Cranial Nerve Pathways and Optic Nerve Anatomy

These -
Receive impulses from the cerebral cortex through the corticobulbar tracts 
Pyramidal cells of the precentral gyrus and the adjacent postcentral gyrus 
Descend through the corona radiata 
Genu of the internal capsule 
Passes through the midbrain in the cerebral peduncles 
Synapse with the lower motor neurons (LMN) on the corresponding brainstem nuclei 

These are lower motor neurons  

These are axons cross the midline, ascend, synapse on other sensory nuclei , and terminate in the cerebral cortex.

Begins with the visual stimuli projected on the retina 
RVF (right visual field) for each eye
Images on the left side of the Retina 
LVF (left visual field) for each eye
Images on the right side of the Retina 
Images are inverted
Superior ½ are projected on the inferior ½ of the Retina 

Photoreceptors 
Rods
Cones
1st order neuron: bipolar cells
2nd order neuron: ganglion cells 
Exit the retina/come together to form the optic nerve 
Become myelinated 

Converge in the optic disc 

Associated with night vision

Associated with day vision

Optic nerve fibers pass directly to the optic chiasm 
Located at the anterior part of the Sella Turcica
Partial decussation of fibers at the Chiasm 
Inner/nasal half of each retina crosses 
Outer/temporal halves do not cross 

Calcarine Fissure (area 17)
Upper ½ = lower visual field 
Lower ½ = upper visual field 

½ of the visual field is lost 

½ of the visual field is lost on the other side
Bitemporal 
Binasal

Both RVF and LVF are lost

Blindness in involved eye 

Injury to decussating fibers of the optic nerve: 
Left eye does not perceive images in the right ½ of the visual field 
Right eye does not perceive images in the right ½ of the visual field 
Bitemporal hemianopia or heteronymous

Loses communication from ½ of each retina 
Right optic tract is destroyed 
Left homonymous hemianopia

Produce a homonymous hemianopia of the opposite side of the visual field 

Masseter
Temproalis 
Lateral and medial pterygoids 
Tensor tympani 
Tensor veli palatini 
Myohyoid 
Anterior belly of the digastric 

Three main branches 
Ophthalmic nerve 
Maxillary nerve 
Mandibular nerve 

V Trigeminal Nerve - Sensory Innervation - Ophthalmic nerve

Sensation to the forehead, eyes, and nose

Upper lip mucosa, maxilla, upper teeth, cheeks, palate, and maxillary sinus

Anterior ⅔ tongue, mandible, lower teeth, upper lip, part of the cheek, and part of the external ear 

Palpate 2 cm above and in front of angle of mandible. Bite downelax 

 Above the ear while chewing; temples can appear shrunken if atrophied

Place a hand on the tip of the mandible as the jaw is held open while the other hand is on the forehead. Ask the patient to bite down; they should be able to close with moderate resistance

Ask the patient to open their jaw to resistance and watch for the tip of the mandible lining up between the upper and medial incisors

Have the patient move their jaw to one side and hold it while attempting to push it to the center. Use the other hand to stabilize the opposite cheekbone

Unilateral paralysis (LMN) 
Unilateral paralysis (UMN)
Bilateral paralysis (UMN)

Three Nuclei 
Main motor nucleus 
Part that innervates the lower face gets CB fibers from the opposite hemi 
Part that innervates the upper face gets fibers from both hemis
Parasympathetic nucleus
Nucleus of the tractus solitarius 

Taste fibers have their primary ___ neurons in the geniculate ganglion 
They run in a bundle called the tractus solitarius and are joined by fibers from IX and X 
They end in the nucleus of the tractus solitarius 
Taste fibers split from the facial nerve in the middle ear
Fibers terminate in the taste buds of the anterior ⅔ of the tongue 
Some also terminate in the taste buds of the hard and soft palate
They ascend from the nucleus solitarius to the ventroposterior thalamus 
They also project to the cortical area for taste at the lower end of the sensory strip

Guard all facial apertures 
Wrinkle the forehead 
Close the eyes tightly
Close the mouth tightly
Pull back the corners of the mouth and tense the cheeks 
Guards the middle ear by innervating the stapedius muscles to protect from loud noises 
Partially responsible for taste 

Observe facial symmetry 
Wrinkle the forehead and observe the symmetry 
LMN lesion 
UMN lesion 
Close the eyes tightly and observe the symmetry 
Smile/pucker “oo-ee” and observe the symmetry 

 Involvement of the entire side of the face on the side of the lesion 
Speech is not significantly impaired 

 CorticoBulbar (CB)
Top half of the face 
Bilateral innervations 
Both CB tracts would need to be affected for paralysis 
Lower half of the face
Contralateral 
Right bottom = innervated by the left hemisphere CB tract
Left bottom - innervated by the right hemisphere CB tract 

Follows the acoustic nerve 
Terminates in the vestibular nucleus on the lateral floor of the 4th ventricle 
In the pons and medulla and on the cerebellum 

Nerve cells and fibers located in the spiral ganglion of the cochlea
Hair cells of the cochlea
Ventral and dorsal cochlear nuclei
Pass through the lateral lemniscus 
Inferior colliculus DECUSSATES 
Medial geniculate bodies 
Project to the auditory cortex of the temporal lobe 

Sensory pathway

Balance, equilibrium, and orientation

Hearing

 Vertigo
Postural deviations
Unsteady walking and standing 
Deviations of the eyes 

Deafness or partial deafness 

Rotating a chair to induce nystagmus 
Caloric test: irrigate the ear with water; nystagmus will develop in the normal ear

Use a vibrating tuning fork and touch it
By each ear 
At the top of the skull 

Motor and Sensory

Origin near the inferior cerebellar peduncles 
Pathway through the jugular foramen 

Taste buds on the upper posterior ⅓ of the tongue 
Upper pharyngeal constrictor muscle 

Loss of sensation and taste in the posterior ⅓ of the tongue 
Unilateral loss of the gag reflex 
Deviation of the uvula to the uninvolved side 
Difficulty in the initial phase of swallowing 

Elicit a gag reflex 
Hyperactive: UMN lesion on the CB tract 
Hyporeflexic: unilateral IX damage 
No reflex: bilateral XI damage 

Provides dual sensory innervation—both special sensory (taste) and general somatic sensory (touch, pain, and temperature)—to the posterior one-third of the tongue

Sensory and motor

External branch
Motor
Internal branch 
Sensory 

Origin: below the larynx 
Ascends to terminate in the larynx 
Innervates all the intrinsic muscles of the larynx except the cricothyroid 
Right and left branches 

Visceral motor nerve
Gut-brain connection 
With V and XI innervates the palatal muscles 
With IX innervates the pharyngeal constrictors 
Innervates the intrinsic muscles of the larynx  

Paralysis of the soft palate (nasality)
Difficulty swallowing 
Variety of voice problems 
Aphonia 
Roughness (unilateral) 
Digestive issues 

Spastic dystonia

left branch of the RLN partial paralysis 
Breathy, soft, hoarseness, flaccid paralysis 

Observe the palate at rest and the symmetry of the arches 
Phonate “ah”. The soft palate should elevate posteriorly
Elicit a gag reflex 
Laryngoscopy for laryngeal fx
Voicing: prolong /ah/. Normal is about 15 sec

Raise and lower the pitch 

Nucleus ambiguus of the medulla
CB fibers from the cerebral hemispheres
Fibers join IX, X, and XI

Spinal nucleus of the anterior gray column of the spinal cord (SC)
Ascends along the spinal cord 
Enters the cranium through the foramen magnum 

Supplies the uvula, levator veli palatini, and joins the RLN

Supplies the sternocleidomastoid and the trapezius muscle

Inability to turn head away from the side of the lesion 
General neck weakness
Laryngeal dysfunction/voice problems 

Size and symmetry of the sternocleidomastoids 
Turn the head to one side and hold it while trying to push it back to the midline 
Thrust the head forward while resisting movement against the forehead 
Shrug the shoulders while pushing down 

Runs under the tongue and controls all tongue movement 
The nucleus is located in the medulla under the lower part of the 4th ventricle 
Receives fibers from both cerebral hemispheres

All intrinsic muscles of the tongue 
3 extrinsic muscles of the tongue: genioglossus, hypoglossus, and the styloglossus

Controls shortening, concaving, narrowing, elongating, and flattening of the tongue 

Genioglossus: tongue protrusion 
Hypoglossus: retraction and depression 
Styloglossus: drawing the tongue up and back 

Open mouth and observe the mouth at rest
Protrude the tongue and observe symmetry 
LMN lesion: the tongue loses symmetry and deviates towards the side of the lesion (ipsilateral) upon protrusion 
UMN lesion: The tongue deviates
away from
the side of the lesion (toward the contralateral or opposite weak side 
Bilateral XII damage: Results in a tongue that appears symmetric, but is profoundly weak or entirely immobile
Lateralize and elevate 
LMN:
UMN:

V, X, XI

IX, X

V, IX, X, XI

X, XI

V, VII, IX