Sound as a stimulus and general structure of the analyzer
Hearing is a distant sensory system that provides sound perception, communication, spatial orientation, and danger warning. The adequate stimulus is mechanical vibrations of particles in an elastic medium (air).
Sound is characterized by two physical parameters:
- Intensity (amplitude) is subjectively perceived as loudness, measured in decibels. The dynamic range of the human ear is about 130 dB.
- Frequency is subjectively perceived as pitch, measured in hertz. The perception range is from 16–20 Hz to 16–20 kHz.
According to I. P. Pavlov, the analyzer consists of three parts:
- Peripheral — outer, middle, and inner ear.
- Pathways — cochlear nerve (VIII cranial nerve), cochlear and vestibular nuclei, superior olives, inferior colliculi, medial geniculate body.
- Cortical — primary and secondary auditory areas, Heschl's gyri in the temporal lobes.
Outer and middle ear: sound conduction
The outer ear captures and pre-processes sound. The auricle directs waves into the external auditory meatus and provides sound source localization in the vertical plane. The auditory canal acts as a resonator, amplifying frequencies of the speech range of 2–5 kHz.
The middle ear overcomes the acoustic barrier: up to 99% of energy is lost during the transition from air to the fluid of the cochlea. Compensation is achieved by two mechanisms:
- The lever system of the auditory ossicles (malleus, incus, stapes) increases wave pressure.
- The area difference between the tympanic membrane and the oval window concentrates energy onto a small area.
Together, these mechanisms amplify sound pressure by approximately 20 times. The acoustic reflex — the reflex contraction of the stapedius muscle in response to sounds above 80 dB — protects the structures of the inner ear from trauma.
Inner ear and mechano-electrical transduction
The cochlea is a fluid-filled bony labyrinth where sound energy is converted into a nerve impulse. Stapes vibrations in the oval window generate a traveling wave on the basilar membrane, reaching a maximum depending on frequency: high frequencies at the narrow, stiff base, and low frequencies at the wide, flexible apex. This is the basis of the tonotopic organization principle (Helmholtz-Békésy place theory).
The organ of Corti, covered by the tectorial membrane, is located on the basilar membrane. The receptors are hair cells:
- Inner (~3500, in a single row) are the main sensory receptors; each contacts ~10 afferent fibers.
- Outer (~12,000, in three rows) amplify membrane vibrations at soft sounds and increase frequency selectivity.
Stages of reception: basilar membrane vibration → deflection of stereocilia relative to the tectorial membrane → opening of mechano-sensitive K⁺/Ca²⁺ channels → cell depolarization → opening of voltage-gated Ca²⁺ channels → glutamate release → generation of an action potential in bipolar neurons of the spiral ganglion.
Pathways of the auditory analyzer
The pathway includes six neurons:
- Spiral (cochlear) ganglion — bipolar neurons forming the cochlear part of the VIII cranial nerve.
- Cochlear nuclei (ventral and dorsal) of the medulla oblongata — primary processing, onset of binaural analysis.
- Superior olivary complex of the pons — key center of binaural hearing, detection of interaural time and intensity differences.
- Inferior colliculi of the midbrain tectum — orienting reflex to sound, descending control of sensitivity.
- Medial geniculate body of the thalamus — relay station, filtering of significant signals.
- Auditory cortex of the temporal lobe — Heschl's gyri (Brodmann areas 41, 42), secondary areas (area 22), including Wernicke's area for speech comprehension.
The primary cortex has a tonotopic organization and analyzes frequency, intensity, and duration; the secondary cortex analyzes sound sequences, timbre, and speech.
Physiological mechanisms of hearing
- Hearing acuity — the ability to perceive sounds of minimal intensity. It is tested by audiometry with pure-tone audiogram plotting, or roughly by whispered speech.
- Dynamic range — about 130 dB, maximal in the 1–4 kHz region. Provided by active amplification by outer hair cells and protective acoustic reflexes.
- Differential sensitivity — frequency discrimination up to 1–2 Hz at 1000 Hz, intensity discrimination — 0.5–1 dB. Provided by the sharp tuning of the basilar membrane and the precision of neurons at all levels.
- Binaural hearing is implemented in the superior olives via interaural time delay (low frequencies) and interaural intensity difference (high frequencies). It provides source localization and binaural release from masking — speech extraction against a noise background.
- Air and bone conduction. Air conduction is the main pathway (outer → middle ear → oval window → cochlear fluid). Bone conduction is through temporal bone vibration, 30–50 dB less efficient. It has diagnostic value in the Rinne and Weber tests: in conductive hearing loss, sound in the Weber test lateralizes to the affected ear, and in the Rinne test, bone conduction prevails over air conduction.
Key points
- The auditory analyzer includes peripheral, pathway, and cortical parts.
- The middle ear compensates for up to 99% of energy loss during sound transition from air to fluid, amplifying pressure by ~20 times.
- Frequency coding is based on tonotopy: high frequencies excite the base of the cochlea, low frequencies excite the apex.
- Inner hair cells are the main receptors; outer cells perform amplifying and tuning functions.
- The neurotransmitter of the hair cell synapse is glutamate; the first neuron of the pathway is located in the spiral ganglion.
- Binaural hearing is provided by the superior olives through the analysis of interaural time and intensity differences.
- Cortical centers are located in Heschl's gyri (areas 41, 42); Wernicke's area is responsible for speech comprehension.
- Comparison of air and bone conduction (Rinne and Weber tests) differentiates conductive and sensorineural hearing loss.