HEARING and BALANCE

External Ear

part extending from the outside of the head to the tympanic membrane, commonly called the eardrum.

Middle Ear

an air-filled chamber medial to the tympanic membrane.

Inner Ear

is a set of fluid-filled chambers medial to the middle ear

Auricle

The fleshy outer part of the external ear that collects sound waves and directs them to the external auditory canal.

External auditory canal

A passageway leading to the eardrum.

Ceruminous glands

produce cerumen (earwax) to protect the tympanic membrane.

Tympanic membrane (eardrum)

A thin membrane separating the external ear from the middle ear that vibrates in response to sound waves.

Oval window and round window

Two openings on the medial side of the middle ear that connect it to the inner ear.

Auditory ossicles (ear bones)

The malleus (hammer), incus (anvil), and stapes (stirrup) form a bony bridge that transmits and amplifies vibrations from the tympanic membrane to the oval window

Auditory tube (Eustachian tube)

A tube that connects the middle ear to the pharynx, allowing for the equalization of air pressure between the middle ear and the outside environment...

Inner ear

Composed of interconnecting tunnels and chambers within the temporal bone, known as the bony labyrinth.

Bony labyrinth

Contains a smaller set of membranous tunnels and chambers called the membranous labyrinth.

Membranous labyrinth

Filled with a clear fluid called endolymph; the space between it and the bony labyrinth is filled with perilymph.

Cochlea

A snail shell-shaped structure involved in hearing, divided into three channels:

Scala vestibuli (scala or stairway)

Extends from the oval window to the apex of the cochlea.

Vestibular membrane

wall of the membranous labyrinth that lines the scala vestibuli

Scala tympani

Extends in parallel with the scala vestibuli, from the apex back to the round window.

Basilar membrane

wall of the membranous labyrinth that lines the scala tympani

Cochlear duct

Formed by the space between the vestibular and basilar membranes, filled with endolymph.

Spiral organ (organ of Corti)

Located inside the cochlear duct, containing hair cells with hairlike microvilli (stereocilia) that detect sound.

Tectorial Membrane

an acellular gelatinous shelf attached to spiral lamina

Hair cells

Specialized sensory cells without axons, associated with axon terminals of sensory neurons located in the cochlear ganglion (spiral ganglion) - A cluster of cell bodies of sensory neurons associated with hair cells in the cochlea; these neurons transmit auditory information to the brain via the cochlear nerve.

Cochlear nerve

Formed by the axons of sensory neurons; joins with the vestibular nerve to become the vestibulocochlear nerve (VIII), carrying action potentials to the brain.

Sound waves

Vibrations that create sound; described by pitch and volume.

Pitch

Frequency or wavelength of sound; higher pitches have shorter wavelengths.

Volume

Related to the amplitude of sound waves; louder sounds have higher amplitudes.

Basilar membrane

Varies in width and density; narrower near the oval window (higher pitch) and wider at the apex (lower pitch).

Hearing impairment

Two main types:

Conduction deafness

Mechanical issues affecting sound transmission.

Sensorineural hearing loss

Damage to the spiral organ or auditory nerves, often from loud sounds.

Balance (Equilibrium)

Comprises two components:

Static equilibrium

Involves the vestibule; evaluates head position relative to gravity.

Dynamic equilibrium

Involves semicircular canals; evaluates changes in head movement direction and rate.

Vestibule

Divided into two chambers:

Utricle

One of the vestibule chambers.

Saccule

The other vestibule chamber.

Maculae

Specialized patches of epithelium in utricle and saccule, containing hair cells; surrounded by endolymph.

Otolithic membrane

Gelatinous mass in which hair cell microvilli tips are embedded; weighted by otoliths (particles of protein and calcium carbonate).

Action potentials

Generated when otolithic membrane moves due to gravity, bending hair cell microvilli; signals sent via vestibular portion of the vestibulocochlear nerve (VIII) to the brain to indicate head position.

Semicircular Canals

Involved in dynamic equilibrium; there are three in each ear, positioned at nearly right angles to detect movement in various directions.

Ampulla

Expanded base of each semicircular canal.

Crista Ampullaris

Specialized epithelial structure within each ampulla; features a ridge of epithelium with a curved gelatinous mass called the cupula.

Cupula

a ridge of epithelium with a curved gelatinous mass and functions similarly to the maculae but contains no otoliths

Endolymph Movement

When the head moves, the endolymph remains stationary momentarily, causing the cupula to be displaced in the opposite direction. Once the head stops moving, the endolymph continues to move, displacing the cupula in the direction of the movement.

Hair Cell Depolarization

Bending of the cupula causes depolarization in the hair cells, initiating action potentials in the vestibular nerves, which combine with cochlear nerves to form the vestibulocochlear nerve (VIII).