1. Oculomotor (CN III)
- Main function is moving the eyeball in all directions
- Adduction (inward movement of the eye) is the most important
action.
- Elevates eyelid (levator palpebrae superioris)
- Constrict pupils (papillary constrictor – short ciliary nerve branch)
- Accommodates the eye (by contracting the ciliary muscle – short
ciliary nerve branch)
2. Trochlear (CN IV)
- Motor to the superior oblique muscles
3. Abducens (CN VI)
- Motor to lateral rectus muscle (all the muscles here are extra
ocular muscles)
- Abducts eye (outwards)
4. Accessory (CN XI)
- Motor to sternocleidomastoid and trapezius
5. Hypoglossal (CN XII)
- Motor to muscles of the tongue
*Oculomotor, trochlear, and abducens nerves will supply the extraocular muscles
CN III: Oculomotor Nerve
- Responsible for lifting the upper eyelid; turning the eye upward,
downward, and medially; constricting the pupil; and
accommodating the eye
- Originates at the mesencephalon or the midbrain
- Has two nuclei – nucleus oculomotorius (motor), Edinger
Westphal (parasympathetic) (*nuclei – nerve cell bodies located
in the midbrain)
- It supplies the muscles of the eye by entering the superior orbital
fissure
- Most of the fibers in cranial nerve III enter the supplementary
orbital fissure and supplies the superior rectus muscle (moves eye
upwards), inferior rectus muscle (moves eye downwards), medial
rectus muscle (moves eye towards the nose), inferior oblique
muscle (moves eye up and out), levator palpebrae (moves eyelid)
- Some fibers will go to the ciliary ganglion that is responsible for
the contraction of ciliary muscle & papillary contractor muscle.
These are parasympathetic functions of oculomotor nerve.
- The oculomotor nerve runs in the lateral wall also of the calculus
sinus together with the cellular nerves
- It exits through the superior orbital fissure before it enters the
orbit
- REMEMBER: all the nerves that innervate the eye, the extraocular
muscles will enter the superior orbital fissure (foramina)
➢ Somatic Motor Neurons
✓ It innervates the extra ocular muscles
✓ Via oculomotor neurons
➢ Preganglionic Parasympathetic Neurons
✓ CN III (Oculomotor), VII (Facial), IX
(Glossopharyngeal), X (Vagus) – Has parasympathetic
functions
✓ Papillary constriction and accommodation
✓ Via Edinger-Westphal nucleus (accessory oculomotor
nucleus)
✓ Superior cranial nerve III fibers – innervates the extra
ocular muscle
✓ All the extraocular muscles are innervated by the
oculomotor nerve except the superior oblique which
is innervated by cranial nerve IV or trochlear nerve
and the lateral rectus which is innervated by cranial
nerve VI or abducens nerve
✓ Oculomotor nerve damage:
❖ Causes deficits in ipsilateral eye, difficulty in
moving eye medially and vertically
❖ Diplopia or double vision (common)
❖ Drooping of the eyelid may occur due to
paralysis of the levator – palpabrae
superioris
❖ The pupillary constriction muscle is
impaired the pupil on the side of damage
may remain dilated a condition known as
mydriasis.
➢ Parasympathetic fibers (Visceromotor function)
✓ Some of the fibers in the cranial nerve perform
parasympathetic function that is the visceral motor
function
✓ Ciliary ganglion – short sensory nerve: sensory muscle
✓ This is through the ciliary ganglion which gives rise to
short ciliary nerve that supplies ciliary muscle, which
makes the lens rounder. So, it varies the shape of lens.
✓ Accommodation (In order that you must focus on
something, that is the ciliary muscle will contract to
make your lens focus on that near object that is getting
near you)
✓ Constricts pupil (pupillary sphincter)
❖ Decreases the diameter of the pupil
❖ Pupillary constriction (meiosis)
➢ Ciliary Muscle
✓ Lens accommodation – varies the lens shape
➢ Pupillary sphincter/Constrictor pupillae
✓ Pupillary constriction
✓ Constrict the pupil to decrease the size of the pupils, it
is called as meiosis
*Both the actions, contraction in ciliary muscle as well as the constriction of pupils
contribute to the regulation of eyeball pressure (measured by tonometer, used by
ophthalmologist)*
➢ Reflex exam
✓ When doing examination, most of the times the cranial
nerve III involves the cranial nerve II because they both
have a relationship in terms of the reflex examination
particularly pupillary like reflex and the
accommodation reflex
✓ Pupillary light reaction - direct and consensual
❖ Consensual light reflex - If you shine a light
on one eye that will result in constriction of
pupil not only on the lighted eye but also on
the other eye
❖ Direct light reflex - If you shine a light only
on the right eye, the left eye will also
constrict
➢ Accommodation
✓ The afferent component of the pupillary reflex is the
optic nerve cranial nerve III
✓ The efferent component to the parasympathetic
component of the cranial nerve III on both sides
(bilaterally)
1. If you shine a light on one eye
2. The conduction of impulses will travel to the optic nerve, which has two
fibers: temporal fibers and nasal fibers
3. Nasal fibers will cross over at the optic chiasm so both conduction will
proceed towards the pretectal nuclei at the back.
4. Pretectal nucleus of the body (pre-tectum and tectum at the midbrain at the
superior colliculus). pretectal nuclei on both sides will have the Edinger
Westphal nucleus that is also located in the mid brain
5. From the Edinger Westphal nucleus it will send that information towards the
oculomotor nerves
6. Via the oculomotor nerves it will travel through the towards the ciliary ganglia
on both sides and goes towards the ciliary nerve to innervate your pupillary
constrictor muscle to affect an action
7. There will be constriction of the pupils on both eyes that is known as concept
direct reflex and a consensual reflex on the other eye
Summary: Shine a light → Optic nerve (afferent component) goes on both sides →
Received on both sides of the Edinger Westphal nucleus → CN III and travel on both eyes
→ Ciliary ganglion → Short ciliary nerve → Pupillary constrictor muscles—constrict the
pupils on both on both eyes right and left
• Accommodation Reflex
- The “near response” of the eye
- Reflex action of the eye in response to focusing on near object, then
looking at distant objects
- Coordinated changes in:
a. Convergence – both eyes will converge to focus on the object
b. Lens shape – changes in shape of lens
c. Pupillary size – changes in pupillary size (pupillary constriction)
- Controlled by the parasympathetic nervous system (via Edinger-Westphal
- If we draw an object near our eyes, both eyes will converge through the
left and right medial rectus
❖ Convergence of the two eyes – this is to make sure the object is focused on
the fovea of each retina. Failure of doing so – for example, when the eye
muscles are weak – would result in double vision. This is because the object
is focused on different parts of the two retinas and the brain sees two images.
❖ Constriction of pupil – this is to reduce spherical aberration. Spherical
aberration occurs when light rays hit the edge of a lens and produce
blurriness. Constricted pupil allows light rays to enter the lens only at the
center where they are best refracted.
❖ Accommodation of the lens – ciliary muscles contract to make the shape of
the lens more convex. This increases the optical power of the lens. It now can
converge the divergent light rays onto the retina.
❖ Presbyopia − is a very common age-associated condition in which the eye
loses the ability to adjust to near vision. In presbyopia, the lens loses its
flexibility with age and becomes stiff. It can no longer change its shape to
accommodate near vision.
✓ Corrected with convex lenses that converge the light rays slightly
before they enter the eye. Presbyopia is not to be confused with
hyperopia, a condition in which the eyeball is too short.
◆ CN III: Looking up and elevating the eyelids by levator palpebrae superioris
➢ Damage to CN III – Ptosis
CN IV: Trochlear Nerve
- Innervates the superior oblique muscle
- Moves eye inferiorly when adducted
- Internally rotates when abducted
Aids more in abduction
- Follows the same course as the CN III
- Nucleus
▪ Found in the anterior part of the gray
matter that surrounds the cerebral
aqueduct of the midbrain
▪ Lies inferior to the oculomotor nucleus at
the level of inferior colliculus
▪ After leaving the nucleus, the nerve fibers
pass posteriorly around the central gray
matter to reach the posterior surface of the
midbrain
▪ Receives corticonuclear fibers from both
cerebral hemispheres
▪ Receives the tectobulbar fibers, which
connect it to the visual cortex through the
superior colliculus
▪ Also receives fibers from the medical
longitudinal fasciculus by which it is
connected to the nuclei of the third, sixth,
and eighth cranial nerve
- Most slender of all cranial nerves
- The only cranial nerve to leave the posterior surface of the
brainstem, emerges from the midbrain and immediately
decussates with the nerve of the opposite side
- Passes forward through the middle cranial fossa in the lateral wall
of the cavernous sinus and enters the orbit through the orbital
CN VI: Abducens Nerve
- Draw the eye toward the side of the head
- It is a motor nerve, supplies the lateral rectus muscle (turning the
eye laterally)
- Enter the orbit through superior orbital fissure
- Innervates the Lateral Rectus muscle
- Abducts the eye
- Abducens nucleus is in the pons. The small motor nucleus is
situated beneath the floor of the upper part of the fourth
ventricle, close to the midline and beneath the colliculus facialis.
- The nucleus receives afferent corticonuclear fibers from both
cerebral hemispheres. It receives the tectobulbar tract from the
superior colliculus, by which the visual cortex is connected to the
nucleus. It also receives fibers from the medial longitudinal
fasciculus, by which it is connected to the nuclei of the third,
fourth, and eighth cranial nerves
- Course of the Abducent Nerve: The fibers of the abducent nerve
pass anteriorly through the pons and emerge in the groove
between the lower border of the pons and the medulla oblongata.
It passes forward through the cavernous sinus, lying below and
lateral to the internal carotid artery.
Extraocular Muscle Movements
➢ CN III: Oculomotor
o Superior (Up), Inferior (Down) and Medial (In-
adduction) rectus
o Inferior oblique – Up and in (upward
adduction)
o Ciliary muscle (Accommodation)
o Levator Palpebrae Superioris (eyelid
elevation)
➢ CN IV: Trochlear
o Superior oblique
o Down and out (downward abduction)
➢ CN VI: Abducens
o Lateral rectus
Conjugate Eye Movement
• Motor coordination of the eyes that allows for bilateral fixation
on a single object.
• Movement of both eyes to maintain binocular gaze
• “Yoked” eye movement
ʘ Cavernous Sinus
- CN (III, IV, V1, V2, VI): Ptosis, diplopia, “Frozen eye”,
Ophthalmoplegia, facial pain
- Occlusion of C5 (venous outflow blockage): Proptosis
(exophthalmus), chemosis
- Carotid artery encasement: Stroke
**Q: What is the first nerve that will be affected if there is sudden development of
aneurysm? A: Abducens**
❖ Types of Conjugate Eye Movements; Site of Control
✓ Saccadic (command): rapid movement from one fixation to
another; Frontal lobe.
✓ Pursuit: the slow eye movement used to maintain fixation to a
moving object; Occipital lobe
✓ Vestibulo-positional (Vestibulo-ocular reflex): eye movement
that compensates for movement of the head to maintain fixation
(Doll’s eye - SIGN THAT YOU’RE STILL ALIVE); Cerebellar Vestibular
nuclei
✓ Convergence: the movement that maintain fixation as an object is
brought close the face; Midbrain
CN XI: Accessory Nerve
- Arises from medulla
- Leaves the skull through the jugular foramen
- Contributions from spinal radices C1-C6 entering the skull in the
foramen magnum
- It has a nucleus known as nucleus ambiguous
- The nucleus receives corticonuclear fibers from both cerebral
hemispheres
- The efferent fibers of the nucleus emerge from the anterior
surface of the medulla oblongata
- between the olive and the inferior cerebellar peduncle.
- The nerve runs laterally in the posterior cranial fossa and joins the
spinal root. The two roots unite and leave the skull through the
jugular foramen.
- The nerve fibers emerge from the spinal cord midway between
the anterior and posterior nerve roots of the cervical spinal
nerves. The fibers form a nerve trunk that ascends into the skull
through the foramen magnum.
- The only cranial nerve to both enters and exits the skull
- Purely motor: Sternocleidomastoid and Trapezius muscles
- Nucleus is innervated from cortex to the contralateral side
(Decussation at the medullary pyramids)
- Bilateral sternocleidomastoid muscle control from cortex
- Somato-motor to the:
▪ Sternocleidomastoid
▪ Trapezius muscle
- Sternocleidomastoid
▪ Tilt, flexion and rotate the head
▪ Contraction turns the head to opposite side
- Trapezius
▪ Shrug the shoulder
▪ Ipsilateral contraction
- Note: Ipsilateral cerebral cortex (UMN) supplies the contralateral
trapezius and sternocleidomastoid plus ipsilateral
sternocleidomastoid muscle thus a single UMN lesion can give rise
to signs of both sides. But the dominant cortex will still be the
contralateral cortex
- The right cerebral cortex will still be dominant control for the Left
sternocleidomastoid and trapezius muscle and vice versa, thus the
dominant weakness will still be on the contralateral side.
Tilt, flexion and rotate the head
✓ Flexion – contract both Sternocleidomastoid
✓ Tilt – Tilting the head will contract the ipsilateral
sternocleidomastoid
o Example: Right head tilt will contract right
• Rotate/Turn side to side – Contraction turns the head to opposite side
turning
o Example: Rotating/Turning towards left will contract the
right sternocleidomastoid.
◆ CN XI Damage
➢ Damage to the right motor cortex
- An acute damage to the UMN (Corticobulbar tract) will cause
predominantly contralateral hemiplegia/paresis of
sternocleidomastoid and trapezius
- Note: Contraction of sternocleidomastoid will turn/rotate the
head to the opposite side, tilt head to the ipsilateral side.
- Weak Left Sternocleidomastoid and Trapezius
✓ Head tilted towards right/side of UMN lesion (Strong
side, which is the right sternocleidomastoid will
dominate – supplied by the intact left motor cortex)
✓ Weak shrug on the left side (Contralateral cortical
innervation of trapezius)
➢ Damage to the right accessory nerve (peripheral nerve)/LMN
- Weak/paralysis turning/rotating head to left side (contraction of
sternocleidomastoid turns head to the opposite side)
- Head tilted towards left (opposite intact accessory nerve will
dominate, cannot tilt head towards right side of LMN lesion)
- Weak/paralysis of shoulder shrug right (trapezius: ipsilateral)
** Q1: Head tilted towards left, plus weak shoulder shrug right, eye deviated towards
left. Where is the lesion? A: Left UMN/Central Left Cortical/Left Corticobulbar damage**
** Q2: Atrophy of right trapezius and sternocleidomastoid and unable to shrug right
shoulder, unable to rotate head towards the left side, head tilted slightly towards left.
Where is the lesion? A: Right LMN/Accessory nerve damage**
CN XII: Hypoglossal Nerve
• Brainstem > Hypoglossal nucleus > Hypoglossal canal > Intrinsic
and Extrinsic tongue muscles
**Note: Palatoglossus is not innervated by CN XII but rather by CN X**
- Purely motor: intrinsic/extrinsic muscles of the tongue
- Supplies all the intrinsic muscles of the tongue as well as the
styloglossus, hyoglossus and the genioglossus
- The nerve is involved in controlling tongue, whistling, movements
required for speech and swallowing, including sticking out the
tongue and moving it from side to side
- Nucleus gets its information from the contralateral motor cortex
- Hypoglossal nucleus is situated close to the midline immediately
beneath the floor of the lower part of the fourth ventricle
The Tongue
• Primary organ of taste
• Manipulate food for mastication
• Phonetic articulation
• Natural “toothbrush”
• 2 groups of muscle (Intrinsic and Extrinsic)
• The extrinsic muscles originate from bone and extend to the tongue. Their
main function is altering the tongue’s position allowing for protrusion,
retraction, and side to side movement
• Innervated by the Hypoglossal Nerve (XII)
➢ Genioglossus: arises from the mandible and protrudes the
tongue, it is also known as the “safety muscle” of the tongue since
it is the only muscle having the forward action
➢ Hyoglossus: arises from the hyoid bone and depresses the tongue
➢ Styloglossus: arises from the styloid process of the temporal bone
and elevates and retracts the tongue
• Innervated by the Vagus Nerve (X)
➢ Palatoglossus: arises from the palatine aponeurosis, and
depresses the soft palate, moves the palatoglossal fold towards
the midline, and elevates the back of the tongue
Tongue Innervation
✓ Motor Innervation:
- CN XII – Intrinsic and Extrinsic Muscles
- CN X – Palatoglossus
✓ Anterior 2/3
- Taste; Chorda tympani branch of CN VII
General sensation (Example: Pain, Temp, Touch,
Vibration): Lingual branch of mandibular nerve
(Trigeminal V3)
✓ Posterior 1/3
- Taste and General sensation: Glossopharyngeal nerve
CN IX
✓ Back of the Tongue (Pharyngeal arch 4)/Palatal area
- Taste and general sensation: Vagus nerve: CN X
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