Auditory System and Sound Transduction

Vocabulary practice flashcards covering the human auditory system, sound physics, cochlear anatomy, hair cell mechanotransduction, tonotopy, and cochlear amplification.

Optogenetics

A methodology that uses viral vectors to insert light-gated ion channel genes (such as channelrhodopsin) into specific neurons, allowing selected neural circuits to be activated or inhibited using specific wavelengths of light.

Channelrhodopsin

A light-sensitive protein discovered in green algae that functions as a light-gated cation channel; when illuminated with blue light, it opens to allow cation influx and depolarize the host cell.

Sound Intensity (Amplitude)

The magnitude of displacement or pressure fluctuation in air caused by a sound wave, measured physically in decibels ($dB$); it corresponds perceptually to loudness.

Sound Frequency

The number of compression and rarefaction cycles of air pressure per second, quantified in Hertz ($Hz$) or cycles per second; it corresponds perceptually to pitch.

Pitch

The perceptual dimension of sound determined primarily by the fundamental sound frequency.

Timbre

The distinct quality or acoustic color of a sound that differentiates sounds of identical pitch and loudness, determined by the frequencies, relative amplitudes, and timing of harmonics or overtones.

Fundamental frequency

The lowest frequency vibration produced by a sound source (such as an instrument or voice), which determines the perceived pitch.

Decibel ($dB$)

A logarithmic unit of measurement for sound intensity, where a $10\,dB$ increase represents a $10\times$ increase in physical sound intensity and sounds approximately twice as loud.

Pinna

The visible external cartilaginous structure of the outer ear that collects and funnels sound waves into the auditory canal and aids in sound localization.

Tympanic membrane

The eardrum; a thin sheet of tissue that vibrates back and forth in response to variations in sound pressure, transmitting vibrations to the middle ear ossicles.

Auditory ossicles

The three smallest bones in the human body located within the air-filled middle ear—the malleus (hammer), incus (anvil), and stapes (stirrup)—which mechanically transmit and amplify sound vibrations to the oval window.

Impedance matching (Middle Ear)

The mechanical transformation of large-amplitude, low-pressure air vibrations at the tympanic membrane into small-amplitude, high-pressure vibrations at the oval window via the ossicles and the $30\times$ area difference, preventing $99.9\%$ of sound energy from being reflected by cochlear fluid.

Attenuation reflex

An involuntary bilateral contraction of the stapedius (and tensor tympani) muscles triggered by loud sounds ($> 70\,dB$) or vocalization that stiffens the ossicles to dampen transmission to the inner ear, mainly at low frequencies.

Cochlea

A coiled, fluid-filled inner ear structure wrapped approximately $2\frac{3}{4}$ turns that contains the sensory apparatus responsible for converting mechanical vibrations into neural signals.

Scala vestibuli

The uppermost perilymph-filled chamber of the cochlea that receives pressure waves from the stapes footplate at the oval window.

Scala media

The central cochlear duct partitioned between Reissner's membrane and the basilar membrane, filled with high-potassium endolymph and housing the Organ of Corti.

Scala tympani

The lower perilymph-filled chamber of the cochlea that terminates at the round window, allowing pressure relief for fluid motion.

Perilymph

The extracellular fluid filling the scala vestibuli and scala tympani, featuring ionic concentrations similar to typical extracellular fluid ($[K^+] = 7\,mM$, $[Na^+] = 140\,mM$).

Endolymph

The specialized fluid filling the scala media, characterized by an intracellular-like ionic composition with exceptionally high potassium ($[K^+] = 140\,mM$) and low sodium ($[Na^+] = 1\,mM$).

Stria vascularis

A vascularized epithelial tissue lining the outer wall of the scala media that actively reabsorbs $Na^+$ and secretes $K^+$ into the endolymph to sustain the endocochlear potential.

Endocochlear potential

The electrical potential difference of roughly $+80\,mV$ maintained by the endolymph relative to the perilymph (the 'cochlear battery'), providing an overall electrical gradient of approximately $120\,mV$ driving $K^+$ into hair cells.

Helicotrema

A narrow opening at the cochlear apex where the scala vestibuli and scala tympani connect, allowing slow fluid shifts to bypass the basilar membrane for very low frequency pressure changes.

Tonotopy

The systematic spatial arrangement and mapping of characteristic sound frequencies along an auditory structure, such as the basilar membrane or auditory cortex.

Basilar membrane mechanical gradient

The structural variation along the basilar membrane wherein the base is narrow and stiff (resonating maximally to high frequencies) while the apex is wide and floppy (resonating maximally to low frequencies).

Organ of Corti

The sensory receptor organ of hearing mounted on the basilar membrane inside the scala media, comprising inner hair cells, outer hair cells, supporting cells, and stereocilia beneath the tectorial membrane.

Inner hair cells (IHCs)

A single row of roughly $4{,}000$ sensory epithelial cells that act as the primary acoustic transducers, synapsing with approximately $95\%$ of all afferent spiral ganglion fibers.

Outer hair cells (OHCs)

Three rows of approximately $12{,}000$ sensory epithelial cells that contact only about $5\%$ of spiral ganglion fibers and function as mechanical amplifiers of basilar membrane motion.

Tip link

A tiny elastic filamentous bridge connecting the top of each stereocilium to the side of its taller neighbor; mechanical tension on these filaments directly pulls open cation channels.

Prestin

A specialized voltage-sensitive motor protein situated in the lateral membranes of outer hair cells that changes length upon depolarization, causing the cells to shorten and expand mechanically.

Cochlear amplifier

The active motor mechanism driven by outer hair cell length fluctuations that amplifies basilar membrane motion by roughly $100\times$ (yielding vibrations $100\times$ larger than passive mechanical response).

Otoacoustic emissions

Acoustic sounds emitted outward by the ear resulting from the active motion of outer hair cells; these occur spontaneously in over $50\%$ of individuals or can be evoked by sound clicks to screen newborn hearing.

Reticular lamina

A rigid structural ceiling of the Organ of Corti formed by supporting cells and hair cell apices, serving as a tight barrier that insulates cell bodies bathed in perilymph from high-$K^+$ endolymph.