otology 1 anatomy and physiology

Ear is divided into three parts

External Ear Middle Ear Transmission of Sound Inner Ear Hearing +Balance

Auricle Embriology

Tragus
Triangular Fossa
Lobule
Antitragus

External Auditory Meatus

Develops from the 1st branchial cleft by medial migration of a solid core of epithelial cells. It becomes completely canalized by the 7th month of gestation.

Extarnal Auditory Meatus

Outer 1/3 Cartilage Thick skin contain adnexial structures (sebaceous glands, hair follicles and ceruminous glands) 3. Skin is tightly adherent to underlying cartilage, therefore inflammation e.g. boil is extremely painful
Inner 2/3 Bony skeleton Thin skin devoid of adnexial structures. 3. The thin skin is tightly adherent to the underlying bone and is easily traumatized



In the adult there is an angle in the meatus. Accordingly the pinna must be pulled upwards, backwards and outwards when using a speculum to examine the ear drum .In children the meatus is shorter and straighter and the tympanic membrane is examined by pulling the pinna just downward. Tympanic membrane lies obliquely at medial end of EAM, , leading to the formation of "meatal recess" anteroinferiorly.
Meatal Recess

Nerve Supply

5th 9th 10th cranial nerves. The medial surface of the auricle is supplied by fibers of the great auricular nerve (C2 and C3) and the lesser occipital nerve (C2).

Relations of EAM

Ant TM Joint
Post Mastoid Antrum And Air Cells
Sup Middle Cranial Fossa
Inf & Ant Parotid Gland

Tympanic Membrane(Eardrum)

Is the lateral wall of middle ear consist of three layers: Outer epithelial layer Middle layer of fibrous tissue Inner layer of mucous membrane It has elliptic, funnel shape, about 8-10 mm in diameter. The ear drum is supported around its periphery by a fibrous thickening, the annulus.

Landmarks of TM

Pars Flaccida
Malleus
Umbo


Pars Tensa
Cone of Light
Right TM
Annulus

Left T.M.

Pars Tensa Pars Flaccida
Lower portion of the tympanic membrane Larger (2/3 ) Thicker In the middle layer there is fibrous tissue Perforation is safe

Upper portion of Tympanic membrane Smaller (1/3) Thinner Deficient of fibrous tissue. Perforation is unsafe

Middle Ear

Middle ear cleft includes; Middle ear (tympanic cavity). Eustachian tube ( opens anteriorly into the nasopharynx) Aditus which leads posteriorly to the mastoid antrum and air cells.

Epitympanum (Attic)

Mesotympanum
Hypotympanum

I C A

Eustachian Tube
Canal for Tensor Tympani Muscle
Aditus
Pyramid
I J V
Promontary
R
O
Facial Nerve
Tegmen Tympani
Ant
Post
Relations of Right Middle Ear
Floor

Aditus leading to mastoid antrum and air cells

Contents of Middle Ear
Air. Ossicles: Malleus Incus Stapes Muscles Tensor tympani Stapedius Nerves; Chorda tympani facial nerve Tympanic plexus

Eustachian Tube

Connects middle ear with the nasopharynx. 36mm in length in adult. In infant the tube is shorter, relatively wider and its course is more horizontal. Respiratory (columnar ciliated) epithelium.

2 Parts: 1) Cartilaginous part (Medial 2/3 of its length). 2) Bony portion (Lateral 1/3). At rest pharyngeal orifice of Eustachian tube is closed. Actively opened during swallowing by contraction of tensor palate muscle and this will equalize air pressure on both sides of the tympanic membrane.

Inner Ear Embryology

Invagination of otic placode to form otic vesicle (otocyst) Chochlea reaches adult Size at 20th wk of gestation!!
12 wk

Inner Ear

Inner ear, or labyrinth, consists of: Bony labyrinth (otic capsule) Membranous labyrinth

Fluids in Inner Ear

PerilymphIn Bony Labirynth↓ K+ ↑ Na+Similar to CSF and ECF EndolymphIn Membranous Labirynth↑ K+ ↓ Na+Similar to ICF


Bony Labirynth( Otic Capsule)
consists of three parts: Cochlea: a snail-like structure lies anteriorly. two and a half turns round its central axis (modulus). Vestibule, in the middle. Three semicircular canals, posteriorly.

Membranous Labyrinth

Cochlear duct (scala media) in bony cochlea Saccule and utricle in Bony Vestibule Membranous semicircular ducts in Bony Semicircular canals

Cross Section in Cochlea

Bony Cochlea Scala Vestibuli Scala Media (Cochlear Duct) Organ of Corti Cochlear Nerve Scala Tympani
Perilymph

Endolymp

Perilymph

Membranous labyrinth lieswithin Bony Labyrinth

Cross Section in Cochlea
S V
S M
S T

Organ ofCorti

Specialized neuro epithelium which is the sense organ of hearing. Rests on basilar memb. Complex arrangement of supporting and hair cells. The terminal fibers of the cochlear division of the 8th cranial nerve end in contact with these hair cells.

Vestibule

Utricle and Saccule lie in the bony vestibule stimulated by linear acceleration. joined by Y – shaped endolymphatic duct which extends to the endolymphatic sac that is probably concerned with absorption of endolymph.

Membranous Semicircular Ducts

occupy , but not filing lumen of corresponding bony canal set at right angles to each other stimulated by angular acceleration. Membranous SCD contain endolumph Bony SCC contain perilymph

The ampullary, utricular and saccular nerves unite to form the vestibular nerve. Vestibular and cochlear nerves together constitute the 8th cranial nerve

Blood Supply of inner ear

labyrinthine artery (internal acoustic artery) which arises from the basilar or anterior inferior cerebellar artery.