Primary Auditory Cortex

• Function: Responsible for processing auditory information.

• Related Condition: Pure word deafness

  • Cause: Damage to Wernicke's area or interruption of auditory input.

Hearing: Pressure Waves in the Air Perceived as Sound

Today's Agenda

• How the ear captures and concentrates sound energy to activate sensory receptors.

• Anatomy of the middle and inner ears.

• Mechanism of how cells in the cochlea convert sound into neural activity.

• Function of the organ of Corti in encoding the frequencies of sounds.

• Neural projections between the cochlea and brain.

• Overview of auditory pathways and brain structures involved in hearing.

• Integration of signals from the left and right ears.

• Mapping of sound frequencies in the auditory system.

Measures of Sound Perception

Human Auditory System

• Detects sounds by two key metrics:

  • Amplitude (Intensity):

  • Measured in decibels (dB).

  • Perceived as loudness.

  • Frequency:

  • Measured in hertz (Hz) - number of cycles per second.

  • Perceived as pitch.

Characteristics of Sound

Pure Tones and Complex Sounds

• Pure Tone: A tone with a single frequency of vibration.

• Complex Sound: Example of sound from a musical instrument.

  • Components:

  • Fundamental: Basic frequency.

  • Harmonics: Multiples of the fundamental frequency.

  • Timbre: The characteristic sound quality of an instrument determined by the intensities of its harmonics.

Function of the External Ear

• Role: Captures, focuses, and filters sound.

• Description of sound:

  • A mechanical force transduced into neural activity.

• Components of the External Ear:

  • Pinna: Collects sound waves.

  • Ear Canal (Auditory Canal): Modifies sound frequency characteristics reaching the middle ear.

Anatomy of the Middle Ear

Middle Ear Function

• Primary Role: Concentrates sound energies.

• Components:

  • Ossicles:

  • Malleus, Incus, Stapes (connect the tympanic membrane (eardrum) to the oval window).

• Muscles of the Middle Ear:

  • Tensor Tympani: Controls volume by stiffening.

  • Stapedius: Reduces sound effects when activated.

Function of the Cochlea

Cochlea's Role in Sound Conversion

• Structure: Spiral-shaped, converts vibrations into neural activity.

• Components:

  • Three parallel canals:

  • Scala Vestibuli (Vestibular Canal)

  • Scala Media (Middle Canal)

  • Scala Tympani (Tympanic Canal)

• Round Window: Separates the tympanic canal from the middle ear, can bulge outward.

The Organ of Corti

Structure and Function

• Function: Converts sound into neural activity.

• Main Structures:

  • Sensory Cells (Hair Cells)

  • Supporting Cells

  • Auditory Nerve Fibers

• Basilar Membrane: Base of the organ of Corti, responsive to sound vibrations.

Deformation of the Basilar Membrane

Frequency Encoding

• Mechanism: Sound vibrations cause the basilar membrane to ripple.

• Frequency Response:

  • High Frequency: Greatest effect at the base (narrow and stiff).

  • Low Frequency: Larger response near the apex (wider and more flexible).

• Illustrative Frequencies: (Visual Representation Required)

  • 200 Hz to 20,000 Hz, spanning both apex and base responses.

Hair Cell Mechanism

Transduction Process

• Structure: Protruding stereocilia (tiny hairs) on hair cells.

• Function of Stereocilia:

  • Connected by tip links which open ion channels upon bending.

  • Result: Depolarization of hair cell and release of neurotransmitters.

Organization of Hair Cells

• Two Groups of Hair Cells:

  • Inner Hair Cells (IHCs): Single row near the central axis.

  • Outer Hair Cells (OHCs): Three rows.

• Vestibulocochlear Nerve (Cranial Nerve VIII): Contacts bases of hair cells.

Neural Connections with Hair Cells

Types of Connections and Neurotransmitters

• Four Neural Connections: Each utilizing different neurotransmitters:

  • IHC Afferents: Release glutamate.

  • IHC Efferents: Release acetylcholine (ACh).

  • OHC Afferents: Release acetylcholine (ACh).

  • OHC Efferents: Release gamma-aminobutyric acid (GABA).

Auditory Signal Pathway

Transmission from Cochlea to Cortex

• Process Flow:

  • Auditory nerve fibers from IHCs terminate in cochlear nuclei.

  • Cochlear Nuclei: Send information to superior olivary nuclei.

  • Superior Olivary Nuclei: Relay information to inferior colliculi (primary auditory centers of midbrain).

  • Outputs of Inferior Colliculi: Go to medial geniculate nuclei of the thalamus.

  • Pathways extend to primary auditory cortex.

Tonotopic Organization of Auditory Pathways

• Definition: Auditory pathways exhibit tonotopic organization, mapping low to high frequency.

• Function: Higher-level auditory neurons excited by certain frequencies while inhibited by neighboring ones, allowing precise frequency discrimination.

Perception of Pitch

Frequency vs Pitch

• Frequency: A physical property of sound.

• Pitch: Subjective perception of sound.

• Methods of Encoding Pitch:

  • Place Coding: Pitch determined by location of activated hair cells.

  • Temporal Coding: Encodes frequency in firing rate of auditory neurons.

Specialized Sensitivity in Animals

Ultrasound and Infrasound

• Definition: Sensitivity to extreme frequencies varies by species.

  • Ultrasound: Frequencies above 20,000 Hz.

  • Infrasound: Frequencies below 20 Hz.

Sound Localization Mechanisms

Binaural Cues for Sound Direction

• Mechanisms include:

  • Intensity Differences: Volume discrepancies caused by differing ear positions or head shadow effects.

  • Latency Differences: Variances in arrival time at each ear.

  • Onset Disparity: Difference in sound onset time.

  • Ongoing Phase Disparity: Difference in phase of continuous sound.

Hearing Loss Types and Issues

Widespread Hearing Loss Problems

• Conduction Deafness: Disorders of outer/middle ear prevent sound from reaching cochlea.

• Sensorineural Deafness: Hair cells fail to respond to basilar membrane movement, resulting in no action potentials. Causes include:

  • Genetic mutations.

  • Infections.

  • Ototoxic drug effects.

  • Exposure to loud sounds.

• Consequences: Damage to hair cells could lead to tinnitus (persistent ringing in ears).

Central Deafness

• Definition: Damage to auditory brain areas from strokes, tumors, or traumatic injuries.

• Types:

  • Word Deafness: Selective difficulty in recognizing normal speech sounds.

  • Cortical Deafness: Difficulty recognizing all complex sounds, verbal or nonverbal; rare condition.

Impact of Loud Noise

Destructive Effects on Cochlea

• Illustration of cochlea under normal conditions vs severe noise damage.

Administrative Note

Mini-quiz #6

• Availability: Today at 1:20 PM.

• Due Date: 10/09 (Thursday) at 11:59 AM.