Chapter 3: Anatomy and physiology of the sensory auditory mechanism

Objectives (1) Anatomy of the inner ear Functions of the cochlear and vestibular systems Three compartments within the cochlea and membranes Branches and function of cranial nerve VIII Function of the hair cells Transitional pathways Otoacoustic emissions, and spontaneous emissions Electrochemical makeup of the cochlea Cochlear electrophysiology Tuning curves

Inner Ear (1) Inner ear: a fluid-filled series of canals Labyrinth because of its mazelike arrangement Cochlea, semicircular canals, vestibule, utricle, saccule, and cochlear duct The inner ear:  Vestibular: the sense of balance and spatial orientation  Cochlear: hearing Oval window and round window Scala vestibuli, scala media, scala tympani Reissner’s membrane and basilar membrane Organ 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 the endocochlear potential, a resting potential that is critical to the function of the inner ear and for the secretion of endolymph.

Inner Ear (3) Difference between IHCs and OHCs 1) IHCs: flask-shaped while OHCs:cylindric shaped 2) IHCs: one low while OHCs: three lows 3) Streocilia of IHCs: -shaped while that of OHCs: V shaped 4) 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 brain Afferent pathway and efferent pathway (1) Afferent (ascending) pathway: from the inner hair cells to the brain (2) Efferent (descending) pathway: from the superior olivary complex 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 Ca 2+ 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, )

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 stria vascularis. 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 of a single neuron using tiny wire electrodes insulated by glass or other nonconducting material. Tuning curve: responsiveness of a single cell to variety of frequencies. Characteristic frequency: Frequency at which the lowest level of stimulation results in an increase in the firing rate.