The Ear: Hearing and Equilibrium

Hearing

The reception of air sound waves that are converted to fluid waves, which ultimately stimulate mechanosensitive hair cells that send impulses to the brain for interpretation.

Sound Waves

Created when an object moves, causing displacement of air molecules and creating areas of high pressure (compression) and low pressure (rarefaction).

Frequency

The number of waves that pass a given point in a given time, with shorter wavelengths indicating higher frequency of sound.

Amplitude

The height of the crests of a sound wave, with higher amplitudes corresponding to louder sounds, measured in decibels (dB).

Tympanic Membrane

Also known as the eardrum, it forms the boundary between the outer ear and middle ear, vibrating in response to sound waves.

Ossicles

Three of the smallest bones in the body (malleus, incus, and stapes) that transfer the vibration of the eardrum to the oval window, amplifying the sound.

Cochlea

A small spiral, conical, bony chamber in the inner ear that contains the cochlear duct, which houses the spiral organ and ends at the cochlear apex (helicotrema).

Spiral Organ

Contains cochlear hair cells arranged in one row of inner hair cells and three rows of outer hair cells, which are stimulated by vibrations of the basilar membrane.

Resonance

The movement of different areas of the basilar membrane in response to a particular frequency, allowing for the mechanical processing of sound before reaching the receptors.

Inner Hair Cells

Deflection of their stereocilia by the movement of the basilar membrane leads to the perception of pitch (sound wave frequency) and loudness.

Localization of Sound

Depends on the relative intensity and timing of sound waves reaching both ears.

Outer Hair Cells

Can contract and stretch, changing the stiffness of the basilar membrane and amplifying the motion of the inner hair cells, protecting them from loud noises.

Equilibrium

Maintained in response to various head movements, relying on input from the inner ear, eyes, and stretch receptors.

Maculae

Sensory receptor organs that monitor static equilibrium, located in the saccule and utricle walls, playing a key role in controlling posture.

Otolith Membrane

Jelly-like mass studded with otoliths (calcite stones) that give the membrane weight and help detect changes in head position.

Receptor for Rotational Acceleration

Crista ampullaris (crista), found at the base of each semicircular canal, excited by rotational movements and detecting all rotational movements of the head.