Chapter 6. b
Motor functions of Cranial Nerves.
Head and neck movements
Mastication
Facial expression
Swallowing tongue movements
Voice (phonation)
Eye movements
Spinal accessory nerve (XI)
Trigeminal (V)
Facial (VII)
Glossopharyngeus (IX)
Vagus (X)
Hypoglossal nerve (XII)
Vagus (X)
Oculomotor nerve (III)
Trochlear nerve (IV)
Abducens nerve (VI)
HEAD AND NECK MOVEMENTS
XI Nerve - Accessory Nerve
The accessory nerve is a purely motor nerve supplying the sternocleidomastoid and trapezius muscles which
have the following functions:
1.
Rotation of head away from the side of the contracting sternocleidomastoid muscle.
2.
Tilting of the head toward the contracting sternocleidomastoid muscle.
3.
Flexion of the neck by both sternocleidomastoid muscles.
4.
Elevation of the shoulder by the trapezius.
5.
Drawing the head back so the face is upward by the trapezius muscles.
Anatomy
The motor neurons of the accessory nerve lie in the intermediate column of gray matter in the upper five
segments of the cervical cord (C1-C5). Fibers that
emerge from these motor neurons pass dorsolaterally
and emerge midway between the anterior and
posterior roots and unite to form an ascending trunk,
which passes through the foramen magnum into the
posterior fossa. The spinal portion of the accessory
nerve then joins the bulbar accessory nerve, which is
the lowest portion of the vagus nerve, and leaves the
posterior fossa through the jugular foramen. The
bulbar portion then joins the vagus nerve while the
spinal portion descends in the neck to terminate in the
sternocleidomastoid and trapezius muscles on the
same side.
Evidence suggests that the motor neurons in
the
upper
cervical
cord
supplying
the
sternocleidomastoid and trapezius muscles have a
segmental distribution, the more rostral cells
supplying the sternocleidomastoid and the caudal
neurons supplying the trapezius.
With weakness or paralysis these functions are
decreased or absent. When the lesion is nuclear or
infranuclear, there is associated muscle atrophy and
fasciculations.
Diagram illustrates the origin and
distribution of the spinal accessory nerve (cranial
nerve XI). Note that the cell bodies of origin are
within the spinal cord, and the nerve fibers innervate the trapezius and sternomastoid muscles. Also shown in this
illustration is the upper motor neuron for this nerve. The cell bodies of origin for this nerve receive a projection from
the cortex that is crossed.
Examination
Head Rotation. Have the patient turn the head all the way to the left. Place your hand on the left side of the
chin and ask the patient to press against your hand while you try to turn the head back to the right, palpating the right
sternocleidomastoid muscle with your other hand at the same time. Repeat the process for rightward head rotation.
Shoulder Elevation. Ask the patient to shrug the shoulders while you resist the movement with your hands.
Spinal Accessory Nerve Lesions
Motor evaluation. Injury to the spinal accessory nerve results in paresis and/or atrophy of the
sternocleidomastoid and trapezius muscles.
Sternocleidomastoid paresis. Manifests as weakness in turning the head to the opposite side. Test by
observing with inspection and palpation while having the patient rotate the head against resistance. Bilateral
sternocleidomastoid paresis results in weak neck flexion.
Trapezius paresis. Manifests as shoulder drop, difficulty raising abducted arm above horizontal. Test by
observing with inspection and palpation while having the patient shrug the shoulders against resistance. Bilateral
trapezius paresis results in weak neck extension. Because of partial innervation of the trapezius from the cervical
plexus, lower trapezius function may be spared in pure accessory palsies.
Clinical Disorders.
Supranuclear lesions (hemispheric lesions resulting in contralateral hemiplegia ) of the eleventh nerve
cause moderate, often transient, impairment of function of the sternocleidomastoid and trapezius muscles, due to the
bilateral innervation.
Nuclear Lesions - rarely cause CN XI palsy in isolation.
• High cervical cord or low medullary lesions (e.g., brainstem infarction, brainstem tumor, syringomyelia)
• Neoplasms (e.g., glomus tumors, schwannomas, meningiomas)
• Trauma
Extracranial Lesions
Isolated lesions of the spinal accessory nerve are rare. Surgical injury is one cause (jatrogenic).
An idiopathic mononeuropathy manifested by pain along the posterior border of the sternocleidomastoid
muscle followed by weakness has been reported.
Radiation can cause injury, with or without involvement of other nerves.
Bilateral weakness suggests muscle or neuromuscular disease.
MASTICATION – V Trigeminal Nerve
Motor component of the TGN
Motor nucleus receives fibers from: Both cerebral hemispheres (firft motor neuron - Corticonuclear fibers)
Motor nucleus (second motor neuron situated in the pons medial to the main sensory nucleus) through
Mandibular branch supplies:
1.
Muscles of mastication (temporalis, masseter, medial and lateral pterygoid)
2.
Tensor tympani
3.
Tensor veli palatine
4.
Mylohyoid
5.
Anterior belly of the digastrics muscle
EXAMINATION
Jaw reflex, masticatorius
Examination Technique:

palpate the temporalis and masseter muscles on either side when
the patient clenches their teeth.

ask the patient to open their mouth and repeat this against
resistance. Observe for any deviation of the jaw to one side.

with their mouth open, ask the patient to protrude their jaw to
either side against resistance.

the jaw-jerk reflex is elicited by the examiner placing their index
finger over the middle of the patient’s chin with the mouth
slightly open and the jaw relaxed. The index finger is then
tapped with a reflex hammer, delivering a downward stroke.
The afferent impulse for this reflex is the sensory portion of the
trigeminal nerve. The efferent limb is through the motor (V3)
branch of the trigeminal nerve.
Normal Response:

the jaw should not deviate to either side.

the jaw-jerk is usually absent or weakly present.
Abnormal Response:

the jaw deviates towards the side of weakness.

the jaw-jerk is exaggerated and pathologically
brisk with lesions affecting the pyramidal pathways above the
5th nerve motor nucleus, especially if the lesions are bilateral.
A lesion of the motor portion of the third branch causes
paralysis of the muscles of mastication. The examiner can
usually easily feel the diminished contraction of the masseter m.
on one side. When the mouth is opened, the jaw deviates toward the paralyzed side because of weakness of the
pterygoid mm.
FACIAL EXPRESSION – the facial nerve, CN VII
The facial nerve, CN VII, is the seventh paired cranial nerve.
Anatomical Course
The course of the facial nerve is very complex. There are many branches, which transmit a combination of
sensory, motor and parasympathetic fibres.
Anatomically, the course of the facial nerve can be divided into two parts:

Intracranial – the course of the nerve through the cranial cavity, and the cranium itself.

Extracranial – the course of the nerve outside the cranium, through the face and neck.
Intracranial
The nerve arises in the pons, an area of the brainstem. It begins as two roots; a large motor root, and a small
sensory root (the part of the facial nerve that arises from the sensory root is sometimes known as the intermediate
nerve).
The two roots travel through the internal acoustic meatus, a 1cm long opening in the petrous part of the
temporal bone. Here, they are in very close proximity to the inner ear.
Still within the temporal bone, the roots leave the internal acoustic meatus, and enter into the facial canal.
The canal is a ‘Z’ shaped structure. Within the facial canal, three important events occur:
1.
Firstly the two roots fuse to form the facial nerve.
2.
Next, the nerve forms the geniculate ganglion (a ganglion is a collection of nerve cell bodies)
3.
Lastly, the nerve gives rise to the greater petrosal nerve (parasympathetic fibres to glands), the
nerve to stapedius (motor fibres to stapedius muscle), and the chorda tympani (special sensory fibres to the anterior
2/3 tongue).
The facial nerve then exits the facial canal (and the cranium) via the stylomastoid foramen. This is an exit
located just posterior to the styloid process of the temporal bone.
Extracranial
After exiting the skull, the facial nerve turns superiorly to run just anterior to the outer ear.
The first extracranial branch to arise is the posterior auricular nerve. It provides motor innervation to the
some of the muscles around the ear. Immediately distal to this, motor branches are sent to the posterior belly of the
digastric muscle and to the stylohyoid muscle.
The main trunk of the nerve, now termed the motor root of the facial nerve, continues anteriorly and
inferiorly into the parotid gland (Note that the facial nerve does not contribute towards the innervation of the parotid
gland). Within the parotid gland, the facial nerve terminates by bifurcating into five motor branches. These innervate
the muscles of facial expression:
1.
Temporal branch - Innervates the frontalis, orbicularis oculi and corrugator supercilii
2.
Zygomatic branch - Innervates the orbicularis oculi.
3.
Buccal branch - Innervates the orbicularis oris, buccinator and zygomaticus muscles.
4.
Marginal Mandibular branch- Innervates the mentalis muscle.
5.
Cervical branch - Innervates the platysma.
Examination
The major role of the facial nerve is to innervate the muscles of facial
expression. These can be observed while taking the history and then more
formally assessed during the neurological examination.

observe for asymmetry – widening of the palpebral fissure or
flattening of the nasolabial fold.

observe for involuntary facial movements (e.g. hemifacial spasm,
orofacial dyskinesia, myokymia, or synkinesis).

ask the patient to wrinkle their forehead by raising their eyebrows and close their eyes tightly. Observe for
asymmetry of ability to burry the eyelashes and palpate for differences of ability to resist eye opening. Ask the
patient to show their teeth, puff out their cheeks and appose their lips.

recall that the efferent limb of the corneal reflex (see trigeminal nerve) is through the 7th cranial nerve.
Normal Response: although patients may have an asymmetric face, there should be no facial weakness.
Abnormal Response:

lower motor neuron weakness causes weakness of the entire side of the face with equal involvement
of upper and lower facial muscles.

an upper motor neuron lesion of the contralateral supranuclear pathway results in weakness primarily
of lower muscles of facial expression. The upper muscles of facial expression (frontalis and orbicularis oculi) are
much less affected because the facial nucleus that innervates them receives partial input from the ipsilateral
hemisphere.
Central innervation of the facial nuclear area in the brainstem.
The portion of the nuclear area controlling the muscles of the forehead is innervated by both cerebral
hemispheres. Thus, a lesion affecting the corticonuclear pathway on one side does not cause weakness of the
forehead muscles. The remainder of the nuclear area, however, is innervated only by the contralateral hemisphere. A
unilateral lesion along the corticonuclear pathway therefore causes contralateral facial weakness with sparing of the
forehead muscles.
What conditions affect the facial nerve?
There are numerous causes of facial nerve disorder:

Trauma such as birth trauma, skull base fractures, facial injuries, middle ear injuries, or surgical
trauma



Nervous system disease including stroke involving the brain stem
Infection of the ear or face, or herpes zoster of the facial nerve (Ramsay Hunt syndrome)
Tumors including acoustic neuroma, schwannoma, cholesteatoma, parotid tumors, and glomus
tumors


Toxins due to alcoholism or carbon monoxide poisoning
Bell's palsy, which is also called idiopathic facial nerve paralysis (see below); this condition is
sometimes associated with diabetes mellitus or pregnancy
Disorders caused by infection
Bell’s palsy
Bell's palsy is the most common cause of facial paralysis and accounts for approximately 80% of all cases. It
is also known as idiopathic unilateral facial paralysis. 15% of patients experience partial facial weakness.
Treatment usually consists of a course of steroids and advice on eye care, oral hygiene and facial
rehabilitation. Complete recovery is expected in 85% of cases in six to nine months.
Intracranial Lesions
The muscles of facial expression will be paralysed or severely
weakened. The other symptoms produced depend on the location of the
lesion, and the branches that are affected:

Chorda tympani – reduced salivation and loss of taste
on the ipsilateral 2/3 of the tongue.

Nerve to stapedius – ipsilateral hyperacusis
(hypersensitive to sound).

Greater petrosal nerve – ipsilateral reduced lacrimal
fluid production.
The most common cause of an intracranial lesion of the facial
nerve is middle ear pathology – such as a tumour or infection. If no
definitive cause can be found, the disease is termed Bell’s palsy.
Extracranial Lesions
Extracranial lesions occur during the extracranial course of the
facial nerve (distal to the stylomastoid foramen). Only the motor
function of the facial nerve is affected, resulting in paralysis or severe
weakness of the muscles of facial expression.
There are various causes of extracranial lesions of the facial
nerve:

Parotid gland pathology – e.g a tumour.

Infection f the nerve - particularly by the herpes virus.

Compression during forceps delivery – the neonatal mastoid process is not fully developed, and
does not provide complete protection of the nerve.

Ideopathic – If no definitive cause can be found, the disease is termed Bell’s palsy.
Ramsay Hunt syndrome
Ramsay Hunt syndrome is caused by a virus in the facial nerve and is a more severe cause of facial paralysis.
Patients may have blisters in the ear and on the roof of the mouth as well as facial weakness. Unlike Bell's palsy, this
syndrome has a complete recovery rate of less than 50%. Initial treatment consists of steroids, antiviral medication,
eye care and facial rehabilitation.
Lyme disease
Lyme disease is a bacterial infection which is transmitted to humans via the bite of infected ticks. There may
have been a target shaped skin rash prior to the presentation of a variety of symptoms such as headache, fever or
weakness. Neurological symptoms may also appear, one of which is facial paralysis.
Once diagnosis has been confirmed, the initial treatment consists of antibiotics and eye care.
Trauma
Traumatic injuries to the head and face represent one of the most common causes of severe permanent facial
paralysis. In particular, fractures through the temporal bone of the skull are commonly associated with injury to the
facial nerve, as well as injury to the labyrinth leading to hearing loss and vertigo. Electrophysiological testing of the
facial nerve and imaging are used to determine the severity of the injury. Surgical decompression or grafting of the
facial nerve is sometimes required. Soft tissue injuries, such as lacerations, can also damage the facial nerve and it is
important that these are explored and repaired correctly for the best chance of rehabilitation.
Iatrogenic injury
Iatrogenic injury can occur during surgical procedures to the head and face. The degree of injury to the facial
nerve will determine what type of treatment may be required. In severe cases a repair to the nerve may be required
and if this is not possible other surgical procedures may be necessary to restore movement and or symmetry to the
face.
Skull base tumours
Tumours within the facial nerve, benign tumours close to and compressing the facial nerve or malignant
tumours invading the facial nerve may result in facial weakness. The tumours most commonly involved are acoustic
neuroma, facial neuroma and tumours in the region of the parotid gland. These should be considered if there is a
progressive or recurrent facial weakness or if there are other symptoms, for example hearing loss or neck mass.
Neurological conditions
Guillain Barre syndrome or a peripheral neuropathy may present with bilateral facial weakness. Stroke may
cause loss of movement to the lower part of the face.
Hemifacial spasm (HFS) is an involuntary twitching or contraction of the facial muscles on one side of the
face. The most common cause is compression of your facial nerve by the anterior inferior cerebellar artery where the
nerve begins at your brainstem. The compression causes the nerve to misfire making your facial muscles contract.
This condition is related to trigeminal neuralgia—an irritation of the fifth cranial nerve that causes severe facial pain.
Both hemifacial spasm and trigeminal neuralgia are caused by nerve compression from a blood vessel, yet differ in
whether the sensory nerve or motor nerve is compressed.
Figure 2. Hemifacial spasm is most often caused by an artery compressing the facial nerve. During surgery, a
sponge is inserted between the artery and the nerve to relieve the pressure and stop the facial muscle spasms.
SWALLOWING – Glossopharyngeal Nerve: IX
The glossopharyngeal or ninth cranial nerve is mixed, containing both motor and sensory fibres.
Branchio-motor fibres to the stylopharyngeus muscle;
The nerve is connected with the following nuclei of the medulla oblongata:
Upper part of the nucleus ambiguus, which gives origin to the branchio-motor fibres;
SWALLOWING – X Vagus nerve
The vagus nerve is the longest cranial nerve. It contains motor and sensory fibers and, because it passes
through the neck and thorax to the abdomen, has the widest distribution in the body. Corticobulbar (bilateral) fibers
descend through the internal capsule to synapse in the nucleus ambiguus. The axons of the lower motor neurons
come out as 8-10 rootlets between the olive and pyramid, exiting the skull through the jugular foramen. They then
divide into 3 main branches: the pharyngeal, superior, and recurrent laryngeal nerves.
Centra
Cell
Components
Function
Peripheral distribution
l connection
bodies
Branchial motor (efferent
Swallowing,
Nucleus
Nucleus
Pharyngeal branches, superior and
special visceral)
phonation
ambiguus
ambiguus
inferior laryngeal nerves
The nucleus ambiguus — which gives rise to the branchial efferent motor fibers of the vagus nerve
TONGUE Movements – XII Hypoglossal Nerve
Description and Physiology
The hypoglossal nerve is a motor nerve
responsible for innervating the muscles of the
tongue. This cranial nerve is also responsible for
the articulation of speech and the act of
swallowing. It originates from the columns of
motor neurons located near the midline in the
dorsal aspect of the medulla. The nerve exits the
ventral side of the medulla as a row of small nerve
rootlets adjacent to the pyramid. After a short
course through the subarachnoid space, the
rootlets come together as a single nerve that passes
through the hypoglossal foramen in the base of the
skull. The hypoglossal nerve innervates all of the
muscles of the tongue except for the palatoglossus,
which is controlled by the vagus nerve.
The three muscles of the tongue controlled by the
hypoglossal nerve are the genioglossus muscle,
the hypoglossus muscle, and styloglossus
muscle.
The genioglossus muscle pushes the
tongue out and can depress the center of the
tongue.
The hypoglossus muscle is responsible for
depressing the entire tongue.
The styloglossus muscle is the muscle that retracts and elevates the tongue.
Of course, the tongue is under voluntary control. Accordingly, corticobulbar pathways activate hypoglossal
motor neurons. As with most cranial nerves, these corticobulbar projections are bilateral, although there is a slight
contralateral predominance. Therefore, large lesions to the corticobulbar system, such as large strokes, can produce
slight weakness of the contralateral tongue.
Examination and Hypoglossal Nerve Lesions
Nerve lesions on the hypoglossal nerve can produce wasting on the opposite side of the tongue and also
fasciculations, or muscle twitches. When the tongue is pushed out of the mouth it deviates toward the side where the
lesion exists. For a central lesion the tongue deviates away from the central lesion. This is the result of a low motor
neuron lesion. Weakness of the tongue is displayed as a slurring of speech. The tongue may feel “thick”, “heavy”, or
“clumsy.” To test the function of the nerve, a person is asked to poke out his/her tongue. If there is a loss of function
on one side, the tongue will point toward the affected side. Weakness of the tongue manifests itself as a slurring of
speech. The patient complains that their tongue feels "thick", "heavy", or "clumsy." Lingual sounds (i.e., l's, t's, d's,
n's, r's, etc.) are slurred and this is obvious in conversation even before direct examination. Examination of the
tongue first involves observation for atrophy and fasciculations.
With supranuclear lesions, weakness, frequently mild, is not accompanied by loss of muscle mass or
fasciculations.
Lesions of the nerve (e.g., hypoglossal neurolemmoma, nasopharyngeal tumor along the base of the skull,
basal skull fracture) or of the nucleus in the brain stem (e.g., medullary stroke, motor neuron disease or bulbar
poliomyelitis) the tongue displays weakness, atrophy and, possibly, fasciculations on the side of the involvement.
Atrophy and fasciculations in combination suggest disease or damage to the motor neurons of the brain stem, but can
be seen with peripheral nerve damage as well. Fasciculations are fine, random, multifocal twitches of muscle. They
are evaluated by observing the tongue while it is at rest in the floor of the mouth. They are best seen along the lateral
aspect of the tongue.
Simply having the patient protrude their tongue in the midline tests strength of the tongue. The normal
vectors of protrusion are illustrated in Figure. When one side of the tongue is weak, it protrudes toward the weakened
side.
A, normal vectors of tongue protrusion.
B, weakness without atrophy of right side of
tongue with left corticobulbar lesion.
C, weakness and atrophy (and fasciculations) with
lesion of hypoglossal nucleus or nerve. The tongue
always deviates toward the weak side whether the
lesion is nuclear or supranuclear.
BULBAR PALSY AND PSEUDOBULBAR PALSY
Bulbar palsy refers to impairment of function of the cranial nerves IX, X, XI and XII, which occurs due to a
lower motor neuron lesion either at nuclear or fascicular level in the medulla oblongata or from lesions of the lower
cranial nerves outside the brainstem.
In contrast, pseudobulbar palsy describes impairment of function of cranial nerves IX-XII due to upper
motor neuron lesions of the corticobulbar tracts in the mid-pons. For clinically evident dysfunction to occur, such
lesions must be bilateral as these cranial nerve nuclei receive bilateral innervation.
Pseudobulbar Palsy
degeneration of corticobulbar pathways to
V,VII,X,XI,XII
Bulbar Palsy
disturbance to X, XI, XII,sometimes VII, rather
than the corticobulbar tracts
lower motor neurone signs absent
lower motor neurone signs present
gag reflex (+/n)
gag reflex (-)
spastic tongue
wasted tongue, fasciculations
jaw jerk (+)
jaw jerk (n)
spastic dysarthria
nasal speech
labile emotions
normal emotions
bilateral UMN
signs in limbs
VOICE – X Vagus nerve
Understandable voice is produced by co-ordinated movements of the tongue, lower jaw and soft palate – the
flexible part of the roof of the mouth. This process is called articulation. Clearly it is a complex system, depending
for its success on sophisticated control. The brain acts as a control centre which receives and sends out signals to
different parts of the body including the diaphragm, muscles of the chest wall, abdomen, larynx, pharynx, oral –
phonatory system.
Divides into left & right branches, then further divides into 3 branches,
1) Pharyngeal
2) Superior laryngeal
3) Recurrent laryngeal
Dysarthria
Dysarthric speech is characterized by problems with articulation (i.e., production of speech sounds), voicing
(i.e., volume and quality of speech) and prosody (i.e., speech rate, rhythm and naturalness). The symptoms include:
"slurred" speech, slow rate of speech, rapid rate of speech with a "mumbling" quality, speaking softly or barely able
to whisper, limited tongue, lip, and jaw movement, abnormal intonation, changes in vocal quality ("nasal" speech or
sounding "stuffy"), hoarseness, breathiness, drooling or poor control of saliva, chewing and swallowing difficulty.
The complete loss of speech is called "anarthria". The type and severity of dysarthria depends on which area of the
nervous system is affected. Dysarthria is a speech disorder resulting from neurological injury including amyotrophic
lateral sclerosis (ALS or Lou Gehrig’s disease), cerebral palsy, Parkinson’s disease, locked-in syndrome, multiple
sclerosis and traumatic brain injury. It may also arise from other diseases with neurological sequelae, including
tumor, postoperative complications, inflammatory and metabolic
Dysphonia
Dysphonia is characterized by altered vocal quality, pitch, loudness, or vocal effort.
Normal voice production depends on power and airflow supplied by the respiratory system; laryngeal muscle
strength, balance, coordination, and stamina; and coordination among these and the supraglottic resonatory structures
(pharynx, oral cavity, nasal cavity).
A disturbance in one of the three subsystems of voice production (i.e., respiratory, laryngeal, and subglottal
vocal tract) or in the physiological balance among the systems may lead to a voice disturbance. Disruptions can be
due to organic, functional, and/or psychogenic causes.
There are several other neurogenic disorders that may affect the larynx and in turn, affect voice quality. In
many cases, these neurogenic voice disorders will be accompanied by other speech and swallowing difficulties such
as dysarthria, dysphagia, apraxia, aphasia, and other cognitive difficulties. Resonance may also be affected by
neurogenic disorders. The range and type of neurologic voice problems that may occur are extremely varied and each
case may present with a different degree of dysphonia and accompanying disorders. Some of the diseases that may
cause voice disorders include (but are not limited to): myasthenia gravis, multiple sclerosis (MS), Parkinson’s
Disease, Amyotrophic Lateral Sclerosis (ALS), Huntington’s Chorea, progressive bulbar palsy.
Cranial Nerves Associated With the Control of Eye Movements
Eye movements
The 3rd 4th and 6th cranial nerves are tested together by examining eye movements. Eye movements are
generated in two main ways each of which should be tested separately. Firstly voluntary movements are generated
from the frontal lobe; they are also called saccadic because of the rapid jumping movement from one point of
fixation to another. These are tested by asking the patient to look rapidly from one side to the other or right and left
and are impaired in cortical brain disease. Secondly and more important clinically are pursuit eye or tracking
movements which are generated from the occipital lobe when the eyes stay on and follow the point of fixation. These
are tested by asking the patient to follow the examiner’s moving finger and are impaired in brain stem and cranial
nerve disorders. Lastly the cerebellum also plays a main role in controlling eye movements in response to body
movements in order to keep the point of fixation. All eye movements are integrated in the brain stem so that the eyes
can move together conjugately in all directions. Eye movement abnormalities are usually noted because the patient
complains of double vision or diplopia and because the eyes appear to the observer be looking in different directions.
When this happens it is called a squint or strabismus. The main causes of diplopia are disorders affecting the function
of the 3rd 4th and 6th cranial nerves. The main sites for these disorders are eye muscles, the neuromuscular junction,
or the individual nerves and their central connections in the brain stem. The most common causes are vascular and
inflammatory disorders affecting the individual nerves and neuromuscular junction respectively.
FIGURE Origin and distribution of cranial nerves (CN) VI, IV, and III, which innervate extraocular eye
muscles. The focus of the upper part of this figure includes the abducens nerve (CN VI) and the general somatic
efferent component of the oculomotor nerve (CN III), which are essential for horizontal gaze. The lower part of this
figure depicts the muscles of the eye and their relationship with CN III, IV, and VI.
Abducens Nerve (Cranial Nerve VI)
Components: The abducens nerve is a pure motor nerve whose principal function is to move the eye
laterally (i.e., abduct the eye). The cell bodies of origin of the abducens nerve lie in the abducens nucleus in the
dorsomedial aspect of the posterior pons. The axons pass ventrally and exit the brain in a medial position at the ponsmedulla border. The abducens nerve courses ventrally into the cavernous sinus and exits the skull through the
superior orbital fissure, as do CN IV and III. Intracranial course segments are: cisternal, petro-clival, cavernous,
orbital. Peripherally, the fibers innervate the lateral rectus muscle on the ipsilateral side. Stimulation of the abducens
nerve results in a contraction of the lateral rectus muscle and causes the eye to be moved laterally.
Clinical Disorders. A peripheral or central lesion involving the abducens nucleus produces paralysis of the
lateral rectus muscle. This results in medial strabismus, which is the inability for both eyes to be focused on the
same object. This is due to the fact that the affected eye cannot abduct and that the affected eye will tend to lay
medially when looking forward. The resulting effect is double vision. To eliminate the double vision, the patient
moves his or her head so that the affected eye is facing the object directly, while the unaffected eye then compensates
for a change in position of the object in the visual field.
The function of this nerve is tested by asking the patient to focus on an object that is placed in the lateral
aspect of his or her visual field without moving his or her head. Because the affected eye cannot move beyond the
midline of the visual field as a result of the unopposed action of the medial rectus muscle, the patient (with the
normal eye closed) will be unable to identify the object located in the lateral aspect of his or her visual field, and the
disorder is thus easily identified.
Trochlear Nerve (Cranial Nerve IV)
Components: The trochlear nerve is unique in that it is the only nerve that exits the brain dorsally and is
also crossed. The cell bodies lie in a medial position just below the midbrain periaqueductal gray at the level of the
inferior colliculus in proximity to the medial longitudinal fasciculus. The fibers pass dorsally and caudally, cross
over to the contralateral side, and emerge from the brain just behind the inferior colliculus. The fibers continue
anteriorly and enter the cavernous sinus. These fibers enter the orbit through the superior orbital fissure and supply
the superior oblique muscle.
The primary action of this muscle is to move the eye downward when it is located in a medial position.
Clinical Disorders. When there is a paralysis of the trochlear nerve, there is an outward rotation of the eye
due to the unopposed action of the inferior oblique muscle. Therefore, when these patients attempt to look downward
and inward, such as when walking down a staircase, they experience double vision and will tend to fall down.
Patients will frequently compensate for this double vision by tilting their heads. Tilting of the head upon downward
gaze thus provides a clue of the presence of a trochlear lesion. Clinically, one could test for fourth nerve lesions by
asking patients to follow an object as it is moved downward within their medial fields of vision without moving their
heads. Again, failure to do so would indicate the likelihood of a fourth nerve lesion.
FIGURE Origin and distribution of the trochlear nerve (cranial nerve IV) to the superior oblique muscle. As
indicated in the cross section of the brainstem, note that this nerve exits the brain from the dorsal aspect, and it is the
only nerve that is crossed. Arrow indicates direction of movement of the bulb downward and inward.
Oculomotor Nerve (Cranial Nerve III)
Components: The oculomotor nerve controls both skeletal and smooth muscles (via a postganglionic
neuron). The general somatic efferent GSE component of the third nerve provides innervation to all of the
extraocular eye muscles (skeletal muscle) except for the lateral rectus and superior oblique muscles. The GVE
component provides pre-ganglionic parasympathetic innervation to the pupillary constrictor and ciliary muscles
(smooth muscle) through connections with postganglionic parasympathetic neurons in the ciliary ganglion.
GSE: Origin, Distribution, and Function. The oculomotor nucleus is located in a medial position just
below the floor of the midbrain periaqueductal gray at the level of the superior colliculus. The nucleus actually
consists of a group of subnuclei, each of which gives rise to axons that innervate skeletal muscle or the ciliary
ganglion. The nerve fibers pass ventrally in the medial aspect of the mid-brain, exit the brain medial to the cerebral
peduncle, pass through the interpeduncular fossa, enter the cavernous sinus, and then enter the orbit through the
superior orbital fissure. The GSE component of the oculomotor nerve supplies the superior, medial, and inferior
rectus muscles as well as the inferior oblique and levator palpebrae superior muscles. The action of the medial rectus
muscle is to move the eye medially; the superior and inferior rectus muscles move the eye up and down,
respectively; the inferior oblique muscle elevates the eye when it is in the medial position; and the levator palpebrae
superior muscle elevates the upper eyelid.
GVE (general visceral efferent=autonomic) Component. As described earlier, the cells that give rise to
the GVE component of the oculomotor nerve, called the Edinger-Westphal nucleus, are located close to the GSE
component and are situated around the midline. Fibers from the Edinger-Westphal nucleus, which constitute
preganglionic parasympathetic neurons, project to the ciliary ganglion. Postganglionic parasympathetic fibers from
the ciliary ganglion then innervate the pupillary constrictor muscles and the ciliary muscles. Note that the pupillary
dilator muscles of the eye receive their innervation from postganglionic sympathetic fibers that arise from the
superior cervical (sympathetic) ganglion.
Contraction of the pupillary constrictor muscles results in a constriction of the size of the pupil. Likewise,
constriction of the ciliary muscles causes a release of tension from the suspensory ligament of the lens, thus causing
it to bulge (i.e., increase its curvature). Accordingly, the GVE component of the oculomotor nerve is capable of
regulating both the size of the pupil (i.e., the amount of light that enters the eye) and the shape of the lens.
In this reflex, light shown into one eye results in a constriction of the pupil in both eyes. Constriction of the
pupil in the same eye that received the light is called the direct light reflex, and the constriction in the other eye is
called the consensual light reflex. The pathway for this reflex involves afferent signals passing in the optic nerve and
optic tract whose fibers terminate, in part, in the pretectal region. Fibers from the pretectal region innervate the
oculomotor nucleus bilaterally. When the oculomotor nucleus on each side is stimulated, impulses are transmitted
along the preganglionic neurons to the pupillary constrictor muscles on each side via postganglionic neurons in the
ciliary ganglion.
A second reflex is called the accommodation reflex. This reflex occurs when an individual attempts to
focus on a near object after looking at more distant objects. The responses that occur include:
(1) pupillary constriction; (2) medial convergence of the eyes by the simultaneous actions of medial recti
muscles; and (3) focusing of the eyes on the near object, which requires contraction of the ciliary muscles, causing
the suspensory ligament to relax and the lens to bulge. This reflex occurs by the activation of the following
pathways: (1) descending cortical fibers from the occipital cortex to the oculomotor complex via a synapse in the
pretectal region;
(2) activation of both somatic motor fibers that cause the medial rectus muscle on each side to contract; and
(3) activation of the visceral motor neurons that stimulate the ciliary ganglion, resulting in both pupillary
constriction and a bulging of the lens, which allows the light rays to properly focus on the near object.
Clinical Disorders. Lesions involving the third nerve can affect both GSE and GVE components.
Concerning the GSE component, lesions will produce lower motor neuron paralysis of the extraocular eye muscles
supplied by this nerve. The most common forms of deficits include: (1) the inability to move the eye inward or
vertically (because of the loss of all of the recti muscles, except the lateral rectus muscle, as well as the loss of the
inferior oblique muscle); (2) lateral strabismus, in which the eye on one side is now not coordinated with the
opposite eye whose extraocular eye muscles are intact, causing diplopia (double vision); and (3) drooping of the
eyelid (called ptosis), which results from damage to the nerves innervating the levator palpe-brae superior muscle.
Lesions of the GVE components will produce the following autonomic effects: (1) loss of the pupillary
light reflex; and (2) accommodation, which includes the convergence reactions. It is also possible that, as a result of
a lesion, the pupil will remain small, but pupillary constriction will be brisk during accommodation. This disorder
results from syphilis and is referred to as the Argyll Robertson pupil, but the locus of the lesion for this disorder
remains unknown.
If lesions involving the third nerve are located within the CNS rather than peripherally, it is likely that a
constellation of deficits will be present. The most typical case involves a lesion located near the ventromedial aspect
of the midbrain. Such a lesion i nvariably affects both fibers of the third nerve and corticospinal fibers contained
within the crus cerebri because of the proximity of one fiber system to the other.In such a condition, the patient
displays both third nerve paralysis as well as upper motor neuron paralysis of the contralateral limbs. This is referred
to as Weber’s syndrome or superior alternating hemiplegia.
FIGURE Origin and distribution of the oculomotor nerve (cranial nerve [CN] III). The anatomical
organization of the general somatic efferent (GSE) cell columns of the oculomotor nerve (CN III) complex, whose
axons innervate all of the extraocular eye muscles except the lateral rectus and superior oblique muscles, is shown.
Supranuclear Oculomotor Disturbances
These disturbances are defined as those in which the voluntary movements and involuntary pursuit
movements of both eyes are simultaneously impaired. The eyes generally remain parallel to each other, but they
cannot be moved together in the horizontal or vertical plane. The lesion lies above the level of the cranial nerve
nuclei and is thus “supranuclear.” In disorders of the brainstem, supranuclear lesions may coexist with nuclear
lesions, so that a skew deviation can also be present.
Horizontal Gaze Palsy ------ A patient with horizontal gaze palsy cannot make a conjugate movement of
the eyes to the right, to the left, or (rarely) in either direction. The causative lesion may be at any of several sites in
the central nervous system:
_ cortical centers generating the impulses for horizontal gaze movements, particularly the frontal eye field
of the frontal lobe;
_ the paramedian pontine reticular formation (PPRF), which receives the impulses from the higher cortical
centers and relays them to the ipsilateral abducens n. nucleus (innervation of the lateral rectus m.) and
simultaneously, by way of interneurons, to the contralateral oculomotor n. nucleus (innervation of the medial rectus
m.). This projection lies within the medial longitudinal fasciculus (MLF, Fig. 11.3). The result is an ipsilateral,
conjugate, horizontal gaze movement (i. e., to the left on activation of the left PPRF and to the right on activation of
the right PPRF);
_ a lesion of the abducens n. nucleus has the same effect as a PPRF lesion, i. e., a conjugate horizontal gaze
palsy to the side of the lesion (see above). Lesions of the frontal eye field. This field occupies area eight in the
middle frontal gyrus. The right eye field generates conjugate gaze movements to the left and the left eye field
generates conjugate gaze movements to the right. When the frontal eye field is affected by an acute lesion, the
influence of the contralateral field predominates for a few hours (or, rarely, days), so that the eyes (and the head)
deviate to the side of the lesion: deviation conjuguee, the patient “looks at the lesion.” Deviation conjuguee is usually
accompanied by contralateral hemiparesis. Active gaze movements toward the midline rapidly become possible
again; so, later, do movements to the opposite side. As contralateral movements begin to reemerge, they are
accompanied by gaze-paretic nystagmus, whose rapid component beats away from the side of the lesion.
Lesions of the posterior hemispheric cortex. Horizontal gaze palsy due to an occipital lesion is often
accompanied by hemianopsia. The gaze palsy is characterized by saccadization of ocular pursuit movements and
optokinetic nystagmus (p. 185) is impaired.
The medial longitudinal fasciculus (MLF) is a pair of crossed fiber tracts (group of axons), one on each
side of the brainstem. These bundles of axons are situated near the midline of the brainstem and are composed of
both ascending and descending fibers that arise from a number of sources and terminate in different areas. MLF is
the main central connection for the oculomotor nerve, trochlear nerve, and abducens nerve. The vertical gaze center
is at the rostral interstitial nucleus (riMLF).
The MLF ascends to the interstitial nucleus of Cajal, which lies in the lateral wall of the third ventricle, just
above the cerebral aqueduct.
Lesions of the paramedian pontine reticular formation (PPRF) affect the last supranuclear “relay station”
for horizontal gaze movements. They usually cause long-lasting or permanent gaze palsy to the side of the lesion.
Lesion of the abducens n. nucleus affects not only the neurons whose axons constitute the sixth cranial
nerve, but also interneurons that connect the nucleus by way of the adjacent medial longitudinal fasciculus (MLF) to
the contralateral oculomotor n. nucleus, which innervates the contralateral medial rectus m. The clinical picture is
initially very similar to that of a PPRF lesion. PPRF lesions, however, spare the vestibule -ocular connections in the
MLF and do not directly involve the cranial nerve nuclei subserving eye movement; thus, in PPRF lesions, the gaze
palsy can be overcome by a vestibular stimulus. In contrast, gaze palsy due to a lesion of the abducens n. nucleus
cannot be overcome either voluntarily or through any kind of reflex.
Vertical Gaze Palsy
------- Impairment of upward or downward conjugate gaze is always due to a
midbrain lesion involving either the rostral interstitial nucleus of the medial longitudinal fasciculus (the
Bьttner−Ennever nucleus) or its efferent fibers (Fig. 11.3). In most patients, both upward and downward gaze are
impaired, but pretectal lesions can cause isolated upward gaze palsy. Vertical gaze palsy is one of the clinical
features of progressive supranuclear palsy (p. 130).
Internuclear Ophthalmoplegia
This condition is caused by a lesion of the medial longitudinal fasciculus (MLF). When the patient
attempts to look away from the side of the lesion, the ipsilateral (adducting) eye cannot fully adduct, and the
contralateral (abducting) eye exhibits end-gaze nystagmus. The inability of the ipsilateral eye to adduct is not due to
a lesion of the oculomotor n. nucleus, as is demonstrated by a preserved ability to adduct (converge) in the near
reflex. Internuclear ophthalmoplegia (INO) can also be bilateral if the MLF is damaged on both sides.