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1. Exploring the Relationship Between Cost Function of Hybrid Traveling Waves and Structural Reflection Coefficient Adapted From Acoustics

   https://doi.org/10.1115/SMASIS2024-140453  

   Gupta, Samikhshak; Malladi, Vijaya_V_N Sriram (September 2024, American Society of Mechanical Engineers)

   Abstract The Basilar Membrane (BM) is the structural component of the mammalian cochlea that transmits auditory information as traveling structural waves, and inner hair cells transduce acoustic waves into electrical impulses in the inner ear. These waves go up towards the cochlea’s apex from its base. The primary structure at the apex of the cochlea that keeps waves from returning to the base is the helicotrema. People can hear continuous sound waves without acoustic reflection or overlap because of this property of the BM. Our research is motivated by this biological phenomenon and aims to comprehend and passively reproduce it in engineering structures. By studying the dynamics of a uniform beam linked to a spring-damper system as a passive absorber, we can use this characteristic of the inner ear to explain some of the observed phenomenological behaviors of the basilar membrane. The spring-damper system’s position separates the beam into two dynamic regions: one with standing waves and the other with non-reflecting traveling waves. This study presents the computational realization of traveling waves co-existing with standing waves in the two different zones of the structure. Moreover, this study aims to establish a correlation between two approaches to analyze the characteristics of the wave profiles: (i) the absorption coefficient approach and (ii) the cost function based on the wave envelope. The Basilar Membrane (BM) serves as the crucial structural conduit for transmitting auditory information through traveling structural waves, with inner hair cells in the inner ear transducing these waves into electrical impulses. These waves ascend from the cochlea’s base towards its apex, and the helicotrema, positioned at the cochlear apex, plays a pivotal role in preventing wave reflection and overlap, thereby facilitating the perception of continuous sound waves. The intrinsic characteristics of the Basilar Membrane (BM) inspire our research as we seek to comprehend and passively replicate this phenomenon in simplified forms. The investigation involves the exploration of the dynamics exhibited by a uniform beam connected to a spring-damper system acting as a passive absorber. This chosen system allows us to take advantage of the unique property of the inner ear, shedding light on some of the observed phenomenological behaviors of the basilar membrane. The positioning of the spring-damper system engenders two distinct dynamic regions within the beam: one characterized by standing waves and the other by non-reflecting traveling waves. The comprehensive analysis incorporates analytical and computational aspects, providing a holistic understanding of the coexistence of traveling and standing waves within these two dynamic zones. 

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2. Mechanical network equivalence between the katydid and mammalian inner ears

   https://doi.org/10.1371/journal.pcbi.1012641  

   Celiker, Emine; Woodrow, Charlie; Guadayol, Òscar; Davranoglou, Leonidas-Romanos; Schlepütz, Christian M; Mortimer, Beth; Taylor, Graham K; Humphries, Stuart; Montealegre-Z, Fernando (December 2024, PLOS Computational Biology)

   Nowotny, Manuela (Ed.)

   Mammalian hearing operates on three basic steps: 1) sound capturing, 2) impedance conversion, and 3) frequency analysis. While these canonical steps are vital for acoustic communication and survival in mammals, they are not unique to them. An equivalent mechanism has been described for katydids (Insecta), and it is unique to this group among invertebrates. The katydid inner ear resembles an uncoiled cochlea, and has a length less than 1 mm. Their inner ears contain a hearing organ,crista acustica, which holds tonotopically arranged sensory cells for frequency mapping via travelling waves. Thecrista acusticais located on a curved triangular surface formed by the dorsal wall of the ear canal. While empirical recordings show tonotopic vibrations in the katydid inner ear for frequency analysis, the biophysical mechanism leading to tonotopy remains elusive due to the small size and complexity of the hearing organ. In this study, robust numerical simulations are developed for anin silicoinvestigation of this process. Simulations are based on the precise katydid inner ear geometry obtained by synchrotron-based micro-computed tomography, and empirically determined inner ear fluid properties for an accurate representation of the underlying mechanism. We demonstrate that the triangular structure below the hearing organ drives the tonotopy and travelling waves in the inner ear, and thus has an equivalent role to the mammalian basilar membrane. This reveals a stronger analogy between the inner ear basic mechanical networks of two organisms with ancient evolutionary differences and independent phylogenetic histories. 

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3. Altered cochlear innervation in developing and mature naked and Damaraland mole rats

   https://doi.org/10.1002/cne.24682  

   Barone, Catherine M.; Douma, Sytse; Reijntjes, Daniël O. J.; Browe, Brigitte M.; Köppl, Christine; Klump, Georg; Park, Thomas J.; Pyott, Sonja J. (March 2019, Journal of Comparative Neurology)

   Abstract Compared to many other rodent species, naked mole rats (Heterocephalus glaber) have elevated auditory thresholds, poor frequency selectivity, and limited ability to localize sound. Because the cochlea is responsible for encoding and relaying auditory signals to the brain, we used immunofluorescence and quantitative image analysis to examine cochlear innervation in mature and developing naked mole rats compared to mice (Mus musculus), gerbils (Meriones unguiculatus), and Damaraland mole rats (Fukomys damarensis), another subterranean rodent. In comparison to mice and gerbils, we observed alterations in afferent and efferent innervation as well as their patterns of developmental refinement in naked and Damaraland mole rats. These alterations were, however, not always shared similarly between naked and Damaraland mole rats. Most conspicuously, in both naked and Damaraland mole rats, inner hair cell (IHC) afferent ribbon density was reduced, whereas outer hair cell afferent ribbon density was increased. Naked and Damaraland mole rats also showed reduced lateral and medial efferent terminal density. Developmentally, naked mole rats showed reduced and prolonged postnatal reorganization of afferent and efferent innervation. Damaraland mole rats showed no evidence of postnatal reorganization. Differences in cochlear innervation specifically between the two subterranean rodents and more broadly among rodents provides insight into the cochlear mechanisms that enhance frequency sensitivity and sound localization, maturation of the auditory system, and the evolutionary adaptations occurring in response to subterranean environments. 

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4. Characterization and Miniaturization of Silver-Nanoparticle Microcoil via Aerosol Jet Printing Techniques for Micromagnetic Cochlear Stimulation

   https://doi.org/10.3390/s20216087  

   Sarreal, Ressa Reneth; Bhatti, Pamela (November 2020, Sensors)

   null (Ed.)

   According to the National Institute of Deafness and other Communication Disorders 2012 report, the number of cochlear implant (CI) users is steadily increasing from 324,000 CI users worldwide. The cochlea, located in the inner ear, is a snail-like structure that exhibits a tonotopic geometry where acoustic waves are filtered spatially according to frequency. Throughout the cochlea, there exist hair cells that transduce sensed acoustic waves into an electrical signal that is carried by the auditory nerve to ultimately reach the auditory cortex of the brain. A cochlear implant bridges the gap if non-functional hair cells are present. Conventional CIs directly inject an electrical current into surrounding tissue via an implanted electrode array and exploit the frequency-to-place mapping of the cochlea. However, the current is dispersed in perilymph, a conductive bodily fluid within the cochlea, causing a spread of excitation. Magnetic fields are more impervious to the effects of the cochlear environment due to the material properties of perilymph and surrounding tissue, demonstrating potential to improve precision. As an alternative to conventional CI electrodes, the development and miniaturization of microcoils intended for micromagnetic stimulation of intracochlear neural elements is described. As a step toward realizing a microcoil array sized for cochlear implantation, human-sized coils were prototyped via aerosol jet printing. The batch reproducible aerosol jet printed microcoils have a diameter of 1800 μm, trace width and trace spacing of 112.5 μm, 12 μm thickness, and inductance values of approximately 15.5 nH. Modelling results indicate that the coils have a combined depolarization–hyperpolarization region that spans 1.5 mm and produce a more restrictive spread of activation when compared with conventional CI. 

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5. Petrosal morphology of the Early Cretaceous triconodontid Astroconodon from the Cloverly Formation (Montana, USA)

   https://doi.org/10.1007/s10914-023-09673-5  

   Hoffmann, Simone; Kirk, E. Christopher; Rowe, Timothy B.; Cifelli, Richard L. (December 2023, Journal of Mammalian Evolution)

   Although several well-preserved crania are known for the Mesozoic Eutriconodonta, three-dimensional reconstructions of the character-rich inner ear and basicranial region based on high-resolution computed tomography scans have previously only been published for the Late Jurassic Priacodon. Here we present a description of the petrosal and inner ear morphology of a triconodontid eutriconodontan from the Lower Cretaceous Cloverly Formation, which we provisionally assign to Astroconodon. The bony labyrinth of Astroconodon is plesiomorphic for mammaliaforms in lacking a primary osseous lamina, cribriform plate, and osseous cochlear ganglion canal. However, as in Priacodon and the zhangheotheriid Origolestes, Astroconodon has a secondary osseous lamina base that extends nearly the complete length of the cochlear canal. The cochlear canal is straighter in Astroconodon and other eutriconodontans compared to several basal mammaliaform clades (e.g., morganucodontans, docodontans), that exhibit varying degrees of cochlear canal curvature. The pars cochlearis of the petrosal was well vascularized in Astroconodon, exhibiting a network of venous canals that crossed the cochlea transversely on its ventral and dorsal aspects. Of particular note are several canals that passed along the base of the secondary osseous lamina. As in Priacodon and Origolestes, those canals do not show the extensive connections to the cochlear labyrinth as seen in the basal mammaliaforms Morganucodon and Borealestes. The inner ear of Astroconodon thus highlights the complex history of the mammaliaform cochlear canal, in which different clades appear to follow independent evolutionary trajectories and various key morphological features (e.g., cochlear canal length, curvature, vascularization and osseous supports for the basilar membrane) exhibit considerable homoplasy. 

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