Hearing and Balance: Anatomy and Physiology of the Ear

Hearing and Balance: Anatomy and Physiology of the Ear

Overview

The human ear is a complex organ responsible for both hearing and balance. It is divided into three main regions: the external ear, middle ear, and internal ear. Each region contains specialized structures that contribute to the detection of sound and the maintenance of equilibrium.

External and Middle Ear

External Ear

• Auricle (Pinna): Composed of elastic cartilage (except the lobule), it funnels sound waves into the external acoustic meatus.

• External Acoustic Meatus: Begins as elastic cartilage, then becomes a canal in the temporal bone lined with skin, hairs, sebaceous, and ceruminous glands.

• Cerumen (Earwax): Produced by ceruminous glands; protects the ear by trapping debris and repelling insects.

• Tympanic Membrane (Eardrum): Thin, translucent membrane marking the boundary between the external and middle ear; vibrates in response to sound waves, transmitting energy to the ossicles.

Middle Ear

• Tympanic Cavity: Air-filled space with the eardrum laterally and a bony wall medially containing the oval and round windows.

• Pharyngotympanic (Auditory) Tube: Connects the middle ear to the nasopharynx, equalizing pressure on both sides of the eardrum.

• Auditory Ossicles: Three small bones (malleus, incus, stapes) transmit vibrations from the eardrum to the oval window.

• Muscles (Tensor Tympani and Stapedius): Contract to protect hearing receptors by limiting ossicle vibration in response to loud noises.

Internal Ear (Labyrinth)

Bony Labyrinth

• System of tortuous channels in the temporal bone, filled with perilymph (similar to CSF).

• Contains three main regions: vestibule, cochlea, and semicircular canals.

Membranous Labyrinth

• Series of membranous sacs and ducts within the bony labyrinth, filled with endolymph (similar to intracellular fluid).

• These fluids conduct sound vibrations (hearing) and respond to mechanical forces (balance).

Vestibule

• Central cavity of the bony labyrinth; contains two sacs: utricle and saccule, suspended in perilymph.

• Saccule: Leads into the cochlea.

• Utricle: Leads into the semicircular canals.

• Both contain equilibrium receptors called maculae that monitor head position and respond to gravity.

Semicircular Canals

• Three canals oriented in three planes of space; each contains a membranous semicircular duct.

• The ampulla is the swollen end of each canal, housing equilibrium receptors in the crista ampullaris.

• These receptors respond to angular (rotational) movements of the head.

Cochlea

• Spiral, bony chamber ("snail-shaped") that coils around a bony pillar called the modiolus.

• Contains the cochlear duct (scala media), which ends at the cochlear apex and houses the spiral organ (organ of Corti), the hearing receptor.

Cochlear Chambers

• Scala Vestibuli: Contains perilymph; continuous with the vestibule and begins at the oval window.

• Scala Media (Cochlear Duct): Contains endolymph; houses the spiral organ.

• Scala Tympani: Contains perilymph; ends at the round window.

• Helicotrema: The apex where scala vestibuli and scala tympani meet, allowing perilymph to be continuous.

Spiral Organ (Organ of Corti)

• Sits on the basilar membrane, which is narrow and thick near the oval window and becomes wider and thinner toward the cochlear apex.

• Consists of supporting cells and cochlear hair cells (hearing receptors): one row of inner hair cells and three rows of outer hair cells.

• Hair cells are sandwiched between the tectorial and basilar membranes.

• Innervated by the cochlear branch of the vestibulocochlear nerve (CN VIII).

Transmission of Sound

Pathway of Sound

• Outer Ear: Pinna to external acoustic meatus to tympanic membrane.

• Middle Ear: Malleus, incus, and stapes transmit vibrations to the oval window.

• Inner Ear: Vibrations travel through the scala vestibuli and tympani, stimulating the spiral organ and generating impulses in the cochlear nerve.

Nature of Sound

• Sound is a pressure disturbance (alternating areas of high and low pressure) propagated by a vibrating object.

• Composed of areas of compression and rarefaction that create sound waves.

• Energy is transferred from molecule to molecule in the direction of the sound wave.

Frequency and Amplitude

• Frequency: Perceived as pitch; measured in hertz (Hz). Human hearing range: 20–20,000 Hz; most sensitive to 1500–4000 Hz.

• Amplitude: Perceived as loudness; measured in decibels (dB), a logarithmic scale. An increase of 10 dB equals a tenfold increase in sound energy, but is perceived as only a doubling of loudness.

• Normal conversation: ~50 dB; noisy restaurant: ~70 dB; amplified concert: ~120 dB; prolonged exposure to >90 dB can cause hearing loss.

Resonance of the Basilar Membrane

• Each frequency of sound has its own pathway through the basilar membrane.

• Fibers near the oval window are short and stiff (respond to high frequencies); fibers near the apex are longer and floppier (respond to low frequencies).

• Only sounds within the hearing range can activate the spiral organ and be perceived as sound.

Sound Transduction

• Inner Hair Cells: The longest stereocilia are embedded in the tectorial membrane; movement of the basilar membrane bends the hairs, opening mechanically gated cation channels (K+ and Ca2+ entry), generating a receptor potential.

• Hair cells release neurotransmitter (glutamate) to excite cochlear nerve fibers.

• Movement in the opposite direction closes channels, causing repolarization.

Outer Hair Cells

• Not directly involved in sound reception; thought to have supportive/protective roles.

• Depolarize and repolarize in response to basilar membrane movement, contracting and stretching to amplify motion and increase responsiveness of inner hair cells.

• Efferent fibers from the brain can cause them to stiffen in response to loud noises, protecting inner hair cells.

Auditory Pathway

• Impulses from the cochlea pass via the spiral ganglion to the cochlear nuclei in the brainstem.

• From there, impulses are sent to the superior olivary nuclei (localization), inferior colliculi (auditory reflex center), and then to the auditory cortex.

• Some auditory pathways decussate (cross over), so both cortices receive input from both ears.

• Pitch is determined by the area of the basilar membrane activated; loudness is determined by the rate of action potentials and neurotransmitter release.

• Localization is perceived by comparing sound intensity and timing between the two ears.

Balance (Equilibrium)

Overview

• Balance organs monitor head movements to maintain orientation and balance in space.

• Balance depends on input from the vestibular apparatus (semicircular canals, utricle, saccule), visual system, and proprioceptive information from muscles and joints.

Static and Dynamic Equilibrium

• Static Equilibrium: Monitored by receptors in the vestibule (utricle and saccule); detects linear acceleration and head position relative to gravity.

• Dynamic Equilibrium: Monitored by receptors in the semicircular canals; detects rotational (angular) movements of the head.

Maculae

• One macula in each utricle and saccule; each consists of supporting cells and hair cells.

• Hair cells have stereocilia and one kinocilium embedded in the otolithic membrane (jellylike mass with calcium carbonate crystals called otoliths).

• Utricular hairs: Respond to horizontal movement or tilting the head.

• Saccular hairs: Respond to vertical movement.

• Vestibular nerve fibers wrap around hair cells to transmit information to the brain.

Mechanism at the Hair Cell Level

• Movement of the otolithic membrane bends the hair cells, altering neurotransmitter release and changing the frequency of action potentials in vestibular nerve fibers.

• Bending toward the kinocilium depolarizes the cell (increases action potentials); bending away hyperpolarizes the cell (decreases action potentials).

Crista Ampullaris (Crista)

• Located in the ampulla of each semicircular canal; detects rotational movements.

• Composed of supporting cells and hair cells whose hairs are embedded in a gel-like mass called the cupula.

• Directional bending of hairs (due to endolymph inertia) increases or decreases the rate of impulses to the brain, providing information about the direction and speed of rotation.

• Hair cells in complementary semicircular ducts have opposite polarity, so one is depolarizing while the other is hyperpolarizing, enhancing detection of rotation direction.

Equilibrium Pathway to the Brain

• Balance information is sent directly to the brainstem for rapid, reflexive responses.

• Integration of vestibular, visual, and somatic sensory input occurs in the vestibular nuclei and cerebellum.

• Commands are sent to motor centers controlling eye, neck, limb, and trunk muscles via vestibulospinal tracts.

Summary Table: Internal Ear Structures and Functions

Bony Labyrinth	Membranous Labyrinth	Function	Receptor Region
Semicircular canals	Semicircular ducts	Equilibrium (rotational/angular acceleration)	Crista ampullaris
Vestibule	Utricle and saccule	Equilibrium (head position relative to gravity, linear acceleration)	Macula
Cochlea	Cochlear duct (scala media)	Hearing	Spiral organ (organ of Corti)

Additional info: The notes above integrate and expand upon the provided lecture slides, including definitions, mechanisms, and clinical relevance where appropriate for a comprehensive review of the anatomy and physiology of hearing and balance.