hearing

Hearing and the Auditory System

Overview


Topic: HearingFocus: The Auditory System

Sound and its Properties

  • Sound Waves: Produced by periodic variations in air pressure; travel at speed of sound (343 m/sec or 767 mi/h).

  • Characteristics of Sound:

    • Frequency: Determines pitch; measured in Hertz (Hz). Human range: 20 Hz to 20 kHz.

    • Ultrasound: Frequencies above 20 kHz, audible to animals (dogs, cats, dolphins).

    • Infrasound: Frequencies below 20 Hz, heard by elephants and whales; related to carsickness.

    • Intensity: Determines loudness, measured in decibels (dB). 0 dB is the threshold; 120 dB is the loudest tolerable sound.

Anatomy and Structure of the Ear

Three Functional Divisions of the Ear:

  • External Ear:

    • Pinna: Cartilaginous sound-capturing funnel (auricle or tragus).

    • External Acoustic Meatus (auditory canal): 2.5 cm long, terminates at the eardrum.

  • Middle Ear (air-filled chamber):

    • Components:

      • Tympanic Membrane: Conical membrane (9 mm diameter).

      • Ossicles: Malleus, Incus, Stapes.

      • Muscles: Tensor tympani and stapedius muscles.

  • Inner Ear (labyrinth, fluid-filled):

    • Components:

      • Cochlea: Spiral tube around the bony modiolus.

      • Vestibule and Semicircular Canals.

Middle Ear Functionality

  • Tympanic Membrane: Displaces due to sound waves; transmits to ossicles.

Role of Ossicles and Eustachian Tube

  • Ossicles: Malleus, Incus, Stapes connected by tiny synovial joints; stapes’ footplate acts on oval window.

  • Eustachian Tube: Connects middle ear to nasopharynx, usually closed by valve.

  • Tensor Tympani and Stapedius Muscles: Increase rigidity of ossicles to dampen loud sounds; reflex takes 50-150 ms.

Pressure Amplification in the Middle Ear


  • Pressure Enhancement: The middle ear amplifies sound pressure by 22 times.2% energy transfer without functioning middle ear vs. 67% with.

  • Mechanisms: Smaller oval window (17x) and leverage from ossicles (footplate of stapes moves 1.3x malleus).

Cochlea Structure

  • Cochlea: Contains oval and round windows; stapes displaces oval window, causing fluid movement.

  • Fluid Absorption: Unabsorbed energy distorts the round window.

Cochlea Chambers

  • Fluid-Filled Chambers: Scala Vestibuli, Scala Media, Scala Tympani.

  • Membranes:

    • Reissner's membrane (between scala vestibuli and media);

    • basilar membrane (between scala media and tympani).

  • Ionic Layout: Scala vestibuli and tympani have perilymph; scala media (cochlear duct) contains endolymph.

Cochlea Overview

  • Layout: Visualization of the oval window, stapes, and fluid chambers.

Basilar Membrane Functionality

  • Basilar Membrane: Supports Organ of Corti; narrow at the base (high frequencies) and wide at the apex (low frequencies).

Frequency Specificity

  • Basilar Membrane Attributes:

    • Narrow, stiff base responds to high frequencies;

    • wide, floppy apex responds to low frequencies.

Frequency Amplitudes

  • High Frequencies: Exhibiting variations in loudness based on the area of the basilar membrane.

Organ of Corti Structure


  • Organ of Corti: Sensory organ for hearing; consists of hair cells aligning with the basilar membrane; covered by the tectorial membrane.Each hair cell has ~100 stereocilia, which are crucial for producing receptor potential.

Hair Cell Arrangement

  • Inner Hair Cells: ~3500 arranged between pillar cells and modiolus; stereocilia interact with endolymph.

  • Outer Hair Cells: ~12000 arranged in rows; also interact with endolymph, involved in sound amplification.

Hair Cell Transduction Process

  • Transduction: Mechanical energy conversion to receptor potential; affected by movement of the basilar membrane.

  • Mechanism:

    • Upward movement causes K channels to open leading to depolarization and neurotransmitter release;

    • downward movement causes hyperpolarization.

Spiral Ganglion Cells

  • Spiral Ganglion Cells: ~30,000 cells; most receive signals from inner hair cells; outer hair cells involved in sound amplification.

Cochlear Nerve Pathway

  • Cochlear Nerve: Axons from spiral ganglion cells, following tonotopic organization, synapse in cochlear nuclei of the medulla.

Cochlear Nuclei Structure

  • Nuclei Types: Two ventral (stellate and bushy cells) and one dorsal (fusiform and tuberculoventral cells) cochlear nuclei, involved in sound encoding and localization.

Hearing Pathway Overview


  • Pathway Steps:Primary auditory cortex, medial geniculate nucleus, inferior colliculus, pons, and other associated structures.

Superior Olivary Nucleus Functions

  • Superior Olivary Nuclei: Processes information for sound localization; medial involved in time delays, lateral in intensity differences between ears.

Inferior Colliculus Role

  • Inferior Colliculus: Integrates auditory and somatosensory information; has a complete tonotopic map.

Medial Geniculate Nucleus

  • Functionality: Receives input from the inferior colliculus, holds somatotopic map; fibers form auditory radiation to primary auditory cortex.

Conductive vs. Sensorineural Hearing Loss

  • Conductive Hearing Loss: Resulting from issues in the middle ear (e.g., otitis media, otosclerosis).

  • Sensorineural Hearing Loss: Caused by loss of cochlear hair cells.

Hearing Tests

  • Weber’s Test: Tests lateralization of sound; conductive loss sound lateralizes to affected ear, sensorineural loss away from affected ear.

  • Rinne’s Test: Conductive loss detection by comparing bone and air conduction of sound.