Anatomy & Physiology of the Ear and Hearing

Major Regions of the Ear

The ear is divided into three main regions, each with distinct anatomical structures and functions related to hearing and balance.

• External Ear: Includes the auricle (pinna) and external auditory canal; collects and directs sound waves toward the tympanic membrane.

• Middle Ear: Contains the tympanic membrane and three auditory ossicles (malleus, incus, stapes); transmits and amplifies sound vibrations.

• Inner Ear: Houses the cochlea (hearing) and vestibular apparatus (balance); converts mechanical vibrations into neural signals.

Organization of the Inner Ear: "Pipe in a Pipe"

The inner ear is organized as a series of fluid-filled tubes within tubes, facilitating both hearing and equilibrium.

• Bony Labyrinth: Outer tube filled with perilymph; includes the cochlea, vestibule, and semicircular canals.

• Membranous Labyrinth: Inner tube filled with endolymph; contains sensory organs for hearing and balance.

• Example: The cochlear duct (scala media) is a membranous tube within the bony cochlea.

Anatomy of the Semicircular Canals and Cristae

The semicircular canals are three looped structures oriented at right angles, each containing a crista ampullaris for detecting rotational movements.

• Crista Ampullaris: Sensory organ located in the ampulla of each canal; contains hair cells embedded in a gelatinous cupula.

• Function: Detects angular acceleration and head rotation.

Ampullary Crests and Dynamic Equilibrium

The ampullary crests (cristae) are responsible for sensing changes in head movement, contributing to dynamic equilibrium.

• Dynamic Equilibrium: Maintenance of balance during sudden movements or rotation.

• Mechanism: Movement of endolymph bends the cupula, stimulating hair cells.

Receptors in the Vestibule and Static Equilibrium

The vestibule contains the utricle and saccule, which house maculae for detecting linear acceleration and head position.

• Maculae: Sensory regions with hair cells embedded in an otolithic membrane containing calcium carbonate crystals (otoliths).

• Static Equilibrium: Sensing gravity and head position when stationary.

Neural Pathway for Equilibrium

Equilibrium signals are transmitted from the vestibular apparatus to the brain via the vestibulocochlear nerve.

• Pathway: Hair cells → Vestibular nerve fibers → Vestibulocochlear nerve (CN VIII) → Vestibular nuclei in the brainstem → Cerebellum and cerebral cortex.

Organization of the Cochlea

The cochlea is a spiral-shaped organ responsible for converting sound vibrations into neural signals.

• Scala Vestibuli: Upper chamber filled with perilymph.

• Scala Media (Cochlear Duct): Middle chamber filled with endolymph; contains the organ of Corti.

• Scala Tympani: Lower chamber filled with perilymph.

Physics of Sound: Vibration and Sound Waves

Sound is produced when an object vibrates, creating pressure waves in the surrounding medium.

• Vibration: Rapid back-and-forth movement of an object.

• Sound Wave: Longitudinal wave of alternating high and low pressure.

• Example: Tuning fork vibrations generate sound waves in air.

Volume of Sounds: Wave Representation

Volume (loudness) is determined by the amplitude of sound waves. Higher amplitude means louder sound.

• Draw: Large amplitude wave = loud sound; small amplitude wave = soft sound.

Pitch: High vs. Low Sound

Pitch is determined by the frequency of sound waves. Higher frequency produces a higher pitch.

• Low Pitch: Fewer waves per second (low frequency).

• High Pitch: More waves per second (high frequency).

• Draw: Low pitch = widely spaced waves; high pitch = closely spaced waves.

Volume and Frequency: Sound Wave Differences

Sound waves can vary in both amplitude (volume) and frequency (pitch).

• Volume: Related to amplitude.

• Frequency: Related to pitch.

• Example: Loud, high-pitched sound = large amplitude, high frequency.

Auditory Ossicles

The three auditory ossicles transmit and amplify sound from the tympanic membrane to the inner ear.

• Malleus (Hammer): Attached to the tympanic membrane.

• Incus (Anvil): Middle ossicle connecting malleus and stapes.

• Stapes (Stirrup): Transmits vibrations to the oval window of the cochlea.

Hair Cells of the Cochlea

Hair cells are the sensory receptors for hearing, located in the organ of Corti within the cochlear duct.

• Function: Convert mechanical vibrations into electrical signals.

• Stimulation: Movement of the basilar membrane bends hair cell stereocilia, opening ion channels.

Function of the Round Window

The round window allows for pressure relief in the cochlea, enabling fluid movement and proper vibration transmission.

• Role: Prevents pressure buildup by bulging outward as the stapes pushes on the oval window.

Cochlea and Hearing Process

The cochlea transforms sound waves into neural impulses through a series of mechanical and electrical events.

• Sound Transmission: Vibrations travel from the oval window through cochlear fluids, moving the basilar membrane.

• Hair Cell Activation: Stereocilia bend, leading to neurotransmitter release and nerve impulse generation.

Auditory Signal Pathway

Auditory signals travel from the cochlea to the brain via the vestibulocochlear nerve.

• Pathway: Hair cells → Cochlear nerve fibers → Vestibulocochlear nerve (CN VIII) → Cochlear nuclei in the brainstem → Thalamus → Auditory cortex.

Summary Table: Ear Structures and Functions

Key Equations

• Frequency of Sound:

• Decibel Level (Intensity):