Chapter 3: Anatomy and physiology of the sensory auditory mechanism

Objectives (1)Anatomy of the inner earFunctions of the cochlear and vestibular systemsThree compartments within the cochlea and membranesBranches and function of cranial nerve VIIIFunction of the hair cellsTransitional pathwaysOtoacoustic emissions, and spontaneous emissionsElectrochemical makeup of the cochleaCochlear electrophysiologyTuning curves

Inner Ear (1)Inner ear: a fluid-filled series of canalsLabyrinth because of its mazelike arrangementCochlea, semicircular canals, vestibule, utricle, saccule, and cochlear ductThe inner ear:

(1) Vestibular: the sense of balance and spatial orientation

(2) Cochlear: hearingOval window and round windowScala vestibuli, scala media, scala tympaniReissner’s membrane and basilar membraneOrgan of Corti (outer hair cell and inner hair cells)

Inner Ear (2)Function of the Cochlea: to convert mechanical energy to electrical energy (electrochemical energy)

Three compartments of the cochlea, the fluid, and the chemical component

(1) Scala vestibuli-----perilymph-----high sodium and low potassium

(2) Scala media-----endolymph-----low sodium and high potassium

(3) Scala tympani-----perilymph------high sodium and low potassium

Stria vascularis: responsible for the generation of theendocochlear potential, a resting potential that is critical tothe function of the inner ear and for the secretion ofendolymph.

Inner Ear (3)Difference between IHCs and OHCs

1) IHCs: flask-shaped while OHCs:cylindric shaped2) IHCs: one low while OHCs: three lows3) Streocilia of IHCs: -shaped while that of OHCs: V shaped4) IHCs role: transmit auditory information to the brain

(ascending=afferent) while OHCs role: receives auditory information from the brain (descending=efferent) and terminates the efferent pathways from the brainAfferent pathway and efferent pathway

(1) Afferent (ascending) pathway: from the inner hair cells to the brain

(2) Efferent (descending) pathway: from the superior olivarycomplex of the brain to the outer hair cells of the cochlea

Inner Ear (4)Structures and Functions of the Basilar Membrane:

(1) The width of the BM increases from base to apex (approximately tenfold)

(2) The thickness of the BM decreases from base to apex (thin at base to thicker at apex)

(3) Its mass increases with the width because of an increase in the size and number of supporting cells

(4) The flexibility of the partition changes drastically stiff at the base and becomes progressively more elastic toward the apex (more than one hundredfold)

(5) Stiffness-dominating at the base and mass-dominating at the apex(6) High frequency stimuli generate the maximum wave amplitude at the

base of the cochlea while low frequency stimuli produce the maximal amplitude displacement at the apex of the cochlea.

(7) Dead animals: broadly tuned while living animals: sharply tuned

Fig. 5. Structure and protein composition of the stereociliary bundle (Mechano-electrical transduction) (Fettiplace R and Hackney CM (2006) The sensory and motor roles of auditory hair cells, Nat. Rev. Neuro. 7: 19–29)

Mechano-Electrical Transduction (MET)

Deflection towards the largest stereocilia-K+

and Ca2+ entering-transduction channels opening (depolarization)-receptor potentials increasing.Deflection towards the opposite direction-the channels closing (hyperpolarization)-receptor potential decreasing.In Vitro MET is measured by output (receptor potential)/input (stereocilia displacement).

In vivo MET is measured by the cochlear microphonic (CM). Output (CM)/input (acoustic signal)MET is mainly found in nonmammalian and mammalian species.MET is affected by noise exposure and TRPA channel blockers such as gadolinium, amiloride, gentamicin, ruthenium red, icilin, and allyl isothiocyanate (AITC).

Fig. 6. The Electromotility (Electro-Mechanical Transduction) of the OHC (Brownell, W. E., Bader, C. R., Bertrand, D., and de Ribaupierre, Y. (1985).

"Evoked mechanical responses of isolated cochlear outer hair cells." Science, 227, 194-6.)

Fig. 7. The putative motors of outer hair cells (Fettiplace R and Hackney CM (2006) The sensory and motor roles of auditory hair cells, Nat. Rev. Neuro. 7:

19–29)

Electro-Mechanical Transduction (EMT)

Receptor potential increment-OHC shorten (thick)-Force generation-BM moves up.Receptor potential decrement-OHC elongate (thin)-BM moves down.In vivo Measurement: (1) Output (BM movement by laser Doppler)/Input (electrical stimulus). (2) Electrically evoked cochlear emissions. Source: the lateral wall of the OHC

EMT is mainly found in mammalian species.Prestin (motor protein) is required for electromotility of the outer hair cell and for the cochlear amplifier.EMT is affected by chlorpromazine and salicylate.

Fig. 8. Feedback system involved in outer hair cell motility

Fig. 9. Frequency responses of the basilar membrane (BM)

Active and passive cochlear amplifier

Fig. 10. Changes in BM velocity (tuning) as a function of frequency.

Active indicates cochlea in good condition while passive one in poor

condition

Fig. 11. Changes in phase as a function of frequency

Inner Ear (5)Cochlear Electrophysiology:

(1) Endocochlear potential: 80 mV, maintained by a combination of active ionic pumps and selectively permeable ion channels located in the cells of the striavascularis. These pumps and channels are also responsible for the unique chemical composition of the endolymph.

(2) Resting potentials of OHCs: -70mV while those of the IHCs: -40 mV(3) Potential differences between endolymph the inside of OHCs or IHCs: 150

mV or 120 mV(4) Cochlear microphonic (CM) and summating potential (SP): placing

electrodes on either side of the cochlear duct (one in the scala tympani and one in the scala vestibuli) or a single electrode in the organ of Corti space (scala media), CM is a pattern of voltage fluctuation while SP is a sizable shift in the baseline voltage level.

(5) Both potentials reach a maximum at a particular best frequency, which corresponds to the displacement peak of the traveling wave envelope. Below the best frequency the potentials decline slowly whereas above they exhibit a rapid decrease in amplitude.

(6) CM originates from OHCs and SP also appears to be primarily the product of OHCs, at least in the low-frequency region of the cochlea. But recent studies shos that SP originates from IHCs (Choi et al., 2004; 2010).

Single-Cell Electrical Activity

It is possible to record the electrical activity ofa single neuron using tiny wire electrodesinsulated by glass or other nonconductingmaterial.Tuning curve: responsiveness of a single cellto variety of frequencies.Characteristic frequency: Frequency at whichthe lowest level of stimulation results in anincrease in the firing rate.

Chapter 4: Anatomy and physiology of the central auditory mechanism

Objectives (1)Two subsystems of the human auditory systemThree main functional mechanisms of the peripheral auditory systemMajor outer ear structures and their functions in the auditory systemThree layers of tissue and two types of fibers in the TMStructures of the middle earSpecific nerves and musclesDifference between epitympanic recess and tympanic cavity proper(tympanum)