Cranial Nerves I
Introduction
• The head and neck are not innervated by
spinal nerves; rather, they receive sensory
information and send motor information via
the 12 cranial nerves located in the brain.
Although they are located within the skull,
cranial nerves are considered part of the
peripheral nervous system because they
convey messages to and from the body's
muscles and glands.
Introduction
• The cranial nerves function as modified spinal
nerves. While all the spinal nerves convey both
sensory and motor information, cranial nerves
may carry solely sensory information, solely
motor information, or both. Cranial nerves that
have only a sensory or motor component can be
called pure nerves ; nerves that have both
components can be called mixed nerves .
Introduction
• The cranial nerve routes for sensory and motor circuits
have different neuroanatomical connections. Sensory
pathways are composed of 3 major neurons: the
primary, the secondary, and the tertiary (see Figure 2).
The cell bodies of primary neurons are usually located
outside the CNS in sensory ganglia. They are
homologous with the dorsal root ganglia of the spinal
cord. The cell bodies of secondary neurons are in the
dorsal gray matter of the brain stem, and their axons
usually cross the midline and project to the thalamus.
These are the actual sensory components of the cranial
nerves. The cell bodies of the tertiary neurons are in the
thalamus, and their axons project to the sensory cerebral
cortex. There are two exceptions to this neuroanatomical
scheme. Cranial nerves I and II (olfactory and optic) are
special cases; the afferent fibers of their primary sensory
neurons enter the brain stem and terminate on the
secondary sensory neurons.
Introduction
• Motor pathways are composed of only two
major neurons: the upper motor neuron and
the lower motor neuron. The upper motor
neuron is usually located in the cerebral cortex.
It's axon projects caudally to contact the lower
motor neuron. Most, but not all, of the motor
pathways that terminate in the brain stem project
bilaterally to contact lower motor neurons on
both sides of the midline. The lower motor
neuron is located in the brain stem. These are
the actual motor components of the cranial
nerves.
Cranial Nerves
• There are various mnemonics floating
around to help students learn the cranial
nerves. A few are shown below. Help for
remembering the functions of each nerve
are more mnemonics in the second table
below.
I Olfactory
On
Oh
II Optic
Old
Oh
III Oculomotor
Olympus
Oh
IV Trochlear
Towering
To
V Trigeminal
Tops
Touch
VI Abducens
A
And
VII Facial
Frenchman
Feel
VIII Auditory &/or Vestibulococchlear
And
Very
IX Glossopharyngeal
German
Green
X Vagus
Viewed
Vegetables
XI Spinal/Accessory
Some
Ah
XII Hypoglossal
Hops
Heaven
Cranial Nerves - S=Sensory; M=Motor; B=Both
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
Some
Say
Marry
Money
But
My
Brother
Says
Big
Business
Makes
Money
Some
Say
Marilyn
Monroe
But
My
Brother
Says
Bridget
Bardoe
Mmm
Mmm
I
Cranial Nerve I: Olfactory Nerve
• CN-I is a purely sensory nerve and serves
the sense of smell. It originates in the
olfactory cells of the nasal mucosa.
Olfactory neurons are bipolar neurons that
serve as the olfactory receptor. They are
bathed by a layer of mucus where odorous
substance dissolve into.
Olfactory Nerve
• Bundles of fibers collect information and
pass it to the olfactory bulb, which
continues in a caudal direction to the
olfactory tract. The olfactory tract extends
into the olfactory trigone, where olfactory
tract splits into lateral olfactory stria and
medial olfactory stria. Most of the olfactory
tract pass into the lateral olfactory stria,
which run into lateral olfactory area.
Olfactory Nerve
• Lateral olfactory area consists of the paleocortex of uncus,
cortex of the entorhinal area (the anterior part of the
parahippocampal gyrus) and the cortex in the region of the
limen insulae. The uncus, cortex of the entorhinal area and
cortex in the region of the limen insulae. The three parts are
generally referred as pyriform cortex (pear shaped). Part of
the amygdaloid body (amygdala) also is included in the
lateral olfactory area. Lateral olfactory area is the principal
region for the awareness of olfactory stimuli, so it is the
primary olfactory area.
• Because olfactory system shares entorhinal cortex with
limbic system, which has extensive connections with the
septal area and hypothalamus. The medial forebrain bundle
contains fibers that connect septal area and hypothalamus
with autonomic nuclei. Descending fibers from
hypothalamus proceed to autonomic nuclei in the brain
stem and spinal cord. So smell of food --> saliva secretion
Olfactory Nerve
• Fracture of the floor of anterior fossa could
damage the olfactory nerves and causing
anosmia (loss of smell), leakage of CSF
from nose (cerebrospinal fluid rhinorrhea).
Tumor can also block the olfactory
pathway, likely to be unilateral.
CN II: Optic Nerve
• CN-II is also a purely sensory nerve serving the sense of
vision. Photoreceptor, Rods, and cones receive light
stimuli and transmitted as action potential to series
layers of cells to ganglion cells. Rods is twenty times
more than the number of cones. Rods is absent in the
central fovea and gets more and more peripherally. It is
more sensitive to dim light and responsible for peripheral
vision. Cones is more sensitive to bright light and
responsible for color vision.
• Note that light and action potential travel at opposite
directions. Action potentials generated from visual
receptors pass via bipolar cells to ganglion cells, which
axons bundle together to form the optic nerve.
Optic Nerve
• Optic nerve originates in the ganglion cells
of the retina and travels to the optic
chiasm , where the fibers from the medial
half of each retina cross to the opposite
side while those from the lateral half of the
retina remain on the same side.
Retinal projections
• 1. Right half of two retinas terminate in the right lateral
geniculate body and to right hemisphere. Left half of two
retinas terminate in the left lateral geniculate body and to
left hemisphere
• 2. Upper quadrants peripheral to the macula (yellow
spot) end in medial part of lateral geniculate, then project
to anterior two thirds of visual cortex above calcrine
sulcus
• 3. lower quadrant peripheral to the macula end in lateral
part of lateral geniculate, then project to anterior two
thirds of visual cortex below calcrine sulcus.
• 4. macula projects to a relatively large area on lateral
geniculate body. Upper quadrant --> medial, lower
quadrant --> lateral, posterior one third of the visual
cortex
Visual Field Projection
• Visual field:
• Retinal image in the visual field is inverted and
reversed from right to left (can be trained the
other way around)
• 1. Left visual field is represented in the right
lateral geniculate body and in the visual cortex
of the right brain.
• 2. Upper half of the visual field is represented in
the lateral portion of the lateral geniculate body,
below calcrine sulcus.
• 3. lower half of the visual field is represented in
the medial portion of the lateral geniculate body,
above the calcrine sulcus
Images projected from the visual field to the retina are inverted
and reversed.
Upper visual field goes to the lower retina, lower visual field goes
to the upper retina
Right visual field projects to the left
hemiretina in each eye, left visual
field projects to the right hemiretina
Central fixation point for each eye falls
on the fovea where highest visual acuity
occurs
Optic nerve
•
•
surrounded by extension of meninges, so
increased intracranial pressure can be
presented by edema of optic disk
(papilledema)
Partial crossing:
– determines binocular vision. Left visual field
is represented in the right hemisphere
Normal
Papilledema
Lateral geniculate
• located in thalamus. Note that lateral
geniculate body has six layers of cells and
crossed and uncrossed optic tracts
terminate into different layers of cells.
Crossed fibers 1, 4, 6 and uncrossed
fibers 2, 3, and 5.
Geniculocalcarine tract
• originate from lateral geniculate , two branches
for upper visual field and lower visual field,
terminate into visual cortex, which is located on
the upper and lower lips of calcrine sulcus.
There is a detailed point to point projection of
the retina on the lateral geniculate body and on
the visual cortex. Therefore, a right temporal
lesion in the brain will affect the right lower
quadrants of both eyes (retina or visual fields ?),
and left upper visual fields will be affected. What
about a right parietal lesion?
Temporal lobe lesions can
cause a contralateral
homonymous superior
quadrantopia visual field
defect.
Parietal lobe lesions can
cause a contralateral
homonymous inferior
quadrantopia visual field
defect
Visual defect
• Macular sparing
– This is due to the fact that input from the
center of the retina (the macula) is spread
over a large portion of the optic radiation and
primary visual cortex.
Visual reflexes
• Small bundle of fibers from optic nerve
bypass lateral geniculate body and enter
the superior brachium which connects with
superior colliculus. Information can also be
projected to pretectal area (group of small
nuclei rostral to the superior colliculus.
Pupillary light reflex
• Information collected from retina could be relayed to
olivary pretectal nucleus (one of those nuclei in the
pretectal area) then to Edinger-Westphal, nucleus of
oculomotor complex finally to the sphincter pupillae in
the orbit and contracts pupils. Be aware that both eyes
should reflex to light entering into one eye, that is
because each retina sends fibers to both sides and
pretectal area projects fibers cross to the contralateral
side of the Edinger-Westphal nucleus.
• Some retinal ganglion cells have axons enter the
retinohypothalamic tract, the visual stimuli can
synchronize the intrinsic circadian rhythm of the firing
pattern of the neurons in suprachiasmatic nucleus. Then
influence the antigonadotrophic activity of pineal gland.
CN III, Oculomotor Nerve
• CN-III is a purely motor nerve that serves
to control all of the eye muscles except the
lateral rectus and the superior oblique . It
also contains parasympathetic fibers that
control the ciliary muscle and pupillary
constrictor muscles; when this part of the
tract is damages, light shone in the
affected eye does not cause constriction of
the pupil.
Somatic motor
(general somatic efferent)
Supplies four of the six
extraocular muscles of
the eye and the levator
palpebrae superioris
muscle of the upper
eyelid.
Visceral motor
(general visceral efferent)
Parasympathetic
innervation of the
constrictor pupillae
and ciliary muscles.
Eye Movements and Extraocular muscles
Rectus Muscles
Superior and Inferior Oblique
Dual Actions of Superior Rectus
Dual Actions of Superior Oblique
Oculomotor nerve
• Oculomotor nerve originates from oculomotor
nucleus, which is located in the periaquductal
gray matter of the midbrain (ventral to the
aqueduct) at the level of superior colliculus.
Oculomotor fibers are partially crossed and
partially uncrossed. Current theory suggests that
there are different nucleus groups to innervate
different eye muscles. For instance, only
uncrossed fibers innervate inferior rectus,
inferior oblique, and medial rectus muscles.
Superior rectus only innervated by crossed
fibers
Nerve Nuclei and Connections
• There are two motor nuclei for the oculomotor
nerve
• The main motor nucleus is located in the
anterior part of the grey matter surrounding the
cerebral aqueduct of the midbrain at the level of
the superior colliculus. All the extrinsic muscles
of the eye except the superior oblique and the
lateral rectus are supplied by this nucleus. The
superior rectus muscle is supplied by the
contralateral oculomotor nucleus, the remaining
extraocular muscles are supplied ipsilaterally
and the Lavator Palpebrae Superioris muscle of
both sides are supplied by a single central group
of cells called the central caudal nucleus.
Nuclei and Connections
• The accessory parasympathetic nucleus (EdingerWestphal nucleus) is located posteriorly to the main
oculomotor nucleus
• The preganglionic nerve cells from the Edinger-Westphal
nucleus follow the same path as the other oculomotor
fibres to the orbit where they synapse in the ciliary
ganglion. The postganglionic fibres leave the ciliary
ganglion and pass through the short ciliary nerves to the
constrictor pupillae of the iris and the ciliary muscles.
• The Edinger-Westphal nucleus receives fibres from the
pretectal nucleus for the direct and consensual light
reflex and corticonuclear fibres for the accommodation
reflex.
CN III
• Oculomotor fibers are partially crossed and
partially uncrossed. So damage to one side will
not cause paralysis of all extraocular muscles.
Interruption of the parasympathetic fibers from
Edinger-Westphal nucleus, causes dilation of
pupil. Light response and accommodation
response will also be severed. The most
common scenario is losing light reflex after head
injury, because the oculomotor nerve is
compressed over the tentorium cerebelli.
Damage of CNIII
• Ptosis or drooping of the upper eyelid
• Dilation of the pupil. No reaction to light or
accommodation
• Overaction of the lateral rectus, superior
oblique, inferior oblique
• abnormal head posture
Cranial Nerve IV: Trochlear Nerve
• CN-IV is a small purely motor nerve which
serves to control the superior oblique muscles of
the eye. Paralysis of the superior oblique muscle
results in extortion (outward rotation) of the
affected eye, which is attributable to the
unopposed action of the inferior oblique muscle.
This gives rise to diplopia (double vision) and
weakness of downward gaze; patients with
trochlear nerve palsies often complain of visual
difficulties when going down stairs
Trochlear Nerve
• The fourth cranial nerve innervates superior
oblique muscle, which intorts, depresses, and
abducts the globe
• Trochlear Nerve Palsy
– Patients often adopt a characteristic head tilt, away
from affected side to reduce their diplopia.
Interestingly, some patients develop head tilt toward
side of lesion. This so-called paradoxic head tilt is
used to create a wider separation of images, which
allows the patient to suppress or ignore one image
Cranial Nerve VI: Abducens Nerve
• CN-IV is a purely motor nerve that serves
to control the lateral rectus muscle of the
eye. Paralysis of the lateral rectus muscle
can result in strabismus , the inability to
direct both eyes towards the same object.
A patient with strabismus has diplopia, but
can obtain normal binocular vision by
moving the head so that the fixed, affected
eye is brought into line with the object of
interest.
Common defects
• Third cranial nerve palsy
• 1. Drooping of the upper eyelid (ptosis)
2. Lateral strabismus: due to unopposed
action from the lateral rectus muscle
3. Inability to direct the eye medially or
vertically
Common defects
• Fourth Cranial Nerve palsy
– diplopia going downstairs
• Sixth cranial Nerve Palsy
– 1. medial strabismus:
– 2. inability to direct affected eye laterally
• Positions of the eye in
strabismus. A. Eyes are
straight (orthophoria). B. Right
eye turned in (right esotropia).
C. Right eye turned up (right
hypertropia). D. Right eye
turned out (right exo-tropia). E.
Right eye turned down (right
hypotropia; however,
convention calls for labeling up
and down turns by the
elevated eye, thus this would
be properly termed left
hypertropia).
Cranial Nerve V: Trigeminal Nerve
• CN-V is a large mixed nerve. The sensory
component carries impulses from touch,
pain, heat, and cold receptors of the facial
area, the scalp, and the mucous
membranes. The motor component
innervates the muscles of mastication
(chewing).
Sensory components
• Trigeminal nerve has three large branches,
ophthalmic, maxillary and mandibular. They
innervate the skin of face (no overlapping),
scalp, mucus membrane of oral and nasal
cavities, paranasal sinuses and teeth.
• Primary sensory neurons are in the trigeminal
ganglion, others are in the mesencephalic
trigeminal nucleus. The peripheral processes of
trigeminal ganglion cells constitute the
ophthalmic and maxillary nerves (sensory only)
and the sensory component of the mandibular
nerve (mixed).
Trigeminal nerve
• Axons of the trigeminal ganglion cells
enter the pons and terminate in the
pontine and spinal trigeminal nuclei.
• Pontine trigeminal nucleus: large sensory
root for discriminative touch
• Spinal trigeminal nuclei: light touch
Spinal trigeminal tract and its nucleus
• Spinal trigeminal tract:
– Medium and fine fibers that carry touch, pain, and
temperature info form the spinal trigeminal tract. Fibers
from facial, glossopharyngeal, and vagus nerves are
also joining the tract carrying in general somatic
sensation from external ears, posterior part of the
tongue, pharynx, and larynx.
– Fibers of spinal trigeminal tract then terminate onto
adjacent spinal trigeminal nucleus which is subdivided
into pars caudalis (receives fibers for pain and
temperature), pars interpolaris (light touch, pain
maybe), and pars oralis (discrimitive touch) and
reticular formation (source of cutaneous stimuli to
maintain arousal and alertness)
Projections from above nucleus
• ventral trigeminothalamic tract: crossed
– small number of fibers from pontine nucleus (both crossed and
uncrossed) proceed to thalamus via dorsal trigeminothalamic
tract
• The combined ventral and dorsal trigeminal
trigeminothalamic tracts form the trigemical lemniscus.
These axons end in the medial division of Ventral
Posterior Nucleus of Thalamus, then project to primary
somatosensory region
• Some efferent fibers from sensory trigeminal nuclei
terminate into motor nuclei of other cranial nerves, like
facial, and hypoglossal nerves. They are responsible for
reflexes, i.e. corneal reflex: touch on cornea transmits
info via opthamic nerve of trigeminal, the efferent fiber,
however, is in facial nerve.
Mesencephalic trigeminal nucleus
• Extending from the potine trigeminal nucleus
into the midbrain. The axons from these nucleus
form the mesencephalic root. Each axon splits
into peripheral and central branch. The
peripheral branch runs with mandibular nerve
collecting proprioceptive info and ending in
muscle spindles. Central branch terminates in
the motor nuclei of the trigeminal nerve. The
connection controls the force of mastication.
Motor component of trigeminal nerve
• Generate from trigeminal motor nucleus, joins
mandibular branch, supplies muscles of
mastication (masseter, temporalis, lateral and
medial pterygoid muscles) and smaller muscles
like tensor tympani, digastric anterior, and
mylohoid. Descending fibers come from
corticobulbar tract. Most of the fibers are
crossed for the contralateral control, however,
significant number of fibers do not cross.
Therefore, damage of one side of the
corticobulbar tract will only cause contralateral
weakness of chewing, but not completely
paralyzed n.
Clinical significance
• Trigeminal neuralgia (tic douloureux)
– Tic douloureux or trigeminal neuralgia is a
severe, stabbing pain to one side of the face.
It stems from one or more branches of the
nerve that supplies sensation to the face, the
trigeminal nerve. It is considered one of the
most painful conditions to affect people.
– Pain is usually triggered by a light touch of the
face or mouth on the same side as the pain
Trigeminal neuralgia (tic douloureux)
• Cause unknown
– compression of the trigeminal nerve
– Tumors and bony abnormalities of the skull
– Trauma, infections, and multiple sclerosis can
also cause damage to the trigeminal nerve
• Treatment
– Pain management
– Surgery
Shingles (Herpes Zoster)
• Shingles
– The ophthalmic division of
the trigeminal nerve is
affected
– Caused by varicella-zoster
virus, the same virus that
causes chickenpox. After
an attack of chickenpox,
the virus lies dormant in the
nerve tissue. As we get
older, it is possible for the
virus to reappear in the
form of shingles