Outer and Middle Ear

Two main divisions of the auditory system

Peripheral auditory system and central auditory system

Specific skull bone region housing the ear structures

Petrous portion of the temporal bone

Components of the Central Auditory System

Cranial Nerve VIII (Vestibulocochlear), brainstem pathways, and the auditory cortex

Five main anatomical landmarks of the Pinna (Auricle)

Helix, concha, lobule, tragus, and antitragus

External Auditory Meatus (EAM) structure and dimensions

An S-shaped tube approximately $2.5\,cm$ long and $7\,mm$ in diameter in adults, with a cartilaginous outer third and a bony inner two-thirds

Three structural layers of the Tympanic Membrane

Outermost epithelial layer, middle fibrous layer, and innermost mucous membrane layer

Pars tensa vs. Pars flaccida

Two regions of the tympanic membrane; the former contains all 3 structural layers, whereas the latter contains only 2 layers

Anatomical boundaries of the middle ear cavity

Roof formed by tegmen tympani, floor above the jugular bulb, anterior wall containing the Eustachian tube, and superior posterior wall containing the oval and round windows

Eustachian tube primary functions and key muscle

Pressure equalization, middle ear ventilation, and drainage; forcibly opened by the tensor palatini muscle

Three auditory ossicles in sequential order from external to internal

Malleus, Incus, and Stapes

Annular ligament

Ring-shaped ligament that holds the footplate of the stapes inside the oval window

Stapedius muscle

A $6\,mm$ muscle originating from the posterior wall of the middle ear and inserting into the neck of the stapes; innervated by CN VII (facial nerve)

Tensor tympani muscle

A $25\,mm$ muscle originating from the anterior wall of the middle ear and inserting into the upper manubrium of the malleus; innervated by CN V (trigeminal nerve)

Two main divisions of the auditory system

Peripheral auditory system and central auditory system

Specific skull bone region housing the ear structures

Petrous portion of the temporal bone

Components of the Central Auditory System

Cranial Nerve VIII (Vestibulocochlear), brainstem pathways, and the auditory cortex

Five main anatomical landmarks of the Pinna (Auricle)

Helix, concha, lobule, tragus, and antitragus

External Auditory Meatus (EAM) structure and dimensions

An S-shaped tube approximately $2.5\,cm$ long and $7\,mm$ in diameter in adults, with a cartilaginous outer third and a bony inner two-thirds

Three structural layers of the Tympanic Membrane

Outermost epithelial layer, middle fibrous layer, and innermost mucous membrane layer

Pars tensa vs. Pars flaccida

Two regions of the tympanic membrane; the former contains all 3 structural layers, whereas the latter contains only 2 layers

Anatomical boundaries of the middle ear cavity

Roof formed by tegmen tympani, floor above the jugular bulb, anterior wall containing the Eustachian tube, and superior posterior wall containing the oval and round windows

Eustachian tube primary functions and key muscle

Pressure equalization, middle ear ventilation, and drainage; forcibly opened by the tensor palatini muscle

Three auditory ossicles in sequential order from external to internal

Malleus, Incus, and Stapes

Annular ligament

Ring-shaped ligament that holds the footplate of the stapes inside the oval window

Stapedius muscle

A $6\,mm$ muscle originating from the posterior wall of the middle ear and inserting into the neck of the stapes; innervated by CN VII (facial nerve)

Tensor tympani muscle

A $25\,mm$ muscle originating from the anterior wall of the middle ear and inserting into the upper manubrium of the malleus; innervated by CN V (trigeminal nerve)

Two main functional systems of the inner ear

Vestibular system (responsible for balance) and cochlear system (responsible for hearing)

Two major structural labyrinths of the inner ear

Osseous (bony) labyrinth and membranous labyrinth

Three main anatomical parts of the inner ear

Semicircular canals, vestibule, and cochlea

Sensory organs for balance within the utricle and saccule

Maculae

Orientation and function of the three semicircular canals

Oriented at $90^\circ$ angles to each other (superior, posterior, horizontal/lateral) to code body movement and balance in 3D space

Modiolus and Apex of the cochlea

The modiolus is the central bony core of the cochlea, and the apex is its innermost tip

Three fluid-filled chambers (scalae) of the cochlea

Scala vestibuli, scala media (cochlear duct), and scala tympani

Scala vestibuli fluid composition and boundary structures

Bounded by bone (roof) and Reissner's membrane (floor), contains the oval window, and is filled with perilymph (high $Na^+$, low $K^+$)

Scala media (cochlear duct) fluid composition and boundary structures

Bounded by Reissner's membrane (roof) and basilar membrane (floor), and is filled with endolymph (low $Na^+$, high $K^+$)

Scala tympani fluid composition and boundary structures

Bounded by the basilar membrane (roof) and bone (floor), contains the round window, and is filled with perilymph

Organ of Corti

The end organ for hearing resting on the basilar membrane within the scala media, containing inner and outer hair cells

Comparison of Inner Hair Cells (IHCs) vs. Outer Hair Cells (OHCs)

IHCs: ~$3,500$ in 1 row, teardrop-shaped, ~$50$ stereocilia each, heavily innervated. OHCs: ~$12,000$ in 3 rows ('W' shape), test-tube-shaped, ~$150$ stereocilia each

Tunnel of Corti

A triangular canal formed by the inner and outer pillar cells that separates the single row of IHCs from the three rows of OHCs

Tectorial membrane

A gelatinous structure ($90\%$ water) arising from the spiral limbus that overlays hair cells, with outer hair cell stereocilia embedded in its underside

Stria vascularis

A vascular tissue attached to the spiral ligament along the outer wall of the cochlear duct that produces endolymph and supplies oxygen to the cochlea