Untitled Quiz

Ear Anatomy

• The ear is composed of three regions: external, middle, and inner.
• The external ear consists of the pinna (auricle) and external auditory canal.
• The pinna is made of fibroelastic cartilage, covered by skin, and connected to surrounding parts by ligaments and muscles.
• Landmarks on the pinna include the helix, antihelix, lobule, tragus, concha, scaphoid fossa, and triangular fossa.
• The external auditory canal (meatus) is initially cartilaginous and then bony.
• At birth, only the cartilaginous portion is present, the bony portion develops later.
• The bony portion comprises two-thirds of the canal's length, while the cartilaginous component forms the outer third.
• The canal's length is 24 mm in total, with the outer cartilaginous part measuring 8 mm, and the inner bony portion measuring 16 mm.
• The external auditory canal has two constrictions: one near the medial end of the cartilaginous part and the other 5 mm short of the tympanic membrane, in the osseous portion, called isthmus.
• The floor and anterior wall are longer than the roof and posterior part of the meatus due to the oblique positioning of the tympanic membrane.
• The bony walls are formed by the tympanic part of the temporal bone.
• The roof and part of the posterior wall are formed by the squamous part of the temporal bone.
• The foramen of Luschke is a deficiency in the anteroinferior part of the bony meatus that closes around the age of four.
• Fissures of Santorini are deficiencies in the cartilaginous portion of the external auditory canal.
• The nerve supply for the anterior and superior walls is the auriculotemporal branch of the mandibular nerve, whereas the posterior and inferior walls are supplied by the auricular branch of the vagus nerve.
• A sensory branch of the facial nerve may supply a portion of the posterior wall of the meatus.
• The blood supply for the pinna includes the branches of the posterior and anterior auricular, as well as a branch of the occipital artery.
• Veins corresponding to these arteries drain into the external jugular and common facial vein.
• Lymphatic supply drains into parotid, upper deep cervical, preauricular, and postauricular lymph nodes.

Middle Ear

• The middle ear cleft comprises the following components:
• Eustachian tube
• Tympanic cavity
• Mastoid antrum
• Aditus ad antrum
• Mastoid air cells
• The tympanic cavity is a six-walled cavity that measures 15 mm antero-posteriorly, 2–6 mm from side to side, and 15 mm in height.
• The lateral wall is mainly composed of the tympanic membrane.
• The middle ear is connected anteriorly to the nasopharynx via the auditory tube.
• The middle ear is connected posteriorly to the mastoid air cells via the mastoid antrum.
• The middle ear walls are lined by columnar ciliated and pavement epithelium.

Middle Ear Spaces

• The middle ear cleft is wider than the tympanic membrane and is divided into spaces in relation to the annulus.
• Epitympanum is located superior to the tympanic membrane and contains the body of the incus and head of the malleus. It communicates with the mastoid via the aditus.
• Mesotympanum is at the same level as the tympanic membrane. it contains the long process of the incus which articulates with the stapes. The facial nerve passes through the space. The oval and round windows are also in this area.
• Hypotympanum is beneath the tympanic membrane. It is usually covered by bone but can be dehiscent.

Prussak's Space

• Also known as the superior recess of the tympanic membrane, it is a space between the neck of the malleus and pars flaccida.
• It is frequently the location of cholesteatoma formation.

Mesotympanum

• Occupies the same level as the tympanic membrane,
• Contains the oval and round windows,
• The incus articulates with the stapes, and the facial nerve runs through the area.

Other Parts of Tympanum

• Protympanum- Area of middle ear close to Eustachian Tube. This tube runs in close proximity to the carotid artery.
• Hypotympanum- The jugular bulb curves through the hypotympanum, usually covered by bone.

Walls of the Middle Ear

•  Medial wall: Separates the middle ear from the inner ear and is primarily formed by the basal turn of the bony cochlea (promontory).
•  Oval window: Lies above and behind the promontory, is covered by the footplate of the stapes. 
•  Round window: Lies below and behind the promontory, covered by the secondary tympanic membrane. 
•  Horizontal part of the facial nerve: Runs in a bony canal above the oval window.
•  Superior wall: Separates the middle ear from the middle cranial fossa and temporal lobe of the brain (tegmen tympani).
•  Inferior wall: Separates the middle ear from the bulb of the internal jugular vein.
•  Anterior wall: Separates the middle ear from the internal carotid artery. It has openings for the Eustachian tube, canals for the chorda tympani and tensor tympani muscle, and anterior malleolar ligament.
•  Posterior wall: Separates the middle ear from the mastoid. It has the aditus ad antrum and the stapedius muscle passing through this bony ridge.

Tympanic Membrane (Eardrum)

• The tympanic membrane is a roughly circular structure.
•  It is almost translucent and has a capillary blood supply.
• It is located between the external auditory canal and the middle ear cavity.
• It encloses the bottom part of the external acoustic meatus.
• This structure's oval shape is somewhat broader above than below.
• The membrane joins, or is joined by, the annular meatus, and its circumference.
• The membrane extends toward the malleus in a manner that creates a concave surface on its lateral aspect.
• The umbo is the most depressed part of this concavity. The fibrous layer thickens and forms an incomplete ring that is attached to the surrounding bone, called the annulus. It's taut here so is termed "pars tensa".
• Superior to the lateral process of the malleus, this ring is deficient, creating the "pars flaccida".  
• The pars flaccida, it is superior to the suspensory ligaments of the malleus, lacks the fibrous layer and is more prone to complications.
• The drum lies at angle of 55˚ away from the floor of the ear.

Mastoid Bone

• The mastoid air cells comprise a honeycomb-like structure of air cells situated behind, above, and below the ear.  
• These cells are lined with a thin mucous membrane. 
• The largest air cell is the mastoid antrum.
• The arrangement and size of the other cells varies by cellularity.
• Mastoid process forms as traction on temporal bone by cervical muscles due to head support.
• The mastoid process is absent in newborns and develops after head support.
• Air cells function in pressure regulation and decreasing skull weight.

Eustachian Tube (Pharyngotympanic Tube)

• The Eustachian tube connects the middle ear cavity to the nasopharynx.
• Its function is to equalize pressure between the atmosphere and middle ear.
• Mucous drainage.
• It's approximately 36mm long and directed medially, forward and downward.
• The cartilaginous portion lies under the mucous membrane of the nasal part of the pharynx and forms the torus tubarius.
• The fossa of Rosenmuller lies behind this. It is the main site of nasopharyngeal malignancy.

Internal Ear

• The internal ear consists of a bony labyrinth that contains the membranous labyrinth.
• The otic capsule develops from 14 centers.
• The internal ear is the most important area for hearing and equilibrium.

Vestibule

• A central ovoid structure (5mm x 3mm) of the inner ear containing the saccule, cochlear recess, and utricle.
•  It has openings from the 3 semicircular canals. 

Cochlea

• Shaped like a conical snail shell, measures 35mm (long) x 5mm base to apex, and 9mm cross base.
• It has a prominent central conical axis called the modiolus.
•  The spiral canal turns 2.5 to 2.75 around it. 

Cochlear Duct (Scala Media)

• The cochlear duct is a canal enclosed between the scala vestibuli and scala tympani. It has the sensory organ, the organ of Corti, that's triangular in cross-section.
• The basilar membrane forms the horizontal limb, Reissner's membrane forms the superior limb, and the stria vascularis forms the vertical limb. 

Organ of Corti

• The organ of Corti is composed of inner and outer hair cells, as well as supporting cells (Deiters' cells and Hensen's cells) and a tectorial membrane.

Membranous Labyrinth

• Lies within the bony labyrinth and fills with endolymphatic fluid.
• Contains the utricle, saccule, and semicircular ducts.

Utricle

• Irregular oblong structure (2-5mm in diameter) inside the elliptical recess of the vestibule
• Has a thickened area (utricular macula) of 3x2mm on the lateral wall and floor.
• Innervated by utricular fibres from the vestibular nerve

Saccule

• Globular structure (1-1.5mm in diameter) within the vestibular spherical recess near the opening of the scala vestibuli.
• Anterior wall has macula set at right angle to that of the utricle
• Connected to the utricle via Y-shaped tube.
• Lies under dura mater.
• Ductus reuniens extends inferiorly and enters cochlea's basal end.

Semicircular Canals

• Superior, Posterior, and Lateral semicircular canals (SCC's).
• 0.8mm in diameter
• Each ends in an ampulla which opens into the vestibule The three SCCs are at 90° relative to each other.

Semicircular Ducts

• Open into the utricle by five openings.
• The ampullary cristae, which are sensory organs, are found here, sensitive to endolymph movements (respond to angular acceleration).
• The septum transversum is a transverse elevation in the ampullary part of each duct, shaped like an hour glass.

Sensory Epithelium

• Ampullary crests have sensory hair cells of type I and type II, along with Hensen's supporting cells.
• Type I cells are pyriform, type II cells are cylindrical, and both have stereocilia (40-100) or microvilli on their apical surfaces.
• A long kinocilium is attached to each hair cell.
• Maculae are related to static balance, ampullary crests to kinetic balance.
• Endolymph has high potassium, low sodium concentrations, similar to intracellular fluid.

Vestibulocochlear Nerve

• The 8th cranial nerve.
•  Divides deep in the internal acoustic meatus into anterior cochlear and posterior vestibular nerves.  
• Oorts anastomosis is a vestibulo-cochlear anastomosis.
• Cochlear nerve branches into filaments at the modiolar base, supplying inner (95%) and outer (5%) hair cells.

Vestibular Nerve

• Supplies utricle, saccule, and ampulla of semicircular ducts,
• Scarpa's ganglion is the site of origin, located at the internal acoustic meatus, and it divides into superior, inferior, and posterior vestibular branches.

Arteries of the Labyrinth

• Internal auditory artery (branch of anterior-inferior cerebellar artery) provides branches to cochlear and vestibular areas.
• Other contributors are the stylomastoid branch of the occipital and posterior auricular artery.