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vestibuli meet at the apex of the cochlea at an area called the helicotrema. The scala vestibuli
and media are divided by Reissner’s (vestibular) membrane while the scala media and tympani
are divided by the basilar membrane.
n
Basilar membrane
Contains organ of Corti (see below)
Supported medially by the spiral lamina (where spiral ganglion nerve fibers pass through to
the modiolus) and laterally by the spiral ligament
Wider (more mass) at apex than at the base (however, note that the cochlear shell is wider at
the base of the cochlea than at the apex)
Stiffer and thicker at base than at apex
Tonotopically organized: high frequencies at base, low frequencies at apex
The organ of Corti runs the length of the scala media and is the true sensory organ of the inner
ear. Here, hydrodynamic forces are translated into electrochemical energy and sent to the auditory
nerve fibers. There are numerous cells and components to the organ of Corti, but arguably the most
important are the inner and outer hair cells (IHCs and OHCs, respectively), the tops of which are
embedded in the reticular lamina and contain stereocilia.
n
Outer hair cells (OHCs)
Supported by Deiter’s cells; distal from the modiolus
Three to five rows
Cylindrical-shaped
~12,000 per cochlea
Resting charge of –70 mV; K+ channels
Taller/longer and double the mass at cochlear apex than at base
Contain contractile proteins for active cochlear mechanism
Few afferent connections (one afferent nerve fiber to many OHCs); Type II fibers
(unmyelinated)
Many efferent connections (myelinated)
Stereocilia
●
50–150 per OHC
●
Embedded in tectorial membrane
●
W-shaped
●
Connected by tip-links and cross-links
●
Shorn by movement of the tectorial membrane
n
Inner hair cells (IHCs)
Tightly supported by supporting cells (stronger HCs); proximal to modiolus
One row
Flask-shaped
3,500 per ear
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Resting charge of –40 mV; K+ and Na+ channels
Majority (90%–95%) of afferent connections (many afferent nerve fibers to one IHC);
Type I fibers (myelinated)
Few efferent connections (unmyelinated)
Stereocilia
●
50–75 per IHC
●
Curved line
●
Longer at apex than base
●
Connected by tip-links and cross-links
●
Moved by endolymph
While fine structure is beyond the scope of this text, grossly the cochlea is supplied with blood
from the labyrinthine artery, which branches off either the basilar artery or the anterior inferior cerebellar artery (AICA) traveling along the brainstem. The cochlea is innervated by CN VIII.
Q & A
Question: The macromechanics of the basilar membrane can be seen as two
distinct mechanical actions. What are those two specific types of actions?
Answer:
Passive mechanics — membrane mechanics are powered purely by simple sound
propagation and affected by the mechanical properties of the material that the
sound is passing through. The basal portion of the basilar membrane is stiffness
limited, whereas the apical portion is mass limited. Both stiffness and mass will
affect the acoustic wave in varying degrees.
Active mechanics — refers to vibrations that are powered by additional energy.
With the assistance of OHC activity, a mechanical energy is injected into a
passive (traveling) wave. The basilar membrane moves up and down, and a sheer
force is created at the apical end of the OHCs between the tectorial membrane
and stereocilia. This action is often referred to as the cochlear amplifier.
Both the stereocilia and tectorial membrane are the most compromised structures within the organ
of Corti.
n
Stereocilia and HCs physically interact with the tectorial membrane and are considered
mechanoelectric organelles found within the organ of Corti. Through its very nature, the
hydromechanical action (initiated by sound waves along the basilar membrane) will be
converted into a mechanical form of energy from the stereocilia tips of the OHCs and IHCs
that are either embedded or touching/tapping the tectorial membrane. Next, the electrical
transduction of energy will aid in modulating a neurotransmitter release (glutamate) at the
base of the HCs. Each component of the chain reaction is sensitive to and selectively responds
to frequency and intensity of the external signal received in the auditory system.
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n
The tectorial membrane is one continuous gelatinous structure that runs from the base to the
apex of the organ of Corti. The tips of each of the thousands of longer stereocilia erupt out
of the cuticular plate of the HCs and will attach to the underside of the tectorial membrane.
Alternatively, thousands of shorter stereocilia erupt out of the cuticular plate of HCs and rest
on the underside of (without attaching to) the tectorial membrane.
AUDIOLOGY NUGGET
High-frequency hearing loss due to chronic exposure to excessive levels of noise
is most often reported and considered a significant public health problem.
Though it is not possible to obtain a visualization of the gross to fine cochlear
structure from conception through the lifespan of humans, there is some validity
in postulating, from the growing knowledge base about the cochlear anatomy
and physiology, about cause and effect of chronic and excessive sound assaults on
the hearing mechanism. Consider the anatomy of the cochlea: High-frequency
processing has been shown to occur maximally at the basal end of the cochlea.
Coincidentally, the mechanical and hydrodynamic activity could be considered
at their maxima at the oval window, where the stapes articulates and creates the
hydrodynamic energy within the membranous cochlea. It would be reasonable
to believe an unusually intense influx of power would result in an intense hydrodynamic force. Next, consider individuals who work in a noisy environment
without wearing hearing protection. What frequencies would likely be negatively
impacted initially in this scenario? When considering the tonotopic organization
of the human auditory system, the high frequencies would most likely be negatively impacted from the environment. It is currently impossible to definitively
identify and precisely quantify cause and effect of high-frequency hearing loss as
a consequence to the intense hydrodynamics generated at the oval window.
Cochlear Mechanics
As mentioned earlier, when the mechanical/vibratory compression action occurs as a consequence of the
stapes footplate vibrating into the oval window, there will be a reciprocal/vibratory energy expansion at
the round window, which can be visualized as a traveling wave into the cochlea, as discovered by Georg
von Békésy. Such dynamic reciprocal action of expansion and compression at the two separately articulating membranous round and oval windows is the beginning of the cochlear transduction of energy.
n
While Reissner’s membrane moves down (or up, depending on the phase of the signal) along
the cochlea, the basilar membrane also moves; the area of movement is dictated by the mass
and stiffness gradient, which contributes to tonotopic organization.
n
In condensation/compression phases, the TM moves inwardly, pushing the stapes into the oval
window, which forces downward motion of the basilar membrane; this action is inhibitory.
Conversely, for rarefaction stimuli, the TM and stapes footplate move outward, and the basilar
membrane moves upward, leading to excitatory actions.
n
When considering HC movement, condensing stimuli move the HCs toward the stria
vascularis (lateral wall), which shears the stereocilia toward the modiolus (medial); this creates
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hyperpolarization of the HCs and the release of gamma-aminobutyric acid (GABA). In
rarefaction phases, the HCs move toward the modiolus and the stereocilia deflect toward the
stria vascularis, opening the mechanically gated channels and depolarizing the cell, releasing
glutamate (an excitatory neurotransmitter) into the intercellular space.
AUDIOLOGY NUGGET
While an AuD student at NSU, Tara Collela introduced acronyms regarding
the physiology of rarefaction and condensation stimuli: DERP: depolarize,
excitatory, rarefaction, positive auditory effect; CHIN: condensation, hyperpolarizing, inhibition/inward movement, negative auditory effect.
Acoustic signals that generate traveling waves are complex — any given acoustic stimuli can create
several traveling waves that simultaneously generate excitatory responses in multiple areas of the basilar
membrane. The oscillation rate (speed) of the stapes’ movement dictates frequency while the amount of
displacement dictates intensity, but fine tuning is controlled by the metabolic function, neural tuning
curves, and nonlinearity in the cochlea.
n
Metabolic function: unfortunately, von Békésy’s experiments were conducted on cadavers
with no metabolic function, and his proposed traveling wave cannot adequately or definitively
explain the fine tuning and nonlinear actions of the cochlea.
Without metabolic function, von Békésy was unable to witness the effects of the cochlear
amplifier, which provides gain to low-level sound inputs and is in effect for acoustic stimuli
around or below 40 dB SPL.
●
The cochlear amplifier stimulates motor proteins (namely, prestin) of the OHC to
shorten or lengthen, effectively enhancing the flow of endolymph across the IHC
stereocilia.
●
For more intense sounds (i.e., >40 dB), the cochlear amplifier is not necessary.
n
Tuning curves: neural tuning curves can depict the preferential frequency of HCs as well as
the frequency range to which a given HC will respond.
The tips of the curves are representative of frequencies eliciting a response at the lowest
intensity presented.
n
Cochlear nonlinearity: at greater intensities, larger portions of the basilar membrane (and
more HCs) are stimulated despite the stapes’ relatively linear movement.
Large areas of the basilar membrane are excited with greater intensities, which were
observed by von Békésy.
●
Due to the cochlear amplifier, smaller areas of the basilar membrane can react to acoustic
stimuli at lower intensities and lead to more discrete tuning (allowing for improved
frequency selectivity as discussed in Chapter 3).
In addition to cochlear mechanics, it is important to understand the cochlear chemical makeup. It
is the interaction between both sodium (Na+) rich and potassium (K+) rich molecules that facilitates
the electrical charges within the cochlear “battery.” Depending upon the cochlear regions, there are
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relative differences in nutrients, fluids, and ionic charges: 0 mV in perilymph, +80 mV in endolymph,
−70 mV in OHCs, and −45 mV in IHCs (Carlson, 1986; Musiek & Baran, 2020), providing nutrients
to inner ear structures.
n
K+ enters the HCs via channels on the stereocilia and is then expelled under the reticular
lamina.
n
Electrical synapses move K+ into fluid surrounding HCs, then through the basilar membrane
into the scala tympani.
n
K+ is picked up by fibrocytes of the spiral ligament, which leads to the stria vascularis.
K+ moves through basal cells, then intermediate cells, through marginal cells, and back into
the endolymph of the scala media.
Sodium (Na+) is also transported through the stria vascularis but is removed from entering
the endolymph (for the most part).
Genetic mutations such as Connexin 26 can disrupt the gap junctions between the basal
and intermediate cells, preventing K+ transport, leading to damage to or destruction of the
organ of Corti (Musiek & Baran, 2020).
Theories of Frequency Encoding
Within microseconds of acoustic signals entering the ear canal, the cochlea is able to process the signal:
first as sound pressure waves (that are simultaneously processed according to signal frequency and
intensity) and next synapse within the efferent and afferent nerves of the inner and/or outer hair cells
to immediately begin the sequential neural impulses to enable synapsing at the auditory nerve. These
unique sequential synapses, secondary to auditory stimulation, continue delivery via neural input to
the central auditory system. As such, the auditory nerve fiber is discriminately stimulated by various
frequencies within a wide range of intensities.
n
Characteristic Frequency: the frequency at which the lowest amount of sound intensity
elicits a neural response (threshold) within specific nerve fibers. There are other frequencies
that a nerve fiber will respond to, but not to the same low intensity as the characteristic
frequency (CF). It is possible to map the specific CF of each nerve fiber within the cochlea
to identify sharp or broad tuning curves. Most of the evidence with frequency coding of the
auditory nerve has clearly indicated that the basal portion of the human cochlea (i.e., the
point proximal to the oval window and middle ear) encodes the high frequencies. Conversely,
when neural signals ascend from the cochlea to the apical aspect of the human cochlea, it is
presumed that low frequencies are encoded.
n
Temporal Coding: related to the firing rate and phase-locking ability of the auditory nerve.
It is conceivable that both coding processes work simultaneously and parallel to each other,
which results in more accuracy of frequency coding. Conversely, temporal frequency coding is
still not well understood.
n
Intensity Coding: the auditory nerve fibers are dependent upon generated spontaneous
firing rates (SFRs) as well as the number of neurons activated by stimulus frequencies and
intensities.
Firing rates can be described as low, medium, or high. Those neurons with high SFRs are
most sensitive to responding to low-intensity signals, whereas those with low SFRs are most
sensitive to firing to higher intensities.
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It is presumed that the dichotomy of coding mechanisms is the neural correlates of
perceived loudness, which allow listeners to effectively process and perceive a wide range
of intensities.
Neurons with low SFRs usually have a wider dynamic range and those with high SFRs have
a smaller dynamic range in terms of the intensities they are capable of encoding. Together,
these two fiber types are thought to account for the significant dynamic range for intensity
among human listeners.
Disorders of the Inner Ear
There are a variety of disorders and malformations that can occur at the level of the cochlea, ranging
from part or a complete absence of the cochlea resulting from naturally or medically induced teratogens
or genetic traits. Appendix 2–C provides a summary of inner ear disorders typically attributed to
SNHL in which both bone-conduction and air-conduction thresholds would be abnormal and within
10 dB of each other.
n
There may be occasions in which a patient presents with no attributable physical
manifestation or etiology but has been identified with SNHL. Unfortunately (for the curious),
such a lack of etiology may remain mysterious with unsubstantiated assumptions about the
existence of an unknown genetic trait link or abnormal embryonic or preembryonic formation
but due to lack of scientific evidence cannot be attributed to the hearing loss.
n
More recent translational research and clinical outcomes have provided evidence about
cochlear HCs and the active processes of the cochlea being negatively impacted with potential
permanence from ototoxic agents (described below), intense acoustic sounds, viruses, bacteria,
teratogens during embryologic development, and genetic traits.
Ototoxic drugs fall into many classes: antibiotics (e.g., aminoglycosides [gentamicin,
kanamycin, amikacin, neomycin], amphotericin B, bacitracin, chloramphenicol,
macrolides, nystatin, polymyxin B); chemotherapy drugs (platinum compounds such
as cisplatin, carboplatin, vincristine); nonsteroidal anti-inflammatory drugs (NSAIDs;
ibuprofen, indomethacin, paracetamol, phenylbutazone, salicylates); and other
(antimalarials [quinine], loop diuretics [Lasix]). Note that several of these drugs are also
vestibulotoxic (gentamicin, streptomycin, tobramycin, chemotherapy agents).
Other toxic agents include asphyxiants (carbon monoxide), heavy metals (mercury, lead),
and solvents (benzalkonium chloride, polyethylene glycol, propylene glycol, styrene, toluene,
xylene). Ethyl benzene, styrene, trichloroethylene, and toluene are all vestibulotoxic.
Central Auditory Nervous System: Auditory Nerve to Cortex
Type I and Type II nerve fibers within the cochlea will ultimately become bundled and exit through the
habenula perforata of the osseous spiral lamina, into the modiolus, and onward to the internal auditory
meatus of the temporal bone. At that point, they are bundled as spiral ganglion and will ascend to the
brainstem as the vestibulocochlear nerve (CN VIII; the first auditory bottleneck). As inferred with the
name of the cranial nerve, they remain dedicated to either vestibular or auditory input that ascends
from the cochlea through the subcortical strata to the auditory cortex. For reference, Table 2–3 includes
the 12 human cranial nerves. Importantly, the tonotopic organization that begins in the cochlea is
maintained throughout the central auditory nervous system (CANS).
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TABLE 2–3. Human Cranial Nerves
CRANIAL NERVES
NUMBER/NAMES TYPE FUNCTION
I Olfactory Sensory Olfaction (smell)
II Optic Sensory Vision
III Oculomotor Motor Eyelid and eyeball muscles
IV Trochlear Motor Eyeball movement
V Trigeminal Mixed Sensory:
Motor:
VI Abducens Motor Eyeball movement
VII Facial Mixed Sensory:
Motor: facial muscles and
salivary glands
VIII Vestibulocochlear Sensory Hearing, vestibular, and balance
IX Glossopharyngeal Mixed Sensory:
Motor: swallowing
X Vagus Mixed Sensory:
EAC, visceral sensation, larynx
Motor: parasympathetic nervous
system (PNS)
XI Accessory Motor Swallowing; moving head and
shoulder
XII Hypoglossal Motor Tongue muscles
facial sensation
chewing muscles
taste
taste
taste in the epiglottis,
Anatomically and physiologically, CN VIII, the outermost fibers, represent the high-frequency
information while the innermost fibers are the low frequencies. Cranial nerves VII and VIII travel in
the internal acoustic meatus, divided into four quadrants, twisting as the nerves course toward the
brainstem. The cochlear portion of CN VIII is located in the anterior-inferior section as shown in
Figure 2–3. A simple way to recall the course of each nerve is 7-Up, Coke down, meaning that CN VII
is anterior-superior and the cochlear portion of CN VIII is anterior-inferior. The vestibular portions of
CN VIII (superior and inferior) are located posteriorly and are self-explanatory.
The ascending tract (CSLIMA, as defined below) is as follows and shown in Figure 2–4:
n
Cochlear nucleus (C): the first obligatory synapse of the CANS
Located bilaterally through the cerebellopontine angle in the pontomedullary junction,
inferior to the fourth ventricle and located below the cerebellar peduncle
Divided into anteroventral, posteroventral, and dorsal sections (AVCN, PVCN, and DCN,
respectively)
CN VIII enters via the root entry zone, then its fibers synapse throughout the cochlear nucleus
FIGURE 2–3. Figure of the left internal auditory meatus. Key: Ant:
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anterior; Sup: superior; Post: posterior; Inf: inferior; VII + NI: facial nerve
and nervous intermedius; VIII vs: superior vestibular branch of vestibulocochlear nerve; VIII vi: inferior vestibular branch of vestibulocochlear
nerve; VIIIc: auditory (cochlear) branch of vestibulocochlear nerve.
FIGURE 2–4. Coronal slice of the brainstem and brain illustrating the
CANS. Key: 1: cochlear nucleus; 2: superior olivary complex; 3: trapezoid body; 4: nuclei of the lateral lemniscus; 5: commissure of Probst;
6: inferior colliculus; 7: commissure of inferior colliculus; 8: brachium of
inferior colliculus; 9: medial geniculate body; 10: internal capsule; 11:
insula; 12: Heschl’s gyrus; 13: inferior central gyri of the parietal lobe;
14: corpus callosum. Source: From The Auditory System: Anatomy,
Physiology, and Clinical Correlates, Second Edition (pp. 1–487) by
Musiek, F. E., & Baran, J. A. Copyright © 2020 Plural Publishing, Inc. All
rights reserved.
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55+ different cell types with unique coding patterns for spectral and intensity information
from CN VIII
●
Different cell types project via different pathways within and outside of the cochlear nucleus
●
Octopus cells themselves are tonotopically organized (Oertel et al., 2000)
The acoustic startle reflex is mediated through the cochlear nucleus, which then goes
through the pontine reticular formation, the medial longitudinal fasciculus, and then to
spinal cord neurons
●
This reflex, used in very early newborn hearing screening techniques, occurred after a loud
sound was played. If the infant startled, the hearing screening was judged "Pass." Note:
Use of the startle reflex is not considered best practice for hearing screening purposes.
Location site of auditory brainstem implants
n
Superior olivary complex (S/SOC): the cochlear nuclei project to the bilateral SOC
Located in the caudal pons
Grossly divided into the lateral and medial SOC (LSO and MSO), but also includes
periolivary nuclei and the nuclei of the trapezoid body
●
Trapezoid body contains the largest synapses in the human body — the calyces of Held
First level of bilateral auditory important, which is important for localization
●
MSO: responsible for encoding the low-frequency interaural timing/phase differences
(ITD/IPDs)
●
LSO: responsible for encoding the high-frequency interaural intensity/level differences
(IID/ILD); this is relevant to the head shadow effect
Plays major role in the middle ear muscle reflex (MEMR) or acoustic reflex (AR), which is a
bilateral response
●
Acoustic information enters the outer, middle, and inner ear, travels up CN VIII to the
cochlear nucleus, then the signal goes to the bilateral SOC, to the bilateral facial nerve
nuclei (FNN), down CN VII (facial nerve) to the bilateral stapedius muscles
●
See Figure 5–7 for the reflex arc
AUDIOLOGY NUGGET
The MEMR is an important tool in the audiologist’s toolbox. There are four
conditions in which the testing is completed; these are named for the ear receiving the stimulus. These conditions are right ipsilateral (stimulus and recording in
the right ear), right contralateral (stimulus in the right ear, recording in the left
ear), left ipsilateral (stimulus and recording in the left ear), and left contralateral
(stimulus in the left ear, recording in the right ear). Depending on the pattern of test
result, different pathologies can be indicated. See Chapter 5 for common patterns
of results. For more information, see D. Emanuel’s Acoustic Reflex Threshold
(ART) Patterns: An Interpretation Guide for Students and Supervisors at https://
www.audiologyonline.com/articles/acoustic-reflex-threshold-art-patterns-875
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n
Lateral lemniscus (L/LL): a large, nonobligatory fiber tract along the CANS
Located within the pons
Contains two nuclei: ventral (VNLL) and dorsal (DNLL)
●
Bilateral dorsal nuclei communicate with each other via commissure of Probst
●
VNLL has many cells sensitive to ITDs
●
DNLL has many cells sensitive to ILDs and responsive to binaural input
Receive bilateral and ipsilateral input from cochlear nuclei and SOC, then project to the
ipsilateral inferior colliculus
n
Inferior colliculus (I/IC): essentially an obligatory synapse in the CANS; this is an important
relay point within the CANS as signals travel to the thalamus
Visible on the dorsal surface of the midbrain
Endpoint for the acoustic chiasm
●
Formed with ipsilateral low-frequency fibers and contralateral high-frequency fibers from
the LSO
Comprising several nuclei, projecting mainly ipsilaterally through the brachium to the
thalamus
41
First area with time duration–sensitive neurons, which are important for gap detection
n
Medial geniculate body of the thalamus (M/MGB): located in the central part of the brain,
the MGB projects off of the posterior end of the thalamus
The MGB is further divided into the ventral (MGV), dorsal (MGD), and medial (MGM)
sections
●
All three sections output to areas of the auditory cortex
Important waystation in the nonclassical auditory pathway as well
●
MGM projects directly to the amygdala, prefrontal cortex, and nucleus accumbens,
among others
●
Responsible for emotional responses to auditory stimuli
A lesioned thalamus does not automatically include auditory areas, but the MGB is
responsible for frequency discrimination, localization, the conditioned fear response, and
rapid ordering of acoustic elements
n
Auditory cortex (A): located on the superior temporal plane of the temporal lobe, located in
the Sylvian fissure
Also called Heschl’s gyrus, the auditory cortex has been traditionally thought of being
divided into primary, secondary, and tertiary areas; more recently, these are termed the core,
belt, and parabelt, which radiate from a center source (core)
Posterior to the auditory cortex is the planum temporale, superior to which is the
supramarginal gyrus and posterior to that is the angular gyrus
●
These areas integrate sensory information and are important for activities such as
reading
Deep to the Sylvian fissure, one will find the insular cortex (or insula or Island of Reil); this
is involved in cognitive, emotional, and sensory processes
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