Week 7 Hearing & Vestibular System pt1 PDF

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Lecture notes week 7 pt1...

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Physics of Sound: - Sound is a compressional or longitudinal wave caused by vibrations in ear pressure - The air through which the sound moves oscillates in the direction of propagation of the wave - A sound wave consists of alternating compressions and rarefactions - The distance between adjacent compressions/rarefactions of the sound wave is called the wavelength Note: This is expressed as a sinusoidal wave

Amplitude - The loudness of the sound wave is proportional to the difference in intensity of air within the rarefactions compared to within the compressions Note: The greater the difference in air intensities, the greater the amplitude - Sound amplitude or intensity is usually expressed in decibels

Frequency: - The frequency of the sound wave is the reciprocal of the time taken to complete one oscillation cycle - Human auditory system can detect frequencies from 20hZ to 22,000hZ Note: Most sensitive in the 2000-5000 hZ range - Above 20,000hZ is ultrasound and below 20hZ is infrasound

Anatomy of the Ear: - There are 3 regions to the ear (i.e external, inner and outer ear) - Outer ear consists of the pinna and ear canal Note: Tympanic membrane separates outer ear from middle ear - Middle and inner ear are ear filled (i.e unless blocked by snot) - Middle ear has ear ossicles which consist of stapes, incus and malleus - Within the cochlea is where we get transduction of sound waves into neural signals

External ear: Sound collection Pinna: Visible part of ear outside of your head (i.e skin covered, cartilaginous structure) - Sound is collected by the pinna - Preferentially funnels sound coming from in front - Convolutions in pinna help in localising sound Note: Some animals have muscular control of pinna and can turn to face direction of sound and improve both detection and discrimination - Pinna funnels sound waves into the ear canal and conducts them to tympanic membrane

Middle ear: Sound conduction and amplification - Sound waves displace the tympanic membrane - Small bones or ossicles called the malleus, incus and stapes conduct and amplify the vibrations of the tympanic membrane to the oval window - The oval window is the connection between the air filled middle ear and the fluid filled cochlea Note: Mechanical amplification of sound occurs in the middle ear because….

1: The lever action of the ossicles means that the small vibrations of the eardrum are tuned to larger vibrations in the oval window 2: The oval window is smaller than the eardrum and same force on this smaller area produces great pressure on the fluid filled cochlea Note: Extra amplification is necessary due to water being more dense otr

Inner ear: Cochlea - Cochlea is compact inner spiral structure of the inner ear Note: Comprises 3 long fluid filled compartments - Scala vestibuli (i.e sound is introduced here) - Scala media - Scala tympani Note: Compartments are separated by 2 membranes - Vestibular and basilar membrane Note: Basilar membrane carries the organ of corti - Sound exits here via the round window

Sound transduction: Cochlea - Unlike gasses, liquid cannot be compressed - The variations in air pressure transmitted by the ossicles to the oval window are converted into displacements of the perilymph - The vestibular and basilar membranes are joined close to the tip (i.e helicotrema) of the cochlea so the scala vestibuli and tympani are continuous Note: Pressure on the oval window pushes the fluid around the cochlea and pushes on the round window which is flexible and allows perilymph to move back and forth - Movement of the perilymph cause deflection of the membranes in the cochlea - Deflections of the membranes bending causes bending of stereocilia of sensory hair cells in the organ of corti

Sound transduction: Organ of Corti - Organ of corti is attached to basilar membrane There are two types of hair cells in the organ of corti…. - Inner hair cells - Outer hair cells Note: Both types of hair cell have hair-like processes called stereocilia on their apical surface (i.e surface which faces fluid inside scala media, endolymph)

Hair cells: Inner hair cells: Send auditory information to the brain that is perceived as sound - 3000 to 3500 per cochlea - Synapse with type 1 spiral ganglion cells (bipolar afferent nerves) > CN VIII Outer hair cells: Control cochlear sensitivity and frequency response - 10,000 to 12,000 outer hair cells per cochlea...