Hearing and Equilibrium: Anatomy & Physiology Study Notes 14

Lecture 14: Hearing and Equilibrium

Objectives

• Define anatomical subdivisions of the ear and describe their structure and functions.

• Describe the principal events involved in the physiology of hearing.

• Identify the receptor organs and their neural pathways for equilibrium.

• Describe the maintenance of dynamic and static equilibrium.

• Explain how rotational movements, gravity, and acceleration affect the equilibrium apparatus.

• Describe conductive and sensory deafness.

Anatomy of the Ear

Main Regions of the Ear

The ear is divided into three main regions: the outer ear, middle ear, and inner ear. All regions are involved in hearing, but only the inner ear is responsible for equilibrium.

• Outer Ear: Collects sound waves and channels them inward.

• Middle Ear: Transmits and amplifies vibrations.

• Inner Ear: Contains sensory organs for hearing and equilibrium.

Diagram: Anatomy of the Ear

The ear consists of the auricle (pinna), external auditory canal, tympanic membrane, auditory ossicles (malleus, incus, stapes), semicircular canals, vestibule, cochlea, and associated structures.

Outer Ear

Structure and Function

• Auricle (Pinna): Composed of elastic cartilage except for the lobule; functions to funnel sound waves into the external auditory canal.

• External Auditory Canal: Passes through the temporal bone and ends at the tympanic membrane (eardrum).

Middle Ear

Structure and Function

• Tympanic Membrane: Separates the outer and middle ear; vibrates in response to sound waves.

• Auditory Ossicles: Three small bones (malleus, incus, stapes) connected by synovial joints; transmit and amplify vibrations from the tympanic membrane to the oval window.

• Pharyngotympanic (Auditory) Tube: Connects the middle ear to the nasopharynx; equalizes pressure on both sides of the tympanic membrane.

Inner Ear

Labyrinths and Fluids

• Bony Labyrinth: Outer structure filled with perilymph (similar to extracellular fluid).

• Membranous Labyrinth: Inner structure filled with endolymph (similar to intracellular fluid).

Main Regions

• Vestibule: Medial to the middle ear; contains utricle and saccule, which are involved in equilibrium.

• Semicircular Canals: Three canals oriented at right angles; detect rotational movement of the head.

• Cochlea: Spiral-shaped organ responsible for hearing; contains the cochlear duct (scala media) filled with endolymph.

Physiology of Hearing

Transmission of Sound

• Sound waves are funneled by the auricle into the auditory canal.

• Waves strike the tympanic membrane, causing it to vibrate.

• Vibrations are transmitted and amplified by the auditory ossicles to the oval window.

• Vibration of the oval window produces pressure waves in the perilymph of the scala vestibuli and scala tympani.

Processing of Sound in the Inner Ear

• Pressure waves travel through perilymph and endolymph, causing the basilar membrane to vibrate.

• High-frequency vibrations affect the base of the cochlea (narrow and stiff region), while low-frequency vibrations travel farther to the apex (wider and more flexible region).

• The round window allows fluid movement within the cochlea.

Pitch and Loudness

• Pitch: Determined by the location of basilar membrane vibration.

• Loudness: Determined by the amplitude of basilar membrane vibration.

Spiral Organ (Organ of Corti)

• Contains receptor cells for hearing (hair cells) located on the basilar membrane.

• Inner hair cells detect sound; outer hair cells increase sensitivity.

• Each hair cell has microvilli called stereocilia that extend into the endolymph.

• Stereocilia are connected by elastic filaments called tip links.

Sequence of Events in Sound Transduction

1. Basilar membrane moves up toward the tectorial membrane, bending stereocilia toward the tallest stereocilium.

2. Bending opens K+ channels, depolarizing the hair cell.

3. Depolarized hair cell releases neurotransmitters, triggering action potentials in the cochlear nerve.

Relaxation of Inner Hair Cells

• Stereocilia bend in the opposite direction as the basilar membrane moves away from the tectorial membrane.

• Tip links are no longer stretched; K+ channels close, hyperpolarizing the hair cell and stopping neurotransmitter release.

Auditory Pathway

• Neural signals travel from the cochlear nerve (part of CN VIII) to the cochlear nucleus (medulla-pons junction), then to the superior olivary nucleus (pons), inferior colliculus (midbrain), medial geniculate nucleus (thalamus), and finally to the primary auditory cortex (temporal lobe).

Deafness

Types of Hearing Loss

• Conduction Hearing Loss: Problems in the outer or middle ear prevent sound waves from reaching the inner ear.

• Sensorineural Hearing Loss: Deficits in the cochlea or neural pathways (cochlear nerve or CNS).

Equilibrium and Vestibular Sensation

Sources of Equilibrium

• Equilibrium is maintained by input from the visual system, proprioceptors, and the vestibular system.

• The vestibular system consists of the vestibule and semicircular canals.

Static Equilibrium

• Maintaining balance when the head is tilted; monitored by the utricle and saccule.

• Each contains a macula with hair cells (stereocilia and kinocilium) embedded in a gelatinous otolithic membrane containing otoliths (CaCO3 crystals).

• Bending of stereocilia toward kinocilium depolarizes hair cells, increasing glutamate release and action potentials; bending away hyperpolarizes hair cells, reducing glutamate release.

Dynamic Equilibrium

• Maintaining balance during rotational or angular motions and linear acceleration.

• Linear acceleration (e.g., riding in a car or elevator) affects the utricle (horizontal movement) or saccule (vertical movement).

• Angular or rotational movement is detected by the anterior, posterior, and lateral semicircular ducts, each with an ampulla containing the crista ampullaris (hair cells embedded in the cupula).

• Movement of endolymph bends stereocilia, altering glutamate release and activity in the vestibular nerve.

Static vs. Dynamic Equilibrium Table

Type	Stimulus	Receptor
Static Equilibrium	Head tilt, up/down movement	Utricle, Saccule
Dynamic Equilibrium	Linear acceleration, angular rotation	Utricle, Saccule, Ampulla (semicircular ducts)

Vestibular Sensation Pathway

• Vestibular signals travel via the vestibular nerve to the vestibular nuclei (medulla-pons junction), then to the thalamus, inferior parietal lobe, oculomotor nuclei, cerebellum, and spinal cord.

• Produces changes in posture, cognitive awareness, and compensatory eye movements.

Key Terms and Definitions

• Auricle (Pinna): External part of the ear that collects sound.

• Tympanic Membrane: Eardrum; vibrates in response to sound.

• Ossicles: Malleus, incus, stapes; transmit sound vibrations.

• Cochlea: Spiral organ for hearing.

• Vestibule: Central part of the inner ear for equilibrium.

• Semicircular Canals: Detect rotational movement.

• Endolymph/Perilymph: Fluids in the inner ear.

• Organ of Corti: Sensory organ for hearing.

• Macula: Sensory region for static equilibrium.

• Crista Ampullaris: Sensory region for dynamic equilibrium.

• Otoliths: Calcium carbonate crystals in the otolithic membrane.

• Stereocilia/Tip Links: Structures on hair cells involved in mechanotransduction.

Relevant Equations

• Sound frequency and pitch: $f = \frac{v}{\lambda}$

• Action potential generation: $\text{Depolarization} \rightarrow \text{Neurotransmitter Release} \rightarrow \text{Action Potential}$

Example: When riding in an elevator, the saccule detects vertical acceleration, while the utricle detects horizontal acceleration when riding in a car.

Additional info: Academic context and terminology have been expanded for clarity and completeness.