The ear's anatomy and physiology are crucial for understanding how we perceive sound and maintain balance. From the outer ear's sound collection to the inner ear's complex mechanics, each part plays a vital role in transforming sound waves into neural signals.

The auditory system's intricate design allows for precise frequency mapping and sound localization. Meanwhile, the vestibular system works in tandem with other sensory inputs to keep us balanced and spatially oriented, highlighting the ear's multifaceted functions beyond just hearing.

Outer ear structure

• The outer ear is the visible portion of the ear that collects and funnels sound waves towards the tympanic membrane
• It plays a crucial role in localization of sound sources by modifying the incoming sound based on the direction it's coming from

Pinna shape and function

• The pinna is the external flap of the ear made of cartilage and skin
• Its convoluted shape helps collect and amplify sound waves, especially high frequencies (2-5 kHz)
• The pinna's ridges and folds cause sound reflections that provide cues for vertical sound localization
• Pinna shape varies between individuals (round, triangular, oval, etc.)

External auditory canal

• The external auditory canal (ear canal) is a tube that extends from the pinna to the tympanic membrane
• Approximately 2.5 cm long in adults and 0.7 cm in diameter
• Lined with skin that contains hair follicles and sebaceous glands
• Slight "S" shape helps prevent foreign objects from reaching the tympanic membrane

Ceruminous glands

• Ceruminous glands are specialized sweat glands located in the skin of the outer one-third of the ear canal
• Secrete cerumen (earwax), a waxy substance that protects and lubricates the ear canal
• Cerumen has antibacterial and antifungal properties to prevent infections
• Traps dust, debris, and small insects, preventing them from reaching the tympanic membrane

Middle ear anatomy

• The middle ear is an air-filled cavity located between the outer and inner ear
• Contains the three smallest bones in the human body (ossicles) that transmit vibrations from the tympanic membrane to the inner ear

Tympanic membrane

• The tympanic membrane (eardrum) is a thin, cone-shaped membrane that separates the outer ear from the middle ear
• Approximately 8-10 mm in diameter and 0.1 mm thick
• Vibrates in response to sound waves, converting them into mechanical energy
• Has three layers: outer epidermal, middle fibrous, and inner mucosal

Ossicles: malleus, incus, stapes

• The ossicles are three tiny bones (malleus, incus, and stapes) that form a chain connecting the tympanic membrane to the oval window of the inner ear
• Malleus (hammer) attaches to the tympanic membrane, incus (anvil) connects the malleus and stapes, and stapes (stirrup) attaches to the oval window
• Ossicles amplify the vibrations from the tympanic membrane, overcoming the impedance mismatch between air and fluid
• Smallest bones in the human body (stapes is about 3 mm long)

Eustachian tube

• The Eustachian tube (auditory tube) connects the middle ear cavity to the nasopharynx
• Approximately 3-4 cm long in adults
• Equalizes pressure between the middle ear and the atmosphere, ensuring proper vibration of the tympanic membrane
• Normally closed, but opens during swallowing, yawning, or sneezing

Tensor tympani and stapedius muscles

• The tensor tympani and stapedius are the two smallest skeletal muscles in the human body
• Tensor tympani attaches to the malleus, while stapedius attaches to the stapes
• Contract reflexively in response to loud sounds (acoustic reflex) to dampen ossicular vibrations and protect the inner ear from damage
• Also contract during vocalization and chewing to reduce self-generated noise

Inner ear structure

• The inner ear is a complex, fluid-filled structure responsible for hearing and balance
• Consists of the bony labyrinth (hard, dense bone) and the membranous labyrinth (soft tissue)

Bony labyrinth vs membranous labyrinth

• The bony labyrinth is a series of cavities within the temporal bone that houses the membranous labyrinth
• Divided into three parts: cochlea (hearing), vestibule (balance), and semicircular canals (balance)
• The membranous labyrinth is a series of interconnected ducts and sacs that float within the bony labyrinth
• Contains endolymph, a potassium-rich fluid that bathes the sensory hair cells

Cochlea anatomy and function

• The cochlea is a snail-shaped, fluid-filled structure responsible for hearing
• Divided into three compartments: scala vestibuli, scala media, and scala tympani
• Scala media (cochlear duct) contains endolymph and the organ of Corti, the sensory organ for hearing
• Basilar membrane runs along the length of the cochlea, supporting the organ of Corti

Vestibular system: semicircular canals, utricle, saccule

• The vestibular system is responsible for maintaining balance and spatial orientation
• Semicircular canals detect rotational movements of the head (angular acceleration)
• Utricle and saccule (otolith organs) detect linear acceleration and head position relative to gravity
• Vestibular hair cells in these structures transduce mechanical stimuli into neural signals

Round and oval windows

• The oval window is a membrane-covered opening between the middle ear and the scala vestibuli of the cochlea
• Stapes footplate attaches to the oval window, transmitting vibrations into the cochlear fluids
• The round window is a membrane-covered opening between the middle ear and the scala tympani of the cochlea
• Allows for the release of pressure waves in the cochlear fluids, enabling fluid motion

Cochlear mechanics

• Cochlear mechanics involves the conversion of mechanical vibrations into neural signals for hearing
• Requires precise interactions between the basilar membrane, organ of Corti, and cochlear fluids

Basilar membrane and organ of Corti

• The basilar membrane is a fibrous structure that runs along the length of the cochlea
• Varies in width and stiffness, allowing for frequency-specific vibrations (tonotopy)
• The organ of Corti sits on top of the basilar membrane and contains the sensory hair cells for hearing
• Inner hair cells (IHCs) are the primary sensory receptors, while outer hair cells (OHCs) amplify and tune the vibrations

Hair cells: inner vs outer

• Hair cells are the sensory receptors for hearing, with stereocilia (hair-like projections) on their apical surface
• Inner hair cells (IHCs) are arranged in a single row and are responsible for transmitting sound information to the brain via the cochlear nerve
• Outer hair cells (OHCs) are arranged in three rows and have electromotile properties, allowing them to amplify and sharpen the vibrations of the basilar membrane
• OHCs are innervated by efferent fibers that modulate their activity

Cochlear fluid dynamics

• The cochlea contains two types of fluid: endolymph (high in potassium) and perilymph (similar to extracellular fluid)
• Vibrations from the stapes create pressure waves in the perilymph of the scala vestibuli
• These waves cause the basilar membrane to vibrate, creating a traveling wave that peaks at frequency-specific locations
• Vibrations are transmitted to the endolymph in the scala media, stimulating the hair cells

Frequency mapping and tonotopy

• The basilar membrane is tonotopically organized, meaning that different frequencies are mapped to specific locations along its length
• High frequencies cause maximum vibration at the base of the cochlea, while low frequencies peak near the apex
• This spatial separation of frequencies allows for the discrimination of pitch
• Tonotopic organization is maintained throughout the auditory pathway, from the cochlea to the auditory cortex

Neural processing of sound

• The neural processing of sound involves the conversion of hair cell activity into neural signals and the transmission of this information to the brain
• Involves both afferent (ascending) and efferent (descending) pathways

Spiral ganglion and cochlear nerve

• The spiral ganglion contains the cell bodies of the primary auditory neurons (bipolar neurons)
• These neurons have one dendrite that synapses with inner hair cells and one axon that projects to the cochlear nucleus in the brainstem
• The cochlear nerve (part of cranial nerve VIII) is formed by the axons of the spiral ganglion neurons
• Transmits auditory information from the cochlea to the brainstem

Afferent vs efferent auditory pathways

• The afferent auditory pathway carries sound information from the cochlea to the brain
• Involves multiple relay stations, including the cochlear nuclei, superior olivary complex, inferior colliculus, and medial geniculate nucleus of the thalamus
• The efferent auditory pathway carries feedback signals from the brain to the cochlea
• Originates in the superior olivary complex and terminates on the outer hair cells, modulating their activity

Brainstem auditory nuclei

• The cochlear nuclei are the first relay stations in the auditory pathway, receiving input from the cochlear nerve
• The superior olivary complex is involved in sound localization based on interaural time and level differences
• The inferior colliculus is a major integrative center, receiving input from multiple brainstem nuclei and projecting to the medial geniculate nucleus
• The medial geniculate nucleus is the thalamic relay for auditory information, projecting to the primary auditory cortex

Primary auditory cortex

• The primary auditory cortex (A1) is located in the temporal lobe (Heschl's gyrus) and receives input from the medial geniculate nucleus
• Tonotopically organized, with different frequencies mapped to specific cortical regions
• Involved in the perception of pitch, timbre, and loudness
• Processes complex sound features and integrates auditory information with other sensory modalities and cognitive functions

Vestibular system function

• The vestibular system is responsible for maintaining balance, spatial orientation, and gaze stabilization
• Detects head movements and position relative to gravity, providing crucial information for motor control and perception

Otolith organs: saccule and utricle

• The saccule and utricle are the otolith organs, responsible for detecting linear acceleration and head tilt
• Contain a sensory epithelium (macula) with hair cells embedded in a gelatinous matrix (otolithic membrane)
• Calcium carbonate crystals (otoconia) sit on top of the otolithic membrane, providing inertial mass
• When the head tilts or accelerates, the otoconia lag behind, bending the hair cells and generating a neural signal

Semicircular canal mechanics

• The semicircular canals (anterior, posterior, and horizontal) detect rotational movements of the head (angular acceleration)
• Each canal has an enlarged end (ampulla) containing a sensory epithelium (crista) with hair cells
• The hair cells are embedded in a gelatinous structure (cupula) that spans the diameter of the ampulla
• When the head rotates, the endolymph lags behind, deflecting the cupula and bending the hair cells

Vestibular hair cell transduction

• Vestibular hair cells have stereocilia (hair-like projections) that are arranged in a staircase pattern, with the tallest stereocilia adjacent to a single kinocilium
• When the stereocilia are bent towards the kinocilium (excitatory direction), ion channels open, allowing potassium to enter the hair cell and depolarize it
• When the stereocilia are bent away from the kinocilium (inhibitory direction), the hair cell hyperpolarizes
• These changes in hair cell membrane potential modulate the release of neurotransmitter onto the vestibular afferent neurons

Vestibular nuclei and reflexes

• The vestibular nuclei (superior, medial, lateral, and descending) are located in the brainstem and receive input from the vestibular nerve
• Process and integrate vestibular, visual, and proprioceptive information to control balance and eye movements
• The vestibulo-ocular reflex (VOR) stabilizes gaze during head movements by generating compensatory eye movements in the opposite direction
• The vestibulospinal reflex (VSR) controls posture and balance by modulating the activity of spinal motor neurons
• The vestibulo-collic reflex (VCR) stabilizes the head on the body by contracting neck muscles in response to vestibular input