Recording-2025-03-13T15:36:49.324Z

Introduction to Hearing

• Understanding sound perception through frequency, intensity, and phase.

• The human ear can perceive sounds between 20 Hz (low frequency) and 20,000 Hz (high frequency).

• Normal hearing tend to decline with age, particularly in higher frequencies:

  • By age 40: 20 Hz to 15,000 Hz

  • By age 50: 20 Hz to 10,000 Hz

Pitch Perception

• The brain's capability to distinguish pitches, even subtle differences, exemplified in musical training.

• Human beings can distinguish pitches as close as 1/10th of a half step.

• Online frequency range tests can help assess individual hearing levels.

Loudness Perception

• The loudness scale is measured in decibels (dB):

  • Human sensitivity spans from 0 dB to 40 dB.

  • Sounds can become painful at approximately 20 dB-40 dB.

• The upper threshold of audible sounds is around 100 dB.

• It is essential to clarify that 0 dB does not represent total silence, as ambient noises may still be present.

Anatomy of the Ear

Outer Ear

• The pinna serves to:

  • Direct sound into the ear canal.

  • Enhance sounds around 4500 Hz, assisting in sound localization.

• External auditory canal acts as a resonator for sounds between 3000 Hz and 4500 Hz, amplifying these frequencies before reaching the eardrum.

Middle Ear

• Structures involved include:

  • Eardrum (tympanic membrane)

  • Ossicles: malleus, incus, stapes

  • Eustachian tube

• Functions of the middle ear:

  • Boosts sound intensity for efficient transfer to the inner ear.

  • Equalizes air pressure to avoid middle ear infections.

  • Controls ossicular movement reflexively to protect against loud sounds.

• Key muscles:

  • Tensor tympani (innervated by cranial nerve V)

  • Stapedius muscle (innervated by cranial nerve VII)

Inner Ear

Cochlea

• The cochlea's physical structure is crucial for hearing:

  • Spiral shape and tonotopic organization: high frequencies at the base and low frequencies at the apex.

• Scala within cochlea:

  • Scala media (endolymph) is critical for sensory cell function.

  • Scala vestibuli and tympani (filled with perilymph) maintain chemical balances and protect sensory cells.

Organ of Corti

• Contains hair cells responsible for sound transduction:

  • Inner hair cells: transmit auditory information.

  • Outer hair cells: amplify soft sounds via contraction, influenced by brain signals.

• Process of hair cell activation:

  • Movement of stereocilia opens ion channels leading to depolarization and nerve stimulation.

Neural Transmission

• Auditory signals travel from inner ear via cochlear nerve to auditory brainstem:

  • Coding frequencies by activation along the basilar membrane.

  • Intensity coded by the firing rate of nerve fibers.

• Localization of sound involves:

  • Comparing signals from both ears (timing and intensity differences).

Vestibular System

• Works in tandem with hearing but focuses on balance and spatial awareness:

  • Inputs required: visual system, vestibular system, proprioceptive system.

• Semicircular canals detect rotational movements.

• Utricle and saccule detect gravitational forces, including tilts and accelerations.

Reflexive Mechanisms

• Vestibulo-ocular reflex: stabilizes vision during head movements.

• Vestibulospinal reflex: manages postural corrections during balance disruptions.

Summary

• The ear's anatomy allows for the intricate processes involved in hearing, from sound wave collection to neural transmission.

• Aging impacts frequency perception, while outer, middle, and inner ear structures each play vital roles in detecting and processing sound.

• Understanding these mechanisms is essential for recognizing how balance and hearing interact.