Auditory System: From Vibration to Electrical Signals

Flashcards covering key vocabulary and concepts related to the auditory system's function, from sound vibration to electrical signal transduction.

Frequency Analysis Program

A program that analyzes microphone recordings to display frequency components (low to high) as a function of time, with intensity shown by brightness.

Cochlea

The organ responsible for converting sound vibrations into electrical signals in the auditory system.

Basilar Membrane

A membrane within the cochlea that runs along its length, vibrating up and down in response to sound.

Organ of Corti

A complex epithelial structure located on the basilar membrane, containing specialized sensory cells (hair cells) responsible for auditory transduction.

Reissner's Membrane

A membrane located above the basilar membrane within the cochlea, separating fluid spaces.

Tectorial Membrane

A membrane that typically sits atop the Organ of Corti epithelium.

Cochlear Epithelium

A specialized barrier lining structures within the cochlea, which separates different ionic environments essential for hearing.

Inner Hair Cells

Specialized sensory cells in the Organ of Corti (approx. 3,500 per cochlea) that primarily transmit auditory information to the brain via nerve fibers.

Outer Hair Cells

Specialized sensory cells in the Organ of Corti (approx. 14,000 per cochlea) that act as micromechanical amplifiers, sharpening the tuning of the basilar membrane.

Hair Cells

Sensory receptor cells found in the cochlea, characterized by hair-like projections (stereocilia) that transduce mechanical vibrations into electrical signals.

Stereocilia

The hair-like projections on the surface of hair cells which are crucial for mechanotransduction.

Tip Link

A small linker connecting stereocilia on a hair bundle, acting as a gate to open ion channels when stretched by mechanical movement.

Cadherin 23 & Protein Cadherin 50

Two proteins that form the tip links in hair cells; mutations in their encoding genes are associated with familial deafness.

TMC1 & TMC2

Ion channels located on stereocilia that open when tip links are stretched, allowing ions to flow into the hair cell and initiate electrical signaling.

Sensory Transduction (Auditory)

The process in the cochlea where mechanical sound vibrations are converted into electrical signals by the flow of ions into hair cells.

Excitation (Hair Cell)

The movement of hair bundles towards the tallest stereocilia, leading to depolarization due to potassium ion influx into the cell.

Inhibition (Hair Cell)

The movement of hair bundles in the opposite direction, causing hyperpolarization due to the closing of ion channels.

Depolarization (Hair Cell)

An electrical change in a hair cell where an influx of positive ions (potassium) makes the cell's internal voltage more positive, activating nerve fibers.

Hyperpolarization (Hair Cell)

An electrical change in a hair cell where its internal voltage becomes more negative, typically by closing ion channels that allow positive current flow.

Synapse (Auditory System)

The junction where hair cells connect to neurons, releasing neurotransmitters to activate nerve fibers and transmit auditory information to the brain.

Basal End (Cochlea)

The narrow, stiff end of the basilar membrane located near the oval window, responsible for processing high-frequency sounds.

Apical End (Cochlea)

The wider, floppier end of the basilar membrane, responsible for processing low-frequency sounds.

Tonotopic Map

A spatial organization within the auditory system where different frequencies are represented at distinct locations, from the basilar membrane to the auditory cortex.

Cochlear Implantation

A medical procedure involving placing electrodes into the cochlea to directly stimulate auditory nerve fibers, thereby restoring a sense of hearing.

Modiolus

The central bony pillar of the cochlea through which the auditory nerve fibers pass after connecting to hair cells.

Auditory Nerve

A bundle of nerve fibers that transmits electrical signals from the cochlea to the brainstem for further auditory processing.

Type I Afferent Fibers

Auditory nerve fibers (95% of total) that originate from inner hair cells, primarily responsible for transmitting detailed acoustic information to the brain.

Type II Afferent Fibers

Auditory nerve fibers (5% of total) that originate from outer hair cells, connecting to multiple outer hair cells.

Spiral Ganglion

A cluster of nerve cell bodies (bipolar neurons) located within the cochlea, whose dendrites connect to hair cells and axons form the auditory nerve.

Characteristic Frequency

The specific sound frequency to which an individual auditory nerve fiber or hair cell is most sensitive, requiring the lowest stimulus level to elicit a response.

Tuning Curve

A graphical representation showing the sensitivity of an auditory nerve fiber or hair cell across a range of frequencies and stimulus levels.

Audiogram

A clinical graph that plots a person's hearing threshold levels (sensitivity) across different frequencies, often showing a U-shaped curve for normal human hearing.

Phase Locking

A temporal coding mechanism in auditory nerve fibers where action potentials fire in synchrony with a specific phase of the sound wave, primarily for low-frequency sounds.

Cochlear Amplification

An active process, primarily driven by outer hair cells, that enhances the movement of the basilar membrane at specific frequency locations, leading to sharper frequency tuning and increased sensitivity.

Outer Hair Cell Motility

The ability of outer hair cells to rapidly contract and lengthen in response to electrical signals, actively pushing and pulling the basilar membrane to amplify sound vibrations.

Age-Related Hearing Loss (Presbycusis)

A common condition often associated with the loss of outer hair cells and synapses, leading to a degraded ability to hear, particularly in noisy environments.

Cochlea's primary role

To convert mechanical vibrations into electrical signals for audition.

Basilar Membrane

A key structure within the cochlea, running along its length and crucial for frequency analysis.

Organ of Corti

Specialized structure on the basilar membrane containing cells responsible for mechanotransduction.

Reissner's Membrane

A membrane within the cochlea, situated above the basilar membrane and Organ of Corti.

Tectorial Membrane

Sits atop the Organ of Corti's hair cells; involved in shearing stereocilia.

Epithelium of the Organ of Corti

The most complex epithelium in the body, specialized with unusual fluid spaces and crucial for maintaining distinct ionic environments.

Endolymph

The fluid space above the Organ of Corti, characterized by a high potassium (K+) environment.

Perilymph

The fluid space below the Organ of Corti and within the cells, distinct from endolymph.

Inner Hair Cells (IHCs)

One row of approximately 3,500 hair cells per cochlea, primarily responsible for transmitting auditory information to the brain.

Outer Hair Cells (OHCs)

Three rows of approximately 14,000 hair cells per cochlea, possessing motility crucial for cochlear amplification.

Stereocilia

Finger-like projections on the surface of hair cells that bend to trigger mechanotransduction.

Tip Links

Fine filament structures connecting the tip of a shorter stereocilium to the side of a taller adjacent stereocilium, composed of cadherin 23 and cadherin 50.

Ion Channels (in stereocilia)

Believed to be TMC1 and TMC2 proteins, located near stereocilia tips; they open when tip links stretch.

Depolarization (in hair cells)

Occurs when positive potassium ions (K+) flow into the hair cell, making its membrane potential more positive and converting mechanical energy into an electrical signal.

Synaptic Transmission (from hair cells)

Depolarization triggers calcium (Ca2+) influx, leading to neurotransmitter release, which generates action potentials in afferent nerve fibers.

Type I Afferent Fibers

Constitute 95\% of auditory nerve fibers, connecting exclusively to inner hair cells with dense innervation.

Type II Afferent Fibers

Constitute 5\% of auditory nerve fibers, connecting to outer hair cells with a broader, less specific innervation pattern.

Modiolus

The central core of the cochlea where all nerve fibers converge and exit to form the auditory nerve.

Spiral Ganglion

Contains the cell bodies of the bipolar auditory neurons (Type I and Type II afferents).

Tonotopic Map

The place-coding of frequency along the basilar membrane, where the basal end responds to high frequencies and the apical end to low frequencies.

Characteristic Frequency (CF)

The frequency at which an auditory nerve fiber is most sensitive, constant for a given fiber and reflecting its basilar membrane location.

Phase Locking

Auditory nerve fibers synchronizing their action potentials to the phase of the sound wave, particularly at low frequencies (1-2 \text{ kHz}), providing temporal frequency information.

Cochlear Amplification

The process by which outer hair cell motility boosts the movement of the