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Chapter 11

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University of Toronto Scarborough

Neuroscience : Chapter 11a The Auditory Systems The Nature of Sound Sounds are audible variations in air pressure (compressed air followed by rarefied air) - almost anything that can move air molecules can produce sound. Cycle: The distance (in cm or other units of measurement) between successive compressed patches of air (equivalent to a wavelength) Frequency: The number of cycles per second; expressed in a unit of measurement called hertz (Hz). The tone of a sound (pitch) is determined by frequency Intensity: The difference in pressure between compressed and rarefied patches of air (similar to amplitude). Loudness is determined by intensity The human auditory system can respond to frequencies ranging from 20 20,000 Hz The Structure of the Auditory System Pinna: The visible portion of the ear; consists of cartilage covered by skin; forming a sort of funnel which helps collect sound Auditory Canal: The entrance into the internal ear; it extends about 1 inch inside the skull Tympanic Membrane: The eardrum Ossicles: 3 tiny bones (smallest bones in the body) which transfer movements of the eardrum to a second membrane called the oval window Cochlea: The apparatus for transforming physical motion at the oval window into neuronal responses There are 3 main divisions of the ear: The Outer Ear (from the pinna to the tympanic membrane), The Middle Ear (from the tympanic membrane to the oval window), and the Outer Ear (every more medial than the oval window) The Middle Ear The middle ear consists of the tympanic membrane, the ossicles, and two tiny muscles attached to the ossicles There are three ossicles bones: 1. Malleus (resembles a hammer but not really) 2. Incus (resembles an anvil again, not really) 3. Stapes (resembles a stirrup - ?) At the end of the Stapes is a flat portion called the footplate, which moves in and out of the oval window like a piston (thus transmitting vibrations into the cochlea) The Malleus and Incus are firmly attached together, whereas the Incus is loosely connected to the Stapes Eustachian Tube: a tube in the middle ear that is continuous with the nasal cavity; it is usually plugged by a valve. The feeling of your ears popping is caused by the opening of the Eustachian tube. When you ascend or descend rapidly, the air pressure on the outside either decreases or increases, respectively, compared to inside your ear. This causes a difference in the air pressure in the middle ear compared to the outside. This difference causes ear drum to bulge out, which causes discomfort. This can be resolved by swallowing or yawning, which opens the Eustachian tube, and equalizes the air pressure in the middle ear and the outside. Sound Amplification by the Ossicles The main purpose of the ossicles are to amplify sound; because the cochlea is filled with fluid and not air, the vibrations of the eardrum are inefficient in moving the fluid in any noticeable way. Fluid is very good at reflecting sound (which is why its so quiet underwater), without the ossicles to amplify the vibrations of the eardrum, 99.9% of the vibrations would be reflected by the fluid in the cochlea In order for the vibrations to be properly transferred to the fluid, two things must occur: 1. The force on the oval window membrane must be greater than that on the tympanic membrane 2. The surface area of the oval window must be smaller than the surface area of the tympanic membrane The ossicles act as levers, converting the large movements of the tympanic membrane into smaller, stronger vibrations on the oval window. The pressure at the oval window is about 20x greater than at the tympanic membrane The Attenuation Reflex There are two muscles attached to the ossicles that have a role in altering vibrations:1. The Tensor Tympani Muscle: attached to the Malleus at one end, and to bone on the cavity of the middle ear at the other end 2. The Stapedius Muscle: attached to the Stapes at one end, and to the bone on the cavity of the middle ear on the other end The contraction of these muscles cause the ossicles, as a whole, to become more rigid, reducing the ability of the ossicles to amplify the vibrations of the eardrum Attenuation Reflex: The contraction of these two muscles in response to a loud sound; occurs more strongly at low frequencies than at high frequencies 1. One reason for the attenuation reflex is to adapt the ear to loud noises, that would otherwise saturate the receptors in the inner ear By reducing the vibrations to the cochlea, the ear can reduce the noise to a level below the saturation point of the receptors. this would make the ears more sensitive to a greater range of sounds 2. Another r
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