Audiology: Outer, Middle, and Inner Ear Review

Comprehensive vocabulary flashcards covering peripheral and central auditory processing, cochlear physiology, and psychoacoustics.

Sound

Vibrating particles travelling through a medium (usually air) at approximately $343\,m/s$.

Frequency

Physical property measuring how often particles oscillate in $Hz$; its perceptual correlate is pitch.

Amplitude

The magnitude of particle displacement and it correlates with loudness.

Pinna

The outer ear structure that funnels sound, provides passive amplification, and assists in vertical sound localisation through asymmetric folds.

Ear Canal

A tube-shaped passage that applies frequency-dependent gain (approximately $12\,dB$), peaking at mid-frequencies around $3\,kHz$.

Ossicles

The three smallest bones in the middle ear: the Malleus (attached to the eardrum), Incus (middle bone), and Stapes (pushes on the oval window).

Eustachian Tube

A passage connecting to the nasopharynx that maintains air pressure and drains fluid; it is less efficient in infants due to its size and angle.

Impedance Matching

The ossicles' solution to the air-to-fluid transition, achieved via a $20:1$ area ratio between the eardrum and stapes footplate and lever action.

Afferent

Signals travelling from the periphery (the ear) to the central nervous system (the brain).

Efferent

Signals travelling away from the central nervous system to the periphery; often used for protection from loud noises.

Tonotopy

The spatial frequency organisation established in the cochlea (high frequencies at the base, low frequencies at the apex) and preserved throughout the auditory pathway.

Perilymph

Fluid filling the scala vestibuli and scala tympani, characterised by high $Na^+$ and low $K^+$ concentrations.

Endolymph

Fluid filling the scala media (cochlear duct) with a unique ionic composition of high $K^+$ and low $Na^+$ critical for hair cell function.

Stria Vascularis

Specialised tissue lining the outer wall of the scala media that acts as a 'charger' to maintain the ionic composition and electrical potential of endolymph.

Endocochlear Potential (EP)

The electrical charge generated by ionic separation between endolymph and perilymph, measuring approximately $+70$ to $+100\,mV$.

Inner Hair Cells (IHCs)

Receptive cells (~3,500) that convert mechanical vibrations into neural signals; they connect to $95\%$ of afferent auditory nerve fibers.

Outer Hair Cells (OHCs)

Cylinder-shaped cells (~12,000 to 20,000) that mechanically pre-amplify low-level sounds via electromotility.

Prestin

The motor protein located in the lateral wall membrane of Outer Hair Cells that allows them to change length in response to depolarisation.

Mechanoelectrical Transduction (MET)

The process where mechanical movement of the hair bundle (stereocilia) is converted into an electrical signal inside the hair cell.

Tip links

Filaments between stereocilia that stretch during bundle deflection to open MET channels, allowing $K^+$ and $Ca^{2+}$ to flow into the cell.

Ribbon Synapse

A specialised junction between the IHC and auditory nerve adapted for speed and reliability, using glutamate as a neurotransmitter.

Otoacoustic Emissions (OAEs)

Sounds produced by the cochlea's OHC electromotility that can be measured in the ear canal with a sensitive microphone.

Loudness Recruitment

The abnormally rapid growth of loudness with increasing sound level caused by the loss of OHC compressive non-linearity in sensorineural hearing loss.

Wide Dynamic Range Compression (WDRC)

A hearing aid technology that applies more gain to soft sounds and less gain to loud sounds to restore the listener's dynamic range.

Cochlear Nucleus (CN)

The first relay station in the brainstem where all auditory nerve fibers project and bifurcate into ventral and dorsal divisions.

Superior Olivary Complex (SOC)

A brainstem structure that receives input from both ears and is the first site of binaural convergence for sound localisation.

Interaural Time Differences (ITDs)

Cues used for low-frequency sound localisation (below $1.5\,kHz$) based on the time difference of sound arrival at each ear.

Interaural Level Differences (ILDs)

Cues used for high-frequency sound localisation (above $1.5\,kHz$) based on intensity differences caused by the head shadow effect.

Duplex Theory of Binaural Hearing

Lord Rayleigh's theory stating low frequencies are localised using ITDs and high frequencies are localised using ILDs.

Jeffress Model

A neural model of sound localisation where Medial Superior Olive (MSO) neurons act as coincidence detectors with varying delay path lengths.

MOC Reflex

A feedback mechanism where the Medial Olivo-Cochlear fibers trigger OHC hyperpolarisation to reduce sensitivity and control gain.

The 'Missing Fundamental'

The perceptual phenomenon where a series of harmonics generates the pitch percept of their common fundamental frequency, even if it is not present.