Our senses of hearing and balance have evolved over more than 300 million years and are essential for the performance of everyday tasks. Animals also depend upon the normal functioning of cochlear and vestibular sensory receptors located within their membranous labyrinths for daily activities and survival. It is important to understand the anatomy and biology of these complex sensory receptors and to explore the development of neural prostheses that may replace and/or augment the input from malfunctioning cochlear and vestibular sensory receptors of the human membranous labyrinth.
My interest in inner ear biology (Ruben et al., 1986), genetics of hearing impairment (Ruben et al., 1991), the use of growth factors as drugs for the treatment of sensory disorders (Bock et al., 1996), the application of these areas of research to clinical aspects of hearing disorders (Van De Water et al., 1996), and the clinical and basic sciences that underlie the practice of Otolaryngology (Van De Water and Staecker, 2005) have developed and changed over my research and teaching career in the fields of Sensory Neuroscience and Otology. It therefore seemed appropriate for me to organize and assemble the authors and subject matter for this Special Issue of The Anatomical Record that is focused on the anatomy and biology of hearing and balance and the development and application of prosthetic devices that attempt to augment and/or replace these critical senses.
ANATOMY OF THE MEMBRANOUS LABYRINTH
Early physician-anatomist Andreas Vesalius in his work entitled “De humani coporis fabrica” (Vesalius, 1543) and Bartolome Eustachi in his work entitled “Epistola de auditus organis” (Eustachi, 1564) provided early but incomplete descriptions of human inner ear anatomy and both of these physician-scientists' supported the theory postulated by Aristotle (Ross, 1906) and later by Galen (Galen, 1542) that the inner ear was filled with a type of purified air, that is, “aer ingenitus.” In 1740, Antonio Valsalva published his anatomical observations (Valsalva, 1740) on the human auditory system in which he pointed out the importance of the ossicular chain and the oval window for hearing and also observed that the innervation target for the auditory nerve was not the osseous spiral lamina as previously suggested by Professor Claude Perrault (Hawkins, 1988), but was instead the membranous portions of the cochlea and that these areas of sensory epithelium represented, in the opinion of Professor Valsalva, the true receptors of sound. It was the discovery of Professor Domenico Cotugno who dissected cochleae from fresh temporal bones and published in his report entitled “De aquaeductus auris humanae internae anatomica dissertatio” not only the anatomical structure of both the cochlear and vestibular aqueducts but also his important observation that the cochlea contained a liquid and not air as maintained by both Aristotle and Galen (Cotugno, 1775) thereby breaking with the centuries old concept of “aer ingenitus.” This liquid within the bony labyrinth that Professor Cotugno observed was termed “liquor Cotunni” to honor his discovery of this watery fluid and later became known as perilymph. Because Cotugno did not observe the inner membranous component of the cochlea, his observation of liquid within the inner ear only addressed perilymph located within the outer chambers of the cochlea, that is, scala tympani and scala vestibuli. It was Professor Cotugno's contention that there was a neural tissue partition suspended in the labyrinth's perilymph and that the acoustic nerves were like strings that oscillated within this perilymph and transmitted the sensation of hearing to the auditory centers of the brain. The observation of a liquid present within the cochlea's inner membranous compartment, that is, scala media, would have to wait for the sharp observational skills of one of his anatomical colleagues, that is, Professor Antonio Scarpa. The name of Antonio Scarpa is most closely associated with Scarpa's Ganglion which is the peripheral ganglion of the vestibular sensory epithelial receptors and so named to honor the anatomical contributions of Professor Scarpa to inner ear anatomy. The name of this famous 18th century physician-anatomist has also been closely associated with the early descriptive anatomy of the bony and membranous labyrinths with the first identification and detailed description of the human bony and membranous labyrinths published by Professor Scarpa in 1789. His work entitled “Anatomicae disquisitions de auditu et olfactu” (Scarpa, 1789) was published while he was Professor and Chair of Anatomy at the University of Pavia. In this publication, Scarpa described in detail the anatomical features of dissected human membranous labyrinths aided by his dissection of the inner ears of animals and birds. Antonio Scarpa's descriptive anatomical work on the vestibular portion of the human inner ear with three curvilinear canals located in the bony portion of the vestibular labyrinth encasing three membranous semicircular canals with associated ampullae was presented in his original 1789 publication. He noted the attachment of these semicircular ducts to the mucosal lining cells that invest the walls of the bony canals and the association of these semicircular canals with a utricle (termed by Scarpa as a common cavity in relationship to the semicircular ducts) and the presence of a saccule (referred to by Scarpa as a small spherical vestibular pouch). Innervation of the three ampullae and the maculae of both the utricle and the saccule were described as occurring via fibers emanating from the acoustic nerve. He described the vestibular (Scarpa's) ganglion as a small, plump, reddish chamber enclosed within the middle of the acoustic nerve. Scarpa based on his anatomical observations of the innervation of the inner ear by what he understood to be various branches of the auditory nerve mistakenly attributed the sense of hearing to all the sensory receptor structures that he had observed to form the membranous labyrinth which included all the vestibular sensory receptors. In Scarpa's anatomical studies of the cochlea, he describes in detail the osseous spiral lamina, the series of fine nerve fibers that emanate from the cochlear nerve, and the presence of the three scala, that is, media, vestibuli, and tympani, with a connection between the scalae tympani and vestibuli via a small apical turn area of communication termed the helicotrema. It was Antonio Scarpa who also noted the presence of a clear fluid within the semicircular ducts of the canals and also present within the cochlea's scala media which he called “Scarpa's fluid,” now known as endolymph. This represented a major advance that would at a later date aid in our understanding of cochlear function. Antonio Scarpa was a gifted anatomist and also a gifted artist with all his descriptive narrative of inner ear anatomy accompanied by his excellent drawings that depicted dissected specimens of cadaver temporal bones revealing the structure of both the bony and membranous labyrinths. His very important contributions to the early understanding of the anatomical structure of the human inner ear and a description of his professional life can be found in a more recent publication by Canalis et al. (2001). Another important contribution to inner ear anatomy by Antonio Scarpa occurred when he was still at the University of Moderna, Italy and was performed prior to his descriptive work on the membranous labyrinth (Scarpa, 1772). This work involved the anatomical aspects of the human round window membrane and addressed the structure–function of this membrane with a translation of this work found in a paper by Sellers and Anson (1962). Professor Scarpa suggests in his book on “Anatomical Observations on the Round Window” that it was Professor Fallopia at the University of Padua who first described both the oval and the round windows and was responsible for the naming of both of these inner ear structures (Fallopia, 1562; Sellers and Anson, 1962). Antonio Scarpa referred to the oval window as the secondary tympanum and in addition to a detailed description of its anatomy; he suggested that this membrane covering the round window opening acted along with the oval window as a transmitter of sound energy into the cochlea hence his reference to this structure as the secondary tympanum. Antonio Scarpa provided a detailed anatomical description of both the round window membrane and its attachments as well as a similar detailed description of the niche in which it is located. According to Scarpa, Valsalva was a strong proponent of only the oval window in cooperation with the tympanic membrane and the ossicular chain for the transmission of sound energy into the cochlea (Valsalva, 1740) while Scarpa developed a strong argument for an additional contribution from the round window via the sound waves created within the tympanic cavity (Scarpa, 1772). It has now been shown that indeed the path of sound transmission proposed by Valsalva was correct with ossicular coupling via the oval window providing the major conversion of sound wave energy into fluid wave energy within the cochlea. Acoustic coupling that transmits sound energy to both the round and oval windows is now known to provide only a very small input from the sound energy within the middle ear cavity, therefore in a normal functioning middle and inner ear, the dominant transfer of sound energy occurs through the actions of the tympanic membrane/ossicular chain and oval window-stapedial footplate route (Rosowski and Merchant, 2005). It is important to note that all of Antonio Scarpa's descriptive anatomies of the bony and membranous labyrinths were accomplished without the aid of either advanced histological techniques or a compound microscope and that all the illustrations which accompanied his descriptive text were his own hand drawn illustrations.
With the development of advanced histological techniques and compound microscopes came advances in the understanding of the anatomical structures of the sensory receptor epithelium located within the membranous labyrinth. The Marquis Alphonse Corti while working in the laboratory of Professor Albert von Kölliker in Würzburg, Germany performed his anatomical-histological investigations of the organ of hearing (Corti, 1851) which later became known as the organ of Corti when Professor von Kölliker referred to the inner and outer pillar cells as the rods of Corti and the intervening tunnel as the tunnel of Corti (von Kölliker, 1852) with the entire structure of the organ of hearing eventually being referred to as Corti's organ (Fig. 1). Professor Corti was an Italian nobleman-physician and scientist working in the anatomical-histology laboratory of a German Professor, that is, von Kölliker, and who published his original report of the fine structure of the organ of hearing in French. This pivotal paper by Corti (who retired from scientific investigation the year after his reporting the description of the organ of Corti to assume his new role as Baron Corti following the death of his father) was the first histological description of the fine structure of cochlear receptor epithelium. His descriptive anatomy of the human organ of Corti was soon followed by papers on the descriptive anatomy of the cochlear receptor by Professors Deiters (1860), Claudius (1856), Hensen (1863), Boettscher (1869), and Nuel (1872). An earlier paper examining the avian ear structure described cells in the bird auditory receptor as auditory teeth which became known in the spiral limbus of the mammalian cochlea as the teeth of Huschke (1835). Each one of these early anatomist had some unique cell type or cell free space within the organ of Corti named after them and most of these cell types are seen and labeled in an excellent anatomical drawing of a radial section through Corti's organ from the second turn of a 6-day-old rabbit cochlea (Fig. 2) that was sketched by the artistically gifted Professor Hans Held (1909). In that same publication, Professor Held depicted the macula of the utricle in a drawing of this structure that roughly depicted the relationship between the vestibular sensory hair cells, the supporting cells and the otolithic membrane which he termed a cupula (depicted without otoliths) without reference to the presence of two different types of hair cells that are present within the vestibular sensory epithelium, that is, Types I and II vestibular hair cells (Fig. 3). An accurate description of the two different types of vestibular hair cells would have to wait for the development of the transmission electron microscope by Max Knoll and Ernst Ruska in 1931 and its application to electron microscopic ultrastructural analysis of cells by Porter (1945). In 1956, one of the most thorough ultrastructural analyses of the anatomical differences in the two different types of vestibular sensory hair cells and their pattern of afferent and efferent innervation was provided by the elegant and thorough ultrastructural study of the cristae ampulares from the vestibular labyrinths' of guinea pigs (Wersäll, 1956). This was the Docent thesis of Jan Wersäll as he studied in the famous Histology Department of the Karolinska Institute and this was accomplished in the same laboratory where Magnus Gustav Retzius performed his exceptional anatomical studies on the comparative anatomy of inner ear sensory receptors and reported on the anatomical structure of the human auditory receptor (Retzius, 1881, 1882, 1884). Docent Wersäll summarized his characterization of the two different type of vestibular hair cell that he observed in the guinea pig crista in a simple but elegant schematic drawing found in Fig. 9 of his thesis that was published as Supplement #126 in Acta Oto-Laryngologica (Wersäll, 1956). This schematic drawing depicts the major differences in both the shape differences of Type I and Type II vestibular hair cells as well as the differences in the characteristics of their afferent innervation (differences in the pattern of efferent innervation of Types I and II vestibular hair cells were not included) see Fig. 4. Later high resolution, low magnification electron micrographs (Harada, 1988) clearly show what Docent Wersäll had depicted in his informative schematic showing the different characteristics of innervation that characterize these two distinct types of vestibular hair cells (Fig. 5). The depiction of anatomical structure with drawings was well documented in the superb illustrations done by Professor Magnus Gustaf Retzius in his studies of the comparative anatomy of the inner ear of many different species of animals and birds, “Das Gehörorgan der Wirbelthiere” in two volumes (Retzius, 1981, 1984) while at the Histology Department of the Karolinska Institute. He is also known for his anatomy drawings depicting the structural features of the human membranous labyrinth, “Die Gestalt des membranosen Gehörorgans des Menschen” performed also at the Histology Department (Retzius, 1882). One of Retzius' most reproduced drawings is his elegant rendition of the human membranous labyrinth as it appeared after 6 months of gestational development (Fig. 6). Of particular note in this inner ear anatomy drawing, that is, Fig. 6, is Retzius' anatomical representation of the pattern of innervation for both the vestibular and auditory sensory receptors. A pioneer in the use of medical art drawings to depict complex anatomy was Max Brödel (Fig. 7) as he founded and was the first director the first academic Department of Medical Illustration, that is, Art as Applied to Medicine, Johns Hopkins University. During his academic career at Johns Hopkins Medical School working first in the Department of Anatomy at the invitation and with the encouragement of Professor Franklin Mall (Crosby and Cody, 1991), Professor Brödel used knowledge that he gained through many hours of careful anatomical dissections to artistically depict the complex anatomy of the human body including the relationship between the outer, middle and inner ears (Brödel, 1940, 1946). Some of his medical drawings of ear anatomy are presented in Figs. 8–10. For otolaryngologist perhaps one of his best known medical illustrations is found in Figure 10 which was completed in 1941 and depicts the relationship between the cochlea, middle ear cavity with its ossicular chain and the tympanic membrane (an anatomic area of great importance to Neurotologist for the insertion of an electrode array during the process of cochlear implantation, see Bas et al., 2012; Eshraghi et al., 2012; Rask-Andersen et al., 2012). The last of the Brödel inner ear drawings was actually completed after the death of Professor Brödel by one of his former students, that is, P. D. Malone, 1945, based in large part on the preliminary sketches and anatomic studies performed at an earlier date by Max Brödel (Fig. 11A, B; Brödel, 1946). These early medical illustrations of ear anatomy by Brödel have provided important insights into cochlea structure for both Otologists and Neurotologists.
Professor Sir Charles Oakley while at the Engineering Department of Cambridge University in the late 1940s along with his students is considered one of the perfectors of the Scanning Electron Microscope, that is, SEM. SEM depends upon critical point drying of biologic specimens then coating these processed specimen with a heavy metal in a partial vacuum (e.g., sputter coating with gold–palladium) and then bombarding the coated specimen with electrons in a partial vacuum so that secondary electrons emitted from the heavy metal coating of the specimen can be collected and provide a representation of the surface topography. One of the early investigators to take advantage of this technique of ultrastructural imaging for the documentation of inner ear surface anatomy was Professor David Lim (Lim, 1969, 1986; Lim and Lane, 1969a, b; Lim and Anniko, 1985). Most of David Lim's SEM ultrastructural observations of inner ear sensory receptor surface anatomy were performed in rodents with many of his studies focused on the morphology of the inner ear structures of adult guinea pigs (Lim, 2005). Professor David Lim was kind enough to provide me with some SEM images of both vestibular sensory receptor epithelia (Figs. 12–14) and the auditory sensory receptor epithelium (Figs. 15–17). In this special issue of the anatomical record Professor Helga Rask-Andersen and colleagues have provided ultrastructural images of the human auditory receptor and related their observations to the process of cochlear implantation (Rask-Andersen et al., 2012). It was the pioneering work of Professors Wersall, Lim, their students and other colleagues that encouraged the ultrastructural exploration of the auditory and vestibular receptors and the analyses of their structure–function using these ultrastructural techniques. Another pioneering investigator of ultrastructural studies was Professor Heinrich Spoendlin at the Ear Clinic, University of Zurich with his characterization of spiral ganglion neurons and the afferent and efferent innervation of the cochlear sensory receptor, that is, organ of Corti. Prior to the ultrastructural observations of auditory hair cell innervation it had been thought that outer hair cells (Figs. 15, 17) were the primary type of sensory hair cell responsible for audition. Professor Spoendlin demonstrated the two types of neurons present within the spiral ganglion, that is, Type I and Type II, and that the predominate neuronal type were the Type I neurons with only a small number of the Type II neurons present within this ganglion in several different species of laboratory animals (Fig. 18). Spoendlin's observations (Spoendlin, 1967, 1969, 1972, 1979a, b, 1981) revealed that the Type I neurons had large, myelinated cell bodies and in adult animals only innervated the inner hair cells (Figs. 15, 16) while the Type II neurons possessed small, unmyelinated cell bodies and only innervated the outer hair cells (Figs. 15, 17) in adult animals. It was further noted as a result of these ultrastructural observations that each individual inner hair was innervated by many, that is, >10, Type I afferent neurons and that a single Type II afferent neuron would innervate en passant several, that is, >5, outer hair cells which Spoendlin summarized in his schematic drawing presented in Fig. 19. These observations of Spoendlin caused quite a reaction and ended up changing the perception of the auditory research community to now consider that the primary sensory receptor cell within the auditory receptor was the inner hair cell and not the outer hair cell. The Type I spiral ganglion neurons and their peripheral neuronal projections are the primary target of the electric pulses produced in a tonotopic pattern by the cochlear implant and its electrode array (Eshraghi et al., 2012; Rask-Andersen et al., 2012; Green et al., 2012; Budenz et al., 2012). Several years later the outer hair cells were found to posses evoked mechanical responses to intracellular currents (Brownell, et al., 1985) and then in several more years outer hair cell motility was found to depend upon a cytoskeletal meshwork attached to a motor protein (Kalinec et al., 1992; Zheng et al., 2000) that modulated their mechanical responses, that is, elongation or shortening of outer hair cell length) in auditory frequency range to electrical stimulation. These observations showed that although the outer hair cells were not the primary sensory cells of audition they were still very important in the modulation of hearing sensitivity and in providing discrimination (Liberman, 2005).
BIOLOGY OF THE MEMBRANOUS LABYRINTH
The functional inner ear consists of the vestibular and auditory divisions of the membranous labyrinth, the perilymphatic spaces that surround the membranous labyrinth, the protective bony labyrinth and the auditory and vestibular ganglia of the VIIIth cranial nerve that carry impulses generated by the inner ear sensory receptors to the appropriate vestibular and auditory pathways of the central nervous system (Van De Water and Staecker, 2005).
Vestibular System: Biology of Balance
The biology of the vestibular system and its role in balance, perception of motion and spacial orientation has long been studied. Scott (1929) refers to studies of the phenomena of vertigo published 100 years before his studies and discusses how not all vertigo cases are of labyrinthine origin. Scott also developed a new definition for vertigo which he described as “the state of consciousness of a false sense of orientation of ourselves in relation to our environment.”
The vestibular system is constituted by the three semicircular canals with cristae located within their ampullae (Fig. 12) and the two otolith organs with associated maculae (e.g., macula of the saccule, Fig. 13). The semicircular canals were thought to respond only to angular accelerations and the otolith organs to the linear accelerations with the coordinated function of this complex arrangement of vestibular receptors responsible for maintaining a sense of balance; Goldberg and Fernandez (1975) have discussed the response of vestibular receptor epithelium to angular and linear and accelerations, respectively. As explained by Professor Highstein (Highstein, 1991; see also Highstein and Holstein, 2012), these organs send to the vestibular pathways of the brain information about head movement and position with respect to gravity in the context of the external world. Paton (1932) studied the vestibulo-ocular reflex, that is, VOR, neural pathways and already stated that besides acoustic functions and ocular control, the preservation of the body's equilibrium depends on the vestibular labyrinth. Thus, the vestibular system provides us with three-dimensional space displacement information which when processed by the vestibular pathways of the brain helps us to retain control of our balance. Head rotation produces image movement in the retina; however the VOR prevents this process by changing the position of the eye according in opposition to the change in position of the head in space. Robinson (1972) postulated that a brainstem neural network would integrate the modulation of the vestibular nuclei and the final eye movement and that the main function of that system is to stabilize the images and prevent nystagmus (Fig. 20). In this regard, Professor Clark (1979) discussed the importance of vestibular function in relation to spacial disorientation of pilots during flights and its contribution to the cause of accidents due to pilot error. In his work Professor Clark summarized a series of angular and linear tests and suggested the use of Coriolis stimulation in the selection and training of pilots.
The presence of the vestibular organs in all vertebrate organisms demonstrates the importance of these balance organs and how the information provided from them dictates motor actions, and can influence most sensory and cognitive activities. Professor Highstein (1991) discussed the different functions of the efferent vestibular system throughout different vertebrate organisms and their similarities in the auditory olivocochlear efferent system. While in the rest of the organisms the encapsulation of the balance organs into the rigid otic capsule prevents the ambient pressure transmission, interestingly the sense of depth in aquatic vertebrates is due to specialized openings in the labyrinth which allow for the transmission of changes in ambient water pressure to the vestibular receptors (Carey and Amin, 2006).
Each of the semicircular ducts contains a structure described as an ampulla. The base of each ampullae (i.e., cristae) and the otolith organs (i.e., maculae) of the utricle and saccule contain a patch of innervated hair cells (Figs. 4, 5, 12–14). Movement of the head displaces the endolymph fluid in each vestibular sensory compartment which causes the deflection of stereocilia of the resident vestibular hair cells thereby transducing this stimulation initiated by mechanical forces into neuro-electrical signals. The hair cells are innervated by primary afferent and efferent nerve fibers (Highstein, 1991; Highstein and Holstein, 2012; Figs. 4, 5). The vestibular system works in a fine balance between the output of the cristae and when the left semicircular canals are activated by rotary stimuli, then the canals of the right side receive an efferent stimulation that causes an inhibition of their afferent nerves and vice versa. This is due to the differences in the directional polarization of the sensory hair cells within the receptor epithelium of the different cristae with this first noted in an ultrastructural study of the semicircular canals and their ampullae/cristae in the ray, that is, Raja clavata, by Lowenstein and Wersäll (Lowenstein and Wersäll, 1959; Fig. 14). These inhibitory connections enhance the sensitivity of the vestibular hair cells and provide the mechanism for achieving balance (Goldberg and Fernandez, 1975; see also Highstein and Holstein, 2012). With such a complex interaction of stimulation and inhibition need to maintain balance (e.g., the directional polarization of the vestibular hair cells of the cristae of the semicircular ducts and the maculae of the utricle and saccule are fundamentally different; Harada, 1988), it is a considerable challenge and task to design and test a prosthetic devise that will aid and at least partially replace a damaged and malfunctioning vestibular system (see Highstein and Holstein, 2012; Fridman and Della Santina, 2012).
Cochlea: Biology of Hearing
The function of the sensorineural receptor organ of the auditory system is to convert airborne mechanical waveforms into electrochemical signals that can be transmitted and processed by the auditory pathways of the brain. During the 19th century there were two main theories put forth to explain the processes involved in the perception of sound, that is, the resonance and the frequency theories. The resonance theory conceived and put forth by Professor Helmholtz described the basilar membrane of the cochlea as a series of mechanical resonators, varying in their tuning from high frequencies represented at the base of the cochlea to low frequencies present at the apex (Gray, 1900). In contrast, the frequency theory (also known as the telephone theory of hearing) stated that the basilar membrane moves as a whole, in unison with the fluid wave transmitted from the perilymph of the scala tympani. This last (i.e., frequency) theory implies that the cochlea does not itself directly analyze sound, instead this analysis is accomplished mainly as a function of the auditory centers within the brain (Best and Tylor, 1952). In support of the frequency theory of hearing, von Békésy (Fig. 21; von Békésy, 1964, 1974) demonstrated the presence of a traveling wave along the cochlea's basilar membrane and that this wave oscillated at the frequency of stimulation. Quoting von Békésy from his Nobel Prize lecture in 1961, “Thus it became clear that the way to decide which of the four theories was representative of the inner ear was simply to measure the volume elasticity, since it is the magnitude of this variable that determines the pattern. These measurements, which are not difficult to make, confirm the existence of travelling waves along the basilar membrane” (von Békésy, 1964). Each position along the basilar membrane is most sensitive to a particular frequency of stimulation, arranged tonotopically along the membrane, where the higher frequencies stimulate the hair cells located in the basal area of the cochlea and the lower frequencies stimulate those hair cells located in the area of the cochlear apex. Traveling waves propagate from the base of the basilar membrane toward the apex of the cochlea. In the case of complex sound stimuli, the amplitude of the traveling wave at the basilar membrane will be maximal at the position whose resonant frequency matches the different frequencies of the stimulus, exciting the hair cells at that position (Hudspeth, 1989). However, this model seemed too simple and other mechanisms necessary for the cochlea's frequency selectivity and sensitivity to sound stimuli were proposed. The fact that von Békésy used cochlear specimens obtained from cadavers (von Békésy, 1974) was a limitation in his model, whereas Johnstone and Boyle's results obtained from experiments with live animals (Johnstone and Boyle, 1967) in general supported von Békésy's observations with the basal region of the cochlea more sharply tuned than the apex. Also, Rhode (1980) pointed out two major issues to the models that were being discussed by the neuroscientists studying hearing, first the relative sharpness of neural and mechanical tuning and second the presence of a mechanical nonlinearity, which was demonstrated to disappear upon the death of an animal test subject.
In addition to the graded mechanical properties of the basilar membrane and the selective stimulation of different subsets of hair cell populations by a specific frequency, Corwin and Warchol (1991) addressed two more processes involved in frequency selectivity and that were intrinsic to auditory hair cells, that is, the band-pass characteristics of the hair cell membranes and the mechanical properties of stereociliary bundles.
In an effort to understand the mechanical impedance discontinuities of wave propagation in the basilar membrane, Kemp (1978) hypothesized a traveling wave reflection that would lead to an “echo.” Not knowing the origin of this newly observed phenomena, Kemp described otoacoustic emissions for the first time, as sound energy emitted by the auditory system into the external canal for a period of milliseconds following stimulation by the initial acoustic impulse (Kemp, 1978). Interestingly, Gold (1948) in a previous publication had already postulated that the lost of energy by the viscous damping of the basilar membrane required what he named “the regeneration hypothesis,” meaning that an amplifier device is required to supply a restorative force that can counteract this energy loss. Kemp (2002) explained that von Békésy's work was done in cadaver cochleae and that the broad peaks were due to a high levels of viscous damping, which means a loss in energy caused by the reduced subtectorial space and by energy absorbed by the hair cells to operate. To counteract for this lost of energy, the mammalian cochlea has evolved a mechanism called “the cochlear amplifier.” Authors such as Flock and Strieloff (1984), Brownell et al. (1985), and Zenner (1986) studied outer hair cell motility in response to several different stimuli. Zenner (1986) devised a model to assess the influence of outer hair cell motility on the mechanics of the basilar membrane to control the damping by the basilar membrane. As stated by Lim (1986), the inner hair cells are located on the edge of the osseous spiral lamina (Fig. 15), where the degree of motion is minimum, in contrast the outer hair cells are located on the basilar membrane (Fig. 15), where a greater degree in motion has been demonstrated. While inner hair cells have been described as the transducers of auditory stimuli into the neuro-electrical signals, the outer hair cells mainly act as effectors by the action of their active motile processes and mechanical modulation of the cochlear function (Lim, 1986; Corwin and Warchol, 1991). Also, different investigators have reported and it is widely accepted that outer hair cell stereociliary bundles interact with the tectorial membrane (Hoshino, 1981), but that this interaction with the tectorial membrane does not occur with the stereociliary bundles of the inner hair cells (Rueda et al., 1996; LeMasurier and Gillespie, 2005).
Hair Cells: Transducers of Mechanical Energy into Neuro-Electrical Signals
Both the cochlea and the vestibular receptors rely on the biological activity of their mechanosensory hair cells, even though auditory and vestibular hair cells differ greatly in their specialization and morphology (Figs. 14, 16, 17), the mechanism of mechanotransduction appears to be similar (LeMasurier and Gillespie, 2005). The stereociliary bundles of the hair cells are located on their apical, luminal surfaces and consist of highly specialized, modified microvilli called stereocilia and in the case of vestibular hair cells there is also a single kinocilium. A single kinocilium was also present on developing auditory hair cells but regressed and is not present as the auditory hair cells undergo prenatal maturation with their location marked only by the presence of a intercellular basal body in the mature auditory hair cell. With the aid of electrophysiological and ultrastructural techniques, investigators have collected more information about the mechanotransduction process, however the investigation of this process is ongoing and many questions still remain unanswered.
Based on the observations of Smith et al. (1958) regarding the positive potential of the endolymph, Malcom (1974) suggested that as the endolymph in contact with the outer surface of the stereocilia was positive with respect to the cytosol of the hair cell, an electrical current would flow through the semipermeable membranes of the stereocilia along to the electrochemical gradient existing between the inside and the outside of the cell. Professor Malcom constructed a set of models with plastic tubing to mimic the stereocilia of the hair cells and based on the physical considerations he hypothesized that stimulation of the hair cells was proportional to the velocity of the fluid surrounding the stereocilia. He also suggested that the length of the kinocilium for transduction in vestibular hair cells was important as the fluid farthest from the hair cell had the greatest velocity and produced the highest bending. However, a few years later Hudspeth and Jacobs (1979) performed experiments in the vestibular system of the bullfrog, that is, Rana catesbeiana, which demonstrated that transduction is mediated via the stereocilia and that the kinocilium was not required for the transduction process, suggesting a role for the kinocilium in hair bundle polarization or transmission of stimulation to the stereocilia's transducer. As a result of further investigation, Hudspeth (1982, 1989) suggested that current flow into hair cells occurred through channels located in the tips of the stereocilia and that mechanical displacement of the hair bundle would cause transduction at the position of bending.
Previous to the paper by Pickles et al. (1984), other authors described the existence of cross-links between stereocilia in the vestibular system and in the cochlea (Fig. 22). However, Pickles and et al. (1984) with an appropriate fixation method were first to provide physical evidence for the existence of tip links. Based on their observations, these authors described three types of links between adjacent stereocilia, the first type joined the stereocilia in the same row, the second type also oriented laterally but joined the stereocilia from different rows and the third type stretching from the tips of shorter stereocilia to the sides of taller stereocilia in the next row. Assad et al. (1991) reported that the tip links between stereocilia transmit tension to the transduction channels and suggested that these tip links are the gating springs of these channels. This group (Assad et al., 1991) also demonstrated the integrity of the tip links was dependent on the presence of Ca++. However, the contention that the tip links are gating springs is open to controversy as the results from recent molecular and structural studies suggest otherwise (Gillespie and Muller, 2009). This topic is still arousing the curiosity of scientists who recently have demonstrated the molecular composition of the tip link helix. Siemens et al. (2004) have suggested that cadherin 23 and myosin-1c cooperate to regulate the activity of the mechanotransduction channels in the hair cells. The fact that the elasticity of cadherin 23 is very limited, suggests that rather than the tip links acting as gating springs it is more likely that they are in series with the gating springs (Gillespie and Muller, 2009). A recent review by Corey (2009) states that the helix of stereocilia tip links is formed by a combination of cadherin 23 and protocadherin 15. Deflection of the stereocilia bundle toward the kinocilium (i.e., for vestibular hair cells) or the position of the basal body where there was once a kinocilium (i.e., for auditory hair cells) increases the tension on the tip links and opens gated or mechanotransduction channels. The cations from the K+ rich endolymph can then enter into the hair cells and cause their depolarization (Corwin and Warchol, 1991; Carey and Amin, 2006), while deflection of the stereocilia in the opposite direction away from the position of the kinocilium will cause a hyperpolarization of the hair cells and an inhibitory response. The voltage-sensitive Ca++ channels are then activated in the depolarized hair cell and a rapid influx of calcium leads to the release of the excitatory neurotransmitters (Carey and Amin, 2006). This process results in action potentials in the afferent nerve fibers that synapse with these hair cells, this neuro-electrical generated signal is then received and processed by the appropriate auditory or vestibular pathways of the central nervous system.
Innervation of Hair Cells: the Afferent and Efferent Systems
Heinrich Spoendlin (Spoendlin, 1967) studied the distribution of the afferent and efferent nerve fibers in the organ of Corti of the cat and observed that the bases of outer hair cells were innervated primarily by efferent nerve fibers and that the number of efferent terminals associated with each outer hair cell was greater in the basal turn than in the apex. Iurato et al. (1978), Spoendlin (1979a), and Warr (1980) demonstrated by different approaches that the majority of efferent nerves that innervate the outer hair cells originate from large neurons in the contra-lateral accessory olivary nucleus, whereas the efferent nerves that innervate the inner hair cells originate from small neurons in the homo-lateral main superior olivary nucleus. Dunn and Morest already (Dunn and Morest, 1975) observed that the afferent fibers innervating the outer hair cells had a different function than the afferent nerves that innervate the inner hair cells. The main function of the afferent neurons innervating the inner hair cells (i.e., Type I, myelinated) was to transmit the sound information from the organ of Corti to the auditory pathway of the brain, while the afferents that innervate the outer hair cells (i.e., Type II, unmyelinated) have been thought to provide an integrated afferent feedback loop and amplify cochlear sensitivity and frequency discrimination (Thiers et al., 2008). Only 5% of the afferent neurons was associated with the outer hair cells, a fact that surprised the author as the outer hair cells are around three times as numerous than the inner hair cells (Spoendlin, 1967, 1979b) and also because at that point in time it was believed that the outer hair cells were the main sensory receptor cells of hearing (Fig. 19). Spoendlin observed that the Type II neurons send branches to many outer hair cells meaning that one Type II neuron can innervate multiple outer hair cells, i.e., >5). Conversely, each inner hair cell was innervated but many unbranched nerve fibers from individual Type I auditory neurons (i.e., >10). Based on the synaptic connections seen between the afferent and efferent neurons and the hair cells, Spoendlin (Spoendlin, 1967) suggested that the efferent innervation would be influencing the afferent neurons, however their role in the acoustic code remained uncertain and the only known effect was inhibition of the afferent system. These findings also provided evidence of a primary function of the outer hair cells as effectors in an active motile process and in the modulation of cochlea biomechanics. According to Spoendlin's observations, other authors such as Hans Engström and his daughter Britta (Engstrom and Engstrom, 1972) also found that inner hair cells were highly innervated by afferent nerve fibers and that the outer hair cells were innervated at their basal end by both afferent and efferent nerve fibers with the efferent terminals being more numerous. The efferent endings were found to be large and with a high content of synaptic vesicles, while the afferent endings were less granulated. These authors (Engstrom and Engstrom, 1972) suggested that the outer hair cells to behave more like neurons because of their intracellular synaptic structures and synaptic vesicles. Later, Flock and Russell (Flock and Russell, 1973) in a fish model, that is, Burbot (Lota lota), selectively blocked the efferent synapses of the lateral line organs with an inhibitor, that is, Flaxedil, and observed that the efferent synapses were cholinergic, suggesting that acetylcholine (ACh) may be released from the efferent synapse of the cochlea as well. Recent studies have confirmed that the fast synaptic inhibition of outer hair cells is not due to activation of GABAergic signaling but achieved by activation of nicotinic ACh receptors (Oliver et al., 2000). This is of particular interest because these receptors mediate excitatory postsynaptic responses at the neuro-muscular junction. As recent as only few years ago, Taranda et al. (2009) reported that the activation of the nicotinic ACh receptors at the base of outer hair cells produced an intracellular increase of Ca++ levels that caused the SK2 channels to open, leading to a hyperpolarization, that is, inhibition of outer hair cell motile activity. Conversely, and recently reviewed by Defourny et al. (2011), glutamate has been demonstrated to be the primary neurotransmitter released by the inner hair cells which stimulate the peripheral nerve endings of the Type I auditory nerve fibers, however several reports describe the presence of N-methyl-D-aspartate receptors on the afferent postsynaptic densities.
The historic acquisition of a broad knowledge base on the structure–function of both the vestibular sensory receptors, that is, both cristae and maculae, and the auditory sensory receptor, that is, organ of Corti, has been the key that has allowed for the development and implementation of cochlear implants for the treatment of the deaf patients and patients with severely impaired hearing as well as to form the basis for the design and construction of a vestibular prosthesis that can hopefully be used in the future to treat patients with life-altering balance disorders.
It is good to have a friend, that is, Jeff, who makes me laugh with his consummate New Yorker view of the rest of these United States. His “Kvetching” has most certainly driven the progress of this Special Edition of The Anatomical Record. Thank You Jeff! Thanks are also due to Dr. Esperanza Bas for all her valuable help in the preparation of this manuscript. Thank you Espe!