Auditory Physiology
Required Slide Session Objectives Auditory System: At the end of this session, students will be able to: 1. Characterize the range of normal human hearing. 2. Understand the components of the major divisions of the outer, - middle, and inner ear. 3. Understand auditory transduction and otoacoustic emissions. 4. Understand causes of hearing loss. 5. Understand consequences of hearing loss at early ages. 6. Understand the relationship between hair cells and axons in the 8th nerve. 7. Understand how sound is encoded in the central nervous system. 2
Deafness and hearing impairment About one in every 10 Americans has some degree of hearing loss. The great majority develop hearing loss as they age. Hearing impairment in very early life can prevent normal language development unless corrected or substituted with sign language.
What is sound? It is the vibration of air molecules. Sound can be categorized according to frequency and amplitude.
Sound frequency frequency: vibrations per second (pitch) Typical human speech: about 500-3000 Hz.
Sound amplitude magnitude of vibrations (loudness) measured in units of decibels (db) 0 db - Near total silence 15 db - A whisper 60 db - Normal conversation 90 db - A power lawnmower 120 db - A pop concert or a jet engine
Effects of sound Sound causes the eardrum (tympanum) to move in and out. The eardrum is located at the inner end of the external part of the ear. OUTER EAR
Outer ear pinna (ear lobe)-- funnels and filters sound ear canal (external auditory canal) --funnels sound ear drum (tympanum) -- vibrates in response to sound OUTER EAR
MIDDLE EAR OUTER EAR
The middle ear Components: malleus (hammer) attached to the eardrum malleus
The middle ear Components: malleus (hammer) attached to the eardrum incus (anvil) incus
The middle ear Components: malleus (hammer) attached to the eardrum incus (anvil) stapes (stirrup) attached to the oval window of the inner ear stapes
Functions: Middle ear: ossicles Conduct the vibration of the eardrum to the oval window. Amplify vibration about 25 times. The movements of the tympanum aren t actually very large.
Middle ear: ossicles Humans can detect sounds that involve air movements of less than 1 nm! The ossicles convert the small movements of a large structure, the tympanum, into large movements of a small structure, the stapes.
Middle ear: ossicles The transfer of energy is most efficient in the frequency range of 1-4 khz. Interruption or calcification of the ossicle chain can result in a hearing loss of 60 db.
Middle ear: stapedius and tensor tympani Muscles that protect the cochlea from damage due to very loud noises
The stapedius muscle stapedius
Attached to the stapes. The stapedius muscle Damps movements of ossicles. Contracts just before we speak or chew. Contracts in response to loud external sound. Damage (as during surgery) can produce intolerance to noise and difficulty hearing in noisy surroundings.
Tensor tympani stapedius
The tensor tympani muscle Attached to the malleus Acts reflexively with stapedius Pulls the tympanic membrane inwards and renders it more tense.
MIDDLE EAR OUTER EAR INNER EAR
The structure is where sound energy is transduced into neural signals. A spiral, snail-shaped tube divided into 3 sub-tubes by 2 membranes. The cochlea
Scala vestibuli Cochlear structure Reissner s membrane Scala media Spiral ganglion Scala tympani Basilar membrane
oval window round window
The basilar membrane uncoiled Movements of the ossicles cause the cochlear fluids to move and that leads to vibration of the basilar membrane at the oval window.
The basilar membrane uncoiled The basilar membrane s dimensions & stiffness aren t uniform. The regions near the base vibrate most in response to high frequencies. The regions near the apex vibrate most in response to low frequencies. apex base high freq. mid freq. low freq. narrower stiffest wider less stiff
Tonotopic map
Organ of Corti: tectorial membrane and hair cells tectorial membrane
inner hair cells Organ of Corti: Two types of hair cells
Sound-induced membrane movements Sound causes the tectorial membrane & the basilar membrane to pivot around slightly different points. This movement & the shear in the fluid between the membranes causes hair bundles to bend. tectorial basilar membrane membrane
Inner hair cells (IHCs) 3,500 inner hair cells per cochlea Primarily involved with sending information about sounds to the central nervous system
Two types of hair cells outer hair cells inner hair cells
Outer hair cells About 12,000 OHCs per cochlea Their role is not to bring messages to the brain about what we hear but instead to improve the cochlea s ability to discriminate different sounds.
Outer hair cells How do they do this? They lengthen or contract in response to sound or messages brought to the cochlea from the brain via efferent axons. This motion selectively amplifies or damps the motion of the basilar and tectorial membranes. The process reduces the cochlea s response to constant background noise, allowing a greater response to transient and selectively-attended sounds.
Outer hair cells: Otoacoustic emissions Sounds that are produced by healthy ears in response to acoustic stimulation. They are byproducts of the activity of the outer hair cells in the cochlea. Useful for screening infants for malfunction in the cochlea 1 child in 1000 is born with hearing impairment.
Outer hair cells: Otoacoustic emissions A normal auditory pathway from the hair cells into the brain and back through the efferents will yield otoacoustic emissions.
Hair cell loss If damage or a genetic anomaly has destroyed hair cells but not eighth nerve fibers, then it is possible to implant a cochlear implant that will directly activate the correct nerve endings.
Cochlear implant
Cochlear implant 2. Transmitter 3. Receiver and stimulator 1. Sound receiver and processor 4. Cochlea
1 khz
https://youtu.be/vzlnasvdaga
Critical period for auditory development Growing evidence indicates cut-off of about age 3-4 for normal acquisition of language. Connections to auditory cortex don t develop normally without auditory input.
Critical period for auditory development Test by using EEG to monitor cortical auditory evoked potential. Normal hearing Early implant (age 6 mo.) Late implant (age 6 yrs.)
Genetic causes of deafness More than 40 genes have been identified that cause deafness Some dominant, some recessive No hair cells, incorrect orientation of hair cells, disorganized hair cell bundles, no separation of endolymph and perilymph compartments, no tip links.
Hearing loss later in life Chemically-induced loss of hair cells: drugs such as streptomycin diuretics such as furosemide anticancer chemotherapeutic drugs, such as cisplatin
Short-term noise-induced hearing loss Tip links break easily with exposure to noise. Unlike hair cells, which can t regenerate in humans, tip links repair themselves, mostly within a matter of hours. The breaking of tip links, and their regeneration, is one of the causes of the temporary hearing loss after a loud blast of sound (or a loud concert).
Long-term noise-induced hearing loss Exposure to excessive noise is a common cause of hearing loss. Noise exposure leads to hair cell death by inducing reactive oxygen species.
Long-term noise-induced hearing loss Noise also causes loss of synapses onto outer hair cells (synaptopathy). This loss doesn t change the threshold but it does make it hard to distinguish sounds, especially speech, when the noise level is high. This kind of hearing loss is very common as people age. X
Loss of high frequency hearing with age (presbycusis)
Encoding of loudness tectorial membrane Louder sounds bigger amplitude vibrations bigger changes in depolarization bigger changes in transmitter release bigger changes in firing basilar Because a single fiber can not respond over the full listening range of 120 db, intensity must be coded in populations of eighth nerve fibers with different thresholds. membrane
Encoding of loudness Why bother to have so many 8 th nerve cells contact each hair cell? One reason is to encode different sound intensities (loudness). A single fiber can not respond over the full perceptible intensity range (> a billion-fold).
Encoding of loudness Why bother to have so many 8 th nerve cells contact each hair cell? One reason is to encode different sound intensities (loudness). A single fiber can not respond over the full perceptible intensity range (> a billion-fold). Therefore, loudness is coded in populations of eighth nerve fibers with different thresholds.
Frequency coding: tonotopic map A given hair cell will respond only to sounds of a given range of frequencies. Since the typical 8 th nerve fiber gets input from a single hair cell, 8 th nerve fibers also have a tonotopic map, depending on their cochlear location.
Characteristic frequency The hair cells and axons can respond to other, nearby frequencies too, if they are loud enough. A very quiet sound will activate only a few hair cells & axons, but a louder sound will activate more cells. Line shows threshold, the quietest sound that elicits a response at a particular frequency
Summary of 8th nerve information encoding Eighth nerve activity encodes frequencies of sound (Which axons are firing faster?) loudness of sound (How many axons are firing, and how fast are they firing?)
Response properties of cells in the cochlear nuclei Some cells respond like eighth nerve fibers. They are most sensitive to one characteristic frequency. They will also respond to nearby frequencies, as the sound becomes louder.
Response properties of cells in the cochlear nuclei Some respond to a wide range of frequencies. Some cells in the cochlear nuclei respond to only a very narrow range of frequencies. They are inhibited by higher and lower frequencies.
Response properties of cells in the cochlear nuclei Some respond to a wide range of frequencies. Some cells in the cochlear nuclei respond to only a very narrow range of frequencies. They are inhibited by higher and lower frequencies. Some cells respond only when the sound is changing in loudness.
Auditory responses beyond the cochlear nuclei Many cells respond like those in cochlear nuclei. Many cells at higher levels are more selective for particular frequencies than are cells in the cochlear nuclei. threshold frequency frequency
Auditory responses beyond the cochlear nuclei Many cells respond like those in cochlear nuclei. Many cells at higher levels are more selective for particular frequencies than are cells in the cochlear nuclei. Some show more complex properties, such as selectivity for frequency modulation. (Such cells respond only to changes in frequency.)
Localizing sound sources There are 2 kinds of binaural cues: Timing Intensity
Localizing sound sources The pinna helps in localizing whether a sound is coming from in front of the head or from the back. The pinna acts as a directional amplifier of sound. The frequency profile of a particular sound will be altered depending upon where it comes from.
Sound localization functions of the cortex Many location-selective cells respond best to sounds coming from the opposite side of the head. Some cells are particularly responsive to sounds from moving sources.
Cortical specializations The subdivisions of the auditory regions of cortex are still being sorted out. Strong evidence indicates that there is a what stream and a where stream, but the details are still being worked out.