Three major areas of ear 1. External (outer) ear – hearing only 2. Middle ear (tympanic cavity) – hearing only 3. Internal (inner) ear – hearing and equilibrium Receptors for hearing and balance respond to separate stimuli Are activated independently 1 External Ear Auricle (pinna)Composed of Helix (rim); Lobule (earlobe) Funnels sound waves into auditory canal 2 External acoustic meatus (auditory canal) Short, curved tube lined with skin bearing hairs, sebaceous glands, and ceruminous glands Transmits sound waves to eardrum 3 Tympanic membrane (eardrum) Boundary between external and middle ears Connective tissue membrane that vibrates in response to sound Transfers sound energy to bones of middle ear 4 A small, air-filled, mucosa-lined cavity in temporal bone Flanked laterally by eardrum Flanked medially by bony wall containing oval (vestibular) and round (cochlear) windows 5 Epitympanic recess—superior portion of middle ear Mastoid antrum Canal for communication with mastoid air cells Pharyngotympanic (auditory) tube—connects middle ear to nasopharynx Equalizes pressure in middle ear cavity with external air pressure 6 Three small bones in tympanic cavity: the malleus, incus, and stapes Suspended by ligaments and joined by synovial joints Transmit vibratory motion of eardrum to oval window Tensor tympani and stapedius muscles contract reflexively in response to loud sounds to prevent damage to hearing receptors 7 8 Bony labyrinth Tortuous channels in temporal bone Three regions: vestibule, semicircular canals, and cochlea Filled with perilymph – similar to CSF Membranous labyrinth Series of membranous sacs and ducts Filled with potassium-rich endolymph 9 Blue structure – series of ducts 10 Central egg-shaped cavity of bony labyrinth Contains two membranous sacs 1. Saccule is continuous with cochlear duct 2. Utricle is continuous with semicircular canals These sacs House equilibrium receptor regions (maculae) Respond to gravity and changes in position of head 11 Three canals (anterior, lateral, and posterior) that each define ⅔ circle Lie in three planes of space Membranous semicircular ducts line each canal and communicate with utricle Ampullae of each duct houses equilibrium receptor region called the crista ampullaris Receptors respond to angular (rotational) movements of the head 12 A spiral, conical, bony chamber Size of split pea Extends from vestibule Coils around bony pillar (modiolus) Contains cochlear duct, which houses spiral organ (organ of Corti) and ends at cochlear apex 13 Cavity of cochlea divided into three chambers Scala vestibuli—abuts oval window, contains perilymph Scala media (cochlear duct)—contains endolymph Scala tympani—terminates at round window; contains perilymph Scalae tympani and vestibuli are continuous with each other at helicotrema (apex) 14 The "roof" of cochlear duct is vestibular membrane External wall is stria vascularis – secretes endolymph "Floor" of cochlear duct composed of Bony spiral lamina Basilar membrane, which supports spiral organ The cochlear branch of nerve VIII runs from spiral organ to brain 15 16 EM viewed from tectorial membrane 17 Sound is Pressure disturbance (alternating areas of high and low pressure) produced by vibrating object Sound wave Moves outward in all directions Illustrated as an S-shaped curve or sine wave 18 Frequency Number of waves that pass given point in given time Pure tone has repeating crests and troughs Wavelength Distance between two consecutive crests Shorter wavelength = higher frequency of sound 19 Amplitude Height of crests Amplitude perceived as loudness Subjective interpretation of sound intensity Normal range is 0–120 decibels (dB) Severe hearing loss with prolonged exposure above 90 dB Amplified rock music is 120 dB or more 20 Pitch Perception of different frequencies Normal range 20–20,000 hertz (Hz) Higher frequency = higher pitch Quality Most sounds mixtures of different frequencies Richness and complexity of sounds (music) 21 Sound waves vibrate tympanic membrane Ossicles vibrate and amplify pressure at oval window Cochlear fluid set into wave motion Pressure waves move through perilymph of scala vestibuli 22 Waves with frequencies below threshold of hearing travel through helicotrema and scali tympani to round window Sounds in hearing range go through cochlear duct, vibrating basilar membrane at specific location, according to frequency of sound 23 24 Waves with frequencies below threshold of hearing travel through helicotrema and scali tympani to round window 25 Sounds in hearing range go through cochlear duct, vibrating basilar membrane at specific location, according to frequency of sound 26 Fibers near oval window short and stiff Resonate with high-frequency pressure waves Fibers near cochlear apex longer, more floppy Resonate with lower-frequency pressure waves This mechanically processes sound before signals reach receptors 27 Cells of spiral organ Supporting cells Cochlear hair cells One row of inner hair cells Three rows of outer hair cells Have many stereocilia and one kinocilium Afferent fibers of cochlear nerve coil about bases of hair cells 28 Stereocilia Protrude into endolymph Longest enmeshed in gel-like tectorial membrane Sound bending these toward kinocilium Opens mechanically gated ion channels Inward K+ and Ca2+ current causes graded potential and release of neurotransmitter glutamate Cochlear fibers transmit impulses to brain 29 Stereocilia Protrude into endolymph Longest enmeshed in gel-like tectorial membrane Sound bending these toward kinocilium Opens mechanically gated ion channels Inward K+ and Ca2+ current causes graded potential and release of neurotransmitter glutamate Cochlear fibers transmit impulses to brain 30 Impulses from cochlea pass via spiral ganglion to cochlear nuclei of medulla From there, impulses sent To superior olivary nucleus Via lateral lemniscus to Inferior colliculus (auditory reflex center) From there, impulses pass to 31 medial geniculate nucleus of thalamus, then to primary auditory cortex Auditory pathways decussate so that both cortices receive input from both ears 31 Pitch perceived by impulses from specific hair cells in different positions along basilar membrane Loudness detected by increased numbers of action potentials that result when hair cells experience larger deflections Localization of sound depends on relative intensity and relative timing of sound waves reaching both ears 32 Vestibular apparatus Equilibrium receptors in semicircular canals and vestibule Vestibular receptors monitor static equilibrium Semicircular canal receptors monitor dynamic equilibrium 33 Sensory receptors for static equilibrium One in each saccule wall and one in each utricle wall Monitor the position of head in space, necessary for control of posture Respond to linear acceleration forces, but not rotation Contain supporting cells and hair cells Stereocilia and kinocilia are embedded in the otolith membrane studded with otoliths (tiny CaCO3 stones) 34 35 Maculae in utricle respond to horizontal movements and tilting head side to side Maculae in saccule respond to vertical movements Hair cells synapse with vestibular nerve fibers 36 37 Hair cells release neurotransmitter continuously Movement modifies amount they release 38 Bending of hairs in direction of kinocilia Depolarizes hair cells Increases amount of neurotransmitter release More impulses travel up vestibular nerve to brain 39 Bending away from kinocilium Hyperpolarizes receptors Less neurotransmitter released Reduces rate of impulse generation Thus brain informed of changing position of head 40 Sensory receptor for rotational acceleration One in ampulla of each semicircular canal Major stimuli are rotational movements 41 Sensory receptor for rotational acceleration One in ampulla of each semicircular canal Major stimuli are rotational movements 42 43 Each crista has supporting cells and hair cells that extend into gel-like mass called ampullary cupula Dendrites of vestibular nerve fibers encircle base of hair cells 44 Hair Cell Transduction Mechanosensitive Ion Channels are gated by Cilia displacement; they are associated with tonic release of Glutamate at rest but levels can either increase or decrease depending on direction of Cilia deflection. TOWARD TALLEST CILIA: Channels open when tip links are stretched causing influx of K⁺ and Depolarization. This opens Voltage-Gated Ca²⁺ channels at the Basolateral surface of the Hair Cell triggering ↑ Glutamate release TOWARD SHORTEST CILIA: Tip link relax = ↓ Glutamate release 45 Bending of hairs in the opposite direction causes Hyperpolarizations, and fewer impulses reach the brain Thus brain informed of rotational movements of head 46 Equilibrium information goes to reflex centers in brain stem Allows fast, reflexive responses to imbalance Impulses travel to vestibular nuclei in brain stem or cerebellum, both of which receive other input 47 Equilibrium information goes to reflex centers in brain stem Allows fast, reflexive responses to imbalance Impulses travel to vestibular nuclei in brain stem or cerebellum, both of which receive other input 48 Three modes of input for balance and orientation: Vestibular receptors Visual receptors Somatic receptors 48.