BISC 421 1st Edition Lecture 21 Outline of Current LectureI. Auditory Periphery Current LectureAuditory PeripherySoundwavesCondensationeach sinusoidal wave has a characteristic amplitude and frequency•Soundwaves: pressure waves moving through air•Condensation: compression and pressure•Rarefaction: expansion minus pressure•Peaks taller= louder (amplitude)Types of SoundsPitch (fundamental frequency) and harmonics (multiples of the fundamental frequency).Various frequencies of various amplitudes, all mixed up.• Different types of sounds•Pure tone: doesn't sound that pleasant, we don't encounter in nature, just one sinusoid•Music tone: don't need to know about harmonics and fundamental frequency but this is what we hear in nature• Noise: frequencies of various amplitudes all mixed up.The PinnaThe grooves and hollows of the pinna act like filters and resonance chambers, altering the spectrum of the sound that reaches the cochlea. For example, some frequencies may be amplified while others are damped.•The pinna = the outer ear, important for localizing sound, alter spectrum of sound that reaches cochlea•Don't need to memorize these partsCochlea The Middle EarThe middle ear acts like an impedance matching device. The impedance (~resistance) of the air is small and the impendence of the fluid filled cochlea is large. In order to make sure that this impendence mismatch does not cause too much sound energy to bounce off the ear:A) The ossicles transmit energy from a larger surface (the tympanic membrane) to a smaller one(the oval window). Just think about how much worse it is when someone wearing stiletto heels steps on your toes than when that person has the courtesy to change to sensible shoes before stepping on you.B) The bones (ossicles) of the middle ear, the malleus, incus and stapes, act as levers to increasethe force of the movement of the tympanic membrane to the stapes•The middle ear: middle ear as an impedance matching device: resistance of air is small and resistance of fluid filled cochlea is large…try to make sure this mismatch doesn't cause toomuch energy to bounce ear by..•Ossicles transmit energy from a larger surface (tympanic membrane) to smaller one (oval window)•Ossicles (malleus, incus and stapes) act as levers to increase the force of the movement ofthe tympanic membrane to the stapes•2 functions: impedance mapping, lever idea•AnimationsThe Inner Ear: The Cochlea•The inner ear: the cochlea (greek for small)•A bone•Apex: helicotrema (top)•Base•Oval window: foot of stapes (stirrup)•Round window: releases pressure on the other endThe Structure of the CochleaBasilar MembraneThe cochlear duct, or scala media (1) is isolated from the scala vestibuli (2) and scala tympani (3) by Reissner's (4) and basilar (5) membranes respectively. The organ of Corti is covered by the tectorial membrane (6) floating in the endolymph. The stria vascularis (7) and the fibers (8)going to the spiral ganglion through the bony spiral lamina (9) are also shown.•Memorize these terms:•Scala timpani: low•Scala vestibuli• Scala media: middle•Reisner’s membrane: top•Basilar membrane: bottom•Scala ganglion•Organ of corti• Stria vascularis The Basilar MembraneIn the cochlea, the basilar membrane is tuned to high frequencies nearest the stapes (the membrane is narrow and stiff) and low frequencies nearest the apex, or helicotrema, where the membraneis wide and floppy. Von Bekesy noted that the motion of the basilar membrane was in the form of a traveling wave, as when youflick a rope. The height of the wave changes from position to position depending onthe resonant frequency (think of the different strings on a violin or guitar) of different regions of the basilar membrane.•The basilar membrane: sound transduction; narrow in some places near stapes and wide inother places near apex•From oval window to helicotroma•All membranes as a big flat sheet running through cochlear ductBasilar Membrane Displacement as a Function of Frequency: II•High frequency soundgreater displacement at base•Low frequency soundgreater displacement at apex•Complex sound decomposed in basilar membrane to different frequency regions•Single role of inner hair cells with hairs called stereocilia: take sound energy and convert it to electrical energy•Bathed in endolymph•Pereolymph somewhere else•We think they evolved from this kind of cilia•There is always one tallest stereocilia – staircase organizationDisplacement of the Hair Bundle in VariousDirections and the Influence on Membrane Potential•Displacement of the hair bundle in various directions and the influence on membrane potential•Wiggle cell towards tallest stereocilium or away from it•Wiggle towards it? Get receptor current; depolarization•Opposite direction? Hyperpolarization•Perpendicular to axis? nothingThe Electrochemical Gradient Across the Hair Cell’s Membrane•The electrochemical gradient across the hair cells membrane•GIANT Nernst potential here because very K outside in scala media and lowr K in innerear and even lower in Perilymph – giant electrochemical gradient in hair cells•Endolymph different from perilymph because HIGH POTASSIUM•Diseases that change this really mess up your hearing and your balanceMechanoreceptors at the Tips of the Stereocilia•Hairs tilted towards Tallest depolarization•Spring attached to mechanoreceptor channel- pull on spring and K can come in for depolarizationAnimation of Hair BundlesOuter hair cells contract when they are depolarized•Outer hair cells contract when current is injected- mechanically coupled to other outer hair cellsSummary of Transduction of Sound to the Receptor PotentialThe function of the organ of Corti may be summarised in 5 stages: (1) Sounds waves move the basilar membrane up and down.(2) Stereocilia of the OHCs , in the tectorial membrane, are bent and the cells are depolarized.(3) Depolarized OHCs react by contracting (electromotility): active mechanism.(4)Because of the tight coupling of OHCs with the basilar membrane and reticular lamina, this active mechanism feeds energy back into the organ of Corti and IHCs are excited.(5) The IHC-auditory nerve synapse is activated and a message is sent to the central nervous system.T ontopic organization of the cochleaTonotopy is easy to understand. The mechanical properties of the