Cochlear microanatomy

Perilymph: location, ions, and charge?

Fills scala vestibuli and scala tympani; high Na+; ionic composition similar to plasma/CSF; ~0 mV.

Endolymph: location, ions, and charge?

Fills scala media; high K+ and low Na+; maintained at about +80 mV.

Stria vascularis: two major jobs?

Creates the ionic composition of endolymph and the +80 mV endocochlear potential.

Stria vascularis cell types and roles?

Marginal: Na-K pump; intermediate: melanin; basal: structural support; capillaries supply cochlear tissues.

Basilar membrane: where is it and what sits on it?

Separates scala media from scala tympani; the Organ of Corti sits on top.

What is the basilar membrane made of?

Nonliving network of collagen fibers; extracellular matrix secreted by cochlear mesothelial cells.

Basilar membrane base vs apex?

Base: narrow, stiff, lower mass. Apex: wide, more flaccid, higher mass.

Why is the cochlea tonotopically organized?

Local basilar-membrane stiffness and mass determine each location's resonant/characteristic frequency.

Pillar cells: signature job?

Inner and outer pillar cells form the Tunnel of Corti.

Tunnel of Corti: what forms it and what passes through it?

Formed by inner and outer pillar cells; OHC-connected auditory nerve fibers exit the Organ of Corti through it.

Deiters' cells: signature jobs?

Support OHCs as cups; their phalangeal processes extend upward and help form the reticular lamina.

Hensen's cells: signature jobs?

Form the outer boundary of the reticular lamina and provide lateral support to hair cells.

Reticular lamina: what is it made of?

Mainly hair-cell cuticular plates, with gaps filled by pillar cells, Hensen cells, Deiters' phalangeal processes, and border cells.

Reticular lamina: two key functions?

Holds hair cells in the horizontal plane and prevents endolymph from mixing with cortilymph.

Why is reticular-lamina integrity important?

Endolymph-cortilymph mixing is extremely toxic and can kill hair cells.

What happens in the reticular lamina after OHC death?

Deiters' cell phalangeal processes can fill gaps left by dead OHCs.

Cortilymph: location and ionic composition?

Bathes hair cells below the reticular lamina; perilymph-like, high Na+ and low K+.

Why is cortilymph unusual anatomically?

It is technically in scala media but is more continuous with scala-tympani perilymph than with scala-media endolymph.

Hair cells: overall role?

Key transducers converting cochlear mechanical energy into electrochemical nervous-system energy.

Outer hair cells: shape, rows, number, main role?

Cylinder-shaped; 3 rows; ~12,000; responsible for cochlear amplification.

How much sensitivity do OHCs add?

OHC cochlear amplification makes the ear about 40 dB more sensitive.

Inner hair cells: shape, rows, number, main role?

Pear-shaped; 1 row; ~3,500; transmit sound messages to the auditory nerve.

Afferent innervation: IHCs vs OHCs?

About 85–95% of auditory-nerve fibers connect to IHCs; about 5–15% connect to OHCs.

IHC stereocilia: number and arrangement?

~40 per IHC, arranged in a line or shallow U shape.

OHC stereocilia: number and arrangement?

~150 per OHC, arranged in a V or W shape; inserted into the tectorial membrane.

Tectorial membrane: attachment and location?

Attaches on the modiolar side at the spiral limbus and sits above the OHCs.

Tectorial membrane: why does it matter mechanically?

OHC stereocilia are embedded in it; it is key to the shearing force that deflects OHC stereocilia.

Spiral ganglion: what is it?

Cell bodies of the neurons that synapse with the hair cells.

Where is the spiral ganglion located?

In Rosenthal's canal.

Type I (radial) auditory neurons: innervation pattern?

Several Type I neurons innervate each IHC.

Type II auditory neurons: innervation pattern?

One Type II neuron innervates several OHCs.

Type I vs Type II: which is larger and why?

Type I neurons are larger because they have more myelin.

Auditory-nerve neurons: basic morphology?

Bipolar neurons with cell bodies in spiral ganglion; one side connects to HCs and the long myelinated axon projects to cochlear nucleus.

Auditory nerve: length and CN VIII relationship?

About 22–26 mm; joins the vestibular nerve to form cranial nerve VIII.

Internal auditory canal / internal acoustic meatus: what is it?

Hole in petrous temporal bone allowing CN VIII to travel to brainstem; also houses part of CN VII.

Cerebellopontine angle (CPA): where is it?

Region between the IAM and cochlear nucleus containing CN VIII and CN VII; common vestibular-schwannoma site.

Endbulb of Held: what is it?

Large spoon-shaped terminal ending of an auditory-nerve neuron at the cochlear nucleus with many presynaptic terminals.

Auditory-nerve tonotopy?

Low frequencies occupy the central core; higher frequencies extend toward the outer layers.

Terminal buttons: why are they clinically/anatomically notable?

They are a site of significant fragility in the cochlea.