a+p of hearing

hearing

• transduction of sound

acoustical (sound waves in air ) → mechanical (middle ear bones) → electrochemical (hair cells + auditory nerve) → interpretation (auditory cortex in brain)

outer ear

• collection, shaping, localization of sound

• 2500 Hz = strongest resonance

• pinna, ear canal, tympanic membrane

external auditory meatus (ear canal)

• tubular canal that connects external ear to the eardrum

• 1/3 outer cartilage (courses medially and upward)

• 2/3 bony inner cartilage (courses medially and downward)

tympanic membrane (ear drum)

• separates ear canal from middle ear cavity

• three layers of epithelium

  • outer: continuous with skin of ear canal

  • intermediate: fibrous connective tissue (structural)

    • superficial layer

    • deep layer

  • inner: mucous membrane

middle ear

• impedance matcher between outer ear and inner ear

• transforms air waves into a form that can move through fluid

• ossicles (malleus, incus, stapes)

• stapedius muscle, tensor tympani muscle

• medial, anterior, posterior walls

mechanisms of impedance matching

• surface area ratio

  • tympanic membrane is larger than oval window (17:1)

  • pressure increases; +25 dB of amplification

• lever action of ossicles

  • level action adds +2 dB

• buckling of tympanic membrane

  • reduces velocity; increasing force

  • adds +4-6 dB

• total gain = about 33 dB

ossicular chain

• malleus, incus, stapes

• overcomes the impedance mismatch between air and cochlear fluid

  • surface area ratio → increased pressure oval window = 20 dB

  • lever system → malleus acts as a lever = 2-3 dB

malleus

• attaches to tympanic membrane (via manubrium of malleus)

• head (bulk of bone) articulates with body of incus

• synovial joint = incudomalleolar joint (malleus + incus)

  • vibrations transfer throughout ossicular chain

incus

• body of incus articulates with the head of malleus

  • via the incudomalleolar joint

• body is located in epitympanic recess

• lenticular process = incudostapedial joint (incus + stapes)

  • vibrations transfer throughout ossicular chain

stapes

• smallest ossicle; stirrup

• head of stapes articulates with lenticular process of incus

• lenticular process = incudostapedial joint (incus + stapes)

• footplate of stapes articulates with oval window

  • pushes on oval window → into vestibule (inner ear)

total middle ear gain

• 25 dB (tympanic membrane + footplate)

• 2 dB (ossicle lever benefit)

• 6 dB (buckling of tympanic membrane)

33 dB gain from middle ear

ossicular ligaments

hold the ossicles in place and allow them to move freely

acoustic reflex

tympanic muscles help with acoustic reflex; protect inner ear from acoustic trauma (reducing range of ossicle movement)

stapedius muscle

dampens sound vibrations to protect inner ear from loud noises; innervated by VII facial nerve

tensor tympani muscle

dampens sound vibrations (pulls malleus and tenses tympanic membrane); innervated by V trigeminal nerve

medial wall

• oval window

  • footplate of stapes; transmits sound to inner ear

• round window

  • below oval window; secondary tympanic membrane

• promontory

  • basal turn of cochlea

• prominence of lateral semicircular canals

  • above the oval window

• prominence of VIII nerve

  • near oval window

anterior wall

reveals the entrance of the auditory tube

posterior wall

reveals the prominence of the stapedial pyramid

lateral wall

reveals the tympanic membrane

inner ear

• converts mechanical energy to electrochemical energy

• bony labyrinth: semicircular canals, vestibule, cochlea

• membraneous labyrinth: semicircular duct, utricle, saccule, cochlear duct

cochlea anatomy

• scala vestibuli (perilymph)

• reissner’s or vestibular membrane

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• scala media or cochlear duct (endolymph)

• stria vascularis (vascular supply)

• organ of corti

• basilar membrane (floor of scala media)

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• scala tympani (perilymph)

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• modiolus of cochlea (spiral ganglion cells)

• spiral limbus (connective tissue)

• tectorial membrane (outer hair cells)

• spiral sulcus (stiffness for basilar membrane)

cochlea physiology

• spectrum analyzer

  • separates sound into frequency components by mechanical properties (basilar membrane and organ of corti)

• transducer

  • converts mechanical energy → electrochemical energy

endocochlear potential

• scala vestibuli (high Na, low K)

• scala media (low Na, high K) (+)

• scala tympani (high Na, low K)

excitatory activation

• basilar membrane moves toward scala vestibuli → excitatory

  • stereocilia bend toward kinocilium (depolarization-activation)

  • hair cell → sending sound signal → more action potentials fired

  • depolarization → glutamate release → nerve fires faster

inhibitory activation

• basilar membrane moves toward scala tympani → inhibitory

  • stereocilia bend away from kinocilium (hyperpolarization-inhibition)

  • results in less action potentials fired

  • hair cell → stopping sound signal → less action potentials fired

  • hyperpolarization → less glutamate → nerve fires slower

organ of corti

• contains 4 rows of sensory hair cells

• sit on dieters cells on basilar membrane

  • 3 rows of outer hair cells (amplify sound vibrations)

  • 1 row of inner hair cells (sensory transducers)

• tunnel of corti separates outer and inner hair cells

• tectorial membrane (lies over hair cell bundles)

outer hair cells

• 12000; test-tube shaped

• amplify sound vibrations

• cilia embedded in the tectorial membrane

• basilar membrane moves → shearing action

• lined with stereocilia

inner hair cells

• 3500; tear-drop shaped

• detect sound and send signals to the brain

• cilia not embedded in the tectorial membrane

• relies on endolymph movement by traveling wave

• constriction → increased velocity → turbulence bends cilia

• lined with stereocilia

afferent innervation (organ of corti)

sensory information (hair cells) → VIII vestibulocochlear nerve → brain

• each inner hair cell is connected to 10 VIII nerve fibers

  • many-to-one innervation

  • type I fibers make up 95% of VIII nerve (innervate inner hair cells)

• each outer hair cell shares innervation with 10 other outer hair cells

  • one-to-many innervation

  • type II fibers make up 5% of VIII nerve (innervate outer hair cells)

efferent innervation (organ of corti)

• originate in the superior olivary complex in the brainstem

• oliveocochlear bundle (OCB); project to the cochlea

• inhibitory; reduces activity of hair cells

• crossed OCB = inhibit outer hair cells

• uncrossed OCB = inhibit inner hair cells

high spontaneous (low-threshold) fibers

• fire readily at low sound intensities

• show random firing even without a stimulus

• important for detecting soft sounds near hearing threshold

low spontaneous (high-threshold) fibers

• fire readily at high sound intensities

• no random firing; more precise

• respond to louder sounds (upper intensity of range)

vestibule

• stapes → oval window → vestibule → scala vestibuli (base of cochlea)

• primarily responsible for balance and head position

• contains utricle and saccule

semicircular canals

• each canal contains endolymph, cupula, ampullae, stereocilia

• movement = fluid moves, stereocilia bends, signal rotation

  • bending toward kinocilium = excitatory

  • bending away = inhibitory

• anterior: vertical; forward/backward rotational movements

• posterior: vertical; tilting toward shoulders

• lateral: horizontal; side-to-side head turning

utricle and saccule

• detect linear acceleration

• macula (sensory areas), hair cells in otolithic membrane

• utricle (connects to semicircular canals)

  • forward/backward and side/side on horizontal plane

  • eye movement during head motion

• saccule (connects to cochlea)

  • up/down on vertical plane

  • maintaining posture

proprioceptive sense

signals from semicircular canals, utricle, saccule → awareness of body position in space

tonotopic organization

• cochlea separates sound frequencies along its length

• width, stiffness, thickness, cilia length

• traveling waves moves from base → apex

• base = stiff, thin (less mass), high frequencies

• apex = flaccid, thick (more mass), low frequencies

fluid movement and membrane distortion

• stapes → oval window → compresses perilymph in scala vestibuli

• reissner’s membrane bulges toward scala media

• basilar membrane bulges toward scala tympani

• movement → translated to basilar membrane

auditory pathway

cochlea → cochlear nerve → superior olivary complex → lateral superior olive → inferior colliculus → medial geniculate nucleus → A1 → A2

• cochlea + cochlear nerve

  • mechanical energy is converted into electrochemical signals (hair cells)

• cochlear nucleus

  • medulla; timing + intensity + localization

• superior olivary complex

  • pons; signals from both ears + localization

• lateral superior olive

  • pons → midbrain; timing + intensity

• inferior colliculus

  • midbrain; reflexive responses + localization

• medial geniculate nucleus

  • thalamus; sends signals to auditory cortex

• primary auditory cortex (A1 - heschl’s gyrus)

  • superior temporal gyrus in temporal lobe; conscious perception of sound (pitch, loudness, and location)

• auditory association cortex (A2 - belt/parabelt)

  • superior temporal gyrus in temporal lobe; higher order processing (speech, music, etc.)

cochlear nucleus firing patterns

• primary-like

• onset-sensitive

• offset-sensitive

• steady state

• pausers

• build-up neurons

tuning curve

• shows how sensitive the auditory nerve fiber is to different frequencies

• sharper curve = higher frequency selection