(Labyrinth)

It is located within the petrous part of the temporal bone between the middle ear and the internal auditory canal. Its length is 20 mm and consists of a membranous labyrinth surrounded by a bony labyrinth:

• The bony labyrinth is a hollow bony capsule, which is a part of the petrous portion of the temporal bone.
• The membranous labyrinth is a group of delicate ducts and sacs.

The membranous labyrinth is filled with a fluid called endolymph and is surrounded by a fluid called perilymph, the membranous labyrinth contains the labyrinthine sensory end organs.

1. The Bony Labyrinth consists of:

1. Bony cochlea: A coiled canal that resembles a snail's shell having 2.5 turns.

2. Three bony semi-circular canals (lateral, superior and posterior), each canal forms 2/3 of a circle and they are perpendicular to each other.
3. Vestibule: Between the cochlea and the semi-circular canals.
   

2. The Membranous Labyrinth consists of:

1. Membranous cochlear duct: Within the bony cochlea contains the organ of corti à Cochlear division of the 8^th Cranial nerve.

2. Membranous semi-circular ducts: Within the bony semi-circular canalsà vestibular division of the 8^th Cranial nerve.
3. Utricle, and Saccule: Within the vestibule à vestibular division of the 8^th Cranial nerve.
4. Endolymphatic duct and sac.
   

The Cochlea:

1. The cochlea forms 2.5 turns around an axis called the modiolus.

2. Cochlear nerve fibers leave the cochlea through the modiolus.

3. The cavity of the bony cochlea is divided into three compartments:

• A middle compartment called scala media (cochlear duct), filled with endolymph. The organ of Corti (the sensory end-organ of the cochlea) lies in this compartment. It rests on a membrane called basilar membrane.
• An upper compartment called scala vestibuli, filled with perilymph.
• A lower compartment called scala tympani, filled with perilymph.

The Labyrinthine Sensory End Organs:

They are the sensory end organs of hearing and equilibrium, located within the membranous labyrinth

• The sensory end organ of the cochlea is called: the organ of Corti.
• The sensory end organ of the semi-circular duct is called: the crista.
• The sensory end organ of the utricle and saccule is called: the macula.

Each sensory end organ contains special sensory hair cells.

THE MIDDLE EAR CAVITY (TYMPANIC CAVITY):
It is located in the temporal bone between the external ear and the inner ear. It is a six-walled cavity about 15 mm in height, 15 mm antro-posteriorly and 2-6 mm from side to side. All the walls are bony, except the lateral wall (tympanic membrane) and are lined with mucous membrane, which is ciliated columnar epithelium except its postero-superior part that has flattened squamous epithelium.

• It is divided into three regions:

1- Meso-tympanum (middle ear cavity proper): lies opposite to the tympanic membrane.

2- Epi-tympanum (attic): lies above the tympanic membrane level.

3- Hypo-tympanum: lies below the tympanic membrane level.

• Contents: It is filled with air and contains:

1. Three bones (auditory ossicles): malleus, incus and stapes.

2. Two nerves: The chorda tympani nerve and the tympanic plexus.
3. Two muscles: The tensor tympani muscle and the stapedius muscle.

Walls of the middle ear:

1. Lateral wall:

• It separates the middle ear from the external ear.

• Formed by:

1) The tympanic membrane with bones above and below.

2) The outer attic mass or scutum (part of the squamous temporal bone) forms the upper part of the lateral wall of the attic ( epi-tympanum).

2. Medial wall: It separates the middle ear from the inner ear.

• Formed mainly by:

1. The first (basal) turn of the bony cochlea (of the inner ear) It produces a central rounded bulge called the promontory.
2. Two openings connect the middle ear (functionally) with the inner ear:

a- The oval window: lies above and behind the promontory. It is closed by the footplate of stapes.

b- The round window: lies below and behind the promontory. It is closed by the secondary tympanic membrane.

3. The horizontal part of the facial nerve: runs in a bony canal above the oval window.

3. Superior wall: It separates the middle ear from the middle cranial fossa and temporal lobe of the brain. It is a thin bony plate called the tegmen tympani.
   

4. Inferior wall: A thin bony plate separates the middle ear from the bulb of internal jugular vein.
   

5. Anterior wall: It separates the middle ear from the internal carotid artery. It has an opening for the Eustachian tube. The tensor tympani muscle enters the middle ear through this wall.

6. Posterior wall: It separates the middle ear from the mastoid process.

• It has an opening (the aditus ad antrum) which connects the epitympanum (attic) with the mastoid antrum.

• The stapedius muscle enters the middle ear through this wall through a bony ridge (the pyramid).

• The vertical part of the facial nerve runs in a bony canal in this wall.

THE EUSTACHIAN TUBE (THE AUDITORY TUBE):

The Eustachian tube communicates the middle ear cavity with the nasopharynx where it opens 1.5 cm behind the posterior end of the inferior turbinate. Its length is about 36 mm and is directed medially, forwards and downwards. The tube is shorter, wider, and more horizontal in children than in adults. Its lateral third is bony while its medial two thirds are cartilaginous. It is lined with mucous membrane with ciliated columnar epithelium. The tube is closed at rest. It opens by contraction of the tensor veli palatine muscle during swallowing and yawning to ventilate the middle ear.

THE MASTOID AIR CELLS

They are located within the mastoid process of the temporal bone which is a pyramidal bony projection directed inferiorly behind the auricle. They are small air-filled bony cavities that communicate with each other and lined with flattened squamous epithelium. They communicate anteriorly with the middle ear cavity through the aditus ad antrum. The largest air cell is the mastoid antrum, which lies behind the attic. The number and size of the other cells varies and usually arranged in groups named after the anatomical structure they are near to e.g. retrofacial, tip, perisinus, periantral, subdural, sinodural and zygomatic air cells, according to the degree of cellularity. The mastoid process is not present in the newborn. It forms from traction on the squamous and petrosal parts of the temporal bone by cervical muscles, since the child starts to support his head.


The functions of the mastoid air cells are still unknown but they are thought to share in pressure-regulating mechanism of the middle ear cavity and in decreasing the weight of the skull bones.

PARTS: 1- The auricle.
2- The external auditory canal.
3- The tympanic membrane.

THE AURICLE (Ear pinna):

It is the comma shaped structure attached to the side of the head by ligaments and muscles. It is composed of a sheet of fibro-elastic cartilage. The covering skin is closely adherent to the perichondrium on the lateral surface.

The auricle is devoid of cartilage in two places:

a) The lobule, which is the small lower part and consists of fat.
b) The incisura terminalis, which lies between the tragus and anterior end of helix.

Blood supply: The external carotid artery branches: superficial temporal artery, occipital artery, and posterior auricular artery.

Venous drainage: Correspond to the feeding arteries à internal and external jugular veins.

Nerve supply:

• Motor: The facial nerve to the vestigial auricular muscles
• Sensory:

1. The auriculotemporal nerve (from cranial nerve V) to the upper 2/3 of the lateral surface.

2. The lesser occipital nerve (C2) to the upper 1/3 of the medial surface.
3. The greater auricular nerve (C2, 3) to the lower 1/3 of the lateral and 2/3 or the medial surfaces.
4. The facial nerve to the area around the concha at the external meatus.

Lymphatic drainage: The pre and post auricular lymph nodes.

2- THE EXTERNAL AUDITORY CANAL (Meatus):

It extends from base of the concha (of auricle) to the tympanic membrane (24mm length in adults).

• The lateral third is cartilaginous, directed medially, upwards, and backwards, continuous with the auricular cartilage. Its skin contains hairs, sebaceous and ceruminous glands (secrete wax).
• The medial two thirds are bony, directed medially, downwards and forwards, continuous with the bony walls of the middle ear. Its skin is much thinner than the outer third and contains no hairs or glands.
  

The epithelium of the canal migrate periodically beginning from the center of the tympanic membrane to outside the canal discharging wax and other substances in a self-cleaning action.To examine the external ear, you have to pull the auricle upwards and backwards to straighten the canal and see the drum.

Blood supply:

The external carotid artery branches: superficial temporal artery, occipital artery, posterior auricular artery, and deep auricular branch of the maxillary artery.

Nerve supply: Sensory only

1. Anterior ½: The auriculotemporal nerve.
2. Posterior ½: Arnold’s branch of the vagus nerve.

Lymphatic drainage: The pre and post auricular lymph nodes.

3- THE TYMPANIC MEMBRANE (The Ear drum)

Site:

It is located at the medial end of the external auditory canal, separating the external from the middle ear. It lies at an angle of 55o with the horizontal because the anterior and inferior walls of the external canal are longer than its posterior and superior walls.

Shape:

The drum is oval (about 8 x 10 mm), about 0.1 mm thickness, pearly grey, semi-translucent and concave laterally (towards the external auditory canal) showing the reflected cone of light on examination.

It is divided into two unequal parts:

1. The larger part: called pars tensa which has a thick rolled periphery called “The membranous annulus”.
2. The smaller uppermost part: called pars flaccida or Shrapnell's membrane.

The two parts are separated by two folds: the anterior and posterior malleolar folds.

Minute anatomy:

The ear drum consists of three layers:

• The outer skin layer is continuous with the skin lining of the external auditory canal.
• The middle fibrous layer is well formed in pars tensa and absent in pars flaccida. The fibrous layer has two types of fibers: circular and radial.

The handle (manubrium) of malleus, which is one of the auditory ossicles, is embedded in this layer. Its end lies in the center of the drum called (the umbo).

• The medial mucosal layer is continuous with the mucosal lining of the middle ear.
• For diagnostic purpose the ear drum is divided into 4 sections by two imaginary, line through the handle of malleus and the other passes horizontally through the umbo. (Anterosuperior, anteroinferior, posterosuperior and posteroinferior).

Blood supply: Branches from the external carotid artery.

Nerve supply: Sensory only

• The inner surface: Jacobson’s branch of the glossopharyngeal nerve.
The anterior ½ of the lateral surface: The auriculo temporal nerve.
• 
  
• The posterior ½ of the lateral surface: Arnold’s branch of the vagus nerve.

CRANIAL FOSSA: The cranium is divided into three regions (fossae), according to the cranial lobes that each region houses.

1 Anterior Cranial Fossa: Houses frontal lobes.

o Includes all of Frontal and Ethmoid bones, and the Lesser Wing of the Sphenoid.

2 Middle Cranial Fossa: Houses temporal lobes.

o Includes the Greater Wing of the Sphenoid, Sella Turcica, and the majority of Temporal Bones.

1. Posterior Cranial Fossa: House occipital lobes, cerebellum, and medulla

o Includes the Occipital bone.

DURAL FOLDS: Extensions of Dura Mater that subdivide the cranial cavity and give support to the brain, such as when bungee jumping.

· FALX CEREBRI: Dural fold going sagittally between the cerebral hemispheres.

§ It connects anteriorly to the Crista Galli of the Ethmoid Bone and the Internal Occipital Protuberance posteriorly.

§ The Superior Sagittal Sinus is the superior limit of the Falx Cerebri, within the dura.

§ The Inferior Sagittal Sinus is the inferior limit of the Falx Cerebri, within the dura.

· TENTORIUM CEREBELLI: Lateral dural fold separating the cerebrum from the cerebellum.

§ The Falx Cerebri holds it down posteriorly, where they intersect.

§ The Transverse Sinus runs through it posteriorly.

§ The Superior Petrosal Sinus runs through it anteriorly.

§ INCISURA TENTORIA: The hole in the Tentorium Cerebelli that allows the cerebrum to communicate with the cerebellum.

· DIAPHRAGMA SELLAE: Small donut-shaped dural fold covering the Pituitary Gland, at the Sella Turcica of the Sphenoid Bone.

DURAL SINUSES: Venous sinuses which drain all blood from the brain. They are formed within the dura mater layer of the meninges. The Dura Mater splits into two layer in the cranium: A Periosteal (outer) Layer and a Meningeal (inner) Layer.The dural sinuses form between them.

1. SUPERIOR SAGITTAL SINUS: Runs within superior aspect of the Falx Cerebri.

· ARACHNOID VILLI: Granulations from the Arachnoid Mater project into this sinus, providing a way to empty CerebroSpinal Fluid into the Superior Sagittal Sinus. This is how we recycle cerebrospinal fluid (CSF).

· It drains into the CONFLUENCE OF SINUSES ------> TRANSVERSE SINUS

2. INFERIOR SAGITTAL SINUS: Runs within the inferior aspect of the Falx Cerebri.

· It joins the GREAT VEIN OF GALEN to form the Straight Sinus.

· From there Blood Drainage is as follows: STRAIGHT SINUS ------> TRANSVERSE SINUS

· The Sigmoid Sinus also communicates with the Internal Vertebral Venous Plexus.

3. OCCIPITAL SINUS: Lies in the Falx Cerebellia. It drains blood as follows: OCCIPITAL SINUS ------>CONFLUENCE OF SINUSES ------> TRANSVERSE SINUS
4. CONFLUENCE OF SINUSES: The junction between the Superior Sagittal, Straight, and Transverse Sinuses. It leads into the Transverse Sinus. The occipital sinus is caudal to it.
5. TRANSVERSE SINUS: It receives blood from all three sinuses above: Superior and Inferior Sagittal, and Occipital. It drains blood as follows:

· TRANSVERSE SINUS ------> SIGMOID SINUS ------> INTERNAL JUGULAR VEIN

6. SUPERIOR PETROSAL SINUS: Drains the Cavernous Sinus ------> OCCIPITAL SINUS
7. INFERIOR PETROSAL SINUS: Drains the Cavernous Sinus ------> INTERNAL JUGULAR VEIN directly.
8. CAVERNOUS SINUS: VERY IMPORTANT because it contains lots of nerves and vessels running through it.
• It is located at the Sella Turcica of the Sphenoid, surrounding the Pituitary Gland, within the Diaphragma Sella dural fold.
• It receives blood from:

i. Superior and Inferior Ophthalmic Veins

ii. Superficial Middle Cerebral Vein

iii. Sphenoparietal Sinus

• It drains blood into:

i. Inferior and Superior Petrosal Sinuses.

ii. Pterygoid Plexus.

· It has the following running through it:

§ All of Cranial Nerves III - VI, except V3

· All of the above run, in numerical order, along the lateral wall of the cavernous sinus from superior to inferior, EXCEPT for the Abducens Nerve (VI) which runs directly through the cavernous sinus.

§ The Internal Carotid Artery. This is also completely surrounded within the Cavernous Sinus.

· The Sphenoidal Paranasal Sinus, in the sphenoid bone, is inferior and medial to the cavernous dural sinus.

· SPHENOID FRACTURE: A fracture at the base of the sphenoid could produce a bone fragment that could sever the internal carotid in cavernous sinus ------> dural hematoma.

MENINGEAL ARTERIES: Arteries that supply the meninges and bone -- not the brain per se.

1. Middle Meningeal Artery is a branch off the first part of the Maxillary. It is the principle supplier to the dura.
2. Anterior Meningeal Artery
3. Posterior Meningeal Artery

HEMATOMA: Bleeding within and around the meningeal layers

1. EPIDURAL HEMATOMA: Middle Meningeal Artery is easily cut with a skull fracture. This will put pressure in the epidural space and smush the brain.

· Flow of blood will go through dura superficially. The extent of the hematoma is limited by suture lines.

2. SUBDURAL HEMATOMA: Blood flow will not stop at the skull sutures in this case. However, hematoma usually will not cross the midline.

3. SUBARACHNOID HEMATOMA: Blood in this space intermixes with CSF and can lead to headache, stiff neck, and loss of consciousness.

· Unlike the ones above, these are very painful.

4. INTRACEREBRAL HEMORRHAGE: Results in death or paralysis by stroke.

INTERNAL CAROTID ARTERY: Major supplier of blood to the brain.

Course: Common Carotid ------> Internal Carotid ------> Into the Foramen Lacerum ------> medially to Carotid canal ------> then Anteriorly through Cavernous Sinus ------> Then superiorly again.

Circle Of Willis: The anastomotic arterial connections supplying the cranial cavity. The two main supplies to the brain are the Internal Carotid and Vertebral Arteries, and they communicate through the Circle of Willis

· Branches of the VERTEBRAL ARTERY

o Both Left and Right Vertebrals give off Posterior Inferior Cerebellar arteries

o Then the two Vertebral Arteries converge to form the singular Basilar Artery

o Basilar Artery gives off the Anterior Inferior Cerebellar and Superior Cerebellar arteries.

o The Basilar Artery then becomes the Posterior Cerebral Artery.

· POSTERIOR COMMUNICATING ARTERY connects the Posterior Cerebral (from Basilar) to the Internal Carotid Artery. This is the major anastomosis between the Carotid and Vertebral arterial channels.

· Branches of the INTERNAL CAROTID

o Middle Cerebral Artery
o Anterior Cerebral Artery

· ANTERIOR COMMUNICATING ARTERY: Connects the Anterior Cerebral Arteries to each other. This is the major anastomosis of the Right and Left Internal Carotids with each other.

VENTRICLES OF THE BRAIN: Cerebrospinal fluid is produced in the ventricular system in the brain.

· CHOROID PLEXUS: Specialized tissue within the Lateral, Third, and Fourth Ventricles, that makes cerebrospinal fluid.

· Median and Lateral Apertures: Holes that drain the cerebrospinal fluid down into the spinal column.

· Interventricular Foramen: A hole that connects the Lateral Ventricle to the Third Ventricle.

· Cerebral Aqueduct: A hole that connects the Third Ventricle to the Fourth Ventricle.

· Cerebrospinal fluid is continuous with the optic tract, until it reaches the back of the eye.

· Again, cerebrospinal fluid from the ventricles communicates with the Superior Sagittal Sinus via Arachnoid Villi penetrating the sinus.

INTERNAL HYDROCEPHALY: The internal buildup of cerebrospinal fluid. There is no feedback inhibitory mechanism for stopping production of cerebrospinal fluid.

· This could be caused by an obstruction at the cerebral aqueduct.

EXTERNAL HYDROCEPHALY: Cerebrospinal fluid buildup outside the CNS. Caused by excess production of CSF, leading to increased intracranial pressure.

SCALP: Cutaneous cranium superficial to the skull. Its layers fit the acronym SCALP

1. Skin
2. Connective tissue -- highly vascular
3. Aponeurosis -- Galea Aponeurotica = dense connective tissue

§ This is continuous with the Epicranius muscle from all sides, consisting of Frontalis and Occipitalis muscles.

4. Loose connective tissue, upon which the aponeurosis glides
5. Periosteum -- adherent to bony skull

SCALP LESION: The outermost connective tissue is highly vascular, and the loose connective tissue tends to hold cuts open, resulting in profuse bleeding of scalp. It is easily fixed with stitches

DANGER ZONE: If you get past the aponeurosis (i.e. from a cut), then you have hit the danger zone. Bacteria like to breed in the loose connective tissue beneath the Galea Aponeurotica.

1. Diploic Veins: Veins in the cancellous bone of the skull. These are superficial veins. The Emissary Veins flow into them.

2. EMISSARY VEINS: Communicate with the diploic veins superficially. They run through the Loose-Connective Tissue Layer -- the DANGER ZONE.

§ If an infection gets in this area, it can easily be carried to the Cerebrospinal fluid via the Emissary Veins.

§ The general blood flow of these veins is outward -- out of the cranium.

ARTERIAL SUPPLY TO SCALP:

· From the EXTERNAL CAROTID, we have, from anterior to posterior:

o Superficial Temporal Artery anterior to ear

o Posterior Auricular Artery posterior to ear

o Occipital Artery back of neck

· From the INTERNAL CAROTID, we have two branches, which anastomose with the External Carotid via both Facial Artery and Superficial Temporal:

o Supratrochlear Artery: Frontal region medially

o Supraorbital Artery: Frontal region laterally

OPTIC CHIASM: The convergence of the two optic nerves.

1. It occurs at the sella turcica, on the body of the sphenoid bone, right at the Pituitary Gland.
2. Fibers from the medial side of each eye cross at the Optic Chiasm, to join the Optic Tract on the opposite side.

i. So medial fibers from the Left Eye will go to the right side of the brain, and medial fibers from the right eye will go to the left side of the brain.

ii. These fibers are carrying peripheral visual information for both respective eyes.

3. Fibers from the lateral side of each eye do not cross at the Optic Chiasm, but instead join the Optic Tract directly on their own side.
4. TUNNEL VISION: Indicates a lesion at the optic chiasm. Medial fibers have been impinged upon, destroying peripheral vision in both eyes.

i. Pituitary Tumor can impinge on the optic chiasm, causing tunnel vision.

OPTIC TRACT: That portion extending between the Optic Chiasm and the Lateral Geniculate Nucleus in the brainstem. This portion of nerve is not properly called the Optic Nerve.

SUPERIOR DIVISION: Somatic innervation to the superior rectus and levator palpebrae muscles.

INFERIOR DIVISION: Somatic innervation to the medial and inferior rectus muscles.

• The inferior division also carries Parasympathetic Fibers, via the Short Ciliary Nerve, to the Sphincter Pupillae and Ciliary Muscles (constrict eye and accommodate for near vision).

UNILATERAL LESION OF OCULOMOTOR NERVE: Damage would be as follows.

1. "Lateral Strabismus" = the eyeball will point downward and outward.

Unchecked tension of the Lateral Rectus (CN VI) will abduct it (pull it outward)

• Unchecked tension of the Superior Oblique (CN IV) will pull the eye down and outward -- not upward!

1. "Ptosis" = the eyelid will droop.

• This is due to lost innervation of the Levator Palpebrae muscle.
• There may be tonic tension of the Frontalis Muscle as well, in order to compensate for drooping eyelid.

2. "Mydriasis" = The pupil will be dilated.

• Due to lost innervation of the Sphincter Pupillae.

3. Accommodation of the lens will be lost.

• Due to lost innervation to Ciliary Muscle.

PATHWAY IN BRAIN:

• It is the only cranial nerve that originates from the opposite of the brain.
• It is the only cranial nerve that exits from the dorsal side of the cranium, so it has a very long path.
• It is the only cranial nerve associated with the eye, that doesn't enter the orbit through a tendinous ring.

CAVERNOUS SINUS: It runs in the cavernous sinus. If there is ever brain hemorrhage, pressure from excess CSF can damage the Abducens nerve.

INJURY: Injury to the Trochlear Nerve or Superior Oblique Muscle will cause someone to tilt their head slightly in compensation.

TRIGEMINAL GANGLION: It contains the sensory cell bodies for the Trigeminal Nerve. It contains no synapses.

• The Motor cell bodies are in the Masticator Nucleus within the pons.

V1 -- OPHTHALMIC NERVE -- Purely Sensory Nerve.

• It runs through the lateral wall of the Cavernous Sinus.
• It exits through Superior Orbital Fissure to enter the orbit.
• Associated Ganglion: CILIARY GANGLION. It carries parasympathetic motor fibers from the Oculomotor Nerve (CN III), which go on to innervate extrinsic eye muscles.

V2 -- MAXILLARY NERVE -- Purely Sensory Nerve.

• It runs through the lateral wall of the cavernous sinus.
• It exits through Foramen Rotundum, in the Greater Wing of the Sphenoid.
• Associated Ganglion: PTERYGOPALATINE GANGLION. It carries parasympathetic motor fibers from the Facial Nerve (CN VII), which goes on to innervate muscles of facial expression.

V3 -- MANDIBULAR NERVE-- Mixed nerve. The only branch of the Trigeminal to carry any motor innervation.

• It exits through Foramen Ovale, in the Greater Wing of the Sphenoid.
• Associated Ganglia:

• OTIC GANGLION: Carries parasympathetic motor fibers from the Glossopharyngeal Nerve (CN IX), which go on to innervate the Parotid Salivary Gland.
• It is located in the Infratemporal Fossa
• SUBMANDIBULAR GANGLION: Carries parasympathetic motor fibers from the Facial Nerve (CN VII), which go onto innervate the Submandibular and Sublingual glands.
• Motor Innervation: Muscles of mastication; Anterior belly of digastric and mylohyoid; Tensor tympani and tensor veli palati.
• Sensory Innervation: It sends up a meningeal branch to innervate part of the meninges.

TIC DOULOUREUX: A severe split second pain in the cutaneous region of the Trigeminal nerve.

• A pain runs through Mandibular and Maxillary divisions, i.e. region around mouth.

MENINGEAL BRANCHES: All branches of Trigeminal send some meningeal branches to the meninges of the brain. If something is irritating the meninges, the pain information will be sent back through the Trigeminal.

• In all cases, meningeal branch will be given off before each branch exits its respective foramen.

CAVERNOUS SINUS: It runs in the cavernous sinus. If there is ever brain hemorrhage, pressure from excess CSF can damage the Abducens nerve.

STRABISMUS: The inability to direct both eyes toward the same object.

INJURY TO ABDUCENS: The eye on the affected side rotates inward (adducts), due to the unopposed action of the medial rectus muscle.

Motor Innervation: The cell bodies for the motor fibers of all cranial nerves are located in the respective brainstem nuclei (lower motor neurons).

Sensory Innervation: The cell bodies for the sensory fibers of all cranial nerves are located in the parasympathetic ganglia (see below) or special sensory organs.

Sympathetic Innervation: The sympathetic fibers mostly run along the internal and external carotid arteries to get to their target organs.

1. Superior Cervical Ganglion: The highest sympathetic chain ganglia, which contains synapses for all the major sympathetic fibers for the head and neck region.
2. External Carotid Artery: Post-Ganglionic sympathetics run along the external carotid to supply the salivary glands and lower face.
3. Internal Carotid Artery: Larger portion of sympathetics run along internal carotid, forming the internal carotid plexus.
4. INTERNAL CAROTID PLEXUS: In cavernous sinus (probably).
   

1. Deep Petrosal Nerve: Given off of the internal carotid plexus within the cavernous sinus.

o From there it goes to Pterygopalatine Fossa and onto nose, palate, and lacrimal glands.

2. Sympathetic Root of Ciliary Ganglion: The internal carotid plexus also sends a sympathetic branch to the Ciliary Ganglion; where it then goes on to form the Long Ciliary Nerve, which will innervate the dilator pupillae muscle of the eye.
   

Parasympathetic Innervation: All Parasympathetic motor innervation to the head synapses exactly once, in one of the four cranial ganglia listed below. All of these ganglia are distributed along branches of theTrigeminal Nerve (V).

1. CILIARY GANGLION: Carries parasympathetics from Oculomotor Nerve (III)
• Located in the posterior of the orbit.
• GVE (Parasympathetic) innervation is to:

1. Ciliary Muscles (for Accommodation)

2. Sphincter of the Pupil (constriction, or miosis).

• It hangs off of the Ophthalmic Branch of the Trigeminal Nerve (V1)
  

2. PTERYGOPALATINE GANGLION: Carries parasympathetics from Facial Nerve (VII)

1. Located in the Pterygopalatine Fossa in the posterior part of sphenoid bone.

2. GVE (Parasympathetic) innervation is to:

o Nose

o Palate

o Lacrimal Glands

3. It hangs off of the Maxillary Branch of the Trigeminal Nerve (V2).

3. SUBMANDIBULAR GANGLION: Carries parasympathetics from Facial Nerve (VII)

1. Located below and lateral to the tongue.

2. GVE (Parasympathetic) innervation is to:

o Submandibular Gland

o Sublingual Gland

3. It hangs off of the Mandibular Branch of the Trigeminal Nerve (V3).

4. OTIC GANGLION: Carries parasympathetics from Glossopharyngeal Nerve (IX)

1. Located in Intratemporal Fossa, just below Foramen Ovale.

2. GVE (Parasympathetic) innervation is to the Parotid Salivary Gland.

3. It hangs off of the Mandibular Branch of the Trigeminal Nerve (V3)

5. VAGUS NERVE (CN X)

1. There is no parasympathetic ganglion in the head for the vagus nerve.

2. Sends parasympathetic innervation to the thoracic and abdominal viscera.

Three Main Functions:

• Innervate the Muscles of Facial Expression
• Supply special sense of taste to anterior 2/3 of tongue, via Chorda Tympani Nerve.
• To innervate all facial glands (salivary, nasal, lacrimal), EXCEPT the Parotid.
• There is also a small area of somatic sensation carried by the facial nerve, around the ear canal ear drum.
• The GENICULATE GANGLION houses the cell bodies for all fibers of the Facial Nerve. It has no synapses.

PATHWAYS OF THE FACIAL NERVE:

All fibers of the facial nerve exit the cranium at the INTERNAL ACOUSTIC MEATUS in the middle ear. From there, all fibers (except sensory to the ear) go to the GENICULATE GANGLION. From there, the nerve splits into two parts: a special-efferent motor division (to innervate facial-expression muscles) and a parasympathetic motor division (to innervate glands)

1. Motor Fibers of Facial Expression (SVE):They all bend downward and all head out the Stylomastoid Foramen and then pass through the Parotid Gland (but they do not innervate it). Then they divide into five branches.
• All of these muscles are derived from the second branchial arch.
• TO ZANZIBAR BY MOTOR CAR:

1. TEMPORAL BRANCH -- Frontal Muscles

2. ZYGOMATIC BRANCH -- Orbicularis Oculi

3. BUCCAL BRANCH -- Buccinator, Orbicularis Oris, Zygomaticus

4. MANDIBULAR BRANCH -- Orbicularis Oris, Depressor Labii Inferiores

5. CERVICAL BRANCH – Platysma

2. Parasympathetic Secretomotor Fibers (GVE): They split into the Chorda Tympani and Greater Petrosal nerves, and head in two different directions.

1. PATHWAY OF CHORDA TYMPANI: Through Middle Ear ------> Out the Petrotympanic Fissure ------>Joins up with the Lingual Nerve ------> Submandibular Ganglion, where it synapses ------> Sublingual and Submandibular Salivary Glands

· So ultimate innervation is secretomotor innervation to Submandibular and Sublingual glands.

2. PATHWAY OF GREATER PETROSAL: Middle Ear ------> Out the Greater Petrosal Hiatus ------> back in through the foramen lacerum ------> Join with sympathetics from Deep Petrosal Nerve to form the Nerve of Pterygoid Canal ------> Pterygopalatine Ganglion, where it synapses ------> Nasal mucosa and Lacrimal Glands.

· So, ultimate innervation is secretomotor innervation to lacrimal glands and nasal mucosa.

BELL'S PALSY: "Paralysis of the Facial Nerve for no obvious reason." Or, loss of SVE motor innervation of the Facial Nerve.

• Commonly occurs if the nerve gets impeded or inflamed at the Stylomastoid Foramen.
• Facial paralysis results on the side affected. Patients will be unable to close eyelids, they will drool, and they will have a contorted face due to unopposed muscles on other side.
• An injury here would not affect parasympathetic secretomotor innervation (those are given off before the Stylomastoid foramen) -- it would only affect muscles of facial expression.
• Loss of buccinator muscle can lead to difficulty eating.
• Inability to close eyelid results in damage to cornea due to dryness of the eye, due to no innervation of palpebral part of orbicularis oculi.
  

VESTIBULAR NERVE:

• Innervates the semicircular canals, utricle, and saccule, and is responsible for dynamic and static balance.

COCHLEAR NERVE:

• Innervates the cochlea and is responsible for hearing.

INTERNAL ACOUSTIC MEATUS: Cranial Nerve VIII exits out the internal acoustic meatus, along with the Facial Nerve.

ACOUSTIC NEUROMAS: A neuroma from a Schwann Cell in the Vestibulocochlear may impinge upon the Vestibular and Cochlear nerves in the Internal Acoustic Meatus. If it is large enough, it may impinge on the Facial Nerve, too.

Five Functions of Glossopharyngeal Nerve:

• Innervates Stylopharyngeus Muscle.
• Special sense of taste to posterior 1/3 of tongue
• General sensation to posterior 1/3 of tongue, lateral oropharyngeal wall, part of soft palate
• Parasympathetic secretomotor to Parotid Gland
• Visceral Sensation from the Baroreceptors ("blood-pressure" receptors) at the Carotid Body (Sinus), at the bifurcation of the Carotids.

• These guys carry special chemoreceptors and stretch receptors. They provide feedback to the heart to control heart rate and modulate blood pressure.

TYMPANIC NERVE: Is given off at the Jugular Foramen from the Glossopharyngeal. It carries Parasympathetic innervation to the Parotid, via the Lesser Petrosal.

• It travels through the middle ear (inside the cranium), where it forms the Tympanic Plexus.
• It exits the cranium at the Lesser Petrosal Hiatus of the Temporal bone.
• After it exits back out, it is known as the Lesser Petrosal Nerve.
• Lesser Petrosal Nerve: The continuation of the Tympanic; it synapses at the Otic Ganglion and then continues to carry parasympathetic innervation to the Parotid Gland.

GAG REFLEX:

Glossopharyngeal can be tested by eliciting a gag reflex. Touch the posterior 1/3 of tongue or palate region to elicit response. · One should also be able to see symmetric raising of the soft palate during swallowing to verify functionality of the nerve.

OVERALL PATH OF GLOSSOPHARYNGEUS TO PHARYNX:

• Through JUGULAR FORAMEN
• Down posterior wall of pharynx to innervate the stylopharyngeus muscle
• Penetrate gap between Superior and Middle Pharyngeal Constrictors, adhered to the Stylopharyngeus muscle.
• Finally, innervation to the posterior 1/3 of tongue.

Type of Innervation:

1. BRANCHIAL MOTOR (SVE): All muscles of the larynx, pharynx, and palate, EXCEPT the Stylopharyngeus (IX) and Tensor Palati (V3).

• In this role, the Vagus is "stealing" some of the innervation from the Spinal Accessory (XI). Hence in this case the Vagus is actually innervating striated rather than smooth muscle.

2. VISCERAL MOTOR (GVE): Parasympathetics to the Thoracic and Abdominal viscera.

• The Vagus serves no Parasympathetic function in the head and neck region -- only the thorax and abdomen.

3. VISCERAL SENSORY (GVA): Sensory info from tongue, pharynx and larynx, heart and lungs, CAROTID SINUS (along with CN IX), stomach and intestine.
4. GENERAL SENSORY (GSA): General sensory from larynx, pharynx, and a small portion of the outer ear and tympanic membrane.

• General Sensory innervation may include the eardrum itself. These individuals may elicit a gag reflex when cleaning their ears -- due to sensory response from the Vagus!

5. SPECIAL SENSORY (SVA): Maybe a few taste buds carry taste information via the Vagus.

TACHYCARDIA:

To treat a racing heart rate, you can give the patient a Carotid Body Massage, at the bifurcation of the Carotids, in attempt to stimulate the visceral sensory components of the Vagus and Glossopharyngeal, to try to slow heart rate by increasing the Vagal stimulation of the heart.

UNILATERAL LESIONS OF THE VAGUS: Lesions of the vagus lead to the following symptoms.

1. Hoarseness, due to lost laryngeal function on the affected side.
2. Difficulty Swallowing, due to inability to elevate the soft palate on the affected side. This also makes the soft palate droop on that side.
3. The uvula tends to deviate toward the unaffected (intact) side, due to droop of the soft palate on the affected side.
• If the uvula deviates toward the left, then damage is to the right Vagus nerve.

SUPERIOR VAGUS GANGLION: It is within the Jugular Foramen. It houses somatic sensory cell bodies of the Vagus nerve.

INFERIOR VAGUS GANGLION: It is right beneath the Jugular Foramen, right outside the skull. It houses visceral sensory cell bodies for the Vagus nerve.

Two Roots:

1. Spinal Root: Gives SVE (Branchial) innervation to Sternocleidomastoid and Trapezius muscles.
2. Cervical Root: Quickly joins up with Vagus (and could be considered part of Vagus) to form the Recurrent Laryngeal Nerve.

NERVE GRAFTS: The function of Spinal Accessory is somewhat redundant.

• It can be used to replace innervation lost by other muscles. The nerve can be redirected to the muscles of facial expression, e.g., and patients can learn to use the new pathway with physical therapy.

DAMAGE TO SPINAL ACCESSORY:

• Shoulder Droop, due to lost innervation to upper part of Trapezius.
• Difficulty in turning head, due to lost innervation to Sternocleidomastoid -- but other muscles serve this function as well!

MOTOR CELL BODIES: Lie in the spinal cord (C1-C5).

• Fibers run up the spinal chord and into the cranium through the Foramen Magnum, and then back out the posterior cranial fossa through the Jugular Foramen.