Physiology II: Hearing II

15,000 hair cells within each cochlea are arranged in … along the length of the basilar membrane

four parallel rows

… of inner hair cells

one row

…: of outer hair cells

three rows

Hairs: …

stereocilia (microvilli)

Inner hair cells are …

mechanoreceptors

Sound induced fluid movement in the inner ear → surface hair mechanically deformed → … → neural signals to afferent pathway and auditory cortex (transduction of sound to neural signals)

receptor potentials

…: change length in response to changes in membrane potential

outer hair cells

… → enhance stimulation of inner hair cells

amplify the motion of the basilar membrane

Sound transduction - step 1: tip of links stretch and open channels when stereocilia bend .. tallest member

towards

Sound transduction - step 2: .. enters hair cell depolarizes

K+

Sound transduction - step 3: depolarization opens …

voltage gates Ca+ channels

Sound transduction - step 4: Ca2+ entry causes … of neurotransmitter

greater release

Sound transduction - step 5: more neurotransmitter leads to …

higher rate of action potential

No sound transduction - step 1: tips links slacken and close channels when stereocilia bend … tallest member

away from

No sound transduction - step 2: No K+ enters hair cell …

hyperpolarizes

No sound transduction - step 3: Ca+2 channels …

close

No sound transduction - step 4: … released

no neurotransmitter

No sound transduction - step 5: no …

action potentials

…: unlike extracellular fluid everywhere higher concentration of K+ than ICF inside inner hair cell

endolymph

…: K+ entry down its concentration gradient → depolarization

mechanically-gated channels

… → opened/closed channels → alternating depolarization/hyperpolarization at frequency of sound

Back and forth deformation of hairs

Depolarization causes … to be released at the synapse, this increases the firing rate (more action potentials) in the afferent auditory nerve fibers

more glutamate neurotransmitter

Hyperpolarization causes … to be released this decreases the firing rate (fewer action potentials) in the auditory nerve

firing rate

Inner hair cells use … to convey sound intensity (loudness)

graded potentials

Inside the cochlea the louder vibration leads to a … in the basilar membrane especially in the region tuned to that frequency

higher amplitude wave (bigger)

The bigger basilar membrane movement bends the stereocilia (hair bundles) of the inner hair cells …

more

More bending more ion channels open → … of the inner hair cells

greater depolarization

Strong depolarization causes the inner hair cell torelease more glutamate → … in the auditory nerve fibers

more frequent action potentials

The brain interprets more frequent action potentials as …

greater sound intensity

Different regions of the basilar membrane … maximally at different frequencies because of physical properties of the basilar membrane (width, flexibility)

naturally vibrate

Each frequency has its own … at different positions along the membrane

peak vibration

…: differnt sound waves stimulate hair cells at different locations → project to specific different locations in auditory cortex

tonotopy

…: overtones cause many points along basilar membrane to vibrate simultaneously and less intensly than fundememntal tone this contricutes to information discrimination in the CNS

timbre-discrimination

Main stations of the auditory pathway: … (5)

auditory nerve, brainstem, inferior colliculus, medial geniculate nucleus (thalamus), primary auditory cortex

In the brainstem auditory signals from each ear transmitted to both temporal lobes, …

fibers partly cross

Tonotopic organization in the primary auditory cortex: each region of basilar membrane is linked to specific region of primary auditory cortex → cortex response to … of sounds (speech recognition)

temporal pattern

…: responses to complex sounds

associative cortex

…: soundwaves are not adequately conducted (external and middle ear)

conductive hearing loss

Causes of conductive hearing loss: … (4)

blockage, eardrum rupture, middle ear infections, restriction of ossicular movement

…: soundwaves transmitted to inner ear, bit not translated into nerve signals/ not transmitted adequately to CNS

Sensorineural hearing loss

Cause of sensorineural hearing loss: … (3)

defect in cochlea or auditory nerve, presbycusis, partial loss due to overexposure

…: partial hearing loss due to degenerative age related process

presbycusis

…: conductive deafness, increase intensity and may modify sound spectrum tailored to pattern of frequency loss

hearing loss

…: electronic, surgically-implanted devices; transduce sound signals into electrical signals that can directly stimulate the auditory nerve bypassing a defective cochlear system

cochlear implants