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Home , Earlobe, Epitympanic recess, Outer ear, Tip link, Vestibular membrane

Three major areas of 1. External (outer) ear – only 2. () – hearing only 3. Internal (inner) ear – hearing and equilibrium Receptors for hearing and balance respond to separate stimuli Are activated independently

1 External Ear (pinna)Composed of (rim); Lobule () 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

3 Tympanic membrane (eardrum) Boundary between external and middle 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 —superior portion of middle ear 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 , , and Suspended by ligaments and joined by synovial joints Transmit vibratory motion of eardrum to Tensor tympani and stapedius muscles contract reflexively in response to loud sounds to prevent damage to hearing receptors

7 8 Tortuous channels in temporal bone Three regions: vestibule, , and Filled with – similar to CSF Series of membranous sacs and ducts Filled with potassium-rich

9 Blue structure – series of ducts

10 Central egg-shaped cavity of bony labyrinth Contains two membranous sacs 1. is continuous with 2. 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 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 () Contains cochlear duct, which houses spiral organ () 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 ; contains perilymph Scalae tympani and vestibuli are continuous with each other at (apex)

14 The "roof" of cochlear duct is External wall is stria vascularis – secretes endolymph "Floor" of cochlear duct composed of Bony spiral lamina , which supports spiral organ The cochlear branch of VIII runs from spiral organ to brain

15 16 EM viewed from

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 with prolonged exposure above 90 dB Amplified rock music is 120 dB or more

20 Pitch 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 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 and one Afferent fibers of 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 membrane studded with (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 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 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: 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

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