Transcript No. 32

No. 32
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Ⅰ--Ⅵ Pairs of Cranial Nerves
Section 2 The Cranial Nerves
12 pairs of cranial nerves arise from the
brain. They leave or enter the cranial
cavity through a series of foramina or
fissures in the base of the skull.
 Fibrous components of the cranial nerves:
 The cranial nerves consist of seven fibrous
components.
①The general somatic motor fibers
 These fibers supply the striated muscles
from the myotome, e.g. the extrinsic
muscles of the eye and the muscles of
tongue.
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②The general visceral motor fibers
 They belong to the parasympathetic
nervous system. Their cell bodies are
located in certain nuclei of the brain stem
and are small in size. All these fibers end
in a certain parasympathetic ganglion near
the visceral effectors and control the
smooth muscle, cardiac muscle and glands.
③The special visceral motor fibers
 These fibers supply the striated muscles
from the branchiomere, e.g. the muscles
of expression, mastication, pharynx and
larynx.
④The general visceral sensory fibers
 They conduct the general sensory
impulses which originate from the viscera
in the head, neck, thorax and abdomen.
⑤The special visceral sensory fibers
 They distribute taste buds and olfactory
organs.
⑥The general somatic sensory fibers
 They transmit the impulses which
originate from the skin, muscle, muscular
tendon, and most part of oral and nasal
mucous membrane.
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⑦The special somatic sensory fibers
These fibers distribute the vestibulocochlear and
visual organs.
Most of the cranial nerves are mixed nerves
composed of both motor and sensory neurons,
although some cranial nerves carry only sensory
impulses.
With the exception of the vagus nerve, the
cranial nerves supply only structures in the head
and neck. The cranial nerves are numbered
(using Roman numerals) from superior to inferior
in the order in which they leave the brain (Table
1).
Table 1 The Origins and passages
of the Cranial Nerves
Name and order
Superficial origin on the brain
Passage of the skull
ⅠOlfactory Nerve
Olfactory bulb
Cribriform foramina
ⅡOptic Nerve
Optic chiasma
Optic canal
ⅢOculomotor nerve
Medial side of the cerebral peduncle
Sup. Orbital flessure
ⅣTrochlear Nerve
The superior medullary velum
Sup. Orbital flessrue
ⅤTrigeminal Nerve
The junction of basilar part of pons
And the middle cerebellar peduncle
Sup. Orbital fissure
(ophthalmic duvision
foramen rotundum
(maxillary division)
foramen ovale
(mandibular division)
ⅥAbducent Nerve
Medial part of the bulbopontine sulcus
Sup. Orbital flessure
ⅦFacial Nerve
Middle part of the bulbopontine sulcus
Facial canal
ⅧVestibulocochlear Nerve
Lateral part of the bulbopontine sulcus
Int. Acoustic meatus
ⅨGlossopharyngeal Nerve
Upper part of the sulcus behind the
loive
Jugular foramen
ⅩVagus Nerve
Middle part of the sulcus behind the
loive
ⅪAccessory Nerve
Lower part of the sulcus behind the
loive
ⅫHypoglossal Nerve
Anterolateral sulcus of the medulla
oblongata
Hypoglssal canal
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According to the components and functions the
twelve pairs of cranial nerves are divided into
three types:
1. The afferent or sensory nerves
They are olfactory, the optic and
vestibulocochlear nerves.
2. The efferent or motor nerves
They are the oculomotor, the trochlear, the
abducent, the accessory and the hypoglossal
nerves.
3. The mixed nerves
They are the trigeminal, the facial, the
glossopharyngeal and vagus nerves.
Ⅰ. Olfactory Nerve
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The olfactory nerves are entirely sensory,
carrying impulses associated with the sense of
smell.
They arise from receptor cells in the nasal
mucosa. Processes of these receptor cells pass
through the perforations of the cribriform plate of
the ethmoid bone and enter the olfactory bulbs of
the telencephalon portion of the brain. In the
olfactory bulbs, the nerve fibers synapse with
neurons that pass posteriorly in the olfactory
tracts. The fibers of the olfactory tracts enter the
brain, and many of them travel to the cerebral
cortex of the medial sides of the temporal lobes.
In severe injuries of the anterior cranial fossa,
the olfactory nerves may be torn, thus producing
a loss of the smell (anosmia) and the
cerebrospinal fluid leakage into the nasal cavity.
Ⅱ. Optic Nerve
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It is a special somatic sensory nerve and
originates from the ganglionic cell of the retina.
The fibers of the optic nerve converge on the
optic disc and they pierce the sclera. As
traversing the sclera the nerve fibers receive their
medullary sheaths, and run in bundles which are
collected to form the optic nerve.
The optic nerve is enclosed in three layers of
coverings. These coverings and the spaces
between them are continuous with those of the
brain. So that, the meningitis may involve the
eyes. An increase in SF pressure slows venous
return from retina, causing the edema of the
optic disc (papilledema), such as in the case of
the brain tumor, intracranial abscess, hemorrhage,
hypertension, etc.
Ⅲ. Oculomotor Nerve
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Components of fibers:
It is a motor nerve and contains somatic and
general visceral motor fibers.
The somatic motor fibers arise from the
oculomotor nucleus. The general visceral motor
fibers arise from the cells of the EdingerWestphal nucleus.
Course:
The oculomotor nerve emerges from the sulcus
on the medial side of the cerebral peduncle, and
passes within the lateral wall of the cavernous
sinus, where it lies above the trochlear nerve. It
then divides into a superior and an inferior
branches, which enter the orbit through the
superior orbital fissure.
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Branches and distribution:
The superior branch, the smaller, supplies
the superior rectus and the levator
palpebrae superioris.
The inferior branch supplies the inferior
rectus, medial rectus and inferior obliquus.
From the nerve for the inferior obliquus, a
short thick branch is given to the ciliary
ganglion. This branch is the preganglionic
fibers of the parasympathetic nerve which
arises from the cells of the EdingerWestphal nucleus.
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Clinical significance:
Paralysis of the oculomotor nerve leads, when
complete, to
(1) ptosis, or dropping of the upper eyelid;
(2) external strabismus, on account of the
unopposed action of the lateral rectus and
superior obliquus;
(3) dilatation of the pupil and loss of power of
accommodation of exposure to light, when
parasympathetic nerve fiber was damaged at the
same time.
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The ciliary ganglion is a
parasympathetic ganglion.
It is situated between the optic nerve and
the lateral rectus near the apex of the
orbit.
The preganglionic fibers (from the E-W
nucleus) of the parasympathetic nerve are
relayed in the ganglion.
The postganglionic fibers arising from the
ciliary ganglion supply the sphincter
pupillae and ciliary muscles.
Ⅳ. Trochlear Nerve
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It is a motor nerve and supplies the
superior obliquus.
It arise from the trochlear nucleus,
emerges from the superior medullary
velum immediately behind the inferior
colliculus. The nerve winds forwards round
the cerebral peduncle, then passes
forwards in the lateral wall of the
cavernous sinus. Through the superior
orbital fissure, the trochlear nerve passes
medially and finally enters the superior
obliquus.
Ⅴ. Trigeminal Nerve
Ⅰ) Fibrous omponents and nervous roots:
 It is a mixed nerve. It includes two
components i.e. general somatic sensory
fibers and special visceral motor fibers.
 It is attached to the side of the pons by
two roots.
 The sensory root is made up of the
central processes of neurons which are
located in the trigeminal ganglion.
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The trigeminal ganglion is a semilunar
dilatation of the nerve, it occupies the trigeminal
impression near the apex of the petrous part of
the temporal bone and is situated in the side of
the cavernous sinus.
The nerve cells of the ganglion are
pseudounipolar neurons, their peripheral
processes enter each of the three divisions of the
trigeminal nerve, their central processes form the
sensory root to enter the pons and end in the
pontine nucleus of trigeminal nerve and the
spinal nucleus of trigeminal nerve.
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The three divisions of the trigeminal nerve
are:
the ophthalmic nerve,
the maxillary nerve,
the mandibular nerve.
The general somatic sensory fibers of the
three nerves are distributed to the eye,
the skin of the face, the mucous
membranes of the oral cavity, nasal cavity
and the paranasal sinuses as well as the
teeth and the meninges, and conduct the
pain, thermal and crude touch sensation.
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The distribution in the face is limited by
the fissures of the eye and the mouth.
The smaller motor root contains the
special visceral motor fibers which arise
from the motor nucleus of trigeminal
nerve and only take part in the third
division of the trigeminal nerve to supply
the masticatory muscles, mylohyoid,
anterior belly of digastric, tensor veli
palatini and tensor tympani etc.
Ⅱ) Branches and distribution
1. The ophthalmic nerve
 It passes forwards within the lateral wall
of the cavernous sinus; just before
entering the superior orbital fissure, it
divides into three branches, and is
distributed to the cerebral dura mater,
orbit, eyeball, lacrimal gland, conjunctiva,
and partial mucous membrane of nose and
the skin of the forehead, upper eyelid and
the back of nose.
1) The lacrimal nerve
 It supplies the lacrimal gland and the
skin of the upper eyelid.
 The parasympathetic fibers coming
from facial nerve go in the nerve by
the communicating branch of the
zygomatic nerve and control the
secretion of the lacrimal gland.
2) The frontal nerve
 It divides into a supratrochlear and
supraorbital branches.
 They are distributed to the skin of
forehead and anterior part of the
scalp.
3) The nasociliary nerve
 It distributes to the eyeball, the
ethmoidal and sphenoidal sinuses,
and to the skin of the eyelids, the
lacrimal sac, and the skin of the
nasal back and mucous membrane of
the nasal cavity.
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2. The maxillary nerve
It passes horizontally forwards within the lower
part of the lateral wall of the cavernous sinus,
leaves the skull through the foramen rotundum.
It then crosses the upper part of the
pterygopalatine fossa, and enters the orbit
through the inferior orbital fissure. It is now
named the infraorbital nerve.
The maxillary nerve supplies the cerebral dura
mater, the skin between the fissures of the eye
and the mouth and the mucous membranes of
the nasal and oral cavities.
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1) The infraorbital nerve
It is the terminal branch of the maxillary nerve.
It supplies the alae nasi, the lower eyelid and the
upper lip.
2) The pterygopalatine nerves
It supplies the mucous membrane of the nasal
cavity, palate and the pharynx.
3) The zygomatic nerve
It is distributed to the skin over part of the cheek
and temple.
The communication with the lacrimal nerve
conveys parasympathetic postganglionic fibers
from the pterygopalatine ganglion to the lacrimal
gland.
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4) The superior alveolar nerves
It includes the anterior, middle and
posterior branches. These branches
communicate freely with each other and
form the superior dental plexus to supply
the maxillary sinus, the upper gum and
teeth.
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3. The mandibular nerve
It is a mixed nerve and the largest division of the
trigeminal nerve.
It emerges almost immediately through the
foramen ovale.
The motor fibers supply the masticatory muscles,
tensor tympani and tensor veli palatini. The
sensory fibers are distributed to the cerebral dura
mater, mandibular teeth and gum, the mucous
membrane of the floor of the mouth and the
anterior two-thirds of the tongue as well as the
skin over the mandible, part of the external ear
and temporal area.
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1) The auriculotemporal nerve
It generally arises by two roots, which encircle
the middle meningeal artery. It supplies the skin
of the anterior surface of the auricle, the
temporal region and the parotid gland.
2) The buccal nerve
It supplies the skin over the buccinator and the
mucus membrane lining its inner surface.
3) The lingual nerve
It passes anteriorly to the inferior alveolar nerve,
and forwards between the ramus of mandible and
the medial pterygoid. After receiving the chorda
tympani of the facial nerve the lingual nerve is
connected to the submandibular ganglion by two
or three branches.
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The branches of the lingual nerve supply the
mucous membrane of the floor of mouth, and
mucous membrane of the anterior two-thirds of
the tongue. It conducts the common sense of the
mucous membrane.
The special visceral sensory fibers (taste fibers)
of chorda tympani nerve distributed with the
lingual nerve to the anterior 2/3 of tongue.
The general visceral motor fibers of chorda
tympani nerve connect the submandibular
ganglion, and the postganglionic fibers control
the secretion of the submandibular and subligual
glands.
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4) The inferior alveolar nerve
It is a mixed nerve. It enters the
mandibular foramen, passes through the
mandibular canal below the teeth as far as
the mental foramen. The terminal branch
is mental nerve.
It supplies all the lower teeth and gums,
the skin and mucous membrane of the
lower lip, mylohyoid and the anterior belly
of the digastric muscles.
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5) The nerve of maticatory muscles
It is a motor nerve, whose branches supply the
masseter, temporalis, lateral pterygoid and
medial pterygoid.
A lesion of the whole trigeminal nerve causes
anaesthesia of the corresponding anterior half
skin of the scalp, and the face, the cornea and
conjunctiva, and the mucous membrane of the
nose, mouth and anterior two-thirds of the
tongue. Paralysis and atrophy of the masticatory
muscles result in the mandible to thrust over to
the paralyzed side when the mouth is opened.
Ⅵ Abducent Nerve
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It is a somatic motor nerve and supplies the
lateral rectus only.
It arises from the abducent nucleus and passes
forwards through the pons, and emerges from
the medial part of the bulbopontine sulcus. After
leaving the surface of the brain stem, the
abducent nerve runs upward and laterally,
crosses the apex of the petrous part of the
temporal bone. It then traverses the cavernous
sinus, lateral to the internal carotid artery, and
enters the orbital cavity though the superior
orbital fissure and finally sinks into the medial
surface of the lateral rectus.
The result of the damage of this nerve is the
paralysis of the lateral rectus, medial squint.