Anatomy and Physiology of the Ear: Structure, Function, and Hearing

17-6 The Ear

Overview of Ear Anatomy

The ear is a complex sensory organ responsible for hearing and equilibrium. It is divided into three anatomical regions: the external ear, middle ear, and internal ear, each with distinct structures and functions.

• External Ear: Collects and directs sound waves.

• Middle Ear: Transmits sound waves as mechanical energy.

• Internal Ear: Sensory organ for equilibrium and hearing.

External Ear

The external ear consists of structures that capture sound and protect the ear canal.

• Auricle (Pinna): Surrounds and protects the external acoustic meatus; provides directional sensitivity.

• Tympanic Membrane: Thin, semitransparent sheet at the end of the external acoustic meatus; separates external from middle ear and conducts sound to middle ear bones.

• Ceruminous Glands: Integumentary glands along the external acoustic meatus; secrete cerumen (earwax) which helps keep out foreign objects and insects, slows growth of microorganisms, and traps debris with tiny hairs lining the canal.

Middle Ear (Tympanic Cavity)

The middle ear is an air-filled chamber that transmits sound from the external ear to the internal ear.

• Auditory Tube (Eustachian Tube): Connects middle ear to nasopharynx; equalizes pressure on either side of the tympanic membrane; susceptible to infection (otitis media).

• Auditory Ossicles: Three tiny bones—Malleus, Incus, and Stapes—deliver mechanical energy of sound waves to the inner ear.

• Muscles of the Middle Ear:

  • Tensor Tympani: Pulls on malleus, stiffens tympanic membrane; innervated by Trigeminal Nerve (V).

  • Stapedius: Reduces movement of stapes at oval window; innervated by Facial Nerve (VII).

Internal Ear (Labyrinth)

The internal ear is a winding passageway responsible for hearing and equilibrium.

• Bony Labyrinth: Surrounds and protects the membranous labyrinth.

  • Perilymph: Fluid between bony and membranous labyrinths.

  • Endolymph: Fluid within membranous labyrinth.

• Subdivisions:

  • Vestibule: Encloses saccule (continuous with cochlear duct) and utricle (continuous with semicircular canals); receptors detect gravity and linear acceleration.

  • Semicircular Canals: Three canals (anterior, posterior, lateral) contain semicircular ducts; receptors stimulated by head rotation.

  • Cochlea: Contains cochlear duct; receptors provide sense of hearing.

• Windows:

  • Round Window: Thin, membranous partition separating perilymph from air spaces of middle ear.

  • Oval Window: Connected to base of stapes by collagen fibers.

Equilibrium

Equilibrium is the state of physical balance, maintained by receptors in the vestibular complex (vestibule and semicircular canals).

• Hair Cells: Sensory receptors in the inner ear.

  • In saccule and utricle: detect position with respect to gravity and linear acceleration/deceleration.

  • In semicircular ducts: detect rotation of the head.

• Semicircular Ducts:

  • Anterior, posterior, and lateral ducts correspond to different head movements (shaking "no", nodding "yes", tilting side-to-side).

  • Each duct contains an ampulla with a gelatinous ampullary cupula and ampullary crest (hair cells).

  • Each hair cell has 80–100 stereocilia (shorter, thinner) and a single large kinocilium (taller, thicker).

• Utricle and Saccule:

  • Connected with endolymphatic duct ending in endolymphatic sac (secretes and drains endolymph fluid).

  • Maculae: Oval structures where hair cells cluster.

    • Macula of utricle senses horizontal movement (e.g., looking up or down).

    • Macula of saccule senses vertical movement (e.g., acceleration).

  • Otoliths: Densely packed calcium carbonate crystals on surface of gelatinous mass; contribute to sensing gravity and movement.

Pathways for Equilibrium Sensations

Equilibrium information is transmitted via sensory neurons in the vestibular ganglia.

• Fibers from ganglia form the vestibular nerve (part of vestibulocochlear nerve, VIII).

• Synapse within vestibular nuclei at the boundary between pons and medulla oblongata.

• Functions of vestibular nuclei:

  • Integrate sensory information about balance/equilibrium from both sides of head.

  • Relay information to cerebellum and cerebral cortex (conscious sense of head position).

  • Send commands to motor nuclei in brainstem and spinal cord.

• Reflexive motor commands adjust eye, head, and neck movements; instructions descend in vestibulospinal tracts.

Automatic Movements of Eyes

• Directed by superior colliculi of midbrain in response to sensations of motion.

• Attempt to keep gaze focused on a specific point.

• If spinning rapidly, eyes make jerky movements (nystagmus).

• Nystagmus: Trouble controlling eye movements when body is stationary; caused by damage to brainstem or internal ear; symptoms include vertigo, dizziness, nausea.

Process of Hearing

Hearing involves the conversion of sound waves into mechanical and then neural signals.

• Sound waves vibrate the tympanic membrane.

• Auditory ossicles conduct vibrations to the internal ear.

• Vibrations are converted to pressure waves in fluid, detected by hair cells in the cochlear duct.

• Information is sent to the auditory cortex of the brain.

Pressure Waves and Sound Properties

• Wavelength: Distance between two adjacent wave troughs.

• Frequency: Number of waves passing a fixed point per unit time; measured in Hertz (Hz).

• Pitch: Sensory perception of frequency; high frequency = high pitch (short wavelength).

• Amplitude: Intensity of sound wave; measured in decibels (dB).

Equation:

• Frequency (Hz): $f = \frac{1}{T}$, where $T$ is the period (seconds per cycle).

Representative Sound Intensities

Sound intensity is measured in decibels (dB), with higher levels posing greater risk of hearing loss.

Decibel Level	Example	Dangerous Time Exposure
0	Lowest audible sound	-
30	Quiet library, soft whisper	-
50	Light traffic at a distance	-
70	Busy traffic, noisy restaurant	Some damage if continuous
90	Truck traffic, home appliances	Less than 8 hours
120	Rock concert, thunderclap	Immediate danger
140	Gunshot, jet plane	Immediate danger
160	Rocket launching pad	Hearing loss inevitable

The Cochlea

The cochlea is a spiral, conical, bony chamber that houses the organ of Corti, the main receptor for hearing.

• Scala Vestibuli: Abuts the oval window; contains perilymph.

• Scala Media (Cochlear Duct): Contains endolymph and the organ of Corti.

• Scala Tympani: Terminates at the round window; contains perilymph.

• Basilar Membrane: Separates cochlear duct from tympanic duct; supports hair cells.

• Tectorial Membrane: Overlies hair cells; involved in sound transduction.

Auditory Ossicles and Sound Transmission

• Convert pressure fluctuations in air into greater pressure fluctuations in perilymph of cochlea.

• Frequency of sound is determined by which part of cochlear duct is stimulated.

• Intensity (volume) is determined by the number of hair cells stimulated.

Excitation of Hair Cells in the Spiral Organ

• Stereocilia: Protrude into endolymph and are enmeshed in the tectorial membrane.

• Bending stereocilia opens mechanically gated ion channels, allowing inward K+ and Ca2+ current, causing a graded potential and release of neurotransmitter glutamate.

• Cochlear fibers transmit impulses to the brain.

Six Basic Steps in Hearing

1. Sound waves arrive at tympanic membrane.

2. Movement of tympanic membrane displaces auditory ossicles.

3. Movement of stapes at oval window produces pressure waves in perilymph of scala vestibuli.

4. Pressure waves distort basilar membrane on their way to round window of scala tympani.

5. Vibration of basilar membrane causes hair cells to vibrate against tectorial membrane.

6. Information about stimulation is relayed to CNS over cochlear nerve.

Additional info:

• Children have more sensitive ears; damage from loud sounds or injuries accumulates into hearing loss.

• Tympanic membrane gets less flexible with age; articulations between ossicles stiffen; round window may ossify.