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Physiology of Hearing: High-Yield Revision for ENT Exams and Clinical Practice Notes Lecture PPT

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Mechanical Conduction of Sound

  1. Sound Propagation: Sound waves, created by vibrating objects (e.g., a tuning fork), involve the compression and decompression of air molecules, travelling at 344 m/s in air, and faster in liquids and solids.
  2. External Ear Functions: The pinna captures and localises sound, directing it to the external auditory meatus, which conducts waves to the tympanic membrane, a sensitive pressure receiver that vibrates.
  3. Middle Ear Functions: Sound waves reflect at air-liquid interfaces (99.9% loss), necessitating middle ear amplification for efficient cochlear transmission.
    • Impedance Matching: The middle ear overcomes impedance mismatch (air vs. inner ear fluid) via ossicular lever action (malleus-incus, 1.3x force), hydraulic action (tympanic membrane to stapes, 14:1 area ratio), and curved membrane effect, amplifying pressure ~18x at the oval window. Mnemonic: “LHC” (Lever, Hydraulic, Curved).
    • Attenuation Reflex: Loud sounds (>70 dB) trigger tensor tympani and stapedius muscles to stiffen the ossicular chain, protecting the cochlea and masking low-frequency noise. Ineffective against sudden sounds (e.g., gunshots, explosions).
    • Phase Differential: The oval window vibrates in opposite phase to the round window, enabling perilymph movement for hearing. Ossicular and acoustic coupling ensure a 4 dB gain.
    • Natural Resonance: The external ear (3000 Hz), tympanic membrane (800–1600 Hz), and ossicles (500–2000 Hz) amplify conversational frequencies (500–3000 Hz), per the minimum audibility curve.

Transduction process in Cochlea

  1. Auditory Transduction: It is the process of converting mechanical energy into electric energy (nerve impulses) in the hair cells of the organ of Corti. Stapes movement at the oval window creates pressure waves in the perilymph (scala vestibuli), leading to compression of the endolymph (scala media). This compression deflects the basilar membrane against the stationary tectorial membrane, bending the stereocilia of hair cells to open cation channels (K+, Ca+), thereby generating receptor potentials.

Neural Pathways

  1. Auditory Pathway: Sound impulses travel via E COLI-MA mnemonic: Eighth nerve → Cochlear nucleus → Olivary complex → Lateral lemniscus → Inferior colliculus → Medial geniculate body → Auditory cortex (Brodmann’s area 41).
  2. Neural Processing: The auditory system detects 20–20,000 Hz, with peak sensitivity at 1000–3000 Hz. It localises sound using interaural time differences (1° precision). Cortical lesions impair localisation.

  Mechanism of Hearing. Dr. Rahul Bagla ENT Textbook. Physiology of Hearing  

 

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Reference Textbooks.

  • Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
  • Glasscock-Shambaugh, Textbook of Surgery of the Ear.
  • P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
  • Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
  • Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery
  • Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
  • Salah Mansour, Middle Ear Diseases – Advances in Diagnosis and Management.
  • Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
  • Rob and smith, Textbook of Operative surgery.
  • Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
  • Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.

Author:

Dr. Rahul Bagla ENT Textbook

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

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Keywords: PPT Free Download, Mechanism, Flowchart, Diagram, Notes, Auditory physiology, Cochlear function, Basilar membrane movement, Sound transduction, Hair cell depolarization, Auditory neural pathway, Endolymphatic potential, Cochlear microphonics, Tonotopic organization, Middle ear amplification, Oval window vibration, Inner ear hair cells, Otolithic membrane, Stereocilia deflection, Auditory nerve action potential, Brainstem auditory processing, Temporal coding in hearing, Physiology of Hearing