Hearing

Hearing

The Auditory System

• Overview of the auditory system

Sound and its Properties

• Sound waves are generated through periodic air pressure variations.

• Speed of Sound: Travels at 343 m/sec (767 mi/h).

• Frequency: Determines pitch, measured in Hertz (Hz).

  • Human Audible Range: 20 - 20,000 Hz.

  • Ultrasound: > 20 kHz, audible to some animals (dogs, dolphins).

  • Infrasound: < 20 Hz, audible to elephants and whales.

• Intensity: Determines loudness, measured in decibels (dB).

  • Begins at 0 dB; 120 dB is the loudest tolerable sound.

Anatomy and Structure of the Ear

• Three functional divisions:

  1. External Ear:

     • Pinna (auricle): A cartilaginous funnel that captures sound.

     • External acoustic meatus: A 2.5 cm tunnel leading to the eardrum.

  2. Middle Ear:

     • Tympanic membrane: 9mm diameter membrane.

     • Three ossicles: malleus, incus, stapes.

     • Tensor tympani and stapedius muscles.

  3. Inner Ear (Labyrinth):

     • Cochlea: A spiraling tube around the bony modiolus.

     • Vestibule.

     • Semicircular canals.

Structure and Function of the Middle Ear

• Tympanic membrane: Conical shape, displaced by sound waves, transmitting to ossicles.

• Ossicles: Malleus, incus, stapes connected by synovial joints.

  • Stapes: Pistons against the oval window,

  • Eustachian tube: Connects middle ear to the nasopharynx.

  • Muscles: Tensor tympani and stapedius increase ossicle rigidity, dampening loud sounds by approximately 10 dB.

• Function of the Middle Ear: Enhances pressure for sound transfer from air to fluid.

  • Without it, only 2% energy transfer; it amplifies pressure by 22 times via size differences and ossicle mechanics.

Structure and Function of the Cochlea

• Cochlea Base: Two openings (oval and round windows) leading into the middle ear.

  • Fluid displacement in cochlea caused by oval window movements.

• Chambers:

  • Scala vestibuli, scala media, scala tympani (fluid-filled).

  • Membranes: Reissner’s and basilar membranes separate these chambers.

• Ion composition:

  • Scala vestibuli and tympani: perilymph (low K+, high Na+).

  • Scala media: endolymph (high K+, low Na+), regulated by stria vascularis.

The Organ of Corti

• Location: On the basilar membrane, covered by tectorial membrane.

• Structures:

  • Inner and outer hair cells, rods of Corti, supporting cells.

• Each hair cell: ~100 stereocilia bending generates a receptor potential.

• Hair cells synapse with dendrites of bipolar spiral ganglion cells located in the modiolus and whose axons form the cochlear nerve

Hair Cell Transduction

• Mechanism:

  • Movement of basilar membrane affects hair cells.

  • Upward movement bends stereocilia, opening K+ channels, causing depolarization.

  • Release of neurotransmitters (e.g., glutamate) to spiral ganglion cells.

  • Downward movement results in hyperpolarization.

  • When the basilar membrane is affected by the scalas’s fluid

    movement, all the structures in the Organ of Corti move

    either up or down, causing the stereocilia to bend sideways

    one way or the other.

  • The outer hair cells have been implicated with sound

    amplification (by augmenting the movement of inner hair

    cells) in the inner ear since ototoxic antibiotics selectively

    destroy them.

Cochlear Nerve

• Axons from spiral ganglion form cochlear nerve, organized tonotopically by frequency at the basilar membrane.

• Pathway: Travels to medulla, synapses with cochlear nuclei.

Cochlear Nuclei

• Types:

  • Two ventral, one dorsal cochlear nuclei.

• Function:

  • Ventral nuclei: sound frequency processing, localization via onset response. (stellate cells). aid in localization of sound along the horizontal axis

  • Dorsal nuclei: responsive to broad frequencies aiding sound localization. (fusiform cells). aid in sound localization along vertical axis.

    • tuberculoventral

      cells that respond with a delay and inhibit

      echo interference.

Superior Olivary Nucleus

• Function: Bilateral synapses aiding sound localization.

  • Medial superior olivary: processes interaural time delays (important for low frequencies).

  • Lateral superior olivary: processes intensity differences (key for high frequencies).

  • Fibers from the superior olivary nucleus form the lateral

    lemniscus. Some of these fibers are going to synapse with

    the nucleus of the lateral lemniscus but most of them are

    going to synapse with the inferior colliculus

Inferior Colliculus

• Processes both auditory and somatosensory information,

• Contains tonotopically organized central nucleus.

Medial Geniculate Nucleus

• Receives input from inferior colliculus, sends fibers from the auditory radiation to primary auditory cortex (Heschl's gyrus).

  • Involved in sound localization and frequency response.

Conductive vs. Sensorineural Hearing Loss

• Conductive Hearing Loss: Issues with middle ear function (e.g., otitis media, otosclerosis).

• Sensorineural Hearing Loss: Loss of cochlear hair cells.

Hearing Tests

• Weber’s Test: Lateralizes sound toward conductive deafness ear; away from sensorineural deafness ear.

• Rinne’s Test: Placement of tuning fork on mastoid; if not audible near ear, conductive loss suspected.