SLH 313L - HEARING SCIENCE Notes

SLH 313L - HEARING SCIENCE: Auditory Anatomy and Physiology

AUDITORY SYSTEM

  • The auditory system is comprised of two main components:

    • Peripheral auditory system

      1. Outer ear

      2. Middle ear

      3. Inner ear

    • Central auditory system

      • Auditory brainstem

        • Cochlear nucleus

        • Superior olivary complex

        • Nucleus of lateral lemniscus

        • Inferior colliculus

      • Auditory forebrain

        • Medial geniculate body

        • Auditory cortex

ANATOMY OF OUTER EAR

  • Composed of two main components:

    • Pinna: Also known as the auricle; the visible part of the ear that collects sound waves.

    • External Auditory Canal (External auditory meatus): The passage that leads from the outer ear to the tympanic membrane (eardrum).

  • Additional anatomical structures include:

    • Concha

    • Associated muscles (Tensor tympani, Stapedius)

PHYSIOLOGY OF OUTER EAR

Physiological Functions

  1. Transfer Functions of Outer Ear

    • Sound Pressure Gain:

      • Peaked around 2.5 kHz, primarily from contributions of the concha and the external ear canal.

      • amount of amplification an audio device, such as the pinna (funnel), applies to the sound pressure entering it

  2. Sound Localization

    • Cues for localizing sound include:

      • Intensity Difference: Variations in sound signal strength perceived by each ear.

      • Phase (Time) Difference: Variations in the time delay of sound reaching each ear.

    • The pinna plays a significant role in aiding sound localization in the midplane.

ANATOMY OF MIDDLE EAR

Structure Overview

  • Components include:

    • Tympanic Membrane: Vibrates in response to sound waves.

    • Auditory Ossicles:

    • Malleus

    • Incus

    • Stapes

    • Eustachian Tube: Equalizes air pressure in the middle ear.

  • Additional reference to tympanic muscles associated with sound transmission.

PHYSIOLOGY OF MIDDLE EAR

Sound Transduction

  1. Transduction of Sound

    • Converts acoustic signals (air vibrations) to mechanical movement, beginning with tympanic membrane vibration and articulating to ossicular movement.

    • Fluid Motion in the cochlea is achieved through stapes motion at the oval window.

  2. Amplification of Sound

    • Sound intensity amplifies across frequencies, peaking at 1000 Hz, through the middle ear's transfer functions.

  3. Impedance Mismatch Problem

    • Problem arises when transitioning from air (tympanic membrane) to fluid (cochlea).

    • Most acoustic energy reflects due to impedance disparity.

    • Resolutions for this include:

      • Increasing pressure/force at the oval window through structures designed to maximize force transmission.

      • Utilizing impedance matching techniques:

      • Pressure, force relationships: P1 = \frac{F}{A1}, P2 = \frac{F}{A2}

      • Torque relationship: \text{Torque} = F*L

  4. Acoustic Reflexes

    • Protects inner ear by contracting middle ear muscles (tensor tympani, stapedius) in response to loud sounds; reduces transmission of intense sound, maintaining ear safety.

ANATOMY OF INNER EAR

  • Composed of two main parts:

    • Vestibular Apparatus: Involved in balance.

    • Cochlea: Primary organ of hearing; involved in sound transduction.

ANATOMY OF COCHLEA

General Structure

  1. Location and Shape

    • The cochlea is embedded in the temporal bone, medial to the middle ear cavity, featuring a coiled structure with about 2.5 turns surrounding the modiolus.

  2. Dimensions

    • Approximately 35 mm in length, 1 cm wide at the base, tapering to 5 mm at the apex.

  3. Internal Details

    • Contains auditory nerve fibers and spiral ganglion whose cell bodies are housed in the osseous spiral lamina.

Scalae Present in Cochlea

  • Three Scalae:

    • Scala Vestibuli: Upper chamber, filled with perilymph.

    • Scala Media (Cochlear Duct): Middle chamber containing endolymph, where the Organ of Corti is located.

    • Scala Tympani: Lower chamber, also filled with perilymph.

  • Membranes:

    • Reissner’s Membrane: Separates scala vestibuli from scala media.

    • Basilar Membrane: Separates scala media from scala tympani.

    • Helicotrema: Apex opening connecting scala vestibuli and scala tympani.

Functions of Cochlea

  1. Sound Transmission

    • Fluid movement caused by stapes' vibration displaces the basilar membrane leading to auditory signal production.

  2. Organ of Corti Structure**

    • Contains hair cells (one row inner, 3-5 rows outer), approximately 15,000 in total per human ear.

    • Houses supporting cells metabolically assisting inner and outer hair cells.

    • Importance of stereocilia on hair cells for signal transduction.

PHYSIOLOGY OF COCHLEA

Sound Processing Mechanism

  • Traveling Wave

    • Basilar membrane displacement activates hair cells, initiating auditory signal.

  • Physiological Response

    • Characterized by:

      • Frequency selectivity: Ability to distinguish between different frequencies.

      • Nonlinearity in responses: Varies dependent on frequency.

Displacement Patterns

  • BM Amplitude Response:

    • Illustrated by tuning curves, reflecting varying responses at specific frequencies.

AUDITORY NERVE - NEUROANATOMY

  • Establishes synaptic connections between hair cells and cochlear nucleus.

  1. Fiber Count:

    • Approximately 50,000 fibers in felids and 30,000 in humans per ear.

  2. Types of Spiral Ganglion Cells:

    • Type I: Major cell type (90-95%); connects to inner hair cells (many-to-one connection).

    • Type II: Minor cell type (5-10%); connects to outer hair cells (one-to-many connection).

AUDITORY NERVE - PHYSIOLOGY

Neural Responses

  • Firing Patterns:

    • Spontaneous Firing Rates: Correlation between firing rate and sound threshold.

    • Low, medium, and high rates correspond to various threshold levels.

    • Higher spontaneous firing rate enables detection of lower threshold sounds.

Frequency Selectivity

  • Tuning Curves:

    • Exhibit frequency-threshold relationships, showcasing neurons' capacity for precise frequency discrimination and tonotopic organization.

Intensity Resolution

  • Dynamic Range:

    • Typically ranges 20-50 dB for most auditory neurons representing their operational thresholds.

Phase Locking

  • Neural Firing Patterns:

    • Temporally correspond to the phase of sound waves, particularly evident at lower frequencies (up to approximately 4-5 kHz).

NEUROANATOMY OF CENTRAL AUDITORY SYSTEM (CAS)

Overview of CAS

  • Comprised of the auditory brainstem and auditory forebrain:

  1. Auditory Brainstem Components:

    • Cochlear nuclei (AVCN, PVCN, DCN): These nuclei are found at the junction of the pons and medulla. They are the first relay station in the brainstem, receiving direct input from the ipsilateral cochlear nerve, and are involved in complex sound processing.

    • Superior olivary complex (SOC): Located in the pons, the SOC is crucial for integrating bilateral auditory information, which is essential for sound localization. It includes several subdivisions like the medial superior olive (MSO) and lateral superior olive (LSO).

    • Nuclei of lateral lemniscus (NLL): Situated in the pons and midbrain, these nuclei primarily transmit information from the cochlear nuclei and SOC to the inferior colliculus, and are involved in analyzing sound direction and complexity.

    • Inferior colliculus (IC): A prominent structure in the midbrain, the IC acts as a major integrative center for auditory information, playing a role in sound localization, frequency analysis, and initiating startle responses.

  2. Auditory Forebrain Components:

    • Medial geniculate body (MGB): Part of the auditory thalamus, the MGB is located in the dorsal thalamus. It serves as the final subcortical relay, processing detailed auditory input before relaying information to the auditory cortex.

    • Auditory cortex (AC): Residing in the temporal lobe of the cerebrum, the auditory cortex is organized into primary (AI) and secondary (AII) regions. This is where advanced auditory processing, perception, and interpretation of sound occur.

Pathway Overview

  • Detailed examination of auditory pathways including the cochlear nucleus (CN), SOC, NLL, IC, MGB, and AC revealing main functions for each auditory nucleus.

PHYSIOLOGY OF AUDITORY CORTEX

Functionality

  • Single neuron responses exhibit complexity, distinguishing features such as:

    • Frequency-modulation detectors: React to varying sound frequencies over time.

    • Temporal-modulation detectors: Analyze timing aspects of sound.

    • Auditory attention neurons: Focus on novelty in auditory input, adapting rapidly to constant stimulus inputs, highlighting functionality in binaural hearing.

SUMMARY

  • Overview of the Peripheral Auditory System highlighting the importance of outer, middle, and inner ear in sound transduction, amplification, and localization.

  • Distinction between Types of Auditory Nerves focusing on physiological responses like spontaneous activity, intensity resolution, frequency selectivity, and phase locking.

  • Overview of the Central Auditory System, programming auditory pathways, and functional roles of auditory nuclei, and the auditory cortex for full auditory processing and perception.