This document describes the anatomy and function of cranial nerve VIII (vestibulocochlear nerve). It notes that CN VIII has two parts - the cochlear part which carries hearing impulses and the vestibular part which carries equilibrium impulses. The auditory pathways that transmit hearing signals from the inner ear to the brainstem and auditory cortex are then described. Finally, the document discusses some clinical correlations of lesions to CN VIII, including causes of hearing loss.

2.  Vestibulo cochlear n.(CN VIII)
 Type: special sensory n. that convey the special senses
from the inner ear to brain stem.
 formed of 2 parts
(1)cochlear part: carrying hearing impulses from the organ of
corti in the cochlea.
(2)Vestibular part: carrying equilibrium impulses
 Course : is very short, leaves at pontomedullary junction, it
enters internal auditory meatus with facial n. and
labyrinthine artery, where it travels within the auditory
canal of the petrous temporal bone.
• within the petrous bone it split in to:
_the cochlear nerve (formed of spiral ganglion) ends in the
cochlea.
_the vestibular nerve (formed of vestibular ganglion) ends in
the utricle, saccule and the 3 semicircal canals.

4. The auditory pathways:

5. -Each central process of cochlear parts of the eight cranial
divides as it enters the brainstem.
-One branch goes to the dorsal cochlear nucleus and the
other one to the ventral cochlear nucleus.
-The neurons consisting the auditory pathways ascend
from the cochlear nuclei to the inferior colliculi of
midbrain.
-Many neurons make intermediate synaptic connections in
superior olivary nuclei located at level of pontomedullary
junction.
-The superior olivary nuclei determine the delay between
sounds forms each ear and thus determine the direction
of the origin of sound.
-The directly projecting neurons synapse in contra lateral
inferior colliculus

6. -But those that synapse in the superior olivary
nucleus project to both colliculi post_synaptic
neurons project from the inferior colliculus to the
medial geniculate nucleus of thalamus.
-Neurons project from the thalamus via the
sublentiform part of the internal capsule to the
auditory cortex in the temporal lobe of the cerebral
hemisphere.

7.  The vestibular pathways:
- The structures in the inner ear
served by the vestibular components
provides information to CNS, which
plays a major part in maintaining
equilibrium.
- The vestibular parts of
vestibulocochlear nerves
originate from receptor cells in the various parts of
vestibular labyrinth and join with cochlear portion to follow
the same route to brainstem.
- Their cell bodies form vestibular ganglion close to the
lateral end of the internal acoustic meatus, their central
processes constitute the vestibular nerve .
Most of the central processes synapse in vestibular nuclei
of the brainstem which make connections with the
cerebellum, spinal cord brainstem and cerebral cortex,
but some pass directly to the cerebellum.

8.  Clinical correlation of CNVIII
-Due to close relationship of the vestibular nerve and
cochlear nerve, lesions of one usually effect the
other, Leading to both tinnitus, vertigo and impaired
hearing.
-In fact usually the cause of vertigo after head trauma
is a peripheral vestibular nerve lesion.
 There are two forms of hearing loss:
-Conductive hearing loss –CNV lll is working but bone
conduction of sound is impaired, due to the external
auditory canal/middle ear not transmitting sound
waves.
-Sensor neural hearing loss-physical ear structures
are preserved, however there is damage to the
cochlea or neuron that is impairing hearing.

9. GLOSSOPHARYNGEAL NERVES (CNIX)
 Type: Mixed n. containing sensory, motor and
parasympathitic fibers.
 Glossopharyngeal nerve nuclei :
.

10. Origin : exit from post, lateral of
medulla below CNV III.
Course and relation:
_it leaves the cranial cavity through
the middle comportment of
jugular foramen.
_it descends downwards between
I.J.Vand I.C.A(in the upper part of
carotid sheath)
_then it leaves the carotid sheath
and passes forwards with
stylopharyngeus muscle between
the I.CA and the E.C.A.
_then it’s disappears deep to the
post. border of hypoglossus m.
(in the gap between the sup. and
the middle contractor m.) and
ascends deep to hyoglossus to
reach the tongue.

11.  Termination : it ends by dividing in to terminal
branches supplying the mucous membranes of
pharynx, tonsil and post.1/3 of tongue.
 Ganglia of glossopharyngyal : it has 2 sensory
ganglia in the jugular foramen:
(1) sup. Ganglion : small and gives no branch.
(2) Inf. ganglion: large and give tympanic branch.

12.  Branches:
1. Maningeal branch : sensory to the meninges of post. Cranial fossa.
2. Carotid branch : General visceral sensory neurons in the
glossopharyngeal n.supply barorecptors in the carotid sinus and the
chemoreceptors in the carotid body, the cell bodies of these fibers are
situated in the inferior ganglion of the nerve, Their central processes
enter the tractus solitarius and end in caudal part of its nucleus.
3. Nerve to stylopharyngeus n.: derived from the third pharyngeal
arch. The special visceral motor neurons to this muscle begin in the
rostral part of the nucleus ambiguus
4. Tonsillar branch(sensory):shares the lesser palatine n. in supplying
palatine tonsil
5. pharyngeal branches(sensory):they enter in the formation of
pharyngeal plexus on lat. surface of the middle constrictor m. of
pharynx
 the pharyngeal plexus receives also :
- motor branch: the pharyngeal branch of vagus
- sympathatic branch: the pharyngeal branch of the superior cervical
sympathetic ganglion.
6. Terminal lingual branches : supply general and taste sensations to
the post 1/3of tongue.

13. .
(7) Tympanic branch : preganglionic visceral motor fibers
in glossopharyngeal n. begin in the inferior salivatory
nucleus.
- Origin : arise from inf. ganglion of glossopharyngeal n. in
the jugular fossa.
-course:
1- it reaches the middle ear cavity where it breaks to form
the tympanic plexus.
2- lesser S. petrosal nerve arise from the tympanic
plexus and reaches the middle cranial fossa via hiatus
carrying its name.
3- then the lesser S. petrosal n. pass through the
foramen ovale to reach the infra temporal fossa to relay
in the otic ganglion.
4- the postganglionic fibers join the auriculotemporal n. to
reach parotid gland.

15.  Clinical Correlation of CNIX
-The most frequent causes of damage to the nerve
are by compression from space _occupying lesions
in pharynx or posterior tongue or invasion by
malignant tumors in these areas.
-when they occur, taste is absent on the post.13 of
tongue, alongside an absence of the gag reflex.
-the sensory nerves from the pharynx are the afferent
components of the gag reflex which is stimulated by
touching the pharyngeal wall with a blunt object
with the mouth closed.

16.  Vagus Nerve
o Type: mixed nerve containing: parasympathatic, motor and
sensory fibers
o origin: from posterior lateral surface of medulla
o vagus nerve nuclei :
 main motor nucleus
This nucleus lies deep in the reticular formation of medulla
oblongata and is formed by the nucleus ambiguus
 parasympathatic nucleus
This nucleus forms the dorsal nucleus of the vagus
 sensory nucleus
This nucleus is the lower part of the nucleus of tractus
solitarius
sensation of taste travel through the peripheral axons of
nerve cells situated in the inferior ganglion on the vagus
nerve
Affrent information concerning common sensation enter the
brain stem through the superior ganglion of the vagus nerve
but end in the spinal nucleus of trigeminal nerve

17.  Course and relations in the neck :
 it leaves the cranial cavity through
the middle compartment of the
jugular foramen, here it has 2
sensory ganglia:
 sup. Ganglion( in the jugular f.)
 inferior ganglion ( just below
the jugular foramen).
 The cranial root of accessory
nerve joins the inferior ganglion of vagus just below the jugular f.
 The vagus descend vertically downwards inside the carotid
sheath :
-First between I.J.V and I.C.A ( passing with them deep to
styloid apparatus and posterior belly of digastric ) till the
upper border of thyroid cartilage.
-lower than, it descends between the I.J.V and C.C.A
 At the root of the neck, the vagus nerve crosses infront of
first part of subclavian artery to enter the thorax

18.  Branches of the vagus in the neck:
 2 branches arise from the sup. Ganglion:
I. meningeal br. Sensory to the dura matter of the post.
cranial fossa
II. auricular br. Sensory to the skin of medial side of
auricle, post. Wall of external auditory meatus and
tympanic membrane.
 2 branches arise from the inferior ganglion
I. Pharyngeal branch ( motor)
- its fibers are mostly derivesd from the cranial accessory
nerve and supply the pharyngeal m. ( except the
stylopharyngeal m.) + all muscle of the soft palat ( except
the tensor palatine muscle)
II. superiorlaryngeal nerve (mixed)
- its divided into
A. internal laryngeal nerve ( sensory ): accompanies the
sup. Laryngeal artery and supply the mucous membrane
of the upper ½ of larynx.
B. external laryngeal nerve (motor) : supply cricothyroid
muscle.

20.  2 cardiac branches : superior and inferior
(parasympathetic)
 recurrent laryngeal nerve: mixed ( motor and sensory)
I. right recurrent laryngeal nerve
II. Left recurrent laryngeal nerve
- Branches of each recurrent laryngeal nerve :
A. motor: to all laryngeal muscle ( except cricothyroid )
and it also gives a branches to trachea, esophagus
and inferior constrictor muscle of pharynx.
B. Sensory : to the mucous membrane of the lower ½ of
larynx (below focal cords)

21.  Clinical correlation
- The vagus nerve are relatively deep and are affected
mainly by – occupying lesions compressing the nerve or
invasion by malignant tumors
-The major cause of damage to the vagus nerve is usually
through damage to nuclei through brainstem stroke.
-loss of innervations of a laryngeal muscles will produce
dysphonia .
- paralysis of muscle of soft palate will also affect those
components of speech requirement movement of the soft
palate, producing some degree of dysatheria.
-deficiency of muscle activity in the pharynx produces
difficulty with swallowing ( dysphagia ) commonly any
dysphagia can lead to aspiration pneumonia due to
impaired swallowing in an individual and its frequently a
cause of a death.
- Lesion to the recurrent laryngeal nerve lead to horsenence
or absence of voice as a symptom of vagus nerve palsy.

22.  accessory nerve
 Type: purely motor nerve
 origin : it arises by 2 roots : cranial and spinal
o cranial root : formed from the axons of nerve cell of
nucleus ambiguus (arises from posterolateral
sulcus of medulla )
o Spinal root : formed from axon of nerve cells in the
spinal nucleus ( arise from the upper 5 or 6 cervical
segment of the spinal cord)

23.  Course and relation :
1. the spinal root ascends through the foramen magnum to join the
cranial root inside the cranial cavity.
2. the united accessory nerve leave the cranial cavity through the
middle compartment of the jugular foramen.
3. Just below the jugular foramen the 2 roots of accessory nerve
separate from each other
4. The cranial root join the inferior ganglia of vagus nerve and is
distributed with its pharyngeal an laryngeal branches.
5. The spinal root : has the following course in the neck
A. it descends inside the upper part of the carotid sheath deep to styloid
apparatus and between I.J.V and the I.C.A
B. Then it passes backwards between I.J.V and posterior belly of
digastric muscle
C. It pierces the deep surface of sternomastoid muscles, supplying it
then it emerges from its posterior border above its middle
D. It crosses the posterior triangle of the neck
E. Finally it pierces the posterior border of trapezius muscle about 5cm
about the clavicle and ends by supplying it

25.  Branches of accessory nerve
1. the cranial accessory nerve : joins the vagus nerve
and distributed with its :
A. pharyngeal br. : supplying all muscle of pharynx
( except stylopharyngius ) and all muscles of
palate (except tensor palatine muscles)
B. Laryngeal branch : supplying the muscle of the
larynx
2. The spinal accessory nerve: supplies 2 muscles :
A. sternomastoid
B. trapezius

26.  clinical correlation of CNXI
 Lesion of spinal part of accessory nerve may occur
anywhere along its course and may result from tumors
or trauma from stab or gunshot wounds in the neck.
 Lesion of the spinal part of accessory nerve will result in
paralysis of the sternocledomastoid and trapezius
muscles.
 The sternocledomastoid muscle will atrophy and there
will be weakness in turning the head to the opposite
side.
 the trapezius muscle will also atrophy and the shoulder
will drop into that side, there will be also weakness an
difficulty in rising the arm about the horizontal.

27.  Hypoglossal nerve
 Type : purely somatic motor n. supplying all intrinsic and
extrinsic muscles of tongue except palatoglossus.
 Origin:
-hypoglossal nucleus is located in medulla in the grey
matter. axons from the nucleus emerge from the
brainstem between the pyramid and olive(from the
anterolateral sulcus of medulla).
-It leaves cranuim via hypoglossal canal ang receives a
communicating branch from C1.
i

28.  Course and relations:
-it leave the cranial cavity
through the ant. condylar
foramen then forms a half
spiral turn around vagus nerve.
-then it descends vertically
downwards inside the upper
part of carotid sheath superficial
to vagus n. between I.J.V
and I.C.A.
-it descends deep to the
styloid process. styloid muscle and post. belly of
digastrics muscle.
-then it turns forwards crossing the following structures :
(a) I.C.A (b) occipital a. (c) E.C.A (d)the loop of
lingual a.

29. -it disappears deep to the intermediate tendon of digastrics
m. to reach the submandibular region where it runs on
hyoglossus m. and covered superficially by submandibular
gland.
-it disappears deep to mylohyoid m. below the lingual n. and
submandibular duct.
-finally it dips in the interval between the hyoglossus and
genioglossus where it breaks in to the terminal branches
supplying the muscle of the tongue.(except palatoglossus).

30.  Branches:
 arising from hypoglossal n. itself innervates all of
the intrinsic and most of the extrinsic muscle of the
tongue ( genioglossus, styloglossus and
hyoglossus muscles) except palatoglossus m.
 derived from C1 joining hypoglossus.
(1) Meningeal br.(sensory)to dura mater
(2) n. to geniohyoid m
(3) n. to thyrohyoid m.
(4) descendents hypoglossai forming the sup limb of
ansacervicalis (supplying infrahyoid mucsles )

32.  Clinical correlation of CNXII
-upper motor neurons that control tongue movement decussate
within the corticobulbar tracts before arriving at the hypoglossal
nuclei.
-This means UMN lesions of primary motor cortex cause contra
lateral weakness of the tongue, whereas LMN lesions of the
hypoglossal nuclei cause ipsilateral weakness of the tongue.
-Tongue weakness causes
the tongue upon protrusion
to deviate towards the weak
side due to overactive
compensatory action of the
other hypoglassal n.
-CNIII damage also cause
tongue muscle to atrophy.

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