The Ear

External Ear

auricle – outermost portion of the ear; consists of helix, antihelix, tragus, antitragus, and cohcha (innermost portion)

external acoustic meatus – lateral 1/3 is cartilagenous, medial 2/3 is bony

isthmus – bony constriction at the internal end of the tube

innervation: V³ (auricle and outer portion); CN VII and CN X (inner portions and isthmus)

Middle Ear

tympanic membrane – at end of external canal and marks lateral wall of the middle ear

composed of outer skin layer, middle cartilagenous layer, inner mucosa

umbo – depression at the center made by the attachment of the malleus (membrane is cone shaped)

pars flaccida – slack part of the membrane; smaller than the pars tensa; lies at supeior border

pars tensa – tense, part of the membrane; comprises most of the membrane

innervation: outer skin Þ V³, VII, X; inner mucosa Þ IX

arterial supply: deep auricular and anterior tympanic (branches of the maxillary)

ossicles – transfer vibrations of the membrane to the perilymph and then to the endolymph of the cochlea

chorda tympani nerve – branch of the facial that passes over the malleus on its way to innervate the anterior tongue taste; the other branch of the facial in the inner ear is the stapedius nerve

tympanic plexus – overlies the prominatory, formed from tympanic branch of CN IX; continues into the auditory tube

facial canal prominance – on the medial wall; represents bulging of the facial n

lateral canal prominance – formed by the lateral canal on the medial wall, inferior to the facial prominance

eustachian (auditory) tube – anterior 2/3 is cartilagenous; posterior 1/3 is bony

pyramidal eminance – on the medial wall, contains the muscle body of the stapedius

stapedius muscle; tensor tympani muscle – dampen the vibration of the ossicles; hyperexcitation leads to tinnitis

Inner Ear

Bony Part

composed of the cochlea, vestubule, and semicircular canals

contains perilymph (similar to CSF), and is connected to the subarachnoid space by the perilymphatic duct

Within bony part is the membranous labyrinth, which consists of the utricle, saccule, semicircular ducts, and cochlear duct;

contains endolymph; excess endolymph is drained by the endolymphatic duct

saccule and utricle sense linear movement, labyrinth senses rotational movement

otoliths: small bones on the hair cells in the semicircular canals

sound transduction: in the cochlea, vibrations at the oval window are carried by perilymph into the spirals of the cochlea through the scala vestibuli; vibration is transferred to the endolymph and then to the hair cells via the tectorial membrane; vibration leaves the cochlea via the scala tympani and are absorbed at the round window

Embryology

External Ear

auricle: from six auricular hillocks from the 1^st and 2^nd branchial arches, three on either side of the external meatus

external auditory meatus: develops from the first branchial cleft

tympanic membrane: develops from the first branchial membrane, so ectoderm on outside, endoderm on inside

Middle Ear: forms from the first three branchial arches

malleus and incus are from the first arch, stapes is from second

tensor tympani: first arch, so gets innervation from V; stapedius: second arch, so innervation from VII
initially, the ossicles are imbedded in mesenchyme, then endoderm from the third pouch invades the middle ear, pushes out the mesenchyme recanalizing the middle ear; eventually all the structures of the middle ear are covered in mucosa derived from the third pouch (hence, innervation from IX)

Inner Ear

otic placode invaginates to form the otic vesicle which migrates towards the brain where it hooks up with CN VIII
it then migrates to the floor of the hindbrain and becomes the inner ear in all its glory