12. Muscles of Facial Expression
• The muscles of facial expression are located in
the subcutaneous tissue, originating from bone or fascia, and
inserting onto the skin.
• By contracting, the muscles pull on the skin and exert their
effects.
• They are the only group of muscles that insert into skin.
• These muscles have a common embryonic origin – the 2nd
pharyngeal arch.
• They migrate from the arch, taking their nerve supply with
them.
• As such, all the muscles of facial expression are innervated by
the facial nerve.
• The facial muscles can broadly be split into three groups:
orbital, nasal and oral.
82. If the facial nerve is dysfunctional, the oral
muscles can become paralyzed.
The patient may present with difficulty
eating, with food collecting between the teeth
and cheeks.
In addition, the tissue around the mouth and
cheeks sags, and is drawn across to the
opposite side while smiling.
83. The muscles of mastication are associated with
movements of the jaw (temporomandibular joint).
They are one of the major muscle groups in the head –
the other being the muscles of facial expression.
There are four muscles:
Masseter
Temporalis
Medial pterygoid
Lateral pterygoid
84. The muscles of mastication develop from the
first pharyngeal arch. Thus, they are
innervated by a branch of the trigeminal nerve
(CN V), the mandibular nerve.
(NB: It is important to note that all the muscles
mentioned here are bilateral structures).
85. The masseter muscle is the most powerful muscle of mastication.
It is quadrangular in shape and has two parts: deep and superficial.
The entirety of the muscle lies superficially to the pterygoids and
temporalis, covering them.
Attachments: The superficial part originates from maxillary process of
the zygomatic bone.
The deep part originates from the zygomatic arch of the temporal
bone.
Both parts attach to the ramus of the mandible.
Actions: Elevates the mandible, closing the mouth.
Innervation: Mandibular nerve (V3).
86. Temporalis
The temporalis muscle originates from the temporal fossa – a
shallow depression on the lateral aspect of the skull.
The muscle is covered by tough fascia which can be harvested
surgically and used to repair a perforated tympanic membrane (an
operation known as a myringoplasty).
Attachments: Originates from the temporal fossa. It condenses into
a tendon, which inserts onto the coronoid process of the mandible.
Actions: Elevates the mandible, closing the mouth. Also retracts the
mandible, pulling the jaw posteriorly.
Innervation: Mandibular nerve (V3).
94. The Muscles of Mastication
• Medial Pterygoid
• The medial pterygoid muscle has a quadrangular shape with
two heads: deep and superficial.
• It is located inferiorly to the lateral pterygoid.
• Attachments:
• The superficial head originates from the maxillary tuberosity and
the pyramidal process of palatine bone.
• The deep head originates from the medial aspect of the lateral
pterygoid plate of the sphenoid bone.
• Both heads attach to the ramus of the mandible near the angle
of mandible.
• Actions: Elevates the mandible, closing the mouth.
• Innervation: Mandibular nerve (V3).
95. The Muscles of Mastication
• Lateral Pterygoid
• The lateral pterygoid muscle has a triangular shape with two heads:
superior and inferior.
• It has horizontally orientated muscle fibers, and thus is the major protractor
of the mandible.
• Attachments:
• The superior head originates from the greater wing of the sphenoid.
• The inferior head originates from the lateral pterygoid plate of the sphenoid.
• The two heads converge into a tendon which attaches to the neck of the
mandible.
• Actions: Acting bilaterally, the lateral pterygoids protract the mandible,
pushing the jaw forwards. Unilateral action produces the ‘side to side’
movement of the jaw.
• Note: Contraction of the lateral pterygoid will produce lateral movement on
the contralateral side. For example, contraction of left lateral pterygoid will
deviate the mandible to the right.
• Innervation: Mandibular nerve (V3).
97. Cranial Nerves
• The cranial nerves are a set of 12 paired nerves that arise
directly from the brain.
• The first two nerves (olfactory and optic) arise from the
cerebrum, whereas the remaining ten emerge from the brain
stem.
• The names of the cranial nerves relate to their function and
they are also numerically identified in roman numerals (I-XII).
98. Cranial Nerves
• Origin of the Cranial Nerves
• There are twelve cranial nerves in total.
• 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).
101. Modalities
• Each cranial nerve can be described as being sensory, motor or both.
They can more specifically transmit seven types of information; three
are unique to cranial nerves (SSS, SVS and SVM).
• See Table 1 for a summary of the cranial nerves, their modalities and
functions.
• Sensory (afferent) Modalities:
• General somatic sensory (GSS) – general sensation from skin.
• General visceral sensory (GVS) – general sensation from viscera.
• Special somatic sensory (SSS) – senses derived from ectoderm (e.g.
sight, sound, balance).
• Special visceral sensory (SVS) – senses derived from endoderm (e.g.
taste, smell).
• Motor (efferent) Modalities:
• General somatic motor (GSM) – skeletal muscles.
• General visceral motor (GVM) – smooth muscles of gut and autonomic
motor.
• Special visceral motor (SVM) – muscles derived from pharyngeal
arches.
102. The Olfactory Nerve (CN I) and Olfactory
Pathway
• The olfactory nerve (CN I) is the first and shortest cranial nerve.
It is a special visceral afferent nerve, which transmits
information relating to smell.
• Embryologicallly, the olfactory nerve is derived from the
olfactory placode (a thickening of the ectoderm layer), which
also give rise to the glial cells which support the nerve.
• The anatomical course of the olfactory nerve describes the
transmission of special sensory information from the nasal
epithelium to the primary olfactory cortex of the brain.
104. The Optic Nerve (CN II) and Visual
Pathway
• The optic nerve (CN II) is the second cranial nerve, responsible
for transmitting the special sensory information for vision.
• It is developed from the optic vesicle, an outpocketing of the
forebrain.
• The optic nerve can therefore be considered part of the central
nervous system, and examination of the nerve enables an
assessment of intracranial health.
• Due to its unique anatomical relation to the brain, the optic
nerve is surrounded by the cranial meninges (not by epi-, peri-
and endoneurium like most other nerves).
105. The Optic nerve (CN II)
Nerve of sight .
Made up of axons of
cells in ganglionic layer
of retina.
Emerge from eye ball
medial to posterior pole.
Runs posteromedially
and pass through optic
canal to cranial cavity
and forms optic
chiasma.
106. Optic Nerve
• Anatomical Course
• The anatomical course of the optic nerve describes the transmission
of special sensory information from the retina of the eye to the
primary visual cortex of the brain.
• It can be divided into extracranial (outside the cranial cavity) and
intracranial components.
107. Optic Nerve
• Extracranial
• 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 rods and cones).
• After its formation, the nerve leaves the bony orbit via the optic
canal, a passageway through the sphenoid bone.
• It enters the cranial cavity, running along the surface of the
middle cranial fossa (in close proximity to the pituitary gland).
• Intracranial (The Visual Pathway)
• Within the middle cranial fossa, the optic nerves from each eye
unite to form the optic chiasm.
108. The Oculomotor Nerve (CN
III)
• The oculomotor nerve is the third cranial nerve (CN III). It
provides motor and parasympathetic innervation to some of the
structures within the bony orbit.
• Overview
• Motor – Innervates the majority of the extraocular
muscles (levator palpebrae superioris, superior rectus, inferior
rectus, medial rectus and inferior oblique).
• Parasympathetic – Supplies the sphincter pupillae and the
ciliary muscles of the eye.
• Sympathetic – No direct function, but sympathetic fibres run
with the oculomotor nerve to innervate the superior tarsal
muscle (helps to raise the eyelid).
110. • Anatomical Course
• The oculomotor nerve originates from the oculomotor nucleus – located
within the midbrain of the brainstem, ventral to the cerebral aqueduct. 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. Within the cavernous sinus, it receives sympathetic
branches from the internal carotid plexus. These fibres do not combine with
the oculomotor nerve – they merely travel within its sheath.
• The nerve leaves the cranial cavity via the superior orbital fissure. At this
point, it divides into superior and inferior branches:
• Superior branch – provides motor innervation to the superior rectus and
levator palpabrae superioris.
• Sympathetic fibres run with the superior branch to innervate the superior
tarsal muscle.
• 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.
111. The Trochlear Nerve (CN IV)
• The trochlear nerve is the fourth paired cranial nerve. It is the
smallest cranial nerve (by number of axons), yet has the longest
intracranial course. It has a purely somatic motor function.
• Anatomical Course
• The trochlear nerve arises from the trochlear nucleus of the
brain, emerging from the posterior aspect of the midbrain (it is
the only cranial nerve to exit from the posterior 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.
113. The Trigeminal Nerve (CN V)
• The trigeminal nerve, CN V, is the fifth paired cranial nerve. It is also the
largest cranial nerve. In this article, we shall look at the anatomical course of
the nerve, and the motor, sensory and parasympathetic functions of its
terminal branches.
• The trigeminal nerve is associated with derivatives of the 1st pharyngeal
arch.
• Sensory: The three terminal branches of CN V innervate the skin, mucous
membranes and sinuses of the face. Their distribution pattern is similar to
the dermatome supply of spinal nerves (except there is little overlap in the
supply of the divisions).
• Motor: Only the mandibular branch of CN V has motor fibres. It innervates
the muscles of mastication: medial pterygoid, lateral pterygoid, masseter
and temporalis. The mandibular nerve also supplies other 1st pharyngeal
arch derivatives: anterior belly of digastric, mylohyoid, tensor veli palatini
and tensor tympani.
• Parasympathetic Supply: The post-ganglionic neurones of parasympathetic
ganglia travel with branches of the trigeminal nerve. (But note that CN V is
NOT part of the cranial outflow of PNS supply)
122. Trigeminal Nerve
• Anatomical Course
• 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.
• A nucleus (pl. nuclei) is a collection of neurone cell bodies within
the central nervous system.
• At the level of the pons, the sensory nuclei merge to form a sensory
root.
• The motor nucleus continues to form a motor root.
• These roots are analogous to the dorsal and ventral roots of the
spinal cord.
• In the middle cranial fossa, the sensory root expands into the
trigeminal ganglion. A ganglion (pl. ganglia) refers to a collection of
the neurone cell bodies outside the central nervous system.
• The trigeminal ganglion is located lateral to the cavernous sinus, in
a depression of the temporal bone. This depression is known as the
trigeminal cave.
124. The Abducens Nerve (CN VI)
• The abducens nerve is the sixth paired cranial nerve. It has a purely
somatic motor function – providing innervation to the lateral rectus
muscle.
• Anatomical Course
• The abducens nerve arises from the abducens nucleus in the pons
of the brainstem. It 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 (a dural venous
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.
128. Abducens Nerve
• Motor Function
• The abducens nerve provides innervation to the lateral
rectus muscle – one of the extraocular muscles.
• The lateral rectus originates from the lateral part of the
common tendinous ring, and attaches to the anterolateral
aspect of the sclera.
• It acts to abduct the eyeball (i.e. to rotate the gaze away from
the midline).
129. ClinicalRelevance- Examinationof the
AbducensNerve
• The abducens nerve is examined in conjunction with the
oculomotor and trochlear nerves by testing the movements of
the eye.
• The patient is asked to follow a point with their eyes (commonly
the tip of a pen) without moving their head.
• The target is moved in an ‘H-shape’ and the patient is asked to
report any blurring of vision or diplopia (double vision).
130. Clinical Relevance -
Abducens Nerve Palsy
• Abducens nerve palsy can be caused by any structural pathology
which leads to downwards pressure on the brainstem (e.g. space-
occupying lesion).
• This can stretch the nerve from its origin at the junction of the pons
and medulla.
• Other causes include diabetic neuropathy and thrombophlebitis of
the cavernous sinus (in these cases, it is rare for the abducens
nerve to be affected in isolation).
• Clinical features of abducens nerve palsy include diplopia, the
affected eye resting in adduction (due to unopposed activity of the
medial rectus), and inability to abduct the eye.
• The patient may attempt to compensate by rotating their head to
allow the eye to look sideways.
131. The Facial Nerve (CN VII)
• The facial nerve (CN VII) is the seventh paired cranial nerve.
• Overview
• The facial nerve is associated with the derivatives of the second
pharyngeal arch:
• Motor – muscles of facial expression, posterior belly of the
digastric, stylohyoid and stapedius muscles.
• Sensory – a small area around the concha of the external ear.
• Special Sensory – provides special taste sensation to the
anterior 2/3 of the tongue via the chorda tympani
• Parasympathetic – supplies many of the glands of the head and
neck, including:
• Submandibular and sublingual salivary glands.
• Nasal, palatine and pharyngeal mucous glands.
• Lacrimal glands.
132. The Facial Nerve (CN VII)
• Anatomical Course
• The course of the facial nerve is very complex. There are many branches,
which transmit a combination of sensory, motor and parasympathetic
fibres.
• 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.
• Intracranial
• The nerve arises in the pons, an area of the brainstem. It begins as two
roots; a large motor root, and a small sensory root (the part of the facial
nerve that arises from the sensory root is sometimes known as the
intermediate nerve).
• The two roots travel through the internal acoustic meatus, a 1cm long
opening in the petrous part of the temporal bone. Here, they are in very
close proximity to the inner ear.
133. The Facial Nerve (CN VII)
• Still within the temporal bone, the roots leave the internal acoustic meatus,
and enter into the facial canal.
• The canal is a ‘Z’ shaped structure. Within the facial canal, three important
events occur:
• Firstly the two roots fuse to form the facial nerve.
• Next, the nerve forms the geniculate ganglion (a ganglion is a collection of
nerve cell bodies).
• Lastly, the nerve gives rise to:
• Greater petrosal nerve – parasympathetic fibres to mucous glands and
lacrimal gland.
• Nerve to stapedius – motor fibres to stapedius muscle of the middle ear.
• Chorda tympani – special sensory fibres to the anterior 2/3 tongue and
parasympathetic fibres to the submandibular and sublingual glands.
• The facial nerve then exits the facial canal (and the cranium) via the
stylomastoid foramen. This is an exit located just posterior to the styloid
process of the temporal bone.
135. The Facial Nerve (CN VII)
• Extracranial
• 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.
• It 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 (note – the facial
nerve does not contribute towards the innervation of the parotid gland,
which is innervated by the glossopharyngeal nerve).
• 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.
136. The Facial Nerve (CN VII)
• Motor Functions
• Branches of the facial nerve are responsible for innervating many of the
muscles of the head and neck. All these muscles are derivatives of the
second pharyngeal arch.
• The first motor branch arises within the facial canal; the nerve to stapedius.
The nerve passes through the pyramidal eminence to supply the stapedius
muscle in the middle ear.
• Between the stylomastoid foramen, and the parotid gland, three more
motor branches are given off:
• Posterior auricular nerve – Ascends in front of the mastoid process, and
innervates the intrinsic and extrinsic muscles of the outer ear. It also supplies
the occipital part of the occipitofrontalis muscle.
• Nerve to the posterior belly of the digastric muscle – Innervates the
posterior belly of the digastric muscle (a suprahyoid muscle of the neck).
• It is responsible for raising the hyoid bone.
• Nerve to the stylohyoid muscle – Innervates the stylohyoid muscle
(a suprahyoid muscle of the neck). It is responsible for raising the hyoid
bone.
137. The Facial Nerve (CN VII)
• Within the parotid gland, the facial nerve terminates
by bifurcating into five motor branches.
• These innervate the muscles of facial expression:
– Temporal – Innervates the frontalis, orbicularis oculi and
corrugator supercilii.
– Zygomatic – Innervates the orbicularis oculi.
– Buccal – Innervates the orbicularis oris, buccinator and
zygomaticus.
– Marginal mandibular – Innervates the depressor labii
inferioris, depressor anguli oris and mentalis.
– Cervical – Innervates the platysma.
138. • Special Sensory Functions
• The chorda tympani branch of the facial nerve is responsible for
innervating the anterior 2/3 of the tongue with the special
sense of taste.
• The nerve arises in the facial canal, and travels across the bones
of the middle ear, exiting via the petrotympanic fissure, and
entering the infratemporal fossa.
• Within the infratemporal fossa, the chorda tympani ‘hitchhikes’
upon the lingual nerve. The parasympathetic fibres of the
chorda tympani stay with the lingual nerve, but the main body
of the nerve leaves to innervate the anterior 2/3 of the tongue.
139. • Parasympathetic Functions
• The parasympathetic fibres of the facial nerve are carried by the greater
petrosal and chorda tympani branches.
• Greater Petrosal Nerve
• The greater petrosal nerve arises immediately distal to the geniculate
ganglion within the facial canal. It then moves in anteromedial direction,
exiting the temporal bone into the middle cranial fossa. From here, its
travels across (but not through) the foramen lacerum, combining with the
deep petrosal nerve to form the nerve of the pterygoid canal.
• The nerve of pterygoid canal then passes through the pterygoid canal
(Vidian canal) to enter the pterygopalatine fossa, and synapses with
the pterygopalatine ganglion. Branches from this ganglion then go on to
provide parasympathetic innervation to the mucous glands of the oral
cavity, nose and pharynx, and the lacrimal gland.
• Chorda Tympani
• The chorda tympani also carries some parasympathetic fibres. These
combine with the lingual nerve (a branch of the trigeminal nerve) in the
infratemporal fossa and form the submandibular ganglion. Branches from
this ganglion travel to the submandibular and sublingual salivary glands.
140.
141. • Clinical Relevance: Damage to the Facial Nerve
• The facial nerve has a wide range of functions. Thus, damage to the nerve
can produce a varied set of symptoms, depending on the site of the lesion.
• Intracranial Lesions
• Intracranial lesions occur during the intracranial course of the facial nerve
(proximal to the stylomastoid foramen).
• The muscles of facial expression will be paralysed or severely
weakened. The other symptoms produced depend on the location of the
lesion, and the branches that are affected:
• Chorda tympani – reduced salivation and loss of taste on the ipsilateral 2/3
of the tongue.
• Nerve to stapedius – ipsilateral hyperacusis (hypersensitive to sound).
• Greater petrosal nerve – ipsilateral reduced lacrimal fluid production.
• The most common cause of an intracranial lesion of the facial nerve is
infection related to the external or middle ear. If no definitive cause can be
found, the disease is termed Bell’s palsy.
142. • Extracranial Lesions
• Extracranial lesions occur during the extracranial course of the
facial nerve (distal to the stylomastoid foramen). Only the motor
function of the facial nerve is affected, therefore resulting in
paralysis or severe weakness of the muscles of facial expression.
• There are various causes of extracranial lesions of the facial nerve:
• Parotid gland pathology – e.g a tumour, parotitis, surgery.
• Infection of the nerve – particularly by the herpes virus.
• Compression during forceps delivery – the neonatal mastoid
process is not fully developed and does not provide complete
protection of the nerve.
• Idiopathic – If no definitive cause can be found then the disease is
termed Bell’s palsy.
143. The VestibulocochlearNerve (CN
VIII)
• The vestibulocochlear nerve is the eighth paired cranial nerve.
• It is comprised of two parts – vestibular fibres and cochlear
fibers.
• Both have a purely sensory function.
144. • Anatomical Course
• The vestibular and cochlear portions of the vestibulocochlear nerve are
functionally discrete, and so 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 sets of fibres 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.
145. Clinical Relevance: Basilar Skull
Fracture
• A basilar skull fracture is a fracture of the skull base, usually
resulting from major trauma.
• The vestibulocochlear nerve can be damaged within the
internal acoustic meatus, producing symptoms of vestibular and
cochlear nerve damage.
• Patients may also exhibit signs related to the other cranial
nerves, bleeding from the ears and nose, and cerebrospinal
fluid leaking from the ears (CSF otorrhoea) and nose (CSF
rhinorrhoea).
146. • Special Sensory Functions
• The vestibulocochlear nerve is unusual in that it primarily consists of bipolar
neurones. It is responsible for the special senses of hearing (via the
cochlear nerve), and balance (via the vestibular nerve).
• Hearing
• The cochlea detects the magnitude and frequency of sound waves. The
inner hair cells of the organ of Corti activate ion channels in response to
vibrations of the basilar membrane. Action potentials travel from the spiral
ganglia, which house the cell bodies of neurones of the cochlear nerve.
• The magnitude of the sound determines how much the membrane vibrates
and thereby how often action potentials are triggered. Louder sounds
cause the basilar membrane to vibrate more, resulting in action potentials
being transmitted from the spiral ganglia more often, and vice versa. The
frequency of the sound is coded by the position of the activated inner hair
cells along the basilar membrane.
152. The Glossopharyngeal Nerve (CN IX)
• Embryologically, the glossopharyngeal nerve is associated with
the derivatives of the third pharyngeal arch.
• Sensory: Innervates the oropharynx, carotid body and sinus,
posterior 1/3 of the tongue, middle ear cavity and Eustachian
tube.
• Special sensory: Provides taste sensation to the posterior 1/3 of
the tongue.
• Parasympathetic: Provides parasympathetic innervation to the
parotid gland.
• Motor: Innervates the stylopharyngeus muscle of the pharynx.
153. The Glossopharyngeal Nerve (CN IX)
• Anatomical Course
• The glossopharyngeal nerve originates in the medulla oblongata of
the brain.
• It emerges from the anterior aspect of the medulla, moving
laterally in the posterior cranial fossa.
• The nerve leaves the cranium via the jugular foramen. At this point,
the tympanic nerve arises. It has a mixed sensory and
parasympathetic composition.
• Immediately outside the jugular foramen lie two ganglia
(collections of nerve cell bodies).
• They are known as the superior and inferior (or petrous) ganglia –
they contain the cell bodies of the sensory fibres in the
glossopharyngeal nerve.
• Now extracranial, 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.
154. Sensory Functions
The glossopharyngeal nerve provides sensory innervation a variety of
structures in the head and neck.
The tympanic nerve arises as the nerve traverses the jugular foramen. It
penetrates the temporal bone and enters the cavity of the middle ear.
Here, it forms the tympanic plexus – a network of nerves that provide
sensory innervation to the middle ear, internal surface of the tympanic
membrane and Eustachian tube.
At the level of the stylopharyngeus, the carotid sinus nerve arises. It
descends down the neck to innervate both the carotid sinus and carotid
body, which provide information about blood pressure and oxygen
saturation respectively.
The glossopharyngeal nerve terminates by splitting into several sensory
branches:
Pharyngeal branch – combines with fibres of the vagus nerve to form the
pharyngeal plexus. It innervates the mucosa of the oropharynx.
Lingual branch – provides the posterior 1/3 of the tongue with general
and taste sensation
Tonsillar branch – forms a network of nerves, known as the tonsillar
plexus, which innervates the palatine tonsils.
155. • Sensory Functions
• The glossopharyngeal nerve provides sensory innervation a variety of structures
in the head and neck.
• The tympanic nerve arises as the nerve traverses the jugular foramen. It
penetrates the temporal bone and enters the cavity of the middle ear. Here, it
forms the tympanic plexus – a network of nerves that provide sensory
innervation to the middle ear, internal surface of the tympanic membrane and
Eustachian tube.
• At the level of the stylopharyngeus, the carotid sinus nerve arises. It descends
down the neck to innervate both the carotid sinus and carotid body, which
provide information about blood pressure and oxygen saturation respectively.
• The glossopharyngeal nerve terminates by splitting into several sensory
branches:
• Pharyngeal branch – combines with fibres of the vagus nerve to form the
pharyngeal plexus. It innervates the mucosa of the oropharynx.
• Lingual branch – provides the posterior 1/3 of the tongue with general and taste
sensation
• Tonsillar branch – forms a network of nerves, known as the tonsillar plexus,
which innervates the palatine tonsils.
156. The GlossopharyngealNerve(CNIX)
• Special Sensory
• The glossopharyngeal nerve provides taste sensation to the
posterior 1/3 of the tongue, via its lingual branch (Note: not to
be confused with the lingual nerve).
• Motor Functions
• The stylopharyngeus muscle of the pharynx is innervated by
the glossopharyngeal nerve. This muscle acts to shorten and
widen the pharynx and elevate the larynx during swallowing.
157. The GlossopharyngealNerve(CNIX)
• Parasympathetic Functions
• The glossopharyngeal nerve provides parasympathetic
innervation to the parotid gland.
• These fibres originate in the inferior salivatory nucleus of CN IX.
• These fibres travel with the tympanic nerve to the middle ear.
From the ear, the fibres continue as the lesser petrosal nerve,
before synapsing at the otic ganglion.
• The fibres then hitchhike on the auriculotemporal nerve to the
parotid gland, where they have a secretomotor effect.
• Remember – although the facial nerve splits into its five terminal
branches in the parotid gland, it is the glossopharyngeal nerve
that actually supplies the gland.
158. The glossopharyngeal nerve supplies sensory
innervation to the oropharynx, and thus carries the
afferent information for the gag reflex.
When a foreign object touches the back of the
mouth, this stimulates CNIX, beginning the reflex.
The efferent nerve in this process is the vagus
nerve, CNX.
An absent gag reflex signifies damage to the
glossopharyngeal nerve.
160. The Vagus Nerve (CN X)
• he vagus nerve is the 10th cranial nerve (CN X).
• It is a functionally diverse nerve, offering many different
modalities of innervation. It is associated with the derivatives of
the fourth and sixth pharyngeal arches.
• Overview
• Sensory: Innervates the skin of the external acoustic meatus
and the internal surfaces of the laryngopharynx and larynx.
Provides visceral sensation to the heart and abdominal viscera.
• Special Sensory: Provides taste sensation to the epiglottis and
root of the tongue.
• Motor: Provides motor innervation to the majority of the
muscles of the pharynx, soft palate and larynx.
• Parasympathetic: Innervates the smooth muscle of the trachea,
bronchi and gastro-intestinal tract and regulates heart rhythm.
161. • Anatomical Course
• The vagus nerve has the longest course of all the cranial nerves,
extending from the head to the abdomen. Its name is derived
from the Latin ‘vagary’ – meaning wandering. It is sometimes
referred to as the wandering nerve.
• In the Head
• The vagus nerve originates from the medulla of the brainstem.
It exits the cranium via the jugular foramen, with the
glossopharyngeal and accessory nerves (CN IX and XI
respectively).
• Within the cranium, the auricular branch arises. This supplies
sensation to the posterior part of the external auditory canal
and external ear.
164. 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:
• The right vagus nerve passes anterior to the subclavian artery
and posterior to the sternoclavicular joint, entering the thorax.
• The left vagus nerve passes inferiorly between the left common
carotid and left subclavian arteries, posterior to the
sternoclavicular joint, entering the thorax.
165. 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.
166. In the abdomen, the vagal trunks terminate by dividing into
branches that supply the oesophagus, stomach and the small
and large bowel (up to the splenic flexure).
Sensory Functions
There are somatic and visceral components to the sensory
function of the vagus nerve.
Somatic refers to sensation from the skin and muscles.
This is provided by the auricular nerve, which innervates the
skin of the posterior part of the external auditory canal and
external ear.
Viscera sensation is that from the organs of the body.
The vagus nerve innervates:
Laryngopharynx – via the internal laryngeal nerve.
Superior aspect of larynx (above vocal folds) – via the
internal laryngeal nerve.
Heart – via cardiac branches of the vagus nerve.
Gastro-intestinal tract (up to the splenic flexure) – via the
terminal branches of the vagus nerve.
167. Special Sensory Functions
• The vagus nerve has a minor role in taste sensation. It carries
afferent fibres from the root of the tongue and epiglottis.
• (This is not to be confused with the special sensation of the
glossopharyngeal nerve, which provides taste sensation for the
posterior 1/3 of the tongue).
• Motor Functions
• The vagus nerve innervates the majority of the muscles associated
with the pharynx and larynx.
• These muscles are responsible for the initiation of swallowing and
phonation.
• Pharynx
• Most of the muscles of the pharynx are innervated by the
pharyngeal branches of the vagus nerve:
• Superior, middle and inferior pharyngeal constrictor muscles
• Palatopharyngeus
• Salpingopharyngeus
• An additional muscle of the pharynx, the stylopharyngeus, is
innervated by the glossopharyngeal nerve.
168. Larynx
Innervation to the intrinsic muscles of the larynx is achieved via
the recurrent laryngeal nerve and external branch of the superior laryngeal
nerve.
Recurrent laryngeal nerve:
Thyro-arytenoid
Posterior crico-arytenoid
Lateral crico-arytenoid
Transverse and oblique arytenoids
Vocalis
External laryngeal nerve:
Cricothyroid
Other Muscles
In addition to the pharynx and larynx, the vagus nerve also innervates the
palatoglossus of the tongue, and the majority of the muscles of the soft
palate.
169. Parasympathetic Functions
In the thorax and abdomen, the vagus nerve is the main
parasympathetic outflow to the heart and gastro-intestinal organs.
The Heart
Cardiac branches arise in the thorax, conveying parasympathetic
innervation to the sino-atrial and atrio-ventricular nodes of the heart
(For more heart anatomy, see here).
These branches stimulate a reduction in the resting heart rate.
They are constantly active, producing a rhythm of 60 – 80 beats per
minute. If the vagus nerve was lesioned, the resting heart rate would
be around 100 beats per minute.
Gastro-Intestinal System
The vagus nerve provides parasympathetic innervation to the majority
of the abdominal organs. It sends branches to the oesophagus,
stomach and most of the intestinal tract – up to the splenic flexure of
the large colon.
The function of the vagus nerve is to stimulate smooth muscle
contraction and glandular secretions in these organs. For example, in
the stomach, the vagus nerve increases the rate of gastric emptying,
and stimulates acid production.
170. The Accessory Nerve (CN XI)
• The accessory nerve is the eleventh paired cranial nerve. It has a purely somatic
motor function, innervating the sternocleidomastoid and trapezius muscles.
• Anatomical Course
• Traditionally, the accessory nerve is divided into spinal and cranial parts.
• Spinal Component
• The spinal portion arises from neurones of the upper spinal cord, specifically C1-
C5/C6 spinal nerve roots. These fibres coalesce 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
(along with the glossopharyngeal and vagus nerves).
• 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 fibres to the
trapezius.
• Note: The extracranial course of the accessory nerve is relatively superficial (it
runs between the investing and prevertebral layers of fascia), and thus leaves it
vulnerable to damage.
173. Spinal Accessory Nerve
Cranial Component
• The cranial portion is much smaller and arises from the lateral
aspect of the medulla oblongata.
• It leaves the cranium via the jugular foramen, where it briefly
contacts the spinal part of the accessory nerve.
• Immediately after leaving the skull, cranial part combines
with the vagus nerve (CN X) at the inferior ganglion of vagus
nerve (a ganglion is a collection of nerve cell bodies).
• The fibers from the cranial part are then distributed through the
vagus nerve.
• For this reason, the cranial part of the accessory nerve is
considered as part of the vagus nerve.
174. • Motor Function
• The spinal accessory nerve innervates two muscles – the
sternocleidomastoid and trapezius.
• Sternocleidomastoid
• Attachments – Runs from the mastoid process of the temporal bone to the
manubrium (sternal head) and the medial third of the clavicle (clavicular
head).
• Actions – Lateral flexion and rotation of the neck when acting unilaterally,
and extension of the neck at the atlanto-occipital joints when acting
bilaterally.
• Trapezius
• Attachments – Runs from the base of the skull and the spinous processes of
the C7-T12 vertebrae to lateral third of the clavicle and the acromion of the
scapula.
• Actions – It is made up of upper, middle, and lower fibres. The upper fibres
of the trapezius elevate the scapula and rotate it during abduction of the
arm. The middle fibres retract the scapula and the lower fibres pull the
scapula inferiorly.
175. Clinical Relevance
• Examination of the Accessory Nerve
• The accessory nerve is examined by asking the patient to rotate
their head and shrug their shoulders, both normally and against
resistance. Simply observing the patient may also reveal signs of
muscle wasting in the sternocleidomastoid and trapezius in
cases of long-standing nerve damage.
• Palsy of the Accessory Nerve
• The most common cause of accessory nerve damage is
iatrogenic. Procedures such as cervical lymph node excision
biopsy or central line insertion can cause trauma to the nerve.
• Clinical features include muscle wasting and partial paralysis of
the sternocleidomastoid, resulting in the inability to rotate the
head or weakness in shrugging the shoulders. Damage to the
muscles may also result in an asymmetrical neckline.
176. The Hypoglossal Nerve (CN XII)
• The hypoglossal nerve is the twelfth paired cranial nerve.
• Its name is derived from ancient Greek, ‘hypo‘ meaning under,
and ‘glossal‘ meaning tongue.
• The nerve has a purely somatic motor function, innervating all
the extrinsic and intrinsic muscles of the tongue (except the
palatoglossus, innervated by vagus nerve).
177. The Hypoglossal Nerve (CN XII)
Anatomical Course
• The hypoglossal nerve arises from the hypoglossal nucleus in the
medulla oblongata of the brainstem.
• It then passes laterally across the posterior cranial fossa, within the
subarachnoid space.
• The nerve 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.
179. The Hypoglossal Nerve (CN XII)
• Motor Function
• The hypoglossal nerve is responsible for motor innervation of
the vast majority of the muscles of the tongue (except for
palatoglossus). These muscles can be subdivided into two
groups:
• i) Extrinsic muscles
• Genioglossus (makes up the bulk of the tongue)
• Hyoglossus
• Styloglossus
• Palatoglossus (innervated by vagus nerve)
180. The Hypoglossal Nerve (CN XII)
• ii) Intrinsic muscles
• Superior longitudinal
• Inferior longitudinal
• Transverse
• Vertical
• Together, these muscles are responsible for all movements of the tongue.
• Role of the C1/C2 Roots
• The C1/C2 roots that travel with the hypoglossal nerve also have a motor
function.
• They branch off to innervate the geniohyoid (elevates the hyoid bone) and
thyrohyoid (depresses the hyoid bone) muscles.
• Another branch containing C1/C2 fibres descends to supply the ansa
cervicalis – a loop of nerves that is part of the cervical plexus.
• From the ansa cervicalis, nerves arise to innervate the omohyoid,
sternohyoid and sternothyroid muscles. These muscles all act to depress the
hyoid bone.
181. Clinical Relevance
• The hypoglossal nerve is examined by asking the patient to protrude their
tongue.
• Other movements such as asking the patient to push their tongue against
their cheek and feeling for the pressure on the opposite side of the cheek
may also be used if damage is suspected.
• Hypoglossal Nerve Palsy
• Damage to the hypoglossal nerve is a relatively uncommon cranial nerve
palsy. Possible causes include head & neck malignancy and penetrating
traumatic injuries.
• If the symptoms are accompanied by acute pain, a possible cause may be
dissection of the internal carotid artery.
• Patients will present with deviation of the tongue towards the damaged
side on protrusion, as well as possible muscle wasting and fasciculations
(twitching of isolated groups of muscle fibres) on the affected side.
