Ear Canal Anatomy and Physiology

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Questions and Answers

Why does tactile stimulation of the external auditory meatus (EAM) sometimes cause a cough?

Tactile stimulation in the EAM can stimulate the glossopharyngeal nerve (IXth cranial nerve), which can trigger a cough reflex.

Explain why cerumen impaction is more prevalent in older men.

Cerumen impaction is more prevalent in older men due to increased hair growth in the ear canal with aging, which can trap cerumen and lead to impaction.

Describe the difference between sebaceous and ceruminous glands in the external auditory meatus (EAM).

Sebaceous glands produce an oily lubricant through passive cell breakdown, while ceruminous glands produce a wax-like substance. The combination of secretions from both glands creates different cerumen textures.

How does the compliance of the tympanic membrane (TM) affect the resonant properties of the external auditory meatus (EAM)?

The compliance of the TM leads to sound reflection and transmission. This modifies the effective length of the EAM which dampens the resonance effect and allows the EAM to resonate over a wider range of frequencies. Signup and view all the answers

Explain why the external auditory meatus (EAM) acts as a quarter-wave resonator and what frequencies are boosted as a result?

The EAM acts as a quarter-wave resonator because it is a tube with one open and one closed end (the TM). This boosts frequencies with wavelengths approximately four times the length of the EAM, typically in the 3500-4000 Hz range. Signup and view all the answers

How do head shadow effects influence sound localization, and which frequencies are primarily affected?

Head shadow effects create interaural level differences (ILDs), primarily affecting high frequencies because their wavelengths are too short to diffract around the head. This asymmetry aids in sound localization. Signup and view all the answers

Briefly describe the migration pattern of the epidermal lining within the ear canal and its relationship to cerumen.

The outer layers of skin in the ear canal migrate laterally, pushing cerumen out of the ear canal. This process aids in the natural removal of cerumen and debris. Signup and view all the answers

How do the glossopharyngeal and vagus nerves (IXth and Xth cranial nerves) contribute to potential syncope (fainting) due to mechanical stimulation of the external auditory meatus (EAM)?

Mechanical stimulation of the EAM can stimulate the glossopharyngeal and vagus nerves, leading to altered heart and blood circulation. In particularly sensitive individuals, this may cause fainting/syncope. Signup and view all the answers

Describe the boundaries of the middle ear space, including the structures that define each boundary.

The lateral boundary is the TM and squamous portion of the temporal bone, the superior boundary is the tegmen tympani, the inferior boundary is the tympanic plate of the temporal bone, the anterior boundary is the carotid wall, and the medial boundary is the dense portion of the temporal bone that houses the inner ear. Signup and view all the answers

Explain how the small volume of air in the middle ear space affects its acoustic impedance, particularly in relation to frequency.

Due to the small volume of air, the acoustic impedance in the middle ear is governed by stiffness. Stiff objects are responsive to high frequencies, leading to increased resistance and attenuation of low frequencies. Signup and view all the answers

Why is a low-frequency probe tone (e.g., 226 Hz) typically used in tympanometry, and what does it reveal about the condition of the middle ear?

A low-frequency probe tone is used because it is highly sensitive to changes in stiffness within the middle ear. It helps identify the pressure at which the tympanic membrane and middle ear system exhibit maximum compliance, indicating the presence of pathologies such as middle ear fluid. Signup and view all the answers

How does a cholesteatoma impact the middle ear, and what tympanometric results might be expected in a patient with this condition?

A cholesteatoma, a middle ear cyst, often results from long-standing negative pressure, creating a retraction pocket in the TM. Tympanograms typically show negative pressure or reduced compliance, indicating conductive hearing loss (CHL). Signup and view all the answers

Describe the progression of otosclerosis and its effects on middle ear function. What tympanometry results would be expected?

Otosclerosis involves vascular otospongiosis that is reabsorbed and replaced with sclerotic plaques, gradually fixing the stapes. This increases middle ear stiffness, leading to low-frequency conductive hearing loss (CHL) and type As tympanograms, indicating reduced compliance. Signup and view all the answers

Explain the clinical significance of the tegmen tympani and its role in middle ear health.

The tegmen tympani is a thin bone plate that forms the superior boundary of the middle ear, separating it from the cranium. Its integrity is crucial as a defect can lead to intracranial complications if an infection spreads from the middle ear. Signup and view all the answers

How does the location of the round window and oval window within the medial wall of the middle ear contribute to their respective functions in the hearing process?

The oval window connects to the vestibule, receiving vibrations from the stapes to transmit sound into the inner ear. The round window, located below, serves as a pressure release valve, allowing fluid movement within the cochlea necessary for auditory processing. Signup and view all the answers

How is the acoustic impedance of the middle ear related to the effective transmission of sound energy to the inner ear, and what structural features contribute to impedance matching?

The acoustic impedance of the middle ear needs to be matched to that of the inner ear fluid for efficient sound transmission. Structures like the ossicles (malleus, incus, stapes) and the tympanic membrane work together in an impedance-matching system, amplifying sound pressure to overcome the higher impedance of the fluid-filled inner ear. Signup and view all the answers

Explain how a deficiency in Connexin 26 can lead to hearing loss, referencing the specific mechanisms disrupted and their impact on the endocochlear potential.

Connexin 26 deficiency disrupts potassium recycling in the cochlear lymph, leading to a loss of endocochlear potential. This deficiency impairs the normal function of the stria vascularis, ultimately affecting the hair cells' ability to transduce auditory signals. Signup and view all the answers

Describe the roles of both the Na+/K+/ATPase pump and the Na+/2Cl-/K+ cotransporter in maintaining the unique ionic composition of the intrastrial space and endolymph.

The Na+/K+/ATPase pump extrudes 3 Na+ ions and uptakes 2 K+ ions, and the Na+/2Cl-/K+ cotransporter uptakes 3 K+ ions along with Na+ and Cl-. Both mechanisms work together to maintain a low K+ concentration in the intrastrial space and a high K+ concentration in the intermediate cells as well as facilitating K+ transport from the intrastrial space to the endolymph. Signup and view all the answers

Explain how the mechanical vibration of sound is converted into electrical energy within the hair cells, detailing the role of stereocilia and transduction channels.

Mechanical vibration causes shearing of stereocilia, which opens gated ion channels located near the tips of the stereocilia. This allows endolymph, rich in K+ ions, to flow into the hair cells, leading to depolarization and the generation of an electrical signal. Signup and view all the answers

Describe how deflecting the stereocilia towards the kinocilium affects the hair cell's membrane potential and neurotransmitter release.

Deflecting stereocilia towards the kinocilium opens ion channels, causing depolarization of the hair cell. This depolarization leads to the influx of potassium ions, which then triggers the release of neurotransmitters and activation of nerve fibers. Signup and view all the answers

Explain the Davis Battery Theory and its role in auditory transduction, highlighting the voltage differences between the IHC, OHC, and the endolymph.

The Davis Battery Theory posits that the stria vascularis generates a +80 mV direct current into the endolymph. This potential difference, along with the resting potentials of IHCs (-45 mV) and OHCs (-70 mV), creates a total voltage drop of 125-150 mV, which is essential for driving auditory transduction in the hair cells. Signup and view all the answers

Describe how the unique ionic composition of the endolymph (high K+ and low Na+) contributes to the depolarization of hair cells during auditory transduction.

The high concentration of K+ ions in the endolymph, relative to the interior of the hair cells, creates a steep electrochemical gradient. When the transduction channels open, K+ ions rush into the hair cells, causing rapid depolarization due to the influx of positive charge. Signup and view all the answers

Compare and contrast the roles of depolarization and repolarization in hair cells, specifying the primary ion responsible for each process and the mechanisms involved.

Depolarization in hair cells is primarily caused by the influx of K+ ions, making the cell's charge more positive. Repolarization is achieved by closing K+ channels in the hair cells and by K+ efflux, which returns the cell to its resting potential. Signup and view all the answers

Explain how the selective K+ channels on the apical membrane of marginal cells contribute to maintaining the high K+ concentration within the endolymph.

The selective K+ channels on the apical membrane of marginal cells release K+ ions into the endolymph, directly contributing to the high K+ concentration. These channels facilitate the movement of K+ from the marginal cells into the endolymph, ensuring the proper ionic balance for auditory transduction. Signup and view all the answers

How does the influx of K+ ions into hair cells (HCs) contribute to both depolarization and subsequent repolarization of the cell?

Influx of K+ depolarizes the HC, opening Ca2+ channels. Ca2+ influx then activates Ca2+ activated K+ channels, allowing outward flow of K+ and cellular repolarization. Signup and view all the answers

Describe the roles of inner hair cells (IHCs) and outer hair cells (OHCs) in auditory transduction, and identify the primary afferent neurotransmitter released by IHCs.

IHCs are the main sensory cells, while OHCs modulate transduction through mechanical processes. The primary afferent neurotransmitter released by IHCs is glutamate. Signup and view all the answers

How does excessive glutamate accumulation around type I auditory nerve synapses lead to auditory dysfunction, and what specific conditions are associated with this excitotoxicity?

Overaccumulation of glutamate around type I nerves leads to neural swelling/inflammation and excitotoxicity. This is associated with auditory neuropathy spectrum disorder (ANSD), high-frequency tinnitus, and reduced hearing sensitivity. Signup and view all the answers

What is the cochlear microphonic (CM), and which type of hair cells predominantly generates it?

The CM is an alternating current with a frequency that mimics the auditory stimulus, reflecting receptor currents flowing through the hair cells. It is predominantly generated by the OHCs. Signup and view all the answers

How does the summating potential (SP) differ from the cochlear microphonic (CM) in terms of its electrical characteristics, and which hair cell type contributes more to the SP at low to moderate stimulus intensities?

The SP is a direct current (DC) shift that follows the envelope of the stimulus, whereas the CM is an alternating current (AC). IHCs contribute more to the SP at low to moderate intensities. Signup and view all the answers

What is the compound action potential (CAP), and what does its threshold relative to behavioral thresholds suggest about its sensitivity?

The CAP originates from spiral ganglion neurons and represents the synchronous response of auditory nerve fibers. Its threshold is usually 10-20 dB above behavioral thresholds. Signup and view all the answers

In the context of electrocochleography (ECochG), what is the clinical significance of an elevated SP/AP ratio (e.g., 0.4-0.5), and what condition does it typically indicate?

An elevated SP/AP ratio of 0.4-0.5 in ECochG is indicative of Meniere's disease. Signup and view all the answers

What are the two main components of the olivocochlear bundle (OCB), and how do their projections differ in terms of myelination and target hair cells?

The two components are the medial olivocochlear (MOC) and lateral olivocochlear (LOC) systems. MOC fibers are crossed and innervate OHCs, while LOC fibers are uncrossed, unmyelinated and innervate type I auditory neurons at the base of IHCs. Signup and view all the answers

Describe the pathway of the auditory nerve (AN) from its origin in the cochlea to its termination in the brainstem, identifying key anatomical landmarks along its course.

The AN starts where cochlear nerve fibers converge in the modiolus, passes through the habenula perforata, forms the trunk in Rosenthal's canal, goes through the IAM, and terminates at the cochlear nucleus in the pons. Signup and view all the answers

Explain why the auditory nerve is referred to as an 'auditory bottleneck,' and where along the nerve are the myelin sheaths formed by Schwann cells versus oligodendrocytes?

The auditory nerve is an 'auditory bottleneck' because slight damage can have large consequences due to the convergence of many fibers into a small space. The distal IAM is Schwann myelin, while proximal is oligodendrocyte myelin. Signup and view all the answers

Explain how the volley principle extends the range of phase locking in auditory nerve fibers, and what is the upper frequency limit achieved through this mechanism?

The volley principle extends phase locking by recruiting multiple fibers to alternate firing when individual fibers reach their frequency limit. This allows coding of frequencies up to 4-5 kHz. Signup and view all the answers

Describe the phenomenon of two-tone suppression in auditory nerve fibers and how it affects the neural representation of complex sounds.

Two-tone suppression occurs when the response of an auditory nerve fiber to one tone is inhibited by the presence of another tone within a certain frequency range. It sharpens frequency tuning and enhances the neural representation of complex sounds. Signup and view all the answers

How does the tuning of auditory nerve fibers change with stimulus intensity, particularly at higher intensities, and what is the underlying reason for this change?

At higher intensities, the tuning of auditory nerve fibers becomes broader, resulting in reduced frequency selectivity. This is because at high intensities, the outer hair cells (OHCs) contribute less to tuning. Signup and view all the answers

Explain the relationship between spontaneous firing rate and threshold in auditory nerve fibers, and describe how this relationship contributes to intensity coding.

Auditory nerve fibers with higher spontaneous firing rates typically have lower thresholds. This allows them to respond to low-intensity sounds. Conversely, low spontaneous rate fibers have higher thresholds, enabling them to respond to high-intensity sounds without saturating. Signup and view all the answers

Describe the adaptation process in auditory nerve fibers and identify two factors that can influence the extent of adaptation.

Adaptation is the reduction in firing rate of an auditory nerve fiber over time despite a constant stimulus. Adaptation is greater for high frequency sounds and in individuals with hearing loss greater than 50 dB HL. Signup and view all the answers

How do temporary threshold shifts potentially lead to 'hidden hearing loss,' and what specific damage is suspected to cause this condition?

Temporary threshold shifts can cause permanent damage to low spontaneous rate (SR) nerve fibers, leading to decreased suprathreshold measures. The damage is thought to occur at the cochlear nerve (CN) terminals (synaptic ribbons) and auditory nerve synaptic region. Signup and view all the answers

What are the three major subdivisions of the cochlear nucleus (CN), and how does the input from the auditory nerve (AN) reach each of them?

The three major subdivisions of the cochlear nucleus are the dorsal cochlear nucleus (DCN), posterior ventral cochlear nucleus (PVCN), and anterior ventral cochlear nucleus (AVCN). The auditory nerve enters at the root entry zone and bifurcates, sending branches to the AVCN and PVCN, with input then reaching the DCN via the PVCN. Signup and view all the answers

Explain how the cochlear nucleus (CN) enhances frequency specificity compared to the auditory nerve (AN), and why is this important for processing complex signals?

The cochlear nucleus enhances frequency specificity through inhibitory influences that narrow the tuning curves of its neurons, unlike auditory nerve fibers. Narrower tuning curves leads to increased specificity to affect the representation of complex signals. Signup and view all the answers

What is the dynamic range (DR) of most fibers in the cochlear nucleus (CN) for intensity coding, and how does the CN achieve sensitivity to small changes in intensity?

Most fibers in the cochlear nucleus have a dynamic range of 30-40 dB. The CN alters firing rates in a stepwise manner in response to a continuous tone, showing great sensitivity for coding small changes in intensity. Signup and view all the answers

Describe why the cochlear nucleus (CN) responds particularly well to amplitude-modulated (AM) tones, and indicate the optimal modulation rate to which CN fibers are most sensitive.

Cochlear Nucleus fibers follow the envelope of modulation with fluctuating firing rates. The CN is most sensitive to AM frequency around 300 Hz. Signup and view all the answers

How does the varying stiffness and width of the basilar membrane (from base to apex) contribute to frequency discrimination in the cochlea?

The basilar membrane's varying stiffness and width—stiffer and narrower at the base, and floppier and wider at the apex—cause different locations along the membrane to vibrate maximally in response to different frequencies. The base responds best to high frequencies, while the apex responds best to low frequencies. Signup and view all the answers

Explain the role of the reticular lamina in maintaining the distinct ionic compositions of endolymph and cortilymph.

The reticular lamina acts as a barrier, separating the endolymph (high in K+ and +80mV charge) from the cortilymph. This separation is crucial for the proper functioning of the hair cells, as their cilia are bathed in endolymph while their cell bodies are bathed in cortilymph. Signup and view all the answers

Describe how the medial olivocochlear (MOC) efferent neurons influence the function of outer hair cells (OHCs).

MOC efferent neurons synapse with the base of OHCs and inhibit their motility by acting on contractive proteins. This reduces the cochlear amplifier effect, potentially protecting the ear from damage due to loud sounds and improving frequency selectivity in noisy environments. Signup and view all the answers

What is the significance of the endolymph's unique ionic composition (high K+ concentration and +80 mV charge) for auditory transduction?

The high K+ concentration and positive charge of the endolymph create an electrochemical gradient that drives K+ ions into the hair cells when the stereocilia are deflected. This influx of K+ ions depolarises the hair cells and initiates the electrical signals that are sent to the brain. Signup and view all the answers

Explain the role of tip-links in the mechanotransduction process of hair cells.

Tip-links are small filaments that connect to mechanotransducer channels atop stereocilia and between stereocilia. When the stereocilia bend in response to sound vibrations, the tip-links pull open the mechanotransducer channels, allowing K+ ions to enter the hair cell and initiate the transduction process. Signup and view all the answers

How do inner hair cells (IHCs) and outer hair cells (OHCs) differ in their innervation patterns and primary functions within the cochlea?

IHCs are innervated by approximately 20 type I spiral ganglion neurons in a 1:1 ratio and are primarily responsible for transmitting auditory information to the brain. OHCs are innervated by type II spiral ganglion neurons that contact many OHCs and primarily function to amplify and refine the cochlear response through electromotility. Signup and view all the answers

Describe the structure and function of the stria vascularis, and explain its importance for cochlear physiology.

The stria vascularis is a highly vascularized tissue located on the lateral wall of the scala media. It is composed of marginal, intermediate, and basal cells, and its primary function is to maintain the ionic composition of the endolymph and generate the endocochlear potential (+80 mV). Signup and view all the answers

Explain how the Hensen's strip, located on the underside of the tectorial membrane, influences the movement of inner hair cell (IHC) stereocilia.

The Hensen's strip restricts the area between the IHC cilia and the tectorial membrane, leveraging Bernoulli's principle to enhance fluid movement and deflection of the IHC stereocilia. This increases the sensitivity of the IHCs to sound vibrations. Signup and view all the answers

Describe the traveling wave and its characteristics as it propagates from the base to the apex of the cochlea. How does the speed and dispersive nature change and why?

The traveling wave originates at the base and moves towards the apex. As it propagates, its speed decreases and it becomes more dispersive due to the decreasing stiffness and increasing mass of the basilar membrane from base to apex. The wave is 2x faster at the base. Signup and view all the answers

Explain why high-intensity, low-frequency sounds can mask the perception of simultaneous high-frequency sounds. How does this phenomenon relate to hearing aid fitting?

High-intensity, low-frequency sounds mask high-frequency sounds due to the upward spread of masking. This occurs because the traveling wave for low frequencies activates a broader region of the basilar membrane, including the areas that respond to high frequencies. In hearing aids, excessive low-frequency gain can exacerbate this effect, hindering the perception of high-frequency sounds. Signup and view all the answers

Describe several potential mechanisms of pathogenesis/cellular damage resulting from noise induced hearing loss (NIHL).

Several potential mechanisms include reduced blood flow leading to hypoxia, metabolic exhaustion of hair cells, excessive neurotransmitter release leading to nerve fiber damage, reticular lamina fracturing leading to fluid mixing, free radical damage, and synaptic ribbon damage. Signup and view all the answers

Explain how the cochlear amplifier, mediated by outer hair cells (OHCs), enhances frequency tuning, especially at low sound intensities.

The cochlear amplifier sharpens frequency tuning by having OHCs contract on upward basilar membrane movement, expanding the downward deflection, thereby increasing the amplitude of the traveling wave. This active process is particularly important in the frequency tuning at intensities close to threshold. Signup and view all the answers

How are free radicals implicated in both ototoxicity and noise-induced hearing loss (NIHL), and what cellular components are primarily affected?

Both ototoxicity and NIHL can be attributed to free radical generation leading to hair cell death. With noise exposure, high noise levels tax mitochondrial respiration processes, causing free radical formation. Signup and view all the answers

Describe the structural relationship between the tectorial membrane and the stereocilia of the outer hair cells (OHCs).

The tallest stereocilia of the OHCs are embedded in notches on the underside of the tectorial membrane. This articulation allows the tectorial membrane to directly influence the movement of the OHC stereocilia in response to sound vibrations. Signup and view all the answers

The attachments points of the basilar membrane (BM) are critical for its function. What are these attachment points and how might damage to them influence hearing?

The attachment points are the osseous spiral lamina and the spiral ligament. Damage to these structures can disrupt the tension and stability of the BM, affecting its ability to vibrate properly and leading to hearing impairment. It can alter the BM's stiffness and resonant characteristics. Signup and view all the answers

Flashcards

What is Atresia?

Partial or complete absence of the ear canal.

What is Cerumen (earwax)?

Waxy substance secreted in the outer ear canal, moisturizing and protecting the canal.