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Although showing a traditional 9 + 2 axoneme pattern of “motile cilia,” they lack the inner dynein arms and only directionally “move” after the cells sense sound, i.e., passive motion (Kikuchi et al., 1989). Besides, kinocilia are essential for HCs morphogenesis and planar cell polarity (PCP), and further auditory integrity (Sipe and Lu, 2011; Kazmierczak and Muller, 2012; Elliott et al., 2018). Genetic mutations affecting ciliary proteins can lead to diseases in multiple organs, collectively known as ciliopathies. Therefore, maintaining stable morphology and structure of kinocilia is essential to normal physiology and their dysfunction results in corresponding sensory ciliopathies. In this review, we describe the structure, function, and degeneration of kinocilia present in the mammalian auditory system and discuss if they are promising therapeutic targets for hearing deficits.
Radixin modulates the function of outer hair cell stereocilia
Presumably, kinocilia are not needed for auditory signal processing in the cochlea, since the longest stereocilia play a very similar role. As mentioned above, the cochlea is unique to mammals, and its internal mechanical receptors have correspondingly evolved in structure and function. Primitive vertebrates such as fish only have an inner ear, which is mainly used for balance. Moreover, although they have a complete vestibular system, auditory functions must be taken into account (Whitfield, 2020). Amphibians such as frogs have evolved a middle ear with an eardrum (Mason et al., 2015).
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However, it is still a matter of debate whether Ca2+ entering through α9α10 receptors is sufficient to activate the SK2 channels or if alternatively, Ca2+ release from an internal store might also take place. The IHC is innervated by auditory nerve fibers, and every fiber contacts the IHC with an unmyelinated single ending to form a single ribbon synapse (Liberman, 1982). All neuronal activity is driven by transmitter release from IHC (Robertson and Paki, 2002).
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Subsequent studies have revealed the identity of many of the proteins that contribute to tip links, illustrated in Figure 2. The tip link itself is formed by cadherin 23 (CDH23) together with protocadherin 15 (PCDH15) (Siemens et al. 2004; Kazmierczak et al. 2007; Müller 2008; Alagramam et al. 2011). The upper portion of the tip link is a parallel dimer of CDH23, while the lower portion is a parallel dimer of PCDH15 (Narui and Sotomayor 2018).
Afferent synapse
Chloride ions would be predicted to move away from the cell interior during hyperpolarization and toward the cell interior during depolarization. The close coupling between charge movement and cell shortening (actuation) remains unresolved. Prestin has also been reported to contribute to amplification by short hair cells in the bird auditory papilla (Beurg et al. 2013). Through hair cells, the auditory system encodes sound intensity (loudness) and sound frequency (tone). It is straightforward to see how larger amplitudes of oscillation might be induced by louder sounds, and how this could create greater depolarization of the hair cell. Two fundamentally different mechanisms have evolved for tone discrimination; one which employs electrical resonance in the basolateral membrane of the hair cell, while the other makes use of tonotopic differences in the mechanical resonance of the basilar membrane.
The perilymph in the scala tympani has a very low concentration of positive ions. The electrochemical gradient makes the positive ions flow through channels to the perilymph. Hair cells can convert the displacement of the stereociliary bundle into an electrical potential in as little as 10 microseconds; indeed, such speed is required to faithfully transduce high-frequency signals and enable the accurate localization of the source of the sound. Evidently the filamentous structures that connect the tips of adjacent stereocilia, known as tip links, directly open cation-selective transduction channels when stretched, allowing K+ ions to flow into the cell (see Figure 13.7D).
In tunicates, hair cells called coronal cells are present on the velum that rings the inner surface of the oral (incurrent) siphon and may serve a protective function by sensing large particles (Caicci et al. 2007; Rigon et al. 2013). The lateral line system is widely present in aquatic larval forms of frogs and salamanders; in newts it is present during the juvenile aquatic stage, disappears during the terrestrial stage, and then reappears during the adult aquatic stage (Duellman and Trueb 1994). At the presynaptic juncture, there is a distinct presynaptic dense body or ribbon.
Bioinspired magnetic cilia: from materials to applications
In contrast to vestibular hair cells and non-mammalian auditory hair cells, cochlear hair cells of mammals do not display electrical resonance. In the absence of electrical resonance, mammalian inner ears have evolved two complementary mechanical features to enhance tone discrimination and sound sensitivity. The first is that the basilar membrane changes in stiffness along its length, being more stiff toward the basal end and less stiff toward the apical end. This arises from differences in the width of the basilar membrane and also differences in its structure, which is thicker at the base and thinner toward the apex. Traveling waves of this sort have not been observed in fish, amphibians, reptiles, or birds. Another specialized synapse is found in Type I vestibular hair cells, which are surrounded on the basolateral side by a calyx-like expansion of the primary afferent nerve ending (Fig. 4).
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Closer to home in Chordata, Manni and colleagues have described mechanosensitive coronal cells on the oral (incurrent) siphon of tunicates that satisfy multiple criteria for homology with vertebrate hair cells (Caicci et al. 2007; Rigon et al. 2013). Hair cells are the sensory receptors of the auditory system—they transduce mechanical sound waves into electrical energy that the nervous system can understand. Hair cells are located in the organ of Corti within the cochlea of the inner ear, between the basilar and tectorial membranes. The outer hair cells serve other functions, such as sound amplification in the cochlea, and are not discussed in detail here.
The decline in response, and therefore sensitivity, to a sustained stimulus. Adaptation is a property of many sensory receptors that enables them to adjust their sensitivity to prevailing conditions and respond only to changes in stimulus intensity. One of three subtypes (I,II and III) of hereditary disorder characterized by sensorineural deafness of cochlear origin combined with loss of vision due to retinitis pigmentosa. The hair follicles in your skin contain living cells to allow your hair to grow. The shaft—the part of the hair we see—is made up of dead cells and consists of three different layers. As a piece of hair grows, it goes through three phases before it sheds and a new one grows.
Activity of the MOC to IHC input is also inhibitory during this developmental period, and would control the excitability of the hair cells (Glowatzki and Fuchs, 2000; Goutman et al., 2005; Johnson et al., 2011b; Sendin et al., 2014). Hair cells possess a set of 20 – 300 modified microvilli, or stereocilia, located in the cellular apex and presenting a stereotyped array (see Fig. 2A) (Hudspeth, 1989). In mammalian hair cells, stereocilia are arranged in three rows of increasing height. Each stereocilia is formed by a pack of actin filaments and inserts in the apical end of the hair cell within the so called cuticular plate. At the tip, a complex machinery devoted to transduce mechanic vibrations into electrical potentials, is located (Denk et al., 1995).
If a hair cell were as tall as the Eiffel tour, then the displacement of the cilia would only be 1 cm. Like other nicotinic receptors, the receptor in hair cells is a pentamer. Two nicotinic subunit types, alpha-9 and alpha-10, have been identified in hair cells, and when expressed together in Xenopus oocytes they form an ACh receptor that preferentially allows Ca2+ to permeate. In fact, it is about 10 times more permeable to Ca2+ than to Na+ (Weisstaub et al., 2002). The key to the puzzle of hyperpolarization produced by inward current is the presence of Ca2+-activated K+ channels in close proximity to the ACh receptors.
Those at the base are termed ankle links; beyond those are a series of so-called shaft connectors, and distal to the shaft connectors there are horizontal top connectors. The stereocilia adjacent to the kinocilium are linked to it by distinct kinocilial connectors. The functional effect of these multiple links is that the entire hair bundle moves as a single unit in response to mechanical stimuli. Between the tip of one stereocilium and the shaft of its adjacent, taller neighbor there is a different mechanical link, termed a tip link (Goodyear et al. 2005).
This shortening is extremely rapid and can be observed in response to imposed depolarizations at frequencies up to 100 kHz, near the top end of frequency sensitivity in bats and cetaceans (whales and dolphins). Impressively, the response amplitude and response phase of OHCs display high fidelity out to 50 kHz in vitro (Frank et al. 1999). By shortening, the OHCs increase the shearing motion of the tectorial membrane over the surface of the IHCs, thereby amplifying the displacement of the IHC stereocilia bundle. Each of our roughly 16,000 hair cells is dedicated to a narrow frequency range. These cells are ordered along the basilar membrane according to the frequencies they detect.
The physiological significance of this cochlear degeneration is still not fully understood, but we can gain insights through comparison of cochlear kinocilia with those in the vestibular system. The time required for the response of a system to decline to 37% of its initial value. For the cell membrane, this is the product of its capacitance and resistance, setting the timescale over which membrane currents change the voltage.
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The Organ of Conti is inside the cochlea, and is made up of many hair cells. The cells on the side nearest the outer ear are called outer hair cells, and the cells further inside are inner hair cells. The cochlea of the inner ear contains hair cells, sensory receptors that transduce sound waves into neural signals that can be interpreted by the brain. The inner hair cells transform the sound vibrations in the fluids of the cochlea into electrical signals that are then relayed via the auditory nerve to the auditory brainstem and to the auditory cortex. Stereocilia are delicate, hair-like projections that react to cochlea fluid movement.
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