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Structure of the Human Ear
The human ear is a specialized organ that is capable of capturing mechanical waves and transforming them into electrical signals that the brain can use to analyze our surroundings. The ear consists of three regions - the outer ear, the middle ear and the inner ear. When a disturbance in the air creates a mechanical wave, it begins to propagate and eventually hits the outer portion of the ear known as the pinna (also known as auricle). The pinna serves to capture a good portion of the energy stored in the mechanical wave. Due to the large size and surface area of the pinna, it is able to amplify the amount of energy that goes into the ear canal (auditory canal). Once inside the auditory canal, the mechanical wave travels to the ear drum (tympanic membrane) of the middle ear. Due to the small size of the ear drum compared to the size of the pinna, the force that the mechanical wave exerts is greatly amplified (called a mechanical advantage in physics). The vibration of the membrane exerts a force on three bones collectively called the ossicles (malleus, incus and stapes), which are connected to one another. These three bones act as a lever system and by decreasing the lever arm (displacement) as we go from bone to bone, they amplify the force even more. This amplification is required in order to pass the air-liquid boundary that exists in the inner ear. The inner ear consists of a fluid called the perilymph and in order to move the mechanical wave into this fluid, we must overcome a considerable amount of resistance. The stapes bone is connected to the oval window, which is the beginning of the inner ear. As the oval window (a membrane) vibrates, it creates a mechanical wave inside the fluid, which moves through the cochlea. This movement causes another membrane called the round window to vibrate, which causes even more pressure variation inside the fluid. The hair cells found in the organ of Corti inside the cochlea contain extensions called micovilli that depolarize when they feel the pressure variation and send that action potential to the cochlear nerve, which connects with the vestibular nerve and travels up to the brain. The ear also contains a set of three canals called the semicircular canals. These three canals are oriented along the three directions (x, y and x) and also contain hair cells that are capable of depolarizing when the pressure varies inside the fluid (called endolymph). These semicircular canals are responsible for helping us balance and allow us to feel acceleration and deceleration. They send their action potentials to the vestibular nerve.
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