Cranial Nerve I: Olfactory Nerve
• Shortest cranial nerve
• Origin: Basal forebrain
• Sense (afferent) of smell (odors)
• Oldest/Most primitive sensory modality
➢ The “first” cranial nerve
• Derives from the embryonic nasal placode
• Can provoke the fastest response
• > 5 million receptors with the ability to regenerate
• The olfactory receptors and bulb are near the cribriform plate of the
ethmoid bone
• Damage may lead to anosmia (Ipsilateral)
• Anosmia – Loss of smell
• Olfactory bulb > Olfactory tract > Descending pathway to pontine reticular
formation in the brainstem (Salivation – Reflex action)
• Smell and Odorants
➢ Qualitative odor sensation:
✓ Example: Smell of Flower/Perfume
✓ CN I
➢ Somato-sensory overtones of odorants:
✓ Example: Warmth, coolness, sharpness, and irritation
✓ CN V (Ophthalmic and Maxillary division)
❖ Example: Someone is cooking food, you
somehow smelled it. So, you salivate, and it
feels like you can already taste the food just
by smelling. This explains the relation
between olfaction, salivation, and taste.
Note: Olfactory mucosa also gets branches from the Trigeminal nerve.
- Reason why some odors have irritable character
- Explains why even when anosmic, patients can still
detect certain irritating substances (for example is
ammonia) due to CN V (trigeminal nerve)
**Olfactory nerve is responsible for the smell while Trigeminal Nerve is responsible for
general sensation (pain, temperature, touch, vibration)**
Arise from the olfactory receptor nerve cells in the olfactory mucous
membrane located in the upper part of the nasal cavity above the level of the
superior concha
o Olfactory Receptor Nerve Cells - scattered among supporting cells
- consists of a small bipolar nerve cell with a coarse peripheral
process that passes to the surface of the membrane and a fine
central process
o Olfactory Hairs - project into the mucus covering the surface of
the mucous membrane - React to odors in the air and stimulate
the olfactory cells
o Olfactory Nerve Fibers - Form by the fine central process which
pass through the openings of the cribriform plate of the ethmoid
bone to enter the olfactory bulb - Unmyelinated and are covered
with Schwann cells
o Olfactory Bulb - ovoid structure which receives axons from the
contralateral olfactory bulb through the olfactory tract and
possesses several types of nerve cells:
✓ Mitral Cell – Largest
✓ Synaptic Glomeruli – Rounded areas formed when the
incoming olfactory nerve fibers synapse with the
dendrites of the mitral cells
Tufted cells and granular cells - smaller nerve cells which also
synapse with mitral cells
o Olfactory Tract - Narrow band of white matter runs from the
posterior end of the olfactory bulb beneath the inferior surface of
the frontal lobe of the brain - consists of the central axons of the
mitral and tufted cells of the bulb and some centrifugal fibers from
the opposite olfactory bulb
Note: *As the olfactory tract reaches the anterior perforated
substance, it divides into medial and lateral olfactory stria
o Lateral Stria - carries the axons to the olfactory area of the
cerebral cortex: (primary olfactory cortex) periamygdaloid and
prepiriform areas
o Medial olfactory stria carries the fibers that cross the median
plane in the anterior commissure to pass to the olfactory bulb of
the opposite side
Note: *Entorhinal area (area 28) of the Para-hippocampal gyrus,
which receives numerous connections from the primary olfactory
cortex, is called the secondary olfactory cortex
• Primary and Secondary Olfactory cortex are responsible for the appreciation
of olfactory sensations
Cranial Nerve II: Optic Nerve
❖ Vision – Sight
• Pretectal region
- In conjunction with Oculomotor nerve (CN III)
✓ Light reflex
✓ Accommodation reflex
• Primary Neurons
- Receptors in the retina
✓ Cones – Interpret color of light waves
✓ Rods – Interpret the intensity of light
- Transforms light into electrical potentials
• Secondary Neurons
- Bipolar cells
- Link to optic nerve
- Interconnect rods and cones to produce receptive fields
• Tertiary Neurons
- End in lateral geniculate nucleus (LGN) of the thalamus
- Some do not run into the LGN but run medial to pretectal region
✓ Connects with the Edinger-Westphal nucleus of
oculomotor nerve (CN III)
✓ Visual reflexes (example: light reflex, accommodation
reflex)
CN 2 cont…
Optic Radiation
- Fibers from the lateral geniculate body to the primary visual
cortex
✓ Superior parietal fibers – Responsible for the inferior
quadrants of the visual field
✓ Inferior temporal fibers – Responsible for the superior
- Quadratic vision
• Primary Visual Cortex
- Area 17
- Calcarine sulcus of the occipital lobe
- Create:
✓ Images
✓ Colors
✓ Fusion of images from both eyes
- Bumping the head on occipital region – cause blurring of vision –
due to location of PVC (primary visual cortex)
- Cortical blindness – complete damage on occipital lobe
◆ Course of the Optic Nerve
Retina (R&L) > Optic Nerve (R&L) > Optic Chiasm > Optic Tract (R&L) > Lateral
Geniculate Body (R&L) > Optic Radiation (R&L) (Superior-Parietal and Inferior-
Temporal) > Visual Cortex: Area 17 (R&L)
Optic Nerve
- Made up of fibers from the cell axon in the ganglionic layer of the
retina. It measures 3 – 4 mm at the center of its nasal side and
converges at the Optic Disc and exit from the eye.
- Comparable to a tract w/in the CNS:
o Myelinated fibers and the sheaths are formed by
oligodendrocytes sheaths
o It leaves the orbital cavity through the optic canal and
unites with the optic nerve of the opposite side
forming optic chiasma
2. Optic Chiasm
- Its anterolateral angles are continuous with the optic nerves, and
the posterolateral angles are continuous with the optic tracts
- Location: Situated at the junction of the anterior wall and floor of
the third ventricle
- Decussation of fibers:
o In the chiasma, the fibers from the nasal (medial) half
of each retina, including the nasal half of the Macula,
cross the midline and enter the Optic Tract (OT) of the
opposite side. While the fibers from the temporal
(lateral) half of each retina, including the temporal half of the macula, pass posteriorly in the optic tract of the
same side
Optic Tract (OT)
- Location: Emerges from the Optic Chiasma and passes
posterolaterally around the cerebellar peduncle
- Function: Most of the fibers terminate by synapsing with nerve
cell in the LGB. Few fibers pass the Pre-Tectal Nucleus and the
Superior Colliculus of the midbrain and are concerned with light
reflexes
o Some do not run into LGB but run medial to pretectal
region
✓ Connects with the Edinger-Westphal
nucleus of CN III
✓ Visual Reflexes (e.g: light
reflex/accommodation reflex)
✓ PERRLA Pupil Equally Reactive to Light or
Accommodation
Primary Visual Cortex
- Optic Radiation
o From lateral geniculate body to the primary visual
o Description: Its fibers are the axons of the nerve cells
of the LGB
o Function: The tract passes posteriorly through the
retro lenticular part of the internal capsule and
terminates in the Visual Cortex (Area 17)
o Visual Cortex (Area 17) – occupies the upper and lower
lips of the calcarine sulcus on the medial surface of the
cerebral hemisphere
o Visual Association cortex (Area 18 & 19) – Responsible
for the recognition of objects and the perception of
color
Visual Pathway and Binocular Vision Neurons
• Four neurons conducting the visual impulses to Area 17
1. Rods and Cones (RnC) – Specialized receptor in the Retina
2. Bipolar Neurons – Connect the RnC to the ganglion cells
3. Ganglion Cells – its axons pass to the LGB
4. LGB nucleus – Axons passes to the Cerebral Cortex
Binocular Vision
o Left and right fields of vision projected on portion of both
retinae.
o Image in the right field: projected on the nasal half of the
right retina and the left temporal half of the left retina.
o In the OC, axons from the two retinal halves are
combined, forming the left optic tract.
o LGB neurons complete the right field of vision on the
visual cortex of the left hemisphere and visual field (visual
cortex of the right hemisphere).
o The lower the retinal quadrants (upper field vision)
project on the lower wall of the Calcarine sulcus, while
the upper retinal quadrants (lower field of vision) projects
on the upper wall of the sulcus.
o Macula Lutea is represented on the posterior part of Area
17, and the periphery of the retina is represented
anteriorly.
Note:
• Nasal Retinas – Temporal field of vision
• Temporal Retina – Nasal field of vision
❖ Optic chiasm
• Nasal Retinas (Temporal field of vision)
✓ Fibers cross the optic chiasm
• Temporal Retinas (Nasal field of vision)
✓ Fibers do not cross the optic chiasm. Remains ipsilateral
❖ Optic tract/Optic radiation/Visual cortex
• Will carry visual information from:
✓ Ipsilateral Temporal Retinas (Ipsilateral nasal field of vision
✓ Contralateral Nasal Retinas (Contralateral temporal field of vision
❖ Visual Field Concept
• Right visual field – From temporal retina of the left eye plus the nasal retina
of the right eye
• Left visual field – From the nasal retina of the left eye plus the temporal retina
• Both nasal retinas cross over to the other side at the chiasm while the
temporal retinas remain ipsilateral
❖ Anopia/Anopsia
- Sightlessness
- The state of being blind/lack of light
- Unilateral Anopia – complete blindness in one eye
❖ Hemianopia/Hemianopsia
- Loss of vision in half of visual field/one side of vertical midline
- Homonymous hemianopia/Hemianopsia (Optic tract/Visual tract)
o Example: Left Homonymous Hemianopia
❖ Quadrant anopsia/Quadrantanopia
- Optic radiation damage (Parietal/Temporal)
- Loss of a quarter of the field of vision
o Example: Right Superior Homonymous
Quadrantanopia
Application:
• Cutting the right retinal nerves (both nasal and temporal retinas) – Result to
complete loss of vision on the right eye
• Cutting the right temporal retina – Result to loss of vision on the nasal side of
the right eye called “left hemianopia” of the right eye
Cranial Nerve VIII: Vestibulocochlear Nerve
• Maintains balance and hearing
• Purely sensory nerve
• Transmits sound and equilibrium (balance) information from the inner ear
to the brain
• Nuclei in the pontomedullary junction consist mostly of bipolar neurons.
• Two functional divisions:
1. Vestibular nerve (Equilibrium/Balance)
- Lesion will cause: Disequilibrium, vertigo, nystagmus
2. Cochlear nerve (Hearing)
- Destructive lesion will cause: Hearing loss/sensorineural deafness
- Irritative lesion will cause: Tinnitus
❖ Anatomy:
ʘ Emerges from pontomedullary junction at the cerebellopontine angle
ʘ Exits via the Internal acoustic meatus (with CN VII)
- Damage of the internal acoustic meatus due to fracture or tumor
– manifestation on damage of cranial nerve VII (facial) and VIII
(abducens) – problem with facial expression and loss of hearing
ʘ Vestibular apparatus and semicircular canals
- Afferent messages associated with sense of equilibrium/ balance
- Vestibular radix - composed of semicircular canals
ʘ Cochlea
- Afferent impulses associate with hearing
- Cochlear radix - shell shaped
ʘ Cochlear part from cochlear ganglion and vestibular part from vestibular
ganglia, they will form a single nerve called vestibulocochlear nerve and exit
to the cerebellopontine angle and reach brainstem (pons)
◆ General Characteristics of the Cochlear Nerve
• The cochlear part has both ventral and dorsal nucleus. Once the
nerve arrives in the pons they will now synapse with the nucleus.
• The ventral and dorsal nucleus connect with the olivary nucleus
found in the pons wherein there is comparison of left\right ear.
• That information will go to the inferior colliculus where
integration with proprioceptive information coming from the
body.
• From the inferior colliculus it goes to the thalamus and move to
the auditory cortex particularly to temporal gyrus where analysis
of the sound occurs.
◆ Cochlear Part of CN III (Hearing)
- The cochlear nerve travels away from the clay of the inner ear
which starts at spiral ganglia
• Spiral Ganglia – Nuclei consist mostly of bipolar
neuron
• Sensory Organ – Organ of Corti (primary neuron)
✓ Inner hair cells
✓ Primary neuron
✓ Housed in the spiral ganglion
➢ Detects sound waves that vibrate the
eardrums
• Secondary Neuron
✓ Superior olivary nucleus
➢ Information from both ears is
compared to localize the sound source
• Third Neuron
✓ Medial geniculate body (thalamus)
• Temporal Gyrus
✓ Low frequency – Anterolateral cortex
✓ High frequency – Posteromedial cortex
❖ Analysis of Sound
• Associative Auditory Cortex
o Comparison with former experiences of sound is done
• Integration
o Speech – Let Hemisphere (temporal lobe) – Wernicke’s and Broca’s
areas
o Music – Right Temporal Lobe
*Note: Vestibule-Ocular reflex is also known as Doll’s eye movement
ʘ Cranial Nerve VIII: Symptoms of Damage (*Patient with otitis media will
experience all these symptoms)
- Hearing loss (Sensorineural)
- Vertigo
- False sense of motion
- Loss of equilibrium (especially in dark places)
- Motion sickness
- Nystagmus – Irregular movement of eyes; usually happens when
travelling (eye wanders around the surrounding)
- Gaze-evoked tinnitus
◆ Vestibular Part of CN VIII (Balance)
• Vestibular Ganglia
- Nuclei: Cell bodies of bipolar neurons
• Five Sensory Organs:
- 3 cristae (semicircular canals) – Afferent receptor in response to
rotational acceleration
- Otolith Organs:
✓ Maculae of Saccule – Vertical Acceleration
✓ Maculae of the Utricle – Linear/Horizontal
Acceleration
Last changed2 years ago