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Ear Notes

Medicine Notes > Neuroscience 1 Notes

This is an extract of our Ear document, which we sell as part of our Neuroscience 1 Notes collection written by the top tier of Oxford students.

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-Sounds are signal of danger so ability to perceive them is needed for survival- evolutionary necessity-Sounds-general noises and speechmeans of communication- massively aided development of human race
-Sound refers to audible pressure variation that propagates away from a source of vibration such as a loudspeaker cone or tuning folk- consists of compression and rarefraction
-Sound is characterised by frequency (number of cycles per second measured in Hertz)-humans have a range of 20Hz to 20000Hz- the frequency determines the pitch of the sound and by its peak amplitude.
-Simplest sound is pure tone-made of sinusoidal increases and decreases in pressure
-Natural sounds, such as speech, tend to have more complex specracontain several frequency components that vary in amplitude. The spectrum of a more complex sound is related to sound quality
-Very important property of auditory system that underlies ability to understand speech is the capacity to resolve individual components of a complex sound.
-Evolution of ear is tracked through natural history-fossil records show how ossicles developed from bones of the jaw-but as these bones are separated from the jaw they are able to evolve specifically for the function of hearing Anatomy of the ear
-Split into 3 parts: Outer, middle and inner ear
-Outer ear + middle ear: involved in conduction of signals to inner ear
-Inner ear: receptors that can detect sound

The outer ear is green, the middle ear cavity red, and the inner ear purple

Anatomy of the outer ear Pinna
-Visible part of outer ear-pinna/auricle - thin plate of yellow elastic cartilage-covered with skin
-Collects sounds from a wide area ii) Resonance
-Amplitude of different frequencies is modified in different ways depending on location of sound source- this filtering effect is referred to as colouration-spectral cues for sound location- useful in distinguishing sounds in front of and behind the head for localising sounds at different elevations.
-This is why some sounds are heard better than others
-Within the flange/helix of the pinna there is a 2dB pressure gain at 4KhZ, whereas in the concha there is a 9dB increase

-Produces an increase in pressure between 2-7 kHz
-Also allow sounds to be localised to some degree using one ear alone

Localisation of sound in vertical plane
-To impair vertical sound localisation one must place a tube into the auditory canal to bypass the pina
-Sweeping curves of outer ear are essential for assessing elevation of source of sound- bumps and ridges produce reflections of entering soundDelays between direct path and reflected path change as sound source moves vertically
-Sound from below will produce slightly more delayed echo than if the same came from above
-Echoes are produced in the range of 3.2 to 10KhZ-high frequencies are particulary affected-but ability to vertical locate sound deteriotes with age Eliminates segment of frequency spectrum-pinna notch-seen for high frequencies
-High frequencies-sounds reflect off the contours of the pinna- some of the sounds that enter the ear travel directly to the ear, other sounds eflec off the contours of the pinna first- so these enter the ear canal after a slight delay-this delay causes phase cancellation-eliminates frequency component whose wave period is twice the delay period
-Some frequencies are amplified-acoustic gain
-Spectra of notches and gains- processed in the brain-vertical localisation
-Vertical localisation of sound is seriously impaired if convolutions of pinna are covered
-Pinna= specific to each individual in the auditory system and is calibrated to interest spectra patterns based on specific pinna- when pinna are altered system must re-learn how to recognise new spectral patterns for localisation
-In studies in which subjects wore pinna moulds that dramatically altered spectral cues
-Immediately after inserting the moulds, subjects perceived all sounds as coming from roughly the same elevation regardless of the actual elevation
-Overtime, localisation abilities become normal supposedly due to recalibration of spectral cues
-After removal of moulds, the subjects were able to perform normally without a learning period-suggests the pattern recognition representations were maintained separately

-This gives evidence for the importance of pinnae in vertical sound localisationhofman et al 1998

iv) Other animals- pinnae may move- allows detection of sounds from different locations more effectively
-Many mammals can move the pinna with auriculares muscles in order to focus their hearing in a certain direction- most humans unlike other animals- don't have this ability

v) It also has a protective funciton Ear canal
-From the pinna the sound pressure waves move into the ear canal- also known as the external acoustic meatus
-Tube leads inward from the bottom of the auricular and conducts vibration towards tympanic cavity Function:
-Cells of canal are protected by layer of cerumen (Ear wax) - secreted by apocrine sweat glands and sebaceous glands that line canal
-Secretion protects other cells and against pathogens but too much can hinder conduction of sound to other cells
-Amplifies frequencies in range of 3kHz to 12kHz Clinical disorders of external ear
-Preauricular cysts: developmental defects in 1 st/2nd branchial arches- can become chronically infected Genetic syndromes
-Konigsmark syndrome- small ears, atresia of the external auditory canalconducting hearing loss and inherited in autosomal recessive manner
-Treacher Collins syndrome- dysplasia of the auricle, atresia of the bony part of auditory canal, hypoplasia of auditory ossicles and tympanic cavity-mixed deafness

Middle ear
-Middle ear: Air filled Cavity behind the tympanic membrane (ear drum is also part of middle ear)-contains 3 smallest bones in human body (collectively known as ossicles)- malleus, incus and stapes (hammer, anvil

and stirrup respectively)- these bones connect tympanic membrane to flexible membrane (oval window) in wall of fluid filled cochlea Function 1: Impedance matching couple movements of eardrum to the inner ear- crucial process as movements of sound waves through air must be impedance matched to the fluid media in the inner ear
-This prevents loss in sound pressure that would otherwise occur due to higher density/ impedence of cochlear fluids. Pressure is increased due to an increase in force and pressure
-Imepedence matching also occurs by the lever action of the ossicles- malleus is longer so will move more than incus to increase pressure at stapes- so force at the surface area of oval window is greater than at the tympanic membrane
-This is achieved as eardrum has greater surface area than stapes footplate (part attached to oval window of cochlea) leading to an equivalent increase in pressure-without middle ear the sound will be reflected back
-3rd factor- is the buckling of the tympanic membrane:
-Tympanic membrane buckles as it moves-causes the malleus to move with about twice as much force as it would otherwise
-Durrant and lovrinic disceovered in 1977 that if these corrective mechanisms were lacking- only 3% of vibrations from outer ear would be transmitted to inner ear
-In pateints whose ossicles were damaged- trauma, infection, cholestoatomameasured that sound pressure must be increased by a factor of 10-50 before the person achieves the same hearing ability as when the ossicles are present Funciton 2: The footplate of the stapes vibrates at the frequency of incoming sound, serving as a piston that cyclically pushes and pulls upon the liquid in the scala vestibuli. The changes in pressure propagate throughout the liquid of the scala vestibuli at the speed of sound. Because the perilymph that fills the scala vestibuli is virtually incompressible, the major effect of the stapes's motion is to displace the liquid in the direction of the elastic cochlear partition. This pushes onto the scala tympani, increasing its pressure, to cause an outward bowing of the round window. Function3: attenuation reflex

-Middle ear also has two muscles: tensor tympani (attached to malleus), stapedius muscle (attached to stapes)- innervated by trigeminal and facial nerves respectively

-High intensity auditory stimulus initiated-reaches cochlea-neural impulses arise from auditory nerve from both cochlea to ipsilateral ventral cochlea
-One pathway, innervates ipsilateral facial motor nucleus-stapedius muscle via facial nerve-2nd pathway-ventral cochlear nuclei-superior olivary complex-impulses cross at brainstem to innervate both ipsilateral and contralateral facial motor nuclei
-measurement of stapedius reflex-locate injury of facial nerve
-Contraction of stapedius-stiffens middle ear ossicles-tilts stapes away from oval window of cochlea-decreases vibrational energy transmitted to cochlea-Clinically relevant-marked changes in impedance properties of the middle ear
-Innervation of the trigeminal nerve- tensor tympani-pulls malleus away
-attenuation reflex: Contraction of these muscles dampens transmission of loud low frequency sounds by stiffening movements of ossicles-as loud sounds saturate response of receptors in inner ear attenuation increases dynamic range we can hear
-Also This reduces transmission to the inner ear to offer some protection against high intensity sounds and as they contract before a person vocalises they reduce auditory self stimulation during speech- however reflex delay of 50-100msec so not much protection from very sudden loud sounds
-Middle ear reflex- activated by intense (loud), low frequency sounds- so when muscle contract lower frequencies are suppressed Tonic tensor tympani syndrome- tinnitus and hyperacussisthreshold for reflex is lower-continually contracting and relaxinscan lead to acoustic shock Measure: Immittance instrumentation-measures changes in refleced energy- more reflected, more impedence
-Elevated or absent acoustic reflex-midde ear disorder-stapes fixation Function 4: Air pressure equalisation Middle ear cavity is connected to the nasopharynx by Eustachian tubetube allows pressure changes to normalise in the cavity as the pressure must be constant to keep constant movement of the tympanic membrane

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