The Region Where the Scala Vestibuli and Scala Tympani Are Continuous With Each Other is the
Cochlear Transduction and the Molecular Basis of Auditory Pathology
Paul W. Flint MD, FACS , in Cummings Otolaryngology: Head and Neck Surgery , 2021
Passive Cochlear Mechanics
The cochlea, and the organ of Corti in particular, constitute an evolutionary marvel of biologic engineering as intricate, microscaled mechanical systems. Although the hallmark of peripheral auditory function in mammals may be the evolutionary adaptation of a relatively simple, passive resonant system into an active one that consumes energy and efficiently detects and amplifies vibratory energy, it is useful to note that the fundamental nature of cochlear mechanics is evident even in cadavers, as so elegantly demonstrated by Nobel prize winner Georg von Békésy. 23 Because cochlear mechanics at this level of function are independent of other factors—neither requiring nor consuming adenosine triphosphate (ATP), for example—the sound-driven motion of the organ of Corti observed by von Békésy and others is commonly referred to aspassive.
The primary elements of passive cochlear transduction are hydromechanical in character, and the integrated contributions of many structural elements are known to determine the resonant character of cadaveric and therefore mechanically passive cochleae (Fig. 148.1). The cochlear partition is composed of an epithelial sheet known as thetympanic cover layer that supports the fibroelastic basilar membrane, the central structure of the end organ both functionally and anatomically; in turn, this partition supports both the sensory and nonsensory supporting epithelium. The fluid-filled major chambers of the cochlea—specifically the scala vestibuli and scala tympani, which are filled with perilymph, and the scala media, which is filled with endolymph—also play an important role in transduction. These structural elements, together with the tectorial membrane—an extracellular gelatinous structure secreted by nonsensory inner ear epithelial cells, which extends from the spiral limbus radially over the apical surface of the organ of Corti—comprise the principal elements that give rise to passive mechanical transduction. Although the Reissner membrane separates the scala media from the scala vestibuli and therefore plays an important homeostatic role in the maintenance of cochlear electrochemistry, it does not seem to play a significant role as an element of passive cochlear mechanics.
Because the focus of this chapter is on cochlear transduction, we will only remind the reader that sound waves collected in the external ear canal produce vibrations of the tympanic membrane, which, in turn, vibrate middle ear ossicles (Fig. 148.2A). It is, of course, the vibratory motion of the stapedial footplate that transmits the mechanical energy of the ossicular chain directly through the oval window of the cochlea, effectively delivering sound-pressure waves to the scala vestibuli and translating mechanical motion into pressure waves that propagate through the virtually incompressible cochlear fluids at a velocity approximating 1.5 km/s. At this rate of propagation, the spread of the pressure wave throughout the volume of the cochlea is nearly instantaneous. Because the bony walls of the cochlea are rigid, and its fluid contents are incompressible, the higher pressure—or lower pressure, depending on the direction of motion of the stapedial footplate—in the scala vestibuli relative to the scala tympani produces a pressure differential across the cochlear partition that creates intracochlear forces that set the partition into motion (seeFig. 148.2D). As described in more detail in a later section, the physical makeup of the organ of Corti, and of the basilar membrane in particular, establishes a space-frequency map that determines the limits of hearing and the capacity of the subject to resolve frequency differences among vibrations that contain energy in the audible range.
Disorders of Peripheral and Central Auditory Processing
Robert Shepherd , ... James Fallon , in Handbook of Clinical Neurophysiology, 2013
16.3.6 Sequential versus simultaneous stimulation
Neural excitation via an electrode within the scala tympani is spatially broad due to the conductive nature of the fluid filled inner ear ( Black et al., 1981; Van den Honert and Stypulkowski, 1987; Snyder et al., 2004). As a consequence, stimulating channels overlap considerably so that any one SGN can be activated by multiple electrodes. Both monopolar and bipolar stimulations produce significant channel interaction during simultaneous stimulation of two or more electrodes (Black et al., 1981; Shannon, 1983; Townshend and White, 1987). Because channel interaction is unpredictable, often resulting in undesirable perceptual effects including pain, current pulses are typically delivered sequentially to a single electrode at a time (Seligman and Shepherd, 2004). Techniques to improve current focusing using simultaneous stimulation of multiple sites are undergoing clinical trials (Van den Honert and Kelsall, 2007; Saoji et al., 2009).
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Physiology of the Auditory System
Paul W. Flint MD, FACS , in Cummings Otolaryngology: Head and Neck Surgery , 2021
Inner Ear Physiology
The inner ear is enclosed in a bony cavity called theotic capsule, and it has two mobile windows, one oval and one round. The inner ear serves the important functions of hearing and balancing. The portion of the inner ear that deals with hearing is the cochlea, whereas the portion of the inner ear that deals with maintaining balance is collectively known as thevestibular organs: the semicircular canals, utricle, and saccule. The cochlea is shaped like a snail and has a spiral configuration
The scala media contains the organ of Corti, which rests on the basilar membrane. Taken together, the organ of Corti and the basilar membrane are referred to as thecochlear partition. The organ of Corti has an arch at its center, called thearch of Corti, formed by the inner and outer pillar cells. The inner hair cells are flask-shaped cells that rest on the side of the inner pillar cells, whereas the outer hair cells are cylindrical cells that rest on the side of the outer pillar cells. In the human cochlea, about 3000 inner hair cells extend in a single row from the base to the apex, whereas about 12,000 outer hair cells are arranged in three or four rows. The hair cells derive their names from the hair-like projections apparent on their apical surface termedstereocilia, which play an important role in the signal-transduction properties of the hair cells.
The scala vestibuli and scala tympani are filled with perilymph, which has a composition similar to that of the extracellular fluid (high in sodium, and low in potassium;Fig. 128.4A). The scala media is filled with endolymph, which has a similar composition to the intracellular fluid (low in sodium, high in potassium). 11 The unique electrolyte composition of the scala media sets up a large electrochemical gradient, called theendocochlear potential, which is +60 to +100 mV relative to the perilymph (Fig. 128.5). 12 The maintenance of such a large electrochemical gradient is accomplished by thestria vascularis, which resides on the outer wall of the scala media, away from the modiolus. The stria vascularis contains multiple active ion channels and maintains the chemical composition of the endolymph and its positive electrical potential. 13
Sensory Systems
David L. Felten MD, PhD , ... Mary Summo Maida PhD , in Netter's Atlas of Neuroscience (Third Edition), 2016
14.16 Cochlear Receptors
Fluid movement through scala vestibuli, around the helicotrema, and back through the scala tympani differentially moves the basilar membrane on which the organ of Corti and its hair cells reside. Movement of hairs on the apical portion of the hair cells by shearing forces of the tectorial membrane results in their depolarization and the release of neurotransmitters. This release stimulates action potentials in the primary afferent axons of spiral ganglion cells. Efferent axons from the olivocochlear bundle, controlled by descending central auditory pathways, can modulate the excitability of hair cells and the sensory transduction process.
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Neurotology
H. Richard Winn MD , in Youmans and Winn Neurological Surgery , 2017
The Cochlear System
The cochlea (from the Greek word for "snail") is made up of a hollow 33-mm tube that spirals 2.5 times. The bony center, around which the spiral is coiled, assumes the shape of a tapered screw (from the Latin wordmodiolus) if the outer parts of the bony spiral are removed. The long axis of the modiolus has a sloping anterolateral orientation in the head, with its base abutting the anterior part of the fundus of the IAC. Fine canals (habenula perforata) within the modiolus house the dendrites of the cochlear nerve and their bipolar somata, which constitute the spiral ganglion.
The basal turn of the cochlea forms a distinct bony promontory on the medial wall of the mesotympanum (i.e., the middle ear) and is the only portion of the cochlea that is visible during middle ear surgery. The remaining turns of the cochlea are enclosed within the petrous bone. At the posterior aspect of the promontory are two fibrous barriers to communication with the inner ear: the oval window superiorly, which faces laterally and houses the footplate of the stapes, and the round window inferiorly, which faces posteriorly and is contained within the bony round window niche (seeFigs. 9-1 and9-2).
A transverse sectional view through the cochlea shows that it is composed of three compartments: the scala vestibuli, scala media, and scala tympani (see Figs. 9-2 and9-4). The scala media, which contains endolymph, is separated from the scala vestibuli and tympani by Reissner's membrane and the basilar membrane, respectively. The scala vestibuli and scala tympani contain perilymph, and these two compartments communicate with each other at the cochlear apex through an opening at the tip of the basilar membrane called thehelicotrema. At the base of the upper compartment, called thescala vestibuli, is the oval window, and at the base of the lower compartment, called thescala tympani, is the round window.
The basilar membrane has a complex structure resting on it that is best appreciated in cross section. The basilar membrane and the thin, sloping Reissner's membrane form a tube that ends blindly and is sealed at the helicotrema. This is the scala media, or cochlear duct, and it contains endolymph (seeFigs. 9-2 and9-4). Extending along the entire basilar membrane and spiraling with the cochlea is the organ of Corti. It contains the structurally complex sensory epithelium innervated by the cochlear nerve. As viewed from above, the basilar membrane is widest near the helicotrema and narrowest at the base. Maximal high-frequency vibration of the basilar membrane occurs at the base, and maximal low-frequency vibration occurs at the apex, thereby resulting in hair cell transduction of high frequencies at the base and of low frequencies at the apex.
The organ of Corti consists of a single row of inner hair cells and three rows of outer hair cells. The inner and outer hair cells slope toward each other to form a triangular canal (thetunnel of Corti) between them. The tectorial membrane extends in a medial-to-lateral direction within the scala media and above the hair cells, where the hair cell stereocilia are embedded.
VOLUME 2
Blake S. Wilson , Michael F. Dorman , in Neuromodulation, 2009
Electrical Stimulation of the Auditory Nerve
Direct stimulation of the nerve is produced by currents delivered through electrodes placed in the scala tympani (ST), one of three fluid-filled chambers along the length of the cochlea. A cutaway drawing of the implanted cochlea is presented in Figure 58.2. The figure shows a partial insertion of an array of electrodes into the ST. The array is inserted through a drilled opening made by the surgeon in the bony shell of the cochlea overlying the ST (called a "cochleostomy") and close to the base of the cochlea. Alternatively, the array may be inserted through the second flexible membrane of the cochlea, the round window membrane, which also is close to the basal end of the cochlea and ST.
Figure 58.2. Cutaway drawing of the implanted cochlea. Illustrated is the electrode array developed at the University of California at San Francisco (Loeb et al., 1983). That array includes eight pairs of bipolar electrodes, spaced at 2 mm intervals and with the electrodes in each pair oriented in an "offset radial" arrangement with respect to the neural processes peripheral to the ganglion cells in the intact cochlea. Only four of the bipolar pairs are visible in the drawing, as the others are "hidden" by cochlear structures. This array was used in the UCSF/Storz and Clarion 1.0 devices
(Reproduced from Leake and Rebscher (2004) with permission of Springer Science + Business Media)Different electrodes in the implanted array may stimulate different subpopulations of neurons. As described above, neurons at different positions along the length of the cochlea respond to different frequencies of acoustic stimulation in normal hearing. Implant systems attempt to mimic or reproduce this "tonotopic" encoding by stimulating basally situated electrodes (first turn of the cochlea and lower part of the drawing) to indicate the presence of high-frequency sounds, and by stimulating electrodes at more apical positions (deeper into the ST and ascending along the first and second turns in the drawing) to indicate the presence of sounds with lower frequencies. Closely spaced pairs of bipolar electrodes are illustrated here, but arrays of single electrodes that are each referenced to a remote electrode outside the cochlea also may be used. This latter arrangement is called a "monopolar coupling configuration" and is used in all present-day implant systems that are widely applied worldwide.
The spatial specificity of stimulation with an ST electrode most likely depends on a variety of factors, including the orientation and geometric arrangement of the electrodes, the proximity of the electrodes to the target neural structures, and the condition of the implanted cochlea in terms of nerve survival and ossification. An important goal of electrode design is to maximize the number of largely non-overlapping populations of neurons that can be addressed with the electrode array. Present evidence suggests, however, that no more than 4–8 independent sites are available using current designs, even for arrays with as many as 22 electrodes (e.g., Fishman et al., 1997). Most likely, the number of independent sites is limited by substantial overlaps in the electric fields from adjacent (and more distant) electrodes. The overlaps are unavoidable for electrode placements in the ST, as the electrodes are sitting in the highly conductive fluid of the perilymph and additionally are relatively far away from the target neural tissue in the spiral ganglion. A closer apposition of the electrodes next to the inner wall of the ST would move them a bit closer to the target cells (see Figure 58.2), and such placements have been shown in some cases to produce an improvement in the spatial specificity of stimulation (Cohen et al., 2006). However, a large gain in the number of independent sites may well require a fundamentally new type of electrode, or a fundamentally different placement of electrodes. The many issues related to electrode design, along with prospects for the future, are discussed, for example, by Wilson and Dorman (2008a) and Spelman (2006).
Figure 58.2 shows a complete presence of hair cells (in the labeled organ of Corti) and a pristine survival of cochlear neurons. However, the number of hair cells is zero or close to it in cases of total deafness. In addition, survival of neural processes peripheral to the ganglion cells (the "dendrites") is at least unusual in the deafened cochlea, as noted before. Survival of the ganglion cells and central processes (the axons) ranges from sparse to substantial. The pattern of survival is in general not uniform, with reduced or sharply reduced counts of cells in certain regions of the cochlea. In all, the neural substrate or target for a CI can be quite different from one patient to the next. A detailed review of these observations and issues is presented by Leake and Rebscher (2004).
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Extracellular Matrix and Egg Coats
Richard J. Goodyear , Guy P. Richardson , in Current Topics in Developmental Biology, 2018
2 The Cochlea and the Mammalian TM
The cochlea is the mammalian organ of hearing. It is a coiled, fluid-filled tube that is comprised of three chambers, scalae vestibuli, media, and tympani that run along its length. Frequency analysis takes place on the basilar membrane (BM), an extracellular matrix that separates scala media from scala tympani and vibrates in response to sound-induced motions of the cochlear fluids. Movements of the BM are, in turn, detected by the hair cells in the organ of Corti, a neuroepithelium that lies along its medial edge (see Fig. 1A). The BM is graded in stiffness and mass along its length, such that the basal end responds best to high-frequency sounds while the apical end responds best to sounds of low frequency. A sound of a given frequency will set up a traveling wave on the BM, an envelope of vibration that reaches a peak at a position along the cochlea that depends on the frequency. The organ of Corti contains a single row of purely sensory inner hair cells (IHCs) that receive 95% of the cochlear afferent innervation, and three rows of sensorimotor outer hair cells (OHCs) that are innervated by efferent fibers from the brain stem. The OHCs are electromotile and can both detect the sound stimulus and amplify the motion of the BM at low sound pressure levels (Ashmore, 2008; Dallos, 2008). The response of the BM is therefore highly dependent on active feedback from the OHCs; if these cells are lost then cochlear sensitivity decreases by ~ 1000 × fold and the organ is no longer sharply tuned (Dallos & Harris, 1978).
Fig. 1. (A) Diagram illustrating the structure of the mammalian organ of Corti. The tectorial membrane (TM) is colored blue, the basilar membrane (BM) is colored reddish brown. (B) A tectorial membrane from the left cochlea of a 4-week-old mouse that was obtained by dissection and briefly stained with the cationic dye Alcian blue. Scale bar = 500 μm. The tectorial membrane is ~ 5 mm long. (C) Cross-sectional profiles of the mouse tectorial membrane at four different points along its length. Cross-sectional area at each point is indicated. Scale bar = 50 μm.
The TM is a ribbon-like strip of extracellular matrix (Fig. 1B) that spirals along the length of the organ of Corti. It is attached along its medial edge to the surface of the spiral limbus, stretches across the internal spiral sulcus, and lies over the sensory epithelium, attaching along its lower lateral surface to the tips of the hair bundles of the OHCs. In mature animals, the hair bundles of the IHCs are not attached directly to the TM but lie in the narrow, fluid-filled subtectorial space. The properties of the TM, much like those of the BM, vary monotonically along the length of the cochlea (Fig. 1C). In the mouse, the cross-sectional area of the TM increases by more than 10-fold from the basal, high-frequency end to the apical, low-frequency encoding region of the cochlea, while its stiffness, as measured by a probe applied perpendicular to the upper surface, increases by a similar amount in the opposite direction, i.e., from the apex to the base of the cochlea (Teudt & Richter, 2014).
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Cochlear Implants
Jos J. Eggermont , in Hearing Loss, 2017
11.1.1 The Electrode Array
Most CI systems use electrode arrays that extend 1–1.5 turns (3–4 octaves in frequency) from the basal entry point into the scala tympani, but one manufacturer (MED-EL GmbH) uses an electrode array that is considerably longer, but not necessarily extends further into the cochlea. Typically, a CI electrode consists of a linear array of 12–22 metal electrode contacts, depending on the device. An implant channel comprises one active electrode along the cochlear array and one or more return electrodes, which may be inside or outside the cochlea. The active electrode typically delivers a train of biphasic pulses. Most of the current flows along the fluid-filled scala tympani, but some also flows into the less conductive osseous spiral lamina adjacent to it. Within this bone lie the spiral ganglion neurons, whose axons form the auditory nerve. A high enough current triggers a volley of action potentials in the auditory nerve ( Bierer, 2010).
Fig. 11.1 shows micrographs of four of the currently available electrode arrays and their extent of insertion in the cochlea. The MED-EL Flex31 (Fig. 11.1A) and Neurelec Standard Array (Fig. 11.1B) arrays are flexible straight designs that curve during insertion, while the Advanced Bionics Helix (Fig. 11.1C) and Cochlear Contour Advance (Fig. 11.1D) arrays have precurved designs. The Advanced Bionics HiFocus Helix array and the Cochlear Contour Advance array have their active electrodes spaced over the shortest distance (13 and 15 mm, respectively). The MED-EL Flex31 array has the widest "electrode spread" at 26.4 mm (Boyd, 2011).
Figure 11.1. X-ray views of several currently available CI electrode arrays, showing extent of typical insertions. (A) MED-EL Flex31, (B) Neurelec Standard Array, (C) Advanced Bionics Helix, and (D) Cochlear Contour Advance.
Reprinted from Boyd, P.J., 2011. Potential benefits from deeply inserted cochlear implant electrodes. Ear Hear. 32, 411–427.Read full chapter
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Marmosets in Auditory Research
Steven J. Eliades , Joji Tsunada , in The Common Marmoset in Captivity and Biomedical Research, 2019
Cochlear Implants
A cochlear implant is a neuroprosthetic device used for patients with severe to profound hearing loss. Cochlear implants (CIs) consist of an array of electrodes placed within the scala tympani of the cochlea and function to bypass damaged structures and directly stimulate the auditory nerve. Despite decades of very successful use, we still neither understand how the auditory system makes sense of the impoverished sensory input provided by a CI nor how the brain learns and adapts to process these signals. Marmosets have seen recent use as a model to study CIs and the brain's response to CI stimulation [15]. Many neurons in the AC do not respond well to CI stimulation, particularly those in the ipsilateral hemisphere and those with narrow frequency or loudness tuning [17]. Such narrowly tuned neurons often exhibit inhibitory sidebands, as discussed before, and it is likely that the rather coarse CI signal, which stimulates multiple adjacent frequency regions of the cochlea, may be engaging both the primary frequency receptive field as well as the inhibitory sidebands. CI stimulation using time-varying inputs, however, results in similar types of synchronized and nonsynchronized responses as normal A1 activity, and individual A1 neurons respond with similar patterns both to the CI and normal acoustic inputs, suggesting that CIs preserve normal timing mechanisms [168]. However, these studies have only involved marmosets using a CI in the laboratory setting; the animals did not use the CI on a regular daily basis like human CI recipients. It remains unclear, therefore, whether there is cortical plasticity or plasticity anywhere along the auditory pathway, with implant use. The presence of many AC neurons with poor responses to the CI is one potential mechanism for CI-related, experience-dependent learning and plasticity, an area ripe for future study.
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Protection and Repair of Hearing
Richard A. Altschuler , ... Josef M. Miller , in Principles of Tissue Engineering (Fourth Edition), 2014
Prostheses with biopolymers and ex vivo gene transfer
Another approach to delivery from prosthesis is to place cells capable of secreting the substance of interest directly onto the prosthesis, where they can deliver to cochlear fluids after placement into scala tympani. With ex vivo gene transfer, cultured cells are transformed to produce a specific gene product and placed into a specific area of the body for region-specific release. One problem has been that the transformed cells can migrate to a different region. With the use of biopolymers with fibronectin or laminin, transformed cells (e.g., fibroblast and fibronectin; Schwann cells and laminin) can be securely attached by the biopolymer to a neuroprobe or prosthesis, which is then inserted into the region of interest. The cells will then remain in place and have their highly localized action. One such study has recently shown the ability of BDNF, secreted by cells attached to the implant, to enhance survival of spiral ganglion neurons in a deafened guinea pig ear [169]. It may also be possible to place stem cells (discussed in more detail in the next section) on a prostheses, either to release drugs of interest or as new neurons close to the stimulation sites that can create a new connection either directly to the central auditory system or to remaining spiral ganglion neurons.
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