Physiology of Hearing Overview

Physiology of Hearing

• Hearing is divided into outer, middle, and inner ear components.

Functional Anatomy of Hearing

• Outer Ear: Comprises the pinna and tympanic membrane (eardrum).
• Middle Ear: Includes the Eustachian tube, ossicles (malleus, incus, stapes), and tensor tympani and stapedius muscles.
• Inner Ear: Contains the bony labyrinth and membranous labyrinth (cochlea, semicircular canals, otolith organs). The bony labyrinth contains perilymph.

Cochlea

• A 35mm coiled tube within the bony labyrinth.
• Three chambers: Scala vestibuli (upper), Scala tympani (lower), and Scala media (cochlear duct).
• Scala vestibuli and tympani are separated from Scala media by the vestibular membrane and basilar membrane.
• Both Scala vestibuli and tympani contain perilymph, which has a low potassium concentration.
• Scala media contains endolymph, rich in potassium.
• Scala vestibuli and tympani meet at the helicotrema (apex of the cochlea).

Organ of Corti

• Located on the basilar membrane.
• Contains 20,000 outer hair cells, organized in three rows, and 3500 inner hair cells organized in two rows.
• Hair cells are specialized auditory receptors.
• Tips of hair cells are stereocilia, embedded in the tectorial membrane.
• Processes of hair cells are in endolymph, and bases in perilymph.

Sound Transmission

• Sound waves cause the tympanic membrane (eardrum) to vibrate.
• Vibrations are transferred to ossicles (malleus, incus, stapes).
• Ossicles amplify the vibration (~1.3x).
• Stapes transmits vibrations to the oval window, triggering vibrations in the perilymph of the Scala vestibuli.
• This wave moves through the perilymph and cochlea.

How Sound is Perceived

• Wave amplitude corresponds to sound intensity.
• Lower frequencies peak near the apex, higher frequencies near the base of the cochlea.
• Differential movement of the tectorial and basilar membranes exert a shearing force on the hair cells.
• The stimulated area along the basilar membrane determines the frequency of the perceived sound (place principle).

Hair Cell Stimulation

• Mechanically gated channels on stereocilia open when stimulated.
• Calcium and potassium ions enter, causing membrane depolarization.
• Myosin-based motors restore hair cells to their resting position, closing channels, and returning the membrane to resting potential.
• Neurotransmitter release (glutamate) initiates afferent neuron depolarization.

Tympanic Reflex

• Tensor tympani and stapedius muscles contract in response to loud sounds.
• This reduces ossicle vibration, protecting the inner ear from damage.

Auditory Nerve Transmission

• Auditory nerve fiber polarization depends on sound intensity.
• Signals travel from the cochlea via the central auditory pathway.
• Auditory division of cranial nerve VIII (CN VIII) projects to dorsal and ventral cochlear nuclei. Then to the inferior colliculus.
• Further processed in the medial geniculate body, before reaching the auditory cortex in the superior temporal gyrus of the temporal lobe.
• Collateral fibers also project to the reticular activating system to allow loud sounds to activate the entire nervous system.

Olivocochlear Bundle

• Fibers from superior olivary complexes target outer hair cells.
• Help modulate hair cell sensitivity via acetylcholine.

Hearing Disorders

• Otosclerosis: Hardening of ossicles, leading to middle ear stiffness.
• Otitis media and meningitis: Infections that can spread to the inner ear and cerebrospinal fluid.
• Presbyacusis: Age-related hearing loss due to hair cell degeneration in cochlea.

Explore the intricate anatomy and physiology of the human auditory system. This quiz covers the outer, middle, and inner ear components, including their functions and structures. Test your knowledge on the organ of Corti and the cochlea's anatomy.