Module 22: Hearing and Vestibulation

Ear organs

Organs of equilibrium and hearing

Ear regions

External, middle, and inner

External ear

Includes auricle, external auditory meatus, tympanic membrane, collects sound and directs it to deeper structures of the ear and is only involved in hearing

Auricle (pinna)

Large, fleshy structure on the lateral aspect of the head made of elastic cartilage, gathers and amplifies sound to direct towards external acoustic meatus

External acoustic meatus

Auditory canal that enters the skull, formed by cartilage laterally and temporal bone in the medial two thirds

Cerumen

Ear wax, protection for the ear, cleans, lubricates

Tympanic membrane

Thin layer of tissue at the end of the external acoustic meatus that vibrates in response to sound waves and separates the external ear from the middle ear

Middle ear

Contains the air-filled tympanic cavity and is just medial to the tympanic membrane, only involved in hearing, contains the auditory ossicles

Auditory ossicles

Malleus, incus, and stapes (hammer, anvil, and stirrup)

Oval window

Superior opening between the middle and inner ear

Auditory (Eustachian, pharyngotympanic) tube

Connection between middle ear and pharynx that equilibrates air pressure across the tympanic membrane, pops open when muscles of the pharynx contract

Inner ear

Includes cochlea and vestibular complex that serve two sensory functions

Cochlea

Involved in hearing and is attached to the stapes via the oval window

Vestibular complex

Involved in process of vestibulation and consists of the semicircular ducts and vestibule

Semicircular ducts

Sensing rotational acceleration

Vestibule

Consists of utricle and saccule, sensing linear acceleration

Bony labyrinth

Series of canals in inner ear embedded within the temporal bone, contains perilymph and inner membranous labyrinth

Membranous labyrinth

Contains endolymph and is where gelatinous layers and specialized hair cells that produce neural signals are

Vestibulocochlear nerve

Consists of separate fiber bundles traveling together to the brain stem that relay neural signals from the gelatinous layers and specialized hair cells

Round window

Inferior opening connecting the middle and inner ear, covered by membrane containing perilymph that moves as sound waves within the perilymph reach it to dissipate remaining energy and prevent echoes

Hearing

Transduction of sound waves into a neural signal

Cochlear duct structures

Vestibular membrane, which is the roof, tectorial membrane, spiral organ (of Corti), and basilar membrane, which is the floor

Spiral organs

Have thick sensory epithelium with hair cells and hair-like stereocilia from the cells’ apical surfaces, on top of the basilar membrane

Basilar membrane

Side of cochlear duct between the spiral organs and scala tympani

Stereocilia

Array of microvilli-like structures arranged from tallest to shortest, tethered together by protein fibers

Sound transmission

Sound waves enter and travel through the external acoustic meatus, which vibrates the tympanic membrane. The malleus is attached to the tympanic membrane, so it moves and articulates the incus which articulates the stapes. The stapes is connected to the membrane covering the oval window at the beginning of the scala vestibuli, so the membrane moves and sound information is transmitted into the inner ear. The scala vestibuli extends from the oval window traveling above the cochlear duct and becomes the scala tympani when returning to the base of the cochlea and ending at the round window. Scala vestibuli and tympani move in a wave-like motion where the frequency matches the frequency of the sound waves. The membrane covering the round window bulges out or puckers in. Scala vestibuli and tympani run along both sides of the cochlea and the basilar membrane moves in specific spots based on the wave frequency. Higher frequency moves close to the base, lower moves near the tip. Tectorial membrane slides across stereocilia, when basilar membrane moves, which bends the stereocilia and depolarizes the hair cell membrane to trigger nerve impulses that travel down the afferent nerve fibers attached to the hair cells.

Scala vestibuli

Fluid-filled perilymph tube within the cochlea

Cochlear duct

Central cavity of the cochlea that contains the sound-transducting neurons

Hair cells function (audition)

Allow cochlea to separate auditory stimuli by frequency

Hair cells (vestibulation)

Within the vestibule of the inner ear and senses head position, head movement, and whether our bodies are in motion

Utricle and saccule

Largely composed of macula tissue that senses head position

Semicircular canals

Three ring-like extensions of the vestibule that sense head movement, one in the horizontal plane and two in the sagittal and coronal planes

Vestibular ganglion function

Generate neural signals to be transmitted through the vestibulocochlear nerve to the brain stem and cerebellum

Macula

Composed of hair cells surrounded by support cells

Otolithic membrane

Viscous gel that stereocilia of the macula extend into

Otoliths

Layer of calcium carbonate crystals on top of the otolithic membrane that make the otolithic membrane top-heavy

Head position interpretation

Otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity, bending the stereocilia and causing hair cells to depolarize or hyperpolarize. The pattern of hair cell depolarization interprets the position of the head.

Ampulla

Enlarged region where the bases of semicircular canals connect, contains hair cells that response to rotational movements

Cupula

Membrane that attaches to the top of the ampulla where stereocilia extend into

Head rotation interpretation

As the head rotates in a plane parallel to the semicircular canal, the fluid lags and that deflects the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae in horizontal and vertical orientations. The vestibular system compares movements of ampullae to detect direction of head movements.

Auditory pathway

Travels along the vestibulocochlear nerve, which synapses with neurons in the cochlear nuclei of the superior medulla. Input is combined to extract location information from the auditory stimuli within the brain stem by comparing information from both ears. Processing continues on to the inferior colliculus, where axons project to the thalamus and superior colliculus. The medial geniculate nucleus projects the info to the auditory cortex. The superior cortex initiates stimulation of muscles to turn the head toward the auditory stimulus.

Vestibular ganglion

Carries info from utricle, saccule, and semicircular canals

Vestibular system

Nerves are composed of axons from the vestibular ganglion, controls head and neck movements in response to vestibular signals and to maintain visual attention

Vestibular pathways

Most axons terminate in the vestibular nuclei of the medulla but some project from the vestibular ganglion into the cerebellum with no intervening synapse, which initiates movements on the basis of equilibrium info. Neurons in the vestibular nuclei project their axons to targets in the brain stem, which influences respiratory and cardiovascular functions in relation to body movements, or spinal cord, which initiates spinal reflexes involved with posture and balance. Fibers of the vestibular nuclei project to the oculomotor, trochlear, and abducens nuclei, which influences signals sent along the cranial nerves. The vestibular nuclei project to the thalamus to join the proprioceptive pathway, which allows for conscious perception of equilibrium.