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Chapter 11

CHAPTER 11 (pt 1)

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INTRODUCTION The sense of hearing is known as audition and the sense of balance is regulated by the vestibular system. THE NATURE OF SOUND Sounds are audible variations in air pressure. When an object moves toward a patch of air, it compresses the air, increasing the density of the molecules. The air is rarefied when an object moves away on the other hand (see Fig. 11.1). The speed of sound is about 343 m/sec (767 mph) for air at room temperature Frequency of sound is the number of compressed or rarefied patches of air that pass by our ears each second. One cycle of the sound is the distance between successive compressed patches and it is expressed in hertz (Hz). Sound waves all propagate at the same speed so high frequency sound waves have more compressed and rarefied regions packed into the same space than low-frequency waves (see Fig. 11.2) Our auditory system can respond to pressure waves between 20 Hz to 20 000 Hz. Whether a sound is perceived to have a high or low tone (i.e. pitch) is determined by its frequency. Intensity is the difference in pressure between compressed and rarefied patches of air (Fig. 11.2b). Sound intensity determines the loudness we perceive loud sounds having higher intensity. THE STRUCTURE OF THE AUDITORY SYSTEM See Fig, 11.3 The visible portion of the ear consists primarily of cartilage covered by skin forming a funnel called the pinna which collects sounds from a wide area. Its shape makes us more sensitive to sounds coming from ahead than from behind Is more or less fixed in position The entrance to the internal ear is the auditory canal which extends about 2.5 cm inside skull and ends at the tympanic membrane (eardrum). Ossicles are connected to the tympanic membrane to its medial surface and they transfer movements of the tympanic membrane into movements of the oval window. The cochlea is located behind the oval window and it contains the apparatus to transform physical motion into a neural response. Sound waves moves the tympanic membrane tympanic membrane moves the ossicles ossicles move oval window membrane motion at the oval window moves fluid in the cochlea fluid movement causes sensory neurons to respond The pinna to tympanic membrane make the outer ear, the tympanic membrane and ossicles make up the middle ear and the apparatus medial to the oval window make up the inner ear. Once a neural response to sound is generated, the signal is transferred to and processed by a series of nuclei in the brain stem. The output is sent to the medial geniculate nucleus (MGN) which projects to the primary auditory cortex (A1). THE MIDDLE EAR Variations in air pressure are converted into movements of the ossicles. Components of the Middle Ear Tympanic membrane conical in shape Three ossicles Two tiny muscles attached to the ossicles There three ossicles: the malleus is attached to the tympanic membrane and forms a rigid connection with the incus which forms a flexible connection with the stapes. The flat portion of the stapes, footplate, moves like a piston at the oval window and transmits sound vibrations to the fluids of the cochlea. The air in the middle ear is continuous with the air in the nasal cavities via the Eustachian tube. Sound Force Amplification by the Ossicles Why isnt the ear arranged so sound waves directly move the membrane at the oval window? The problem is that the cochlea is filled with fluid (not air) so if sound waves impinged directly on the oval window, the membrane would barely move and all but 0.1% of the sound energy would be reflected The fluid in the inner ear resists being moved much more than air does, so more pressure is needed to vibrate the fluid than air can provide and thus ossicles provide this necessary amplification in pressure. The pressure at the oval window will become greater than the pressure at the tympanic membrane if: The force on the oval window membrane > force on tympanic membrane Surface area of the oval window < area of the tympanic membrane By using both mechanisms, the pressure increases at the oval window and the force at the oval window become greater since the ossicles act like levers. Sound causes large movements of the larger tympanic membrane which transformed into smaller but stronger vibrations of the smaller oval window. The pressure at the oval window is about 20 times greater than at the tympanic membrane and this is sufficient to move the fluid in the inner area. The Attenuation Reflex Two muscles attached to the ossicles effect sound transmission to the inner ear (see Fig. 11.6): Tensor tympani muscle: attaches to the malleus Stapedius muscle: attaches to the stapes When these muscles contract, the chain of ossicles become rigid and sound conduction to the inner ear is greatly diminished. Loud sound causes these muscles to contract -- attenuation reflex which is much greater at low frequencies than at high frequencies. Attenuation reflex protects the ear from loud sounds that would damage it. However, this reflex has a delay of 50-100 msec from the time that sound reaches the ear so it can`t protect the ear from very sudden loud sounds. The reflex also suppresses low frequencies more than higher ones so high frequencies are easier to discern in an environment of a lot of low frequencies. THE INNER EAR Consists of the cochlea (auditory system) and cochlea (vestibular system). Anatomy of the Cochlea (see Fig. 11.6) In the cochlea, the hollow tube has walls made of bone. The central pillar of the cochlea is a conical bony structure, modiolus. The base of the cochlea consists of two membrane-covered holes: the oval window and the round window. The cochlea is divided into three fluid-filled chambers (see Fig. 11.7): the scala vestibuli, the scala media, and the scala tympani. The Reissners membrane divides the scala vestibuli and scala media; the basilar membrane separates the scala tympani form the scala media. The organ of Corti sits on the basilar membrane and contains auditory receptor neurons. Hanging over this structure is the tectorial membrane. The perilymph is the fluid in the scala vestibule and scala tympani. It has an ionic content similar to cerebrospinal fluid: low K (7nM) and high Na (140 nM) c
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