Inner Ear and Cochlea

Divisions of the inner ear

The auditory division and the vestibular division.

Hearing organ

The Cochlea.

Divisions of the cochlea

The Scala Vestibuli, Scala Media, and Scala Tympani.

Endolymph

A fluid rich in potassium (K+), found in the Scala Media.

Perilymph

A fluid high in sodium (Na+), located in the Scala Vestibuli and Scala Tympani.

Division between scala media and scala tympani

The Basilar Membrane.

Division between scala media and scala vestibuli

Reissner's Membrane.

Openings in the cochlea

The Oval Window and the Round Window.

Connection of oval window

The oval window is connected to the stapes.

Organ of Corti

The sensory organ of hearing, located within the scala media, on top of the basilar membrane.

Outer Hair Cells (OHCs) vs Inner Hair Cells (IHCs)

There are more Outer Hair Cells (OHCs).

Tectorial membrane

A gel-like structure that sits on the basilar membrane, with the stereocilia of the Outer Hair Cells embedded in it.

Hair cells change polarization

Through the bending of their stereocilia, caused by the shearing force from the movement of the basilar membrane.

OHCs responsibility

Sound intensity.

Electromotility

The ability of OHCs to change their length, acting as a cochlear amplifier to sharpen the response to sound.

IHCs responsibility

Sound clarity.

Otoacoustic emission

A measurable sound that travels back from the cochlea through the hearing system.

Afferent pathways

Carry signals towards the central nervous system.

Efferent pathways

Carry signals away from the central nervous system.

Hair cell with more afferent connections

Inner Hair Cells (IHCs).

Sound propagation in the cochlea

The stapes' movement on the oval window creates a fluid wave in the perilymph, which creates a transverse wave that travels down the basilar membrane.

Tonotopicity

The spatial arrangement of sound frequencies.

Tonotopically organized structure

The Basilar Membrane.

Location of high and low frequencies

High frequencies are at the base, and low frequencies are at the apex.

Upward spread of masking

When a low-frequency sound makes it harder to hear a higher-frequency sound.

Theories behind encoding of sound

Traveling wave theory, place theory, and volley theory.

Cranial nerve innervating the cochlea

Cranial Nerve VIII, the Vestibulocochlear nerve.

Cranial Nerve VIII

The Vestibulocochlear nerve.

Vestibular organs

5 vestibular organs; a person has a total of 10 vestibular organs (5 on each side).

Semicircular Canals

3 canals: Anterior/Superior, Posterior, and Horizontal/Lateral.

Function of semicircular canals

They detect angular acceleration.

Functional pairs of semicircular canals

Right anterior with left posterior, left anterior with right posterior, and right horizontal with left horizontal.

Ampulla

An enlarged area at the base of each semicircular canal that contains the crista ampullaris.

Cupula

A gelatinous structure within the ampulla that houses the stereocilia of the hair cells.

Hair cells polarization in semicircular canals

The deflection of the cupula bends the stereocilia, causing the hair cells to depolarize or hyperpolarize.

Vestibule

The central part of the bony labyrinth, containing the utricle and saccule.

Hair cells in vestibular organs

Type I hair cells and Type II hair cells.

Utricle function

Detects linear acceleration in the horizontal plane.

Saccule function

Detects linear acceleration in the vertical plane.

Macula

The sensory receptor organ in the utricle and saccule.

Otoconia

Calcium carbonate crystals that sit on top of the macula, adding weight and making the utricle and saccule sensitive to gravity.

Cranial nerve innervating vestibular organs

The vestibular portion of the Vestibulocochlear nerve (CN VIII); it has two divisions: superior and inferior.

Auditory nerve fibers

Nerve fibers that connect to the inner and outer hair cells and form the cochlear portion of CN VIII.

Action potential

An 'all or nothing' neural firing.

CN VIII tonotopic organization

Low frequencies are in the center, while high frequencies are on the outside.

Sound travel to superior olivary complex

From the cochlea to the ipsilateral cochlear nucleus, and then to both the ipsilateral and contralateral superior olivary complex (SOC).

Action at the SOC

Sound localization and speech perception.

Tonotopic organization preservation

It is preserved all the way up to the primary auditory cortex.

Aspects of sound preserved to primary auditory cortex

Frequency (tonotopicity), phase, and latency.

Efferent connections from primary auditory cortex

Responsible for providing inhibitory feedback to reduce background noise through a 'gating function'.

Sound transmission in auditory system

Sound waves are captured by the outer ear, amplified by the middle ear, and converted into electrical signals in the inner ear's cochlea, which then travel up the auditory pathway to the brain.