2. CRANIAL NERVES
• There are twelve cranial nerves
• The olfactory nerve (CN I) and optic nerve (CN II) originate from the cerebrum
• Cranial nerves III – XII arise from the brain stem
• They can arise from a specific part of the brain stem (midbrain, pons or medulla), or from a junction
between two parts:
• Midbrain – the trochlear nerve (IV) comes from the posterior side of the midbrain. It has the longest intracranial
length of all the cranial nerves.
• Midbrain-pontine junction – oculomotor (III).
• Pons – trigeminal (V).
• Pontine-medulla junction – abducens, facial, vestibulocochlear (VI-VIII).
• Medulla oblongata – posterior to the olive: glossopharyngeal, vagus, accessory (IX-XI). Anterior to the olive:
hypoglossal (XII).
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8. OLFACTORY
NERVE
Special sensory information is transmitted from
the nasal epithelium to the primary olfactory
cortex of the brain
The sense of smell is detected by olfactory
receptors located within the nasal epithelium,
then axons penetrate the cribriform plate of the
ethmoid bone and enter the cranial cavity
Once in the cranial cavity, the fibers enter
the olfactory bulb, which lies in the olfactory
groove within the anterior cranial fossa, then the
specialized neurons pass to the olfactory tract
9. OLFACTORY NERVE
The primary olfactory cortex sends nerve fibres to many other areas of the brain: the piriform cortex, the
amygdala, olfactory tubercle and the secondary olfactory cortex. These areas are involved in the memory and
appreciation of olfactory sensations.
The olfactory tract travels posteriorly on the inferior surface of the frontal lobe and as it reaches the anterior
perforated substance in an area at the level of the optic chiasm, it divides into medial and lateral stria:
Lateral stria – carries the axons to the primary olfactory
cortex, located within the uncus of temporal lobe.
Medial stria – carries the axons across the medial plane
of the anterior commissure, where they meet the
olfactory bulb of the opposite side.
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11. OPTIC NERVE
• Special sensory information is transmitted from the retina
of the eye to the primary visual cortex of the brain
• The optic nerve is formed by the convergence of axons
from the retinal ganglion cells. These cells in turn receive
impulses from the photoreceptors of the eye
• The nerve leaves the bony orbit via the optic canal, through
the sphenoid bone then enters the cranial cavity, running
along the surface of the middle cranial fossa
• Within the middle cranial fossa, the optic nerves from each
eye unite to form the optic chiasm.
• At the chiasm, fibers from the nasal (medial) half of each
retina cross over to the contralateral optic tract, while
fibers from the temporal (lateral) halves remain ipsilateral:
• Left optic tract – contains fibers from the left temporal
(lateral) retina, and the right nasal (medial) retina.
• Right optic tract – contains fibers from the right
temporal retina, and the left nasal retina.
12. OPTIC NERVE
• Each optic tract travels to its corresponding cerebral
hemisphere to reach the lateral geniculate nucleus, a relay
system located in the thalamus; the fibres synapse here.
• Axons from the LGN then carry visual information via a
pathway known as the optic radiation. The pathway itself
can be divided into:
• Upper optic radiation – carries fibres from the
superior retinal quadrants (corresponding to the
inferior visual field quadrants). It travels through the
parietal lobe to reach the visual cortex.
• Lower optic radiation – carries fibres from the
inferior retinal quadrants (corresponding to the
superior visual field quadrants). It travels through the
temporal lobe, via a pathway known as Meyers’ loop,
to reach the visual cortex.
• Once at the visual cortex, the brain processes the sensory
data and responds appropriately.
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15. OCULOMOTOR
NERVE
The oculomotor nerve originates from the oculomotor nucleus –
located within the midbrain. It emerges from the anterior aspect of
the midbrain, passing inferiorly to the posterior cerebral artery and
superiorly to the superior cerebellar artery.
The nerve then pierces the dura mater and enters the lateral aspect
of the cavernous sinus.
The nerve leaves the cranial cavity via the superior orbital fissure,
where it divides into superior and inferior branches:
•Superior branch – provides motor innervation to the superior rectus and levator
palpabrae superioris.
•Inferior branch – provides motor innervation to the inferior rectus, medial rectus and
inferior oblique.
•Also supplies pre-ganglionic parasympathetic fibres to the ciliary ganglion, which
ultimately innervates the sphincter pupillae and ciliary muscles.
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17. TROCHLEAR NERVE
• The trochlear nerve arises from the trochlear nucleus of the brain, emerging from the
posterior aspect of the midbrain
• It runs anteriorly and inferiorly within the subarachnoid space before piercing the dura
mater adjacent to the posterior clinoid process of the sphenoid bone
• The nerve then moves along the lateral wall of the cavernous sinus (along with the
oculomotor nerve, the abducens nerve, the ophthalmic and maxillary branches of the
trigeminal nerve and the internal carotid artery) before entering the orbit of the eye
via the superior orbital fissure.
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19. TRIGEMINAL NERVE
• The trigeminal nerve originates from three sensory nuclei (mesencephalic, principal sensory, spinal nuclei of
trigeminal nerve) and one motor nucleus (motor nucleus of the trigeminal nerve) extending from the midbrain
to the medulla.
• At the level of the pons, the sensory nuclei merge to form a sensory root. The motor nucleus continues to form
a motor root.
• In the middle cranial fossa, the sensory root expands into the trigeminal ganglion. The trigeminal ganglion
is located lateral to the cavernous sinus, in a depression of the temporal bone known as the trigeminal cave.
• The peripheral aspect of the trigeminal ganglion gives rise to divisions:
• ophthalmic (V1), maxillary (V2) and mandibular (V3)
• The motor root passes inferiorly to the sensory root, along the floor of the trigeminal cave. Its fibers are only
distributed to the mandibular division.
• The ophthalmic nerve and maxillary nerve travel lateral to the cavernous sinus exiting the cranium via the
superior orbital fissure and foramen rotundum respectively. The mandibular nerve exits via the foramen ovale
entering the infra-temporal fossa.
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21. ABDUCENS
NERVE
The abducens nerve arises from the abducens nucleus in
the pons and exits the brainstem at the junction of the pons and the
medulla
It then enters the subarachnoid space and pierces the dura mater to
travel in an area known as Dorello’s canal
At the tip of petrous temporal bone, the abducens nerve leaves
Dorello’s canal and enters the cavernous sinus. It travels through the
cavernous sinus and enters the bony orbit via the superior orbital
fissure.
Within the bony orbit, the abducens nerve terminates by innervating
the lateral rectus muscle.
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23. FACIAL NERVE
• The course of the facial nerve is very complex. There are many branches, which transmit a combination of sensory, motor and
parasympathetic fibers.
• 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.
• Intracranially, the nerve arises in the pons. It begins as two roots; a large motor root, and a small sensory root
• The two roots travel through the internal acoustic meatus, an opening in the temporal bone. Here, they are in very close proximity
to the inner ear.
• The roots then leave the internal acoustic meatus and enter the facial canal. Within the facial canal, three important events occur:
• the two roots fuse to form the facial nerve
• the nerve forms the geniculate ganglion
• the nerve gives rise to:
• Greater petrosal nerve – parasympathetic fibers to mucous glands and lacrimal gland.
• Nerve to stapedius – motor fibers to stapedius muscle of the middle ear.
• Chorda tympani – special sensory fibers to the anterior 2/3 tongue and parasympathetic fibers to the submandibular
and sublingual glands.
• The facial nerve then exits the facial canal via the stylomastoid foramen located just posterior to the styloid process of the temporal
bone.
24. FACIAL NERVE
• Extracranially, 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 that 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
• Within the parotid gland, the nerve terminates by splitting into five branches:
• Temporal branch
• Zygomatic branch
• Buccal branch
• Marginal mandibular branch
• Cervical branch
• These branches are responsible for innervating the muscles of facial expression.
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26. VESTIBULOCOCHLEAR
NERVE
• The vestibular and cochlear portions of the vestibulocochlear
nerve originate from different nuclei in the brain:
• Vestibular component – arises from the vestibular nuclei
complex in the pons and medulla.
• Cochlear component – arises from the ventral and dorsal
cochlear nuclei, situated in the inferior cerebellar
peduncle.
• Both fibers combine in the pons to form the vestibulocochlear
nerve. The nerve emerges from the brain at
the cerebellopontine angle and exits the cranium via
the internal acoustic meatus of the temporal bone.
• Within the distal aspect of the internal acoustic meatus, the
vestibulocochlear nerve splits, forming the vestibular nerve
and the cochlear nerve.
• The vestibular nerve innervates the vestibular system of the
inner ear, which is responsible for detecting balance.
• The cochlear nerve travels to cochlea of the inner ear, forming
the spiral ganglia which serve the sense of hearing.
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28. GLOSSOPHARYNGEAL NERVE
• The glossopharyngeal nerve originates in the medulla. It emerges from the anterior aspect
of the medulla, moving laterally in the posterior cranial fossa then leaves the cranium via
the jugular foramen.
• Extracranially, the glossopharyngeal nerve descends down the neck, anterolateral to the
internal carotid artery.
• At the inferior margin of the stylopharyngeus, several branches arise to provide motor
innervation to the muscle. It also gives rise to the carotid sinus nerve, which provides
sensation to the carotid sinus and body.
• The nerve enters the pharynx by passing between the superior and middle pharyngeal
constrictors. Within the pharynx, it terminates by dividing into several branches – lingual,
tonsil and pharyngeal.
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30. VAGUS NERVE
• The vagus nerve has the longest course of all the
cranial nerves, extending from the head to the
abdomen.
• The vagus nerve originates from the medulla. It
exits the cranium via the jugular foramen
• Within the cranium, the auricular branch arises
that supplies sensation to the posterior part of
the external auditory canal and external ear.
31. VAGUS
NERVE
In the Neck
In the neck, the vagus nerve passes into the carotid sheath,
travelling inferiorly with the internal jugular vein and common
carotid artery.
At the base of the neck, the right and left nerves have differing
pathways:
•right vagus nerve passes anterior to the subclavian
artery and posterior to the sternoclavicular joint, entering
the thorax.
•left vagus nerve passes inferiorly between the left
common carotid and left subclavian arteries, posterior to
the sternoclavicular joint, entering the thorax.
Several branches arise in the neck:
•Pharyngeal branches – Provides motor innervation to the
majority of the muscles of the pharynx and soft palate.
•Superior laryngeal nerve – Splits into internal and external
branches. The external laryngeal nerve innervates the
cricothyroid muscle of the larynx. The internal laryngeal
provides sensory innervation to the laryngopharynx and
superior part of the larynx.
•Recurrent laryngeal nerve (right side only) – Hooks
underneath the right subclavian artery, then ascends towards
to the larynx. It innervates the majority of the intrinsic muscles
of the larynx.
32. VAGUS
NERVE
In the Thorax
• In the thorax, the right vagus nerve forms the posterior vagal
trunk, and the left forms the anterior vagal trunk.
• Branches from the vagal trunks contribute to the formation of
the esophageal plexus, which innervates the smooth muscle of
the esophagus.
Two other branches arise in the thorax:
•Left recurrent laryngeal nerve – it hooks under the arch of the
aorta, ascending to innervate the majority of the intrinsic muscles
of the larynx.
•Cardiac branches – these innervate regulate heart rate and
provide visceral sensation to the organ.
• The vagal trunks enter the abdomen via the esophageal hiatus,
an opening in the diaphragm.
• In the abdomen, the vagal trunks terminate by dividing into
branches that supply the esophagus, stomach and the small
and large bowel up to the splenic flexure
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34. ACCESSORY
NERVE
The spinal portion arises from neurons of the upper
spinal cord, specifically C1-C5/C6 spinal nerve roots.
These fibers merge to form the spinal part of the
accessory nerve, which then runs superiorly to enter the
cranial cavity via the foramen magnum.
The nerve traverses the posterior cranial fossa to reach
the jugular foramen. It briefly meets the cranial portion
of the accessory nerve, before exiting the skull
Outside the cranium, the spinal part descends along
the internal carotid artery to reach the
sternocleidomastoid muscle, which it innervates. It then
moves across the posterior triangle of the neck to supply
motor fibers to the trapezius.
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36. HYPOGLOSSAL NERVE
The hypoglossal nerve arises from the hypoglossal nucleus in the medulla. It
then passes laterally across the posterior cranial fossa, within the subarachnoid
space. Then exits the cranium via the hypoglossal canal.
Now extracranial, the nerve receives a branch of the cervical plexus that
conducts fibres from C1/C2 spinal nerve roots. These fibres do not combine
with the hypoglossal nerve – they merely travel within its sheath.
It then passes inferiorly to the angle of the mandible, crossing the internal and
external carotid arteries, and moving in an anterior direction to enter the
tongue.
