ENT (Otorhinolaryngology) · MU Sofia

4. Physiology of auditory analyzer.

Explains how the auditory analyser turns sound into neural signals, covering three parts: the peripheral section, the auditory pathway and central processing areas. It follows sound collection and middle-ear impedance matching, then the travelling wave and tonotopy, hair-cell transduction and the cochlear amplifier. The pathway is traced from the spiral ganglion through brainstem nuclei, the superior olivary complex, thalamus and cortex, with localisation cues and a clinical note on otoacoustic emissions.

Inside: 1 section · 1 labelled picture · 15 flashcards · 6 min

Topic 4 of 52 in the MU Sofia syllabus for ENT (Otorhinolaryngology) (Medicine, Year 4). The free AceMed note covers it in 1 section, about 893 words (6 min of reading), with 15 flashcards.

In this note

  1. Physiology of the Auditory Analyser

Physiology of the Auditory Analyser

The auditory analyser is the system that receives sound, converts it into neural impulses, conducts those impulses to the brain, and interprets them as meaningful sound. It has three major components:

  1. 1. Peripheral Section (external ear + middle ear + cochlea): Converts sound energy into a neural impulse.
  2. 2. Auditory Pathway (CN VIII and Brainstem Nuclei): Transmits and processes the signal.
  3. 3. Central auditory processing areas (brainstem nuclei → thalamus → cortex): turns the signal into conscious perception.

I. The Peripheral Section: Sound Transduction - This phase involves collection, impedance matching, and mechanical-to- electrical transduction.

A. Sound Collection (External Ear)

  • Auricle (Pinna): Collects sound waves and helps in vertical sound localization (determining elevation) by filtering high-frequency sounds differently based on their angle of entry.
  • External Auditory Canal: Acts as a quarter-wave acoustic resonator, providing a passive amplification gain of 10 - 15 dB for frequencies between 2 kHz and 5 kHz (critical for speech).

B. Impedance Matching (Middle Ear)

The primary role of the middle ear is to overcome the impedance mismatch that occurs when sound in air meets the fluid-filled cochlea. This is achieved by concentrating force onto a smaller area.

  1. 1. Hydraulic Action (Area Ratio):
  • Area ratio (TM : stapes footplate, 17:1) → +25 dB
  • This concentration of force amplifies pressure by a factor of 17, providing a gain of 25 dB.
  1. 2. Lever Action (Ossicles):
  • The lever arm of the Malleus is longer than the Incus, providing a mechanical leverage advantage of 1.3:1.
  • This contributes an added gain of 2 dB.
  • Total Gain: The total pressure gain is 27 dB, effectively matching the impedance.

C. Mechanical-to-Electrical Transduction (Inner Ear)

This is the process by which mechanical vibrations are converted into neural impulses by the hair cells.

1. The Traveling Wave and Tonotopy

  • The piston-like movement of the stapes in the oval window creates pressure waves in the perilymph of the scala vestibuli.
  • These waves generate a Traveling Wave along the Basilar Membrane (BM) of the cochlea.
  • Tonotopic Organization:
  • The BM is narrow and stiff near the Base (oval window). High Frequencies resonate maximally here.
  • The BM is wide and flexible near the Apex (helicotrema). Low Frequencies resonate maximally here.
  • The peak of the traveling wave determines the perceived pitch of the sound (Place Theory).

2. The Organ of Corti and Hair Cells: The Organ of Corti sits on the BM and contains sensory receptor cells:

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