Chapter 9 Auditory System
External ear (function)
the external ear contributes to sound detection by collecting and channeling the collected sound waves in the external auditory meatus to the TM) the pinna and EAM enhance the peak resonance of the sound and play a role in localization)
frequency
the number of complete wavelengths that pass a point in a given time the speed of particle vibration is measure in cycles per second (frequency) or Hertz (Hz). frequency analysis determines if the sound is pitch (harmonically related tones) or noise (random tones) frequencies important for human speech fall within the range of 250-8,000 Hz, with most sensitive frequencies from 1,000-3,000 Hz
mixed hearing loss
the presence of both conductive and sensorineural hearing loss
Summary of auditory FUNCTIONS and STRUCTURES within the auditory system
(STRUCTURE - FUNCTION) external/middle ear - signal amplification and impedance matching cochlea (hair cells) - fluid motion conversion in neural impulses spiral ganglia - first-order nerve cells sound transmission auditory nerve - signal transmission to CNS cochlear nuclear complex, trapezoid body, superior olivary nucleus, inferior colliculus, brachium of inferior colliculus, medial geniculate body (MGB), primary and association auditory coritices - CNS structures involved with transmitting, integrating, and processing (for a fine perception and recognition) of auditory signals
Rinne test (normal hearing vs. conductive loss vs. sensorineural loss)
(tuning fork placed on mastoid process) NORMAL air better than bone conduction CONDUCTIVE LOSS better hearing through bone conduction than air conduction SENSORINEURAL LOSS reduced sensitivity to both air and bone conduction
Weber test (normal hearing vs. conductive loss vs. sensorineural loss)
(tuning fork placed on scalp) NORMAL lateralized to both ears CONDUCTIVE LOSS sound lateralized to affected ear SENSORINEURAL LOSS sound lateralization to better ear
The human ear is sensitive to an SPL intensity range of ______________________.
0-140 dB (with most spoken communication taking place at 60 dB)
Name the 4 distinctive properties of the auditory system
1.) bilateral auditory representation - multiple crossings of auditory info through ascending interconnections at the levels of the cochlear nuclei - primary auditory cortex in each hemisphere receives input from both ears - lesion at any point along the central auditory pathway extending from the pons to the auditory cortex would NOT result in complete hearing loss, but cause only a mild bilateral attenuation of hearing 2.) tonotopic representation - discrete tonal representation at the cochlear level is maintained throughout central auditory pathway - despite interconnections and crossings, tonal representation from the hair cells is retained throughout the auditory system - tones are represented even in the primary auditory cortex, where a single neuron responds best to certain frequencies! 3.) sound source localization - each ear receives a particular sound at different pints in time; this time difference is known as interaural time delay - the interaural time delay is important in localizing sound source - individuals use loudness as differences to determine sound location (e.g. sound is less intense in ear further from source) 4.) descending auditory projections for the tuning of the receptors - parallel to the ascending projections are descending auditory fibers that exist from the primary auditory cortex to the cochlear hair cells - descending fibers of the feedback circuits refine perception of pitch and loudness and sharpen reception of specific frequencies through the process of lateral inhibition - lateral inhibition improves signal-to-noise ratio - descending connections contribute to better hearing in noisy situations through the suppression of competing background sounds
Sounds greater than ______ dB cause pain.
140
tensor tympani and stapedius
2 muscles that reflexively control the motion of ossicles these muscles reflexively protect the auditory mechanism from damage by controlling the ossicular motion when exposed to high-intensity sounds stapedius = controlled by the facial (CN VIII) nerve; inserts into the stapes and contracts to restrict stapes movement in response to sounds louder than 70-80 dB tensor tympani = controlled by the trigemincal (CN V) nerve; inserts in to the malleus and participates in restricting ossicular movements in response to louder sounds, specifically noises; extent of noise attenuation by tensor tympani is small, as this muscle is more involved with the flexibility of the TM together, both muscles stiffen the ossicular system and attenuate the transmission of energy for high-intensity sounds to the inner ear (attenuation reflex) with a reflex latency time of 50-150 ms; this latency is too long to fully protect the inner ear from any loud noise sound intensity can only be attenuated by only 10-15 dB, which is largely caused by the action of the stapedius muscle and affects only low-frequency sounds, leaving the high frequencies largely unaltered
Frequencies important for human speech fall within the range of ______________, with most sensitive frequencies from 1,000-3,000 Hz
250-8,000 Hz
A sound that is 10 times the power of 20 uPa (level of just an audible sound) has a 20 dB SPL. A sound that is 100 times the power of the reference sound pressure (level of just an audible sound) has a ________________.
40 dB SPL
Most spoken communication takes place as _____ dB.
60
Prolonged and repeated exposure to sounds greater than ____________ dB may cause structural damage to the cochlear hair cells.
90-100 (Sounds greater than 140 dB cause pain)
basilar membrane
A structure that runs the length of the cochlea in the inner ear and holds the auditory receptors, called hair cells known for its tonotopic representation, which ranges from 20-20,000 Hz, with higher frequencies at its base, and lower frequencies at its apex (tip) the base = where higher frequencies are represented = narrower apex (tip) = where lower frequencies are represented = wider as basilar membrane is displaced at its base, the deformation moves toward the apex of the cochlea (where lower frequencies are represented) as a traveling wave; as pressure wave moves, velocity slows, but amplitude increases toward the apex, eventually reaching its maximum different sound frequencies produce different traveling wave patterns, with peak amplitudes at different locations on the cochlea peak amplitude of traveling wave for high-frequency sounds occurs near the base of the basilar membrane peak amplitude of traveling wave for low-frequency sounds occurs near the apex of the basilar membrane (lower frequencies travel further towards the apex) a signal consisting of many frequencies causes a sound wave to have multiple peaks along the basilar membrane
Compare the organization of the auditory pathway and the somatosensory system.
AUDITORY PATHWAY contains multiple synapses between the cochlear nuclei (second-order neurons) and the thalamus (third-order neurons) SOMATOSENSORY SYSTEM 3-neuron organization
inferior colliculus
All lateral lemniscus fibers synapse on nuclei in the inferior colliculus both inferior colliculi are connected through commissural fibers, permitting further axonal crossing and potentially integrating monaural and binaural properties of the auditory inputs; thus potential implications for a more refined localization of "what" and "where" attributes" of signals and their sources primary output is to thalamus (as its projections travel through the brachium to the MGB) part of tectal brainstem circuitry, which provides organism with a 3-dimensional neurologic map of the external environment (eyes movements, head turns, and body rotations occur in repsonse to visual stimuli projects fibers to deep layers of superior colliculus axonally connected to reticular formation in midbrain
High, medium, and low frequencies have different sites of maximum amplitude along the cochlea. Explain.
FREQUENCY - SITE OF MAX AMPLITUDE High frequencies - near oval window Medium frequencies - middle Low frequencies - near helicotreme
central auditory system
Includes: LOWER BRAINSTEM (cochlear nuclei, superior olivary nucleus, and lateral lemniscus), UPPER BRAINSTEM (inferior colliculus and MGB), PRIMARY AUDITORY CORTEX
Does 0dB mean no sound?
No, it means that the measured sound pressure (Px) is equal to the reference sound pressure (Pr)
The scala media contains the ________________.
Organ of Corti (with sensory hair cells which are the primary receptor cells)
primary and secondary auditory cortex
PRIMARY AUDITORY CORTEX Brodman area 41 located in the transversely oriented Hecshl gyri, which are buried in the lateral sylvian sulcus on the dorsal surface of the superior temporal gyrus extends onto the lateral surface of the superior temporal gyrus surrounded by the secondary auditory cortex (Brodman area 42) receives impulses through crossed and uncrossed fibers known to retain the cochlear tonotopic representation, with a representation of higher frequencies in the posteromedial region and lower frequencies in the anterolateral region of the gyri of Heschl; area between receives fibers carrying middle range frequencies not essential for basic frequency discrimination alone, but it does screen for patterned acoustic properties that not only characterize the speech and vocalization, but also play a role in the discrimination of auditory signals (e.g. timing patterns, abrupt onset of vocalization, and sudden shifts in frequencies of auditory events) SECONDARY AUDITORY CORTEX Brodman area 42 receives input from primary auditory cortex as well as some from the thalamic MGB along with the primary auditory cortex, it either processes other essential properties of the signal or undertakes high tier analysis of the properties such as timing patterns, abrupt compositional shifts, an the spatial distribution of vocal energy PRIMARY/SECONDARY CORTEX area posterior to auditory cortical region is the area of planum temporale (temoral planum), the temporal region in the Sylvian fissure hidden by the overlying operculum of the temporal, parietal, and frontal lobes; next to the Wernicke cortex extensive axonal bundle connects the auditory (primary and secondary) cortex to the language association cortex, or Wernicke area (Brodman 22) PATHOLOGICAL INVOLVEMENT OF AUDITORY CORTEX acoustic form of aphasia; impaired ability to perceive and discriminate speech sounds
Review the definition of Pr in the formula for calculating the sound pressure level (SPL) in decibels: SPL (dB) = 20 log Px/Pr
Pr =reference sound pressure =0.0002 dyne/cm squared = 20uPa = SPL required to make a 1,000 Hz sound just audible to the human ear
Regarding information transmission in the central auditory pathway, there are 2 things that apply: 1.) one preserves information 2.) the other processes the transmitted auditory information. Elaborate.
Preserving information involves the retention of the tonotopic details from the cochlear hair cells to the primary auditory cortex, and it also involves the intensity attribute Processing refers to the contralaterality of auditory projections, incorporation of monaural and binaural afferents, and integration of audition with other information, including selective screening and attention in signal processing
Briefly summarize the process of audtion
Sound waves, channeled through the external auditory meatus strike the TM Vibration of TM converts pressure waves into mechanical energy, setting the ossicles into motion Mechanical energy is transformed into hydraulic energy in the cochlear fluid of the inner ear Patterned hydraulic waves stimulate the cochlear hair cells, which send coded impulses through the fibers of the vestibulocochlear nerve (CN VIII) to the cochlear nuclear complex in the brainstem The cochlear complex nuclei transmit nerve impulses to multiple points in the brainstem and thalamus before projecting to the primary auditory cortex (Brodman area 41) (Note that the primary and secondary auditory cortices, located in the Heschl gyri and the area around it on the superior surface of the temporal lobe, may be involved with processing and segmenting different components of the sound signal analysis needed for signal discrimination and sound perception as human speech.) (For speech perception, the next step would involve the projection of perceived sound signals to the posterior-superior temporal lobe (Wernicke area) in the left hemisphere where perceived speech signals are interpreted into language-specific signals.)
Changes in intensity are perceived as changes in loudness, though there is not always a 1:1 relationship. Explain.
There is not always a 1:1 relationship because the human ear is NOT equally sensitive to all sound frequencies; more intensity is required at some frequencies than others E.g. the average normal hearing listener needs 45.5 dB to hear a 125 Hz sound, but requires only 8.5 dB to hear a 2,000 Hz sound
decibel
Unit used to measure sound intensity (loudness) in audiological examinations by calculating the ratio between two sound pressures 4 ways dB can be measured: 1.) IL (intensity level) 2.) SPL (sound pressure level) 3.) HL (hearing level) 4.) SL (sensation level)
internal auditory meatus
a canal in the petrous portion of the temporal bone that opens into the cranial cavity at the side of the junction of the pons and medulla
presbycusis
age-induced SNHL due to the degeneration of hair cells progressive hearing impairment affects perception and discrimination of sound primarily affects high frequencies results from degeneration of hair cells in the first turn of the cochlear duct
neural coding of auditory information
all properties of sound (timing, intensity, frequency) are fully retained throughout the path to the brain in the transmission of audition it is likely information is coded in a variety of ways; frequency represented by the stimulation of the region-specific hair cells along the basilar membrane may only be one way of coding other possible ways information may be coded: -the number of cochlear units responding to a specific frequency - the synchronized firing patterns of auditory nuclei - location of fibers in ascending tract -sound intensity is most likely coded by the number of related neural units stimulated along the basilar membrane, by the intensity of discharging rates of fibers, or by the number of axons involved in the information transmission
membranous labyrinth
arrangement of endolymph-filled cavities that include scala media of the cochlear duct and vestibular labyrinth and contain receptive hair cells the dual-functioning mechanism contains the saccule, utricle, semicircular canals, and the cochlear duct) encased in a series of cavities in the petrous portion of the temporal bone (bony labyrinth) 2 functions: 1.) mediates equilibrium [achieved by the vestibular apparatus (saccule, utricle, and semicircular ducts)] 2.) serves hearing and other special sensory functions [achieved by the cochlear duct (scala media)]
temporal planum (planum temporale)
association language cortex that is located posterior to Heschl gyri on the superior surface of the left temporal lobe; associated with cerebral dominance
organ of Corti
auditory receptor organ that contains hair cells and supporting cells; located in the scala media contains outer and inner hair cells Collectively, outer and inner hair cells embedded in the tectorial membrane are termed the organ of Corti this is the location of sound transduction
Elaborate on what happens in regard to basilar membrane movement and the hair cells
basilar membrane movement produces mechanical displacement of the cilia relative to the tectorial membrane shearing effect bends apical ends of the hair cells, resulting in increased cilia permeability to K+ inward movement of ions leads to a graded (local) depolarization of the hair cell depolarized cell causes synaptic vesicles to release a neurotransmitter in the synaptic cleft between the hair cells and the cochlear nerve fibers; this depolarizes the afferent cochlear nerve terminals, and the generated action potentials travel to the brainstem through the fibers of the vestibulocochlear nerve (CN VIII) (see electrical transduction flashcard for more info)
depolarization
changes in membrane potentials in which the cellular interior changes from negative (resting potential) to positive MORE DETAILED: mechanical deformation of the stereocilia in the cochlea cells opens K+-sensitive pores, allowing the movement of potassium into the cell bodies through the cilia tips in the scala media due to the K+ influx, the depolarized hair cell (local potential) opens the voltage-gated calcium channels, causing calcium ions to move int othe cell; subsequencly, the triggered chemical potential causes synaptic vessels at the base of the hair cell to release glutamate (a fast excitatory neurotransmitter) into the synaptic space, which is picked up by nerve receptors, and thus causing the depolarization of the cochlear nerve terminals action potentials generated in the nerve terminals travel thorugh fibers of CN VIII to the cochlear nuclear complex located at the pontomedullary junction
Meniere disease
chronic condition of progressive hearing loss, vertigo, and tinnitus secondary to pressure changes in the membranous labyrinth chronic condition associated with edema and excessive endolymph pressure in membranous labyrinth marked by progressive and fluctuating hearing loss, sensation of ringing in ears, and vertigo
cochlear nuclear projections
cochlear nuclear complex sends multiple projections to both the ipsilateral and contralateral ascending auditory pathways most cochlear projections cross the midline to project to opposite cortical areas; the cochlear projections that cross the midline travel in three bundles (dorsal acoustic stria, intermediate stria, and trapezoid body); fibers of trapezoid body are by far the most important and largest stria as they cross the midline to terminate in the superior olivary nucleus a small number of cochlear projections remain ipsilateral and ascend on the same side of hte auditory cortex; either send projections to the ipsilateral superior olivary nucleus or bypass it on their way to the ipsilateral lateral lemniscus
scala media
cochlear region lying between the scala vestibuli and the scala tympani Organ of Corti is located in the scala media
modiolus
conical bony structure around which the cochlea is wrapped there are approximately 30,000 spiral ganglia cells in the modiolus (types I and II)
cochlear nuclear complex
considered to have 900,000 functional units fibers of the vestibulocochlear nerve (CN VIII) terminate in the cochlear nuclear complex (after entering the brainstem at the pontomedullary junction) the entering fibers of CN VIII divide into dorsal and ventral branches and synapse onto the respective (dorsal and ventral) cochlear nuclei highly specialized cells, such as bush and multiform, have been identified in the cochlear nuclear complex second-order neuron governed by discrete tonotopic organization fibers carrying low-frequency information from apex of the cochlea terminate at the superficial layers of the cochlear nuclear complex fibers carrying high-frequency information from the base of the cochlea terminate deeper in the cochlear nuclear complex sends multiple projections to both the ipsilateral and contralateral ascending auditory pathways; the cochlear projections that cross the midline travel in three bundles (dorsal acoustic stria, intermediate stria, and trapezoid body)
primary auditory cortex
considered to have over 100,000,000 neurons (indicating its superior ability compared to the cochlear nuclear complex)
Inner ear (function)
consists of dual-functional mechanism for serving the special sensory modalities of audition and equilibrium, which are served by the interconnected fluid-filled membranous labyrinth ducts the dual-functioning mechanism is the membranous labyrinth the dual-function membranous labyrinth (containing the saccule, utricle, semicircular canals, and the cochlear duct) is encased in a series of cavities in the petrous portion of the temporal bone (bony labyrinth) 2 functions of membranous labyrinth: 1.) mediates equilibrium [achieved by the vestibular apparatus (saccule, utricle, and semicircular ducts)] 2.) serves hearing and other special sensory functions [achieved by the cochlear duct (scala media)]
Inner ear (structure)
consists of the bony labyrinth (temporal bone) and the membranous labyrinth (saccule, utricle, semicircular ducts, and cochlear duct) cochlea is a snail-shaped structure coiled 2.5 times around the modiolus (the bony core of the cochlea); cochlea consists of 3 fluid-filled scalae (scala vestibuli, scala media, and scala tympani)
auditory reflexes
coordinate head and eye movements toward sound influence vestibular functions reflex mechanism that involves 3 anatomic pathways: FIRST PATHWAY: includes projections from the inferior colliculus to the superior colliculus and tectum; this integrated the auditory and visual systems and controls extaocular head and neck movements; also involved with startle and attention reflexes SECOND PATHWAY: from the superior olivary nucleus to the medial longitudinal fasciculus coordinates activity of the oculmotor nerve (CN III), trochlear nerve (CN IV), and abducens nerve (CN VI); impulses traveling on the medial longitudinal fasciculus regulate directional ocular movements in response to auditory stimuli THIRD PATHWAY: includes auditory projections to the vestibular nuclear complex in the brainstem and participates in equilibrium
tectorial membrane
covering membrane that extends over the entire Organ of Corti collectively, outer and inner hair cells embedded in the tectorial membrane are termed the organ of Corti As the basilar membrane vibrates, the tiny clusters of hair cells are bend against the tectorial membrane, triggering the hair cells to fire triggers the hair cells of the organ of corti to fire
eustachian tube
duct that connects the middle ear with the nasopharynx; it equalizes air pressure on both sides of the TM (eardrum)
helicotrema
end region of the cochlea that connects the scala vestibuli to the scala tympani
The scala media is filled with __________________.
endolymph
Central Auditory Pathway
extends from the cochlear nuclear complex and its relay - nuclei up to and including the primary auditory cortex Regarding information transmission, there are 2 things that apply: 1.) one preserves information 2.) the other processes the transmitted auditory information structures included in this pathway are: cochlear nuclei (lower brain stem) superior olivary nuclei (lower brain stem) lateral lemniscus (lower brain stem) inferior colliculus (IC) (upper brain stem) brachium of the inferior colliculus medial geniculate body (MGB) (upper brain stem) geniculocortical fibers (auditory radiations) primary auditory cortex (in Heschl gyri)
The process of audition (hearing) begins as the sound waves, channeled through the _________________ strike the ______________.
external auditory meatus tympanic membrane
External ear (structure)
external auditory meatus (a.k.a. ear canal) pinna (the external ear contributes to sound detection by collecting and channeling the collected sound waves in the external auditory meatus to the TM) (the pinna and EAM enhance the peak resonance of the sound and play a role in localization)
endolymph
extracellular fluid that fills the semicircular canals, utricle, and saccule the scala media (cochlear region lying between the scala vestibuli and the scala tympani) is filled with endolymph compositionally similar to the intracellular fluid with a high concentration of potassium (K+) secreted by the stria vascularis (which is located on the outer wall of the cochlea)
lateral lemniscus
fibers projecting auditory impulses between the superior olivary nucleus and the inferior colliculus primary ascending auditory pathway extends from the superior olivary nucleus to the inferior colliculus of the midbrain receives the crossed and uncrossed projections from the dorsal, ventral, and intermediate striae fibers climb laterally in the pontine tegmentum retains a bilateral representation with a stronger representation from the opposite ear; this bilaterality of projections explains why the pathology of the central auditory pathway at any level does not lead to a profound hearing impairment in either of the ears fibers not only project ipsilaterally and bilaterally to the inferior colliculi, but they may also be responsible for additional analysis of signals in terms of selecting and screening information
perilymph
fluid contained in the bony labyrinth (specifically, contained in the scala vestibuli and scala tympani) that protects the membranous labyrinth
cochlea
fluid-filled spirally coiled structure that contains the organ of Corti, the sensory ogran of hearing uniquely equipped to analyze and identify the individual frequency components of a complex sound
sensorineural hearing loss (SNHL)
hearing loss resulting from a dysfunctioning organ of Corti or cochlear nerve (CN VIII) associated with damage to the cochlear hair cells and/or auditory nerve can range from mild to severe characterized by difficulty understanding speech, particularly in noise accompanied by tinnitus and recruitment (abnormally rapid growth of loudness after the hearing threshold is reached) patients usually speak loudly as a result of reduced self-monitoring ability CAUSES: - prolonged exposure to noise - toxicity from accumulation of certain antibiotics in endolymph - Meniere disease - presbycusis - disease, irritation, or pressure on nerve trunk can affect vesibulocochlear nerve (CN VIII) - tumors of the shealth (schwann cells) of the vestibulocochlear nerve (CN VIII); e.g. vestibular schwannoma, acoustic neuroma
conductive hearing loss (CHL)
hearing loss that results form an interrupted transmission of sound through the outer and/or middle ear to the cochlea characterized by fluctuating hearing loss, good word-speech recognition ability, specifically at high intensities; softly spoken speech; impaired auditory reflex; and most importantly AIR BONE GAP. less severe communicative effect CAUSES: -otosclerosis (abnormal bone growth near the over window which impedes the movement of stapes) -otitis media (accumulation of fluid in middle ear causes eustachian tube malfunction)
association cortex (Wernicke area)
includes part of the planum temporale, posterior-superior first temporal gyrus, and the inferior parietal lobe concerned with the comprehension of spoken language
HL (hearing level)
indicated sound intensity in relation to average normal hearing HL of 0 dB denotes an intensity level that is barely heard by the human ear
stria vascularis
located on outer wall of cochlea; located laterally in the scala media secretes endolymph contributes to maintenance of the ionic difference between the endolymph and perilymph fluids, which is essential for optimal functioning of inner ear structural and functional abnormalities related to stria vascularis have an effect on the functioning of cochlear hair cells functioning of stria vascularis is highly susceptible to drug-induced ototoxicity degeneration of stria vascularis causes a proportionate hearing loss involving all frequencies
Changes in intensity are perceived as changes in _____________.
loudness
Low and high frequency sounds are perceived respectively as ____________________________.
low and high-pitched sounds *though the relationship is not always linear
scala tympani
lowermost compartment of the cochlea, which is connected to the scala vestibuli through the helicotrema
incus
middle ear bone
stapes
middle ear ossicle attached to oval windoq
malleus
middle ear ossicle attached to the TM
central auditory impairment
most identifying clinical feature is the near-normal sensitivity/thresholds to auditory stimuli with impaired processing of linguistic and metalinguistic signals **Note that the central auditory system includes: 1.) LOWER BRAINSTEM 2.)UPPER BRAINSTEM 2.)PRIMARY AUDITORY CORTEX Lesions to lower brainstem - minimal effect on hearing sensitivity, profound effect on ability to identify/localize sound source, impaired processing of speech in noise, impaired ability to retain critical linguist information in conversation setting Lesions to upper brainstem: impaired ability toselect and attend to auditory information, impaired ability to regulate processing speed, impaired ability to integrate audition with visual-motor functions for reflexive responses Lesions to auditory cortex: impaired ability to perceive and discriminate speech signals; acoustic aphasia and Wernicke aphasia (in the case of lesions to the language association cortex); difficulty with processing environmental sounds, nonverbal memory, and musical and prosodic interpretation (in the case of lesions to the right hemisphere of auditory cortex)
tensor tympani
muscle of middle ear regulated by the trigeminal nerve (CN V) reflexively contracts to attenuate hearing sensitivity by restricting ossicular movement inserts in to the malleus and participates in restricting ossicular movements in response to louder sounds, specifically noises extent of noise attenuation by tensor tympani is small, as this muscle is more involved with the flexibility of the TM
stapedius
muscle of the middle ear controlled by the facial nerve (CN VIII) reflexively contracts to attenuate hearing sensitivity by restricting ossicular movement inserts into the stapes and contracts to restrict stapes movement in response to sounds louder than 70-80 dB
plasticity in the auditory cortex
neuronal plasticity of auditory cortex continues into adult life electrical stimulation of nucleus basalis in rats (Kilgard and Merznich, 1998) found that auditory neurons were found to respons to tones near 9 kHz later on in life
cochlear implants
newer technique used for assisting children born with congenital deafness used to treat profound SNHL involves implantation of multiple pairs of electrodes to the cochlea integrated speech processing algorithm is used to send signals to electrodes after converting incoming sounds into electrical impulses results in stimulation of different regions of the cochlea if applied early, can promote the development of speech and language functions in individuals born with profound SNHL
superior olivary nuclei
nucleus in the medullary tegmentum that is the first to receive projections from both cochleae and plays an important role in sound localization collection of nuclei in the pons first structure in the neural circuitry to connect with both the ipsilateral and contralateral cochlear nuclei contains binaural cells uniquely equipped to calculate differences in the time and intensity of auditory stimuli from BOTH ears contains two sets of dendrites extending from the opposite sides of its soma; medial dendrite largely receives projections from the contralateral cochlear nuclei; lateral dendritic set of fibers receives input mostly from the ipsilateral cochlear nuclei
Middle ear (function)
ossicles of middle ear connect the TM to the oval window of the inner ear regulates energy transmission to the inner ear using the stiffness of the ossicles to compensate for the difference in impedance between air and fluid, and also compensate for the size discrepancy between the TM (50 mm squared) and the oval window (3.5 mm squared); this compensation is achieved by reducing the movement of the TM and by increasing its force on the cochlear oval window to ensure optimal transmission mass and stiffness of ossicular chain restricts motion and speed, which limits the range of sound frequencies that can be efficiently transmitted through the middle ear; thus, we only hear the sound frequencies that are not dampened by the ossicular motion limitations eustachian tube (which runs from middle ear to nasopharynx) ventilates the middle ear by equalizing middle-ear pressure with the atmospheric pressure
Middle ear (structure)
ossicles: malleus, incus, and stapes air-filled cavity containing 3 bones (ossicles) and two muscles (tensor veli palatine and levatator veli palatine) ossicles are suspended by ligaments motion of ossicles is reflexively controlled by two muscles: tensor tympani and stapedius; these muscles reflexively protect the auditory mechanism from damage by controlling the ossicular motion when exposed to high-intensity sounds) eustachian tube runs from middle ear to nasopharynx
vestibular apparatus
part of the inner ear responsible for detecting head motion
Elaborate on the peak amplitudes of travelling waves in the cochlea
peak amplitude of traveling wave for high-frequency sounds occurs near the base of the basilar membrane peak amplitude of traveling wave for low-frequency sounds occurs near the apex of the basilar membrane (lower frequencies travel further towards the apex) high-frequency sounds cause a small region of the basilar membrane near the stapes to move, while low frequencies cause almost the entire membrane to move a signal consisting of many frequencies causes a sound wave to have multiple peaks along the basilar membrane
The scala vestibuli and scala tympani are filled with ________________.
perilymph
scala vestibuli
perilymph-filled uppermost compartment of the cochlea
word deafness
quasi-language-based syndrome uncommon marked by severe loss of language comprehension speech production, naming, and reading are in tact typically a result of a bilateral temporal lesion which causes an isolation of auditory cortices from the Wernicke area by interrupting the fibers bilaterally radiating from the gyrus of Heschl
SPL (sound pressure level)
ratio of the absolute, Sound Pressure and a reference level (usually the Threshold of Hearing, or the lowest intensity sound that can be heard by most people) SPL is measured in decibels (dB), because of the incredibly broad range of intensities we can hear (internet definition)
Hair cells
receptive cells of the inner ear stretched along the cochlear duct in two rows (the inner and outer hair cells) positioned on a thickened ridge of the basilar membrane three outer hair cells and one inner hair cell upon each segment of the basilar membrane outer hair cells (OHCs) are more numerous (approx. 12,000); projecting from each OHC are 40-50 stereocilia inner hair cells (IHCs) are less numerous (approx 3500); projecting from each IHC are 50-70 stereocilia "hairs" on these cells project toward the inside of the cochlear duct and are embedded in a covering membrane (the tectorial membrane) taller cilia (apical ends) project into overlying tectorial membrane, which extends over the entire Organ of Corti IHCs and OHCs differ in terms of innervation; bases of IHCs are innvervated by the cochlear nerve endings; OHC are connected to the projections from the descending auditory pathways, particularly the olivocochlear bundle (OCB) collectively, these structures are termed the organ of Corti and this is the location of sound transduction
Heschl gyri
refers to short and oblique convolutions in the lateral sulcus that form the primary auditory cortex
attenuation reflex
reflexive contraction of the middle ear muscles causing a decrease in auditory sensitivity
intensity
represented by the amplitude of sound waves strength of molecular movement correlates with perceived loudness; amplitude of energy in sound waves that determines loudness measured in decibels defined as the log of the ration between the measured sound pressure at the TM and a well-defined reference sound pressure waves formula for calculating the sound pressure level (SPL) in decibels is: SPL (dB) = 20 log Px/Pr Px = measured sound pressure Pr = reference sound pressure (always 0.0002 dyne/cm squared, or 20uPa which represents the SPL required to make a 1,000 Hz sound just audible to the human ear) *If Px=Pr, then the SPL is 0dB
bony labyrinth
series of cavities in the petrous portion of the temporal bone that contain the cochlea, semicircular canals and vestibule.
tympanic membrane
soft tissue covering that separates the external ear from the middle ear
tonotopic
systematic organization of frequency distribution in the auditory cortex relating to or being the anatomic organization by which specific sound frequencies are received by specific receptors in the inner ear with nerve impulses traveling along selected pathways to specific sites in the brain (internet definition)
medial geniculate body (MGB)
thalamic nucleus that mediates auditory information to the primary auditory cortex thalamic relay nucleus for the transmission of auditory information located in lateral caudal portion of the lower layer of the thalamic nuclei located between basal ganglia and thalamic nuclei receives tonotopic input from the ipsilateral inferior colliculus no known crossing of impulses directly at level of MGB projection fibers pass ventrally and caudally to the lenticular portion of the internal capsule and terminate in the ipsilateral primary auditory comrtex -- the gyri of Heschl in the superior temporal lobe likely helps to further integrate attentional processes with auditory information, as well as use auditory afferents to regulate visceral functions, emotional expression, and the pain mechanism also contributes to refined signal analysis
electrical transduction
the action potentials (generated by the depolarization of the hair cell) include: 1.) the chemical (ionic) properties of the hair cells 2.)the transmission of the charged ions through the cell membranes mechanical deformation of the stereocilia in the cochlea cells opens K+-sensitive pores, allowing the movement of potassium into the cell bodies through the cilia tips in the scala media due to the K+ influx, the depolarized hair cell (local potential) opens the voltage-gated calcium channels, causing calcium ions to move int othe cell; subsequencly, the triggered chemical potential causes synaptic vessels at the base of the hair cell to release glutamate (a fast excitatory neurotransmitter) into the synaptic space, which is picked up by nerve receptors, and thus causing the depolarization of the cochlear nerve terminals action potentials generated in the nerve terminals travel thorugh fibers of CN VIII to the cochlear nuclear complex located at the pontomedullary junction
spiral ganglia
there are approximately 30,000 spiral ganglia cells in the modiolus (type I and type II) type I - amount to 90% of spiral ganglion cells, respond to a narrow range of frequency by bein connected to only a few select hair cells in the cochlea-selectivity processing type II - axonal processes from these cells synapse with 10 or more hair cells, suggesting their sensitivity to a wide range of frequencies (less precision and less selectivity)
ossicles
three small bones in the middle ear: malleus, incus, and stapes
cochlea (function)
transfers mechanical energy into neural impulses basilar membrane responds to transmitted pressure in the cochlear perilymph by its displacement
retrocochlear (post cochlea) auditory mechanism
transmits signals from the hair cells in the Organ of Corti to the brainstem cochlear nuclear complex hair cells transmit nerve impulses to the peripheral (many unmyelinated) axons of the unipolar spiral ganglia (first-order neuron in the bony modiolus) the central (myelinated) process of the spiral ganglion cells form the acoustic branch of the vestibulocochlear nerve (CN VIII) and pass through the internal auditory meatus (a canal in the petrous portion of the temporal bone that opens into the cranial cavity at the side of the junction of the pons and medulla) afferent axons synapse in the cochlear nuclear complex type I spiral ganglia cells amount to 90% of spiral ganglion cells, and respond to a narrow range of frequency by being connected to only a few select hair cells in the cochlea-selectivity processing type II spiral ganglia cells synapse with 10 or more hair cells, suggesting their sensitivity to a wide range of frequencies
Evaluation of Hearing Disorders
tuning fork tests -Rinne test (stem of vibrating tuning fork is placed on mastoid process and patient is asked to listen to tone by bone conduction; when tone is no longer heard, fork is placed in front of the ear to determine if patient can hear the sound by air conduction) - Weber test (vibrating tuning fork is placed on scalp at the vertex and patient is asked to lateralize the sound by indicating if the tone is louder in one ear than the other) pure tone audiometry - used to establish threshold hearing across the frequency range (250-8,000) that is most important for human communication - audiometer generates pure tones at various frequencies and intensities - hearing is tested determining air and bone conduction thresholds tympanometry - measures compliance (elastic deformation) of TM and middle-ear pressure under conditions of changing air pressure - impaired TM compliance is indicator of middle ear pathology otoacoustic emission - tests the presence of normal pressure waves transmitted through the cochlear fluids - pressure waves can be recorded in the external ear canal - absence of normally emitted signals of OHCs indicates dysfunction of OHCs (could indicate conductive or sensorineural) auditory brainstem response audiometry (ABR) - measures neuronal activity for the brainstem auditory pathway within 10 ms after the onset of controlled stimuli, such as clicks - can be used on infants
Vascular supply to auditory mechanism
vascular supply to auditory circuitry and associated vestibular mechanism comes from brainstem Vascular supply from the internal auditory artery (a branch of the anterior inferior cerebellar artery): -inner ear cochlear mechnism -semicircular canals of the vestibular system - spiral ganglia any vascular disruption involving basilar or anterior inferior cerebellar artery is likely to result in monoaural hearing loss and falling to the affected side due to imbalance; can also result in facial paralysis; ocular movements may be impaired depending on the involvement of the abducens nerve (CN VI) Vascular supply from smaller bilaterally located pontine branches of the basilar artery: - superior olivary nucleus (ascending auditory path) - lateral lemniscus fibers (ascending auditory path) any occlusion of the pontine branches of the basilar artery or superior cerebellar artery is likely to affect the ascending auditory projections from both ears, resulting in binaural attenuation of hearing sensitivity and affecting "what" and "where" attributes of sound signals Vascular supply from thalamogeniculate artery: - thalamic MGB
How is sound created?
when force sets a molecular vibration in the medium (e.g. air), propagating a pressure wave movement of the molecules in the medium transmits sound, which is characterized by 2 major attributes: frequency and intensity
