Auditory System

Peripheral Auditory System: Anatomy

The peripheral parts of the auditory system are located in the os petrosum, which is the petrous part of the temporal bone (the word petrous derives from the Latin word petrosus, meaning stone-like, hard; actually, it is the hardest bone in the human body).

The outer ear consists of the auricle (or pinna), the ear canal, and the outside of the tympanic membrane (or eardrum). Although the outer ear, in particular the auricle itself, is not indispensable for hearing, its characteristic structure is designed to capture sound; higher frequencies are conducted better to the more medial parts of the ear.

The middle ear is located in an air-filled cavity (tympanic cavity), directly behind the ear drum, and includes the three small ear bones (ossicles): maleus (or hammer), incus (or anvil), and stapes (or stirrup). The orifice of the auditory tube (or Eustachian tube) is also located in the middle ear. The ossicles (being the smallest bones in the human body) are connected to form a chain that builds a bridge between the eardrum and the opening to the inner ear (oval window). Along this way, vibrations are transferred—and amplified—from the tympanic membrane (larger diameter) to the stapes footplate, which covers the oval window (smaller diameter). Thus, the middle ear functions as an impedance transformer from the air-filled middle ear to the fluid-filled inner ear.

The inner ear comprises not only the snail-shaped cochlea that contains the sensory organ for hearing but also the labyrinth with the vestibular apparatus. Within the bony structure of the cochlea, there are three fluid-filled canals. Due to their similarity with a circular stair, they are called scala vestibuli, scala tympani, and scala media (Italian: scala = stair). The scala media is embedded between the other two canals; it is separated from the scala vestibuli by Reissner's membrane and from the scala tympani by the basilar membrane. The organ of Corti—representing the actual sensory organ—is located along this basilar membrane. It contains the sensory cells (so-called hair cells) that transform the movement of the basilar membrane into a neural code, which is conducted to the brain via the auditory nerve.

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