Cranial Nerves I, V, VII, IX, X, XI, XII

Learning Objectives

• Describe the functions of the cranial nerves
• Explain the pathway of each cranial nerve
• Describe where the nerve originates from (where cell bodies are located within the CNS)
• Note where axons project to and synapse upon
• Explain how to clinically test the integrity of each cranial nerve
• Identify which cranial nerve has a lesion when given a case study
• Identify where the lesion is located given a case study or symptoms
• List the signs/symptoms a patient will have if a cranial nerve or nucleus is damaged

Outline

• Cranial nerves overview
• Cranial nerve I: olfactory nerve
• Special sensory pathway
• Clinical correlations
• Clinical testing
• Cranial nerve V: trigeminal nerve
• Pathways
• Sensory for pain and temperature
• Tactile, vibratory sensation and proprioceptive pathway
• Motor pathway
• Clinical correlations
• Clinical testing
• Cranial nerve VII: facial nerve
• Pathways
• Motor efferent pathway
• Visceral motor efferent pathway
• General sensory pathway
• Special sensory pathway
• Clinical correlations
• Clinical testing (Bell's palsy vs central facial palsy)
• Cranial nerve IX: glossopharyngeal nerve
• Pathways
• General sensory
• Visceral sensory
• Special sensory
• Branchial motor
• Visceral motor
• Clinical correlations
• Cranial nerve X: vagus nerve
• Pathways
• General sensory
• Visceral sensory
• Branchial motor
• Visceral motor
• Clinical correlations
• Clinical testing: gag reflex
• Cranial nerve XI: spinal accessory nerve
• Pathway
• Clinical correlations and testing
• Cranial nerve XII: hypoglossal nerve
• Pathway
• Clinical correlation and testing

Overview

• There are 12 pairs of cranial nerves that emerge from the cranium
• These nerves provide sensory and motor information for the head and neck
• Six distinct modalities are carried by them
• General sensory
• Visceral sensory
• Special sensory
• Somatic motor
• Branchial motor
• Visceral motor

Cranial Nerve I - Olfactory Nerve

• Relays odorant information from the olfactory epithelium to the pyriform olfactory cortex, providing special sensory function
• Neurons in the olfactory epithelium project through cribriform plate to synapse on olfactory neurons in the olfactory bulbs (cell bodies) and olfactory tracts (axons) which will bifurcate at olfactory trigone into medial and lateral striae to the pyriform area
• Cranial nerve pathways are always ipsilateral

Clinical Correlations of I - Olfactory Nerve

• An anteroposterior skull fracture parallel to the superior sagittal suture can tear olfactory axons in the cribriform plate resulting in ipsilateral loss of smell (anosmia)
• Frontal lobe tumors or meningiomas on the floor of the anterior cranial fossa can interfere with the transmission of olfactory information
• Damage to the primary cortical olfactory area in the temporal lobe from tumors or seizures can result in olfactory hallucinations (phantom smells)

Testing of I - Olfactory Nerve

• Have the patient close their eyes and occlude one nostril
• Place a non-irritating substance, such as coffee or chocolate, under open nostril to test the integrity of the olfactory pathway

Cranial Nerve V: Trigeminal Nerve

• Consists of the ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions
• Two modalities are represented
• General Sensory of the face (touch, vibration, pressure, position, pain and temperature; V1, V2,V3)
• Branchial Motor to the muscles of mastication (V3)

Pathways of Cranial Nerve V

• Touch and vibration information from the ipsilateral face
• 1st order neurons: trigeminal ganglion
• 2nd order neurons: principal sensory nucleus (axons cross midline and ascends as part of the medial lemniscus tract)
• 3rd order neurons: (medial aspect of the ventral posterior nucleus of the) thalamus
• Pain and temperature information from the ipsilateral face
• 1st order neurons: trigeminal ganglion (axons descend caudally, forming the tract of the spinal trigeminal nucleus)
• 2nd order neurons: spinal trigeminal nucleus
• 3rd order neurons: (medial aspect of the ventral posterior nucleus of the) thalamus
• Proprioceptive information from muscles of mastication
• Unusual sensory pathway since the first order neuron is in the CNS
• 1st order neurons: mesencephalic nucleus (only pseudounipolar neuron in the CNS)
• Axons synapse in cerebellum for unconscious priorioception reflex
• Motor pathway to muscles of mastication
• Lower motor neurons in trigeminal motor nucleus
• (UMN in precentral gyrus, face area)
• Jaw jerk reflex (masseter reflex) – (not part of regular neurological exam)
• Monosoynaptic reflex; Mandible is struck downward at chin underneath lips and masseter muscle jerks mandible upwards
• Sensory branch: V3 (mesencephalic)
• Motor branch: V3 (trigeminal motor)

Clinical Considerations of Cranial Nerve V

• Sensory Lesions
• Tic Douloureux (Trigeminal Neuralgia) features episodes of lancinating, severe pain with unknown etiology
• Treatment includes antiepileptics, analgesics, or transection of nerves in trigeminal ganglion
• Motor Lesions
• Lower motor neuron lesion to muscles of mastication is characterized by paralysis, atrophy of muscles, and decreased strength of bite

Clinical Testing of Cranial Nerve V

• To test sensory division of CN V
• Have the patient close their eyes and use a cotton swab to lightly touch each side of the forehead (V1), cheeks (V2), and jaw (V3)
• To test motor division of CN V
• Palpate the masseter and temporalis muscles on each side and have the patient clamp their jaws tightly (contraction of muscles indicate motor division of V is intact)
• Have the patient open their jaw
• If there is a lesion, the jaw will deviate towards the side of the lesion

Cranial Nerve VII: Facial Nerve Divisions

• Branchial motor efferents to the muscles of facial expression (orange in image)
• Visceral motor efferents (parasympathetic) to glands and mucous membranes (blue in image)
• General sensory afferents from the ear (aqua in image)
• Special sensory afferents (taste from anterior 2/3 of the tongue) (green in image)

CN VII: Special Sensory Pathway (Taste)

• Sensory afferents relay taste information from the taste buds on the anterior 2/3 of the tongue.
• 1st order neurons: Cell bodies are located in the geniculate ganglion and axons project to the medulla, forming the solitary tract.
• 2nd order neurons: nucleus solitarius or solitary nucleus
• 3rd order neurons: ventral posterior nucleus of the thalamus

CN VII Motor Efferent Pathway

• The largest division of CN VII, with the portion of the facial nucleus that innervates the upper muscles of facial expression of the forehead receiving bilateral input from both precentral gyri (primary motor cortex)
• The portion of the facial nucleus that controls the lower muscles of facial expression (from the eye and below) receive input only from the contralateral precentral gyrus, with axons of neurons in the facial nucleus innervating ipsilateral muscles of facial expression
• CNVII is also responsible for secretions of the face, i.e. control of glands which secrete tears, mucus, or saliva
• The nucleus involved is the superior salivatory nucleus (not ID'ed in lab), which is located medial to the facial motor nucleus.

CN VII: Clinical Considerations and Testing

• To test motor component of CN VII
• Examine muscles of facial expression by observing the patient's facial expressions while speaking, looking for symmetry in facial expression
• Have the patient raise their eyebrows, close eyes tightly, and press lips firmly together

CN VII: Clinical Considerations

• An upper motor neuron lesion leads to paralysis of voluntary muscles of facial expression in the contralateral lower quadrant, and only partial paralysis in the contralateral upper quadrant
• Muscles in the lower quadrant receive innervation only from the contralateral precentral gyrus, while the muscles in the upper quadrant receive bilateral innervation from both precentral gyri
• Lower motor neuron lesion leads to paralysis of the muscles in the ipsilateral upper and lower quadrants because neurons in the facial nuclei innervate ipsilateral muscles of facial expression
• To test the blink reflex (involves CN V₁ & VII)
• A cotton swab is lightly touched on the cornea, which causes the eyes to blink
• To test the special sensory component of CN VII
• Place a sugary or salty solution on the patient's tongue and have the patient point to a chart indicating what was tasted

Cranial Nerve IX: Glossopharyngeal

• Consists of five components
• General sensory from posterior 1/3 of tongue, skin and external ear, internal surface of tympanic membrane and pharynx
• Visceral sensory from carotid body and sinus
• Special sensory for taste from posterior 1/3 of tongue
• Branchial motor to stylopharyngeus muscle
• Visceral Motor (parasympathetic) to parotid gland
• General sensory information from the external ear, inner surface of the tympanic membrane, posterior 1/3 of the tongue, and upper pharynx is carried by the glossopharyngeal nerve
• Cell bodies are located in the superior and inferior glossopharyngeal ganglia
• Axons project and synapse on the spinal trigeminal nucleus in the brainstem.

Cranial Nerve IX: Visceral Sensory Function

• Chemoreceptors in the carotid body (monitor oxygen in the blood) and baroreceptors in the carotid sinus (monitor arterial blood pressure) project
• Cell bodies are located in the inferior glossopharyngeal ganglion
• Axons project and synapse on the nucleus Solitarius.
• Taste information from the posterior 1/3 of the tongue is carried by the glossopharyngeal nerve
• Cell bodies are located in the inferior glossopharyngeal ganglion
• Axons project and synapse on the nucleus Solitarius in the medulla.

CN IX: Branchial Motor Pathway

• Corticobulbar tracts synapse bilaterally on rostral nucleus ambiguous which contains cell bodies of the lower motor neurons that innervate the stylopharyngeus muscle, which elevates the larynx and pharynx, and dilates the pharynx to permit the passage of a large food bolus, thereby facilitating swallowing
• Visceral Motor Pathway
• Preganglionic parasympathetic cell bodies located in the inferior salivatory nucleus in the medulla project their axons (the lesser petrosal nerve) to synapse on the otic ganglion where the postganglionic neuron will innervate the parotid gland

CN IX: Clinical Considerations

• Glossopharyngeal neuralgia is characterized by a sharp lancinating pain in the tonsil region that radiates to the ear and is triggered by yawning, swallowing, or food in the tonsillar region
• No cause can usually be identified for glossopharyngeal neuralgia, but sometimes is caused by compression of CN IX caused by carotid aneurysms, oropharyngeal malignancies, peritonsillar infections, or lesions at the base of the skull
• The gag reflex is a protective reflex that prevents entry of foreign objects into the alimentary and respiratory passages, initiated by CN IX which relays sensation of the presence of a foreign object from the back of the mouth, and then relayed to cranial nerve nuclei X and XII so that respiratory and alimentary passages are closed and the tongue protrudes to expel the object.
• Clinically, CN IX and X are assessed by testing the gag reflex
• CN IX carries the sensory input and CN X (vagus) carries the motor output
• When assessing the gag reflex, right and left sides of the upper pharynx should be lightly touched with a tongue depressor
• CN IX is considered intact if the pharyngeal wall can be seen to contract after each side has been touched.

CN X: Vagus Nerve

• A "wandering” nerve, innervates visceral organs and relays sensory information from visceral organs to the brainstem and is the largest visceral sensory (afferent) nerve of the cranial nerves
• General sensory from the posterior meninges, external ear, pharynx and larynx
• Visceral sensory from the larynx, trachea, esophagus, thoracic and abdominal visceral, stretch and chemoreceptors in the aortic arch and body
• Branchial motor to muscles of the larynx and pharynx
• Visceral motor to smooth muscles and glands of larynx and pharynx, thoracic and abdominal viscera and cardiac muscle

CN X: Pathway

• General sensory information from the larynx and pharynx travel via internal laryngeal nerve
• Cell bodies are located in the inferior vagal ganglion
• The central processes synapse in the spinal trigeminal nucleus in the brainstem
• Axons of the spinal trigeminal neurons project to the thalamus, which then projects to the postcentral gyrus
• Visceral sensory information
• The visceral sensory component carries visceral sensation from the lower pharynx, larynx, trachea, epiglottis, base of the tongue, esophagus, and thoracic and abdominal viscera, stretch receptors in the aortic arch and chemoreceptors in the aortic bodies adjacent to the arch
• Visceral sensation is not appreciated at a conscious level of awareness other than as “feeling good” or “feeling bad", unlike visceral pain.
• The cell bodies of the visceral sensory afferents are in the inferior vagal ganglion
• Cell bodies enter the brainstem and synapse on nucleus Solitarius
• Parasympathetic preganglionic cell bodies are located in the dorsal motor nucleus of the vagus in the medulla and the axons project to one of the ganglia located on or near the organ
• The nucleus ambiguus also contains cell bodies of the motor efferents of the vagus that leave the brainstem and innervates the muscles of the pharynx and the larynx.

CN X : Clinical Considerations

• Perform a Gag reflex to test integrity of CN IX (sensory component) and CN X (motor component)
• Right and left sides of the pharynx should be lightly touched and if circuitry is intact, the pharyngeal wall will contract when touched
• To test integrity of CN X alone, observe the posterior pharynx at rest and during phonation (say “Ah”; contraction of superior pharyngeal muscle)
• If damaged, the uvula deviates towards the normal side

CN XI: Spinal Accessory Nerve

• A branchial motor efferent with cell bodies located in the ventral horn of spinal cord segments C1-C5
• Axons travel up through the foramen magnum and out the jugular foramen to innervate the sternocleidomastoid and trapezius muscles
• Damage to the spinal accessory nerve causes lower motor neuron symptoms to the ipsilateral trapezius and sternocleidomastoid muscles, so a patient would not be able to raise the affected shoulder (loss of innervation to the trapezius muscle) or turn their neck to the opposite side (loss of innervation to the sternocleidomastoid muscle) if CN XI is damaged

CN XI: Testing

• Verify integrity of CN XI and the sternocleidomastoid muscle: have the patient tilt and turn their chin towards the opposite side
• To test CN XI and trapezius muscles, have the patient shrug their shoulders against resistance, and once the arm is abducted above 90 degrees, apply resistance and instruct the patient to raise their arm
• Observe any muscle wasting or if the scapula is rotated down and laterally

CN XII: Hypoglossal Nerve

• The hypoglossal nucleus located in the medulla contains cell bodies of lower motor neurons that project their axons to innervate the intrinsic and extrinsic tongue muscles
• Lesions to the upper motor neurons or its axons that project to the hypoglossal nucleus would cause muscle weakness on the side contralateral to the lesion, causing the tongue to deviate to the affected side
• Lesions to the hypoglossal nucleus or the axons that project to the extrinsic tongue muscles would cause muscle weakness on the side ipsilateral to the lesion, causing the tongue to deviate to the affected side
• Observing to which side the protruded tongue deviates is insufficient to determine whether the lesion is in the UMN or LMN ,but to verify if there the lesion is in the LMN, observe the tongue for fasciculations and atrophy
• Testing involves inspecting the tongue for fasciculations and atrophy (tongue looks like it has a wrinkled surface) and instructing the patient to stick out their tongue