Basic Anatomy and Physiology of Hearing

Functional Roles of the Outer Ear

• Protection: The outer ear serves to protect the more sensitive structures of the middle and inner ear from physical damage and foreign objects.
• Conduction: It acts as a funnel, conducting sound waves from the external environment toward the middle ear.
• Sound Modification and Localization: The pinna plays a critical role in sound localization by modifying incoming sound waves.
• Amplification via Resonance: The outer ear provides significant gain through resonance.
  • It offers a gain of approximately $15$ to $20\,dB$ within the frequency range of $1500$ to $7000\,Hz$.
  • This frequency range is particularly important as it corresponds to the speech range of human communication.

Anatomy and Function of the Middle Ear

• Middle Ear Structures:
  • Tympanic Membrane (Eardrum): Receives sound vibrations from the outer ear.
  • Ossicular Chain: Composed of the three smallest bones in the human body: the Malleus, Incus, and Stapes. These bones transmit acoustic energy from the tympanic membrane to the inner ear.
  • Eustachian Tube: A canal connecting the middle ear to the nasopharynx, responsible for equalizing pressure.
• Impedance Matching:
  • The primary function of the middle ear is the transmission of sound waves from air (a low-impedance medium) to the fluid-filled cochlea (a high-impedance medium).
  • The ossicular chain acts as a high-impedance connection to ensure efficient energy transfer without excessive loss of signal.

Anatomy of the Inner Ear and Fluid Compartments

• Bony Labyrinth: A series of canals and cavities within the petrous portion of the temporal bone. It contains both the cochlea (responsible for hearing) and the vestibular system organs (responsible for balance, including the semicircular canals, utricle, and saccule).
• Membranous Labyrinth: Suspended within the bony labyrinth and filled with specialized fluids.
  • Perilymph: Fluid found within the scala vestibuli and scala tympani.
  • Endolymph: Fluid found within the scala media (cochlear duct). It is characterized by a high concentration of $K^+$ ions.
• Cochlear Compartments: The membranous labyrinth creates three distinct fluid-filled chambers:
  1. Scala Vestibuli: Connects to the oval window.
  2. Scala Media (Cochlear Duct): Houses the Organ of Corti.
  3. Scala Tympani: Terminating at the round window.
  • The Helicotrema is the opening at the apex of the cochlea where the scala vestibuli and scala tympani meet.

Cellular Structure and Electrophysiology of the Cochlea

• The Organ of Corti: A group of specialized cells situated on top of the basilar membrane. It contains the sensory receptors for hearing.
• Hair Cells (Mechanotransducers): These cells utilize stereocilia (stiff bundles of actin filaments) to convert mechanical motion into electrical signals.
  • Inner Hair Cells (IHCs): Organized in a single row (approximately $3,500$ cells). They receive $95\%$ of afferent innervation, making them the primary sensory transducers.
  • Outer Hair Cells (OHCs): Organized in three rows (approximately $12,000$ cells). They function primarily as the "cochlear amplifier."
  • Tunnel of Corti: A space separating the IHCs and OHCs, formed by the inner and outer pillar cells.
• The Transduction Process:
  • Bending of the stereocilia opens mechano-sensitive channels (MET channels).
  • Because stereocilia are bathed in the endolymph of the scala media (which is high in $K^+$), potassium flows into the hair cell.
• Electrical Potentials:
  • Endocochlear Potential (EP): The scala media maintains an electrical potential of $+90\,mV$ relative to the scala tympani.
  • Intracellular Potential: Hair cells have an internal potential of $-50$ to $-80\,mV$.
  • Total Driving Force: The combined electrical potential gradient between the scala media and the hair cells is approximately $140$ to $170\,mV$.

Basilar Membrane Mechanics and Tonotopy

• Travelling Wave: Sound induces motion in the basilar membrane. Compression and rarefaction of fluid in the cochlea cause the membrane to vibrate.
• Tonotopic Organization: The physical properties of the basilar membrane vary along its length ($33\,mm$ total length), causing different frequencies to vibrate at specific locations:
  • Base of the Cochlea: Narrow and stiff; responds to High Frequencies ($10,000\,Hz$ and above).
  • Apex of the Cochlea: Wide and flexible; responds to Low Frequencies ($100\,Hz$ and below).
• Frequency Representation:
  • Place Code (Tonotopy): The position on the coiled cochlea provides a spatial map of frequency, which is transmitted to the brain via "labelled lines."
  • Temporal Code (Volley Theory): Low-frequency pitch is encoded by the temporal pattern of discharge. Action potentials become phase-locked to the sound stimulus, firing at the same phase of each cycle or different cycles to convey repetition rate.

The Cochlear Amplifier (Outer Hair Cells)

• Electromechanical Motor: OHCs possess a unique motor mechanism involving the motor protein Prestin located in the lateral cell membrane.
• Electromotility: When an OHC is depolarized, it physically contracts. This active movement amplifies the motion of the basilar membrane, specifically near the peak of the travelling wave.
• Functional Impact:
  • Single auditory afferent fibers are sharply tuned to specific frequencies. Pre-calculations of basilar membrane tuning were much broader; the active OHC mechanism explains the sharp sensitivity and tuning seen in neural responses.
  • Active vs. Passive Mechanics: OHC amplification is most critical at low sound intensities. At high intensities, passive linear mechanics dominate as the OHC amplifier becomes saturated.

Intensity and Loudness Coding

• Rate-Level Functions: Individual nerve fibers increase their firing rate as sound intensity (loudness) increases.
• Afferent Diversity: Afferents have different thresholds and are driven by different slopes of the mechanical input-output curve.
• Population Code: As sound intensity increases, a larger area of the basilar membrane is displaced, leading to the recruitment of a larger number of responsive neurons.
• Consequences of OHC Damage:
  • Reduced sensitivity (partial deafness).
  • Reduced frequency discrimination and speech intelligibility (especially in noise).
  • Recruitment: Abnormal growth of perceived loudness where the individual is over-sensitive to suprathreshold sounds despite having a higher initial hearing threshold.

Central Auditory Pathway and Brain Integration

• Ascending Pathway Hierarchy:
  1. Cochlea
  2. Cochlear Nucleus (CN): Includes Dorsal (DCN), Posteroventral (PVCN), and Anteroventral (AVCN).
  3. Superior Olivary Complex (SOC): Includes Lateral Superior Olive (LSO) and Medial Superior Olive (MSO); crucial for sound localization.
  4. Lateral Lemniscus (NLL): The tract carrying information through the brainstem.
  5. Inferior Colliculus (IC): Located in the midbrain; a major integration center.
  6. Medial Geniculate Body (MGB): The auditory relay station of the thalamus.
  7. Auditory Cortex: Located in the temporal lobe; maintains tonotopic organization (core, belt, and parabelt areas).
• Structural Characteristics:
  • The pathway includes multiple decussations (crossings), meaning sound from one ear is represented on both sides of the brain, which has significant pathological significance.
  • Parallel Pathways: Information is processed simultaneously in different streams.
• Systemic Connections:
  • The auditory system is connected to the Limbic System, involving areas for memory and emotion.
  • Sensory Gating: The brain can filter or "gate" auditory information based on relevance and state.