The ear contains structures that provide the senses of hearing and equilibrium. It is divided into three compartments: the external (receives sound waves), middle (converts sound waves into mechanical waves and transmits them to the fluid of the inner ear), and inner ear (converts mechanical movements within fluid into nerve impulses to provide hearing and equilibrium). Review the general organization and functions of the middle and inner ear in the diagram and table below.

Clinical note: Bacterial infections of the middle ear cavity (otitis media) are common complications of colds and upper respiratory tract infections in small children. If such an infection does not respond to antibiotics, the resulting fluid and inflammatory material may be drained through a perforation in the tympanic membrane. In the image to the right the ear drum on the left is healthy, whereas the one on the right is markedly inflamed and must have hurt.

The cochlea is a spiral shaped, bony organ enclosing a membranous region, the cochlear duct. The bony labyrinth (including the scala vestibuli and scala tympani) contains perilymph, whereas the membranous labyrinth (scala media) contains endolymph. The cochlear duct houses the spiral organ of Corti, the structure containing hair cells with an underlying basilar membrane and overlying tectorial membrane. Pressure waves moving through the perilymph and endolymph displace these membranes, bending the stereocilia on the hair cells and producing nerve impulses that provide hearing.

Examine the images below and this section of the inner ear. This specimen was dissected from the skull and sectioned to best demonstrate structures within the conical, spiral-shaped cochlea. Identify the following:

• Spiral ganglion (SG) and cochlear nerve (CN)
• Spiral organ of Corti (SOC)
• Ducts associated with the organ of Corti (scala vestibule SV, scala media SM, scala tympani ST)
• Vestibular membrane (VM)
• Tectorial and basilar membranes (TM, BM)
• Stria vascularis (StV)
• Hair cells (H)

Clinical note: Degeneration of hair cells in the organ of Corti is a common cause of hearing loss in older individuals. Recent research has shown the ability of hair cells to regenerate in certain experimental systems and this is now an important area of research in sensory neuroscience. Click the image for an expanded view.

Watch this YouTube movie, one of the best we've seen, detailing auditory transduction (how we hear). Because it's YouTube, there will be a short commercial message, which within a few minutes you can skip.

Now for the vestibular structures.