Inner Ear Anatomy

Location of the Cochlea

The cochlea is part of the Inner Ear

This figure shows the location of the cochlea with respect to the external ear canal. The cochlea is shown here as the blue spiral structure, which resembles a snail. (The word cochlea is derived from the Greek word (kokhlias) for a snail) The cochlea is responsible for converting sounds which enter the ear canal, from mechanical vibrations into electrical signals. This process, known as transduction, is performed by specialized sensory cells within the cochlea. The electrical signals, which code the sound's characteristics, are carried to the brain by the auditory nerve.

The Inner Ear Fluids

The anatomy of the inner ear is dominated by large fluid-filled spaces. As shown in this figure, the inner ear consists of a complex series of tubes, running through the temporal bone of the skull. The bony tubes (sometimes called the bony labyrinth) are filled with a fluid called perilymph. Perilymph is shown both here and in the picture on the opening page colored orange. Within this bony labyrinth is a second series of tubes made out of delicate cellular structures (called the membranous labyrinth). The fluid inside these membranous structures is called endolymph, and is show both here and in the opening illustration colored blue. The different spaces of the perilymphatic and endolymphatic compartments are each interconnected by a series of ducts, which are described in the next figure. An important feature of the endolymphatic space is that it is completely bounded by tissues and there are no ducts or open connections between perilymph and endolymph.

Nomenclature of the Inner Ear Fluid Compartments

The inner ear is comprised of two main sections. The semi-circular canals, utricle and saccule make up the vestibular system and are involved in balance. The scala tympani, scala media and scala vestibuli make up the cochlea which is involved in hearing.

The perilymphatic chamber of the vestibular system has a wide connection to scala vestibuli, which in turn connects to scala tympani by an opening called the helicotrema at the apex of the cochlea. Scala tympani is then connected to the cerebrospinal fluid (CSF) of the subarachnoid space by the cochlear aqueduct.

The endolymphatic system of the cochlea (scala media) is connected to the saccule by the ductus reuniens and from there connects to the endolymphatic sac, which lies in a bony niche within the cranium. The endolymph of the utricle and semi-circular canals also connects to the endolymphatic sac.

The existence of the many ducts connecting different parts of the inner ear has led to the idea that the cochlear fluids are flowing through the ear. However, unlike other body fluids, such as saliva or lacrymal fluid (tears), the fluids of the inner ear are not secreted and resorbed in volume. This is a widespread misconception, based on old studies which used poor experimental techniques. Studies performed over the past decade in our lab have shown that neither endolymph nor perilymph "flow" along their respective compartments in the normal cochlea. Maintenance of the chemical composition of both fluids is dominated by ion transport processes which are localized in each region.

If the bony capsule of the inner ear is perforated, then perilymph escapes, driven by the hydrostatic pressure of CSF. The escaping fluid is replaced by CSF entering the cochlea through the cochlear aqueduct. In this condition a longitudinal flow will exist between the cochlear aqueduct and the site of the perforation. The chemical composition of perilymph will be disturbed because the perilymph will continually be "washed out" and replaced by CSF. This condition is known as a perilymphatic fistula.

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Page generated by: Alec N. Salt, Ph.D.,
Cochlear Fluids Research Laboratory,
Washington University, St. Louis