The complexity of the middle and inner ear

The most important parts of the auditory system, regarding most forms of tinnitus, are the function of the cochlea, the auditory nerve and the central nervous system. Until recently, the middle ear was considered important for tinnitus when impairments reduced the sound that reaches the cochlea. Now the role of middle ear in tinnitus has been re-evaluated The middle ear has been closely studied since audiology began, but even now its functions are not fully understood. The middle ear is an intricate arrangement of membranes, bones, muscles, and ligaments At hearing threshold, the eardrum moves of the order of picometres . Understanding how such minute movements are transmitted through a delicate system of bones and joints stretches experimental apparatus to its limits. We all know that the middle ear conducts sound to the inner ear. Furthermore, the device is an “impedance adapter”: its function is to match sound passing from the low impedance of the air to the high impedance of the cochlear fluid. The primary function of the middle ear is to recover part of the energy dissipated by the air - liquid passage When a sound wave is transferred from a low-impedance medium (eg, air) to one of high impedance (eg, water), a considerable amount of its energy is reflected and fails to enter the liquid. If no middle ear were present, only 0.1% of the acoustic wave energy traveling through air would enter the fluid of the cochlea and 99.9% would be reflected. (97% if we consider the perilinfa instead of water). The primary function of the middle ear is to recover part of the energy dissipated by the air - liquid passage. Middle ear: a pure transformer? Traditional teaching holds that the middle ear acts as a pure transformer. The middle ear pressure gain, resulting from this transformer action, is about 30 dB. This is derived from two structurally related factors: the area ratio (TM area divided by the stapes footplate area [22:1]) and the o’ssicular lever (the length of the manubrium of the malleus divided by the length of the long process of the incus [1.3:1]). Implied in a transformer analogy is that this gain is independent of frequency. Modifications of this theory have deve’loped because of better understandings of TM motion/velocity, increased laxity of the malleal-incudal joint system above 1 kHz, ossicular coupling and acoustic coupling. The pressure gain provided by the normal middle ear is frequency-dependent. The middle ear has its major gain in the lower frequencies, with a peak near 0.9 kHz. The mean gain was 23.0 dB below 1.0 kHz;; the mean peak gain was 26.6 dB at 1 kHz, the resonant frequency of the middle ear. Above 1.0 kHz, the second pressure gain decreased at a rate of -8.6 dB/octave, with a mean gain of 6.5 dB at 4.0 kHz. Only a small amount of gain was present above 7.0 kHz. Middle ear muscles The 2 smallest str’iated muscles in the body, the tensor tympani and the stapedius, are contained within the middle ear. The tensor tympani, 2 cm long, attaches to the superior portion of the malleus; 5 ° cn The stapedius, only 1 mm long, is attached to the upper portion of the stapes; innervated by the 7° cn When the tensor tympani muscle contracts, it pulls on the malleus and forces the stapes into the oval window, raising the pressure of the incompressible cochlear fluids and distending the round window. When the stapedius contracts, it pushes the ossicular chain towards the middle ear, reducing the inner ear pressure Which is the function of middle ear muscles? Although the nature of the acoustic middle ear muscle reflex has been well established, its physiologic role has remained unclear. The standard answer is protection of the cochlea from loud noises. Contraction of both muscles is activated by acoustic stimulation of 70-90 dB above threshold. Refex contraction stiffens the ossicular chain, causing an increase in mechanical impedance, and hence reducing sound transmission by 5-10 dB However, it is a relatively slow action (the latency of the muscle contraction is more than 10 dB, takes up to 25-35 ms) and cannot protect the ear from a sudden impulsive acoustic trauma like a gunshot. In addition, the degree of attenuation is primarly concentrated in the low frequencies. In ancient ages (no weapons, no industrial noise) which was the function of middle ear muscles? Other possible functions of the middle ear muscles have been proposed, including Improved speech perception The stapedial reflex helps clarify high-frequency verbal recognition by modulating low-frequency sounds that might otherwise mask higher frequency sounds at elevated dB levels. Recent data have shown that speech perception remains unaffected with or without a stapedial response. But another study looking at patients who had undergone argon laser stapedotomy with preservation of the stapedial tendon compared with patients without tendon preservation demonstrated that patients with tendon preservation showed a tendency to better results in the sound-noise ratio. The function of middle ear muscles i hearing accomodation! The middle ear also contains ligaments that connect the little bones to the walls. Among these ligaments, there are the oto-mandibular ligaments. Dissections in humans have established an anatomical link between the TMJ, the middle ear: the oto-mandibular ligaments. The oto-mandibular ligaments may be implicated in tinnitus associated with TMDisorders. A positive correlation has been found between tinnitus and ipsilateral TMJ disorder. It has been proposed that a TMJ disorder may stretch these ligaments, thereby affecting middle ear structure equilibrium The oto-mandibular ligaments are the discomalleolar ligament (DML), and the anterior malleolar ligament.They connect the malleus' anterior process (Gracilis process) with the TMJ disc. The DML is an embryological remnant of the external pterygoid muscle; it is located more laterally to the AML and inserts into the postero-supero-medial TMJ capsule and retrodiscal portion of the TMJ disc.The AML is placed more medially and joins the spheno-mandibular ligament (EML), being accompanied by the chorda tympani nerve. The AML was considered to be a continuation of the EML, an embryological remnant of Meckel's cartilage Eustachian Tube The proper function of the middle ear depends on the presence of a mobile tympanic membrane capable of vibrating in response to a sound wave. For the tympanic membrane to have maximal mobility, the air pressure within the middle ear must equal that of the external environment. The eustachian tube acts as a pressure release valve to adjust the middle ear pressure. The eustachian tube extends from the anterior wall of the tympanic cavity to the lateral wall of the na’sopharynx. In its resting state, the Eustachian tube valve is closed because of the elastic forces of the tube and its supporting structures. The valve area within the nasopharynx d’lates when the tensor veli palatini muscle contracts during pharyngeal swallowing and yawning The ability of the eustachian tube to perform its functions of ventilation, protection, and drainage can be influenced by variations in its own structure and by the conditions of the middle ear and the nasopharynx. The tensor tympani muscle is anatomically and functionally correlated to the tensor veli palatini (external peristalphine) and can be considered a masticatory muscle. Therefore masticatory disorders may result in excessive contraction of the tensor tympani which would result in increased cochlear impedance and excessive fragility of the cochlear structures, particularly at baseline. The inner ear The bony labyrinth is the rigid, bony outer wall of the inner ear in the temporal bone. It is one of the most resistant bones of the human body. It consists of three parts: the vestibule, semicircular canals, and cochlea. They contain a fluid, the perilymph, in which the membranous labyrinth is situated. The saccule is the most mysterious organ of the labyrinth and, for this reason, one of the most fascinating. For anatomic, embryologic and physiologic reasons the saccule has both auditory and vestibular characteristics, being the connecting link between the vibratory energy and the vestibular response: it is an anatomic bridge between the anterior and the posterior labyrinth; it has a common embryologic origin with the cochlea in the pars inferior of the labyrinth; it is the main hearing organ in fish and other ancestral vertebrates. The saccule is an otolith organ involved in vertical linear movement detection and sensing gravitational changes; it controls the tonic components of the antigravity muscles contributing to the postural control. In addition, through the vestibule-sympathetic reflexes, it contributes to the control of blood pressure during movement and through postural changes. The Cochlea The cochlea is a very complex structure with essentially two fluids systems with different I’onic composition. The most important structures for transduction of sound into a neural code in auditory nerve fibers are the sensory cells (hair cells) that are located along the basilar membrane. Inner and outer hair cells There are two kinds of sensory cells (inner and outer hair cells) that are morphologically similar but have complete different functions. The apical portion of all hair cells is similar and contains bundles of actin filaments called stereocilia, a kinocilium.The stereocilia are connected to each other by cross-links and tip-links. There are approximately 3500 inner hair cells arranged in one row and 12,000 outer hair cells in three rows.The structural aspects of the cell bodies differ significantly and reflect their functional differences. The inner hair cells are flask-shaped and contain high concentrations of Golgi bodies and mi’tochondria and vesicles with neurotransmitters. Although highly metabolic, inner hair cells are considered to be passive transducers in the auditory system The inner hair cells are the actual sensory receptors, and 95% of the fibers of the auditory nerve arise from them. Outer hair cells are cylindrical in shape and contain microfilaments and microtubules along the length of the cell that give rise to motile activity. The motile properties have been shown in a classical experiment by Ashmor The terminations on the outer hair cells are almost all from efferent axons that arise from cells in the brain. A short video recorded in the laboratory of Prof. Ashmore at University College London and first seen on the BBC programme called ‘Ear We Go’ in 1987. demonstrates what happens to an isolated guinea pig outer hair cell when it is stimulated electrically by an electrode. Through the pipette, an alternating current signal is injected, and the resulting motor response is observed under a microscope This outer hair cell gets thinner when it gets longer and fatter when it gets shorter. Measuring up these changes indicates that the cell volume stays constant. This supports the idea that the ‘motor’ is a molecule whose job it is to change membrane area. The molecule, discovered in 2000 by Peter Dallos is called ‘prestin’. This is capable of generating a length change of approximately 5%. This discovery has completely changed the cochlea physiology Cochlear hydrodynamics When a sound wave is transmitted to the middle ear, it vibrates the ossicular chain. Vibration of the stapes transmits the sound wave via the oval window to the scala vestibuli, generating fluid waves in the perilymph. The displacement of the perilymph causes a wavelike displacement of the basilar membrane and organ of Corti, causing distention of the round window membrane. Von Békésy Traveling wave theory (1961) The motion of the basilar membrane is generally described as a traveling wave. The traveling wave theory was presented in 1961 by Von Bekesy who gained the Nobel prize for that. The basilar membrane has an anatomical structure that varies from the base to the apex, The basilar membrane is widest (0.42–0.65 mm) and least stiff at the apex of the cochlea, and narrowest (0.08–0.16 mm) and most stiff at the base. High-frequency sounds localize near the base of the cochlea, while low-frequency sounds localize near the apex. Physical properties of the basilar membrane, related to changes in its stiffness and mass along its length, can account for these passive-tuning properties. The amplitude of a sine wave traveling along the basilar membrane increased until it reached a maximum and then declined . The site at which the traveling wave reached maximum amplitude depended on the specific frequency of the stimulus, with the high frequencies peaking toward the base of the cochlea and the lower frequencies more toward the apex. Physical properties of the basilar membrane, related to changes in its stiffness and mass along its length, can account for these passive-tuning properties. The basilar membrane has an anatomical structure that varies from the base to the apex. The basilar membrane is widest and least stiff at the apex of the cochlea, and narrowest and most stiff at the base.[ High-frequency sounds localize near the base of the cochlea, while low-frequency sounds localize near the apex. However, later studies revealed that, in vivo, a gradual rise in amplitude of the wave does not occur as it travels to its point of maximal amplitude. Rather, the wave travels along the basilar membrane, causing minimal displacement until it reaches the site of the membrane that is maximally sensitive to a stimulus of that particular frequency. At this site, the basilar membrane vibrates at the frequency of the stimulus. The basilar membrane behaves as a finely tuned band-pass filter, with each location along its length responding to a specific or characteristic frequency. This fine-tuning mechanism is dependent on active, or energy-dependent, processes and therefore not evident in cadaveric studies. The OHC sharpen the peak! They are the muscles of the cochlea. The role of the outer hair cell is to provide the cochlea with its exqu’isite fine-tuning properties, allowing each specific region along the basilar membrane to be tuned to one specific frequency. I would like to emphasize a last important aspect of the physiology of the ear. We have two systems that protect our ears from sounds: the first one is the acoustic reflex that protects from loud low frequency sounds The CNS can influence cochlear function by the olivocochlear bundles, originating in the superior olivary complex of the brainstem. The bundles are divided into lateral and medial groups, dependent on their site of origin within the superior olivary complex. The medial groups of fibers go primar’ily on the outer hair cells. Their function has been studied much more extensively than that of the lateral fibers, Stimulation of the medial efferent fibers decreases the amplification provided by the outer hair cells. Stimulation of these fibers results in the release of the neurotransmitter ACh. The upper trace shows a normal otoacoustic emission in response to clicks in a quiet room. The lower trace shows the reduced otoacoustic emission in response to the same clicks while broadband noise about as loud as a whisper was delivered to the contralateral ear. Some neurons of the superior olivary nucleus join the vestibular division of the eighth nerve. Near the cochlea, these fibers transfer to the cochlear division and then project bilaterally to the OHC where they make inhibitory sy’napses. These medial olivocochlear neurons form the efferent limb of a reflex that suppresses the contractility of outer hair cells in the presence of high-frequency noise, including the frequencies important for speech perception . This is important because it permit to detecting other sounds, such as speech, in the presence of background noise. Hence the stapedius and medial olivocochlear reflexes may have complementary roles, enhancing the ability of the auditory system to discriminate sounds in the presence of low-frequency and high-frequency noise, respectively..