Auditory System: From Vibration to Electrical Signals

Cochlear Function: From Vibration to Neural Signals

Sound Frequency Analysis and Perception

  • Free Analysis Program: Utilizes laptop microphone data to perform frequency analysis, displaying components from low to high frequency as a function of time, with intensity encoded by brightness.

  • Demonstration of Pure Tones:

    • 400 \text{ Hz} (low frequency)

    • 4 \text{ kHz} (mid frequency)

    • 8 \text{ kHz} (higher frequency)

    • 16 \text{ kHz} (very high frequency, audible to some)

  • Complex Speech Frequencies: Pure tones are simple, but spoken language, like the word "cochlea," involves multiple frequencies and harmonics, which are typically not pure tones.

Conversion of Vibrations to Electrical Signals

  • Critical Interface: The cochlea's primary role is to convert mechanical vibrations into electrical signals, serving as the essential interface between movement and neuronal activity for audition.

Cochlear Anatomy and Membranes

  • Basilar Membrane: A key structure within the cochlea, running along its length in a coil. It is visualized as an unravelled membrane for understanding its function.

  • Organ of Corti: A specialized structure located on the basilar membrane containing the cells responsible for mechanotransduction.

    • Scanning Electron Microscopy: Provides detailed views of the Organ of Corti, revealing its cellular complexity.

  • Reissner's Membrane: Another membrane within the cochlea, situated above the basilar membrane and the Organ of Corti.

  • Tectorial Membrane: Sits atop the Organ of Corti's hair cells.

  • Epithelium of the Organ of Corti:

    • Complexity: Described as the most complicated epithelium in the body, highly specialized with unusual fluid spaces within it, unlike typical tightly packed epithelia.

    • Barrier Function: Like skin or gut lining, epithelia act as barriers. This barrier is crucial for maintaining distinct ionic environments.

  • Ionic Environments: The fluid space above the Organ of Corti (endolymph) has a high potassium (K+) environment, distinct from the fluid below (perilymph) and within the cells, which is critical for transduction.

Hair Cells: The Sensory Receptors

  • Types: The Organ of Corti contains specialized sensory cells called hair cells, with characteristic projections called stereocilia.

  • Inner Hair Cells (IHCs):

    • One row located on the inner side of the cochlear coil.

    • Number approximately 3,500 per cochlea.

    • Primarily responsible for transmitting auditory information to the brain.

    • Can only observe surface hairs in some views; cell body with nucleus is underneath the epithelium.

  • Outer Hair Cells (OHCs):

    • Three rows located on the outer side of the cochlear coil, approximately 14,000 per cochlea, a 3:1 ratio compared to IHCs.

    • Have a V or W-shaped arrangement of stereocilia bundles.

    • Possess motility (ability to contract and lengthen) crucial for cochlear amplification.

  • Supporting Cells: Other cells like Henson cells (function unknown for some parts) and pillar cells exist. Pillar cells act as a fulcrum, allowing differential movement to protect IHCs.

  • Stereocilia: Finger-like projections on the surface of hair cells. They look like cilia but are not true cilia.

    • Approximately 100 stereocilia per hair cell.

    • Varying lengths, arranged in multiple rows.

Mechanotransduction: Converting Movement to Electrical Signals

  • Mechanism: Sound-induced movement of the basilar membrane causes a differential movement between the basilar membrane and the tectorial membrane. This shearing force bends the stereocilia.

  • Tip Links: Fine filament structures connecting the tip of a shorter stereocilium to the side of a taller adjacent stereocilium.

    • Composed of two proteins: cadherin 23 and cadherin 50.

    • Familial deafness is associated with mutations in genes encoding these proteins, highlighting their critical role.

  • Ion Channels: Located near the tips of stereocilia, connected to the tip links.

    • Believed to be TMC1 and TMC2 proteins.

    • When stereocilia are bent, tip links stretch, directly pulling open these ion channels.

  • Ion Influx and Depolarization: When channels open, positively charged potassium ions (K+) from the high-potassium endolymph flow into the hair cell.

    • This influx of positive charge causes depolarization (making the cell's membrane potential more positive), converting mechanical energy into an electrical signal.

    • Resting potential inside the hair cell is typically around -45 \text{ mV} to -70 \text{ mV}.

  • Excitation and Inhibition: Bending stereocilia towards the tallest causes excitation (depolarization), while bending them in the opposite direction causes inhibition (hyperpolarization by closing standing currents).

    • The basilar membrane's sinusoidal movement thus creates cycles of excitation-inhibition.

  • Synaptic Transmission: Hair cells are specialized sensory receptor cells, not neurons. They synapse with neurons.

    • Depolarization of the hair cell triggers calcium (Ca2+) influx, leading to the release of neurotransmitters.

    • Neurotransmitters bind to receptors on afferent nerve fibers, generating action potentials that travel to the brain.

Auditory Nerve Fibers and Tonotopy

  • Number of Fibers: Each cochlea contains approximately 30,000 auditory nerve fibers.

  • Types of Afferent Fibers:

    • Type I Afferent Fibers: Constitute 95\% of auditory nerve fibers.

      • Connect exclusively to inner hair cells.

      • Typically, one Type I fiber connects to one IHC, but one IHC may have connections from up to 20 different Type I fibers, providing dense innervation.

    • Type II Afferent Fibers: Constitute 5\% of auditory nerve fibers.

      • Connect to outer hair cells.

      • One Type II fiber can branch and innervate up to 100 OHCs, indicating a broader, less specific sampling of OHC activity.

  • Modiolus: The central core of the cochlea where all nerve fibers converge and exit to form the auditory nerve, leading to the brainstem.

  • Spiral Ganglion: Contains the cell bodies of the bipolar auditory neurons (Type I and Type II afferents). These neurons have a peripheral dendrite extending to the hair cell and an axon projecting to the brainstem.

  • Tonotopic Map: The basilar membrane functions as a spectral analyzer, mapping distinct frequencies to specific locations.

    • Basal End: Near the oval window, is stiff and thin, and responds optimally to high frequencies.

    • Apical End: The far end, is floppy and wide, and responds optimally to low frequencies.

    • This place-coding of frequency is maintained throughout the auditory pathway, from the cochlea to the auditory cortex.

  • Nerve Fiber Tuning Curves (Characteristic Frequency):

    • Experimental Measurement: By recording from a single auditory nerve fiber and varying the frequency and amplitude of sound, its tuning curve can be determined.

    • Response Pattern: At high sound levels, a nerve fiber responds to a wider range of frequencies (broader tuning) because a larger portion of the basilar membrane moves enough to activate the hair cell.

    • As sound level decreases, the range of frequencies to which the fiber responds narrows, becoming highly selective.

    • Characteristic Frequency (CF): The frequency at which a nerve fiber is most sensitive (requires the lowest sound level for activation). This CF is constant for a given fiber and reflects its specific location on the basilar membrane (e.g., 4.49 \text{ kHz} example).

    • Audiogram Connection: The envelope of the tuning curves across many nerve fibers in an animal like a guinea pig mirrors its audiogram (a U-shaped curve showing hearing sensitivity as a function of frequency). This confirms that audiograms indirectly measure these underlying nerve responses.

  • Phase Locking (Temporal Coding):

    • In addition to place coding, auditory nerve fibers can phase-lock their action potentials to the phase of the sound wave, particularly at low frequencies (up to 1-2 \text{ kHz}).

    • This temporal coding provides complementary frequency information to the tonotopic place map.

  • Auditory Pathway Refinement: The tonotopic map extends from the cochlea all the way to the auditory cortex. Signals undergo refinement as they ascend, and efferent signals (descending from the brain) also modulate cochlear function.

Cochlear Amplification by Outer Hair Cells

  • Von Békésy's Traveling Wave: George von Békésy received a Nobel Prize for demonstrating the traveling wave on the basilar membrane in cadavers, which showed a broad tuning envelope.

  • Need for Amplification: To explain the sensitivity and selectivity of live hearing, an amplification mechanism is required to sharpen this broad tuning.

  • Role of Outer Hair Cells (OHCs):

    • OHC motility (ability to change length) provides this micromechanical amplification.

    • When depolarized, OHCs contract.

    • When hyperpolarized, OHCs lengthen.

    • Mechanism: OHCs effectively boost the movement of the basilar membrane by contracting and lengthening in phase with the sound-induced vibration,