranial Nerve VIII (Vestibulocochlear)

• Sensory only, for hearing and balance.
• Has cochlear (hearing) and vestibular (balance) components.
• Originates from the pontomedullary junction.
• Passes through the internal acoustic meatus.
• Lesions may result in unilateral hearing loss or vertigo.

Clinical Considerations for CN VIII

• Labyrinthitis: inflammation of the membranous labyrinth causing damage to vestibular and cochlear branches; symptoms include hearing loss, tinnitus.

Cranial Nerve IX (Glossopharyngeal)

• Mixed nerve (sensory and motor).
• Sensory: General sensation from the posterior tongue, oropharynx, middle ear, and Eustachian tube; taste from the posterior tongue.
• Motor: controlling the stylopharyngeus muscle, and parasympathetic functions affecting the parotid gland.
• Originates in the medulla.
• Passes through the posterior cranial fossa to exit the jugular foramen.
• Nuclei located within the medulla.
• Tongue taste registers in the insular cortex

Cranial Nerve X (Vagus)

• Mixed nerve (sensory and motor).
• Sensory: larynx, pharynx, thoracic and abdominal organs
• Taste from the epiglottis
• Motor: larynx, pharynx, and soft palate, parasympathetic functions influencing the trachea, bronchi, gastrointestinal tract, and heart rhythm.
• Originates in the medulla.
• Passes through posterior cranial fossa to exit the jugular foramen.
• Nuclei located within medulla.
• Taste registers within the insular cortex

Clinical Considerations for CN X

• Complete CN X lesions are rare.
• Some branches are susceptible to compression (like the recurrent laryngeal nerve).

Cranial Nerve XI (Spinal Accessory)

• Motor only, innervating the sternocleidomastoid and trapezius muscles.
• Originates in the medulla and upper cervical spinal cord.
• Passes through posterior cranial fossa and jugular foramen (spinal part passes through the foramen magnum).
• Nuclei located within the medulla and upper cervical spinal cord.
• Cortex responsible for motor commands is in the frontal lobe.
• Iatrogenic trauma (such as surgery) is a common cause of damage.

Clinical Considerations for CN XI

• Damage often presents as muscle wasting and partial paralysis in the sternocleidomastoid and trapezius muscles.

Cranial Nerve XII (Hypoglossal)

• Motor only, controls muscles of the tongue.
• Innervates genioglossus, hyoglossus, and styloglossus muscles.
• Originates in the medulla.
• Passes through the posterior cranial fossa exiting via the hypoglossal canal.
• Nuclei located in the medulla.
• Cortex responsible for motor commands is in the frontal lobe.

UMN (Central) vs. LMN (Peripheral) Lesions

• UMN lesions (superior to the hypoglossal nucleus) result in contralateral tongue deviation.
• LMN lesions (inferior to the hypoglossal nucleus) result in ipsilateral tongue deviation.