3.2 The anatomy of the cochlea
The cochlea has a spiral shape resembling the shell of a snail (Figure 4a). You can approximate the structure of the cochlea by wrapping a drinking straw 2.5 times around the tip of a sharpened pencil. The hollow tube, represented by the straw, has walls made of bone and the central pillar of the cochlea, represented by the pencil, is a conical bony structure called the modiolus. Unravelled (Figure 4b), the cochlea's hollow tube is about 32 mm long and 2 mm in diameter. The tube of the cochlea is divided into three chambers: the scala vestibuli, the scala media (or cochlear duct) and the scala tympani. The three scalae wrap around inside the cochlea like a spiral staircase (‘scala’ is Latin for ‘stairway’). The scala vestibuli forms the upper chamber and at the base of this chamber is the oval window. The lowermost of the three chambers is the scala tympani. It too has a basal aperture, the round window, which is closed by an elastic membrane. The scala media or cochlear duct separates the other two chambers along most of their length. The start of the cochlea, where the oval and round windows are located is known as the basal end, while the other end, the inner tip is known as the apical end (or apex). The scala vestibuli and the scala tympani communicate with one another via the helicotrema, an opening in the cochlear duct at the apex. Both scala vestibuli and scala tympani are filled with the same fluid, known as perilymph (essentially the same in composition as the extracellular fluid bathing most of the nervous system), while the scala media is filled with endolymph (with very high potassium and low sodium concentrations).
Figure 5 is a cross-section of the cochlea showing the three chambers which run along its length. Between the scala vestibuli and the scala media is a membrane called Reissner's membrane and between the scala tympani and the scala media is the basilar membrane. Lying on top of the basilar membrane within the cochlear duct is the organ of Corti and hanging over the organ of Corti, is the tectorial membrane. The collective term for the partitions of the scala media (the organ of Corti, the basilar membrane and the tectorial membrane) is the cochlear partition.
The following simple activity will help you understand the structure of the cochlea more clearly. Imagine that your empty coffee mug (assuming it's a cylindrical shape, or thereabouts) represents a section of the uncoiled tube-like cochlea. Take a piece of paper that is about the same height as the mug and that is wide enough to wrap halfway around the mug (Figure 6a). Fold it in half and insert it into the middle of the mug so that the fold runs vertically down the side of the mug where the handle is located (Figure 6b). The paper should lie across the middle of the mug. Now (if necessary) separate the two pieces of paper so that they form a V-shape (Figure 6c). You now have the basic structure of the cochlea. If you hold the mug by the handle in your right hand and look directly into it, you have the same view as that shown in Figure 5. Compare Figure 5 to your ‘cochlea’ and answer the following questions:
What structure does the top piece of paper represent?
What does the space above the top piece of paper represent?
The scala vestibuli.
What does the bottom piece of paper represent?
The basilar membrane.
What does the space below the bottom piece of paper represent?
The scala tympani.
What does the V-shaped space represent?
The scala media (or cochlear duct).
The scala media houses the organ of Corti. The organ of Corti and all its associated structures (including the hair cells, see below) runs the length of the basilar membrane (from the top to the bottom of your mug) as does the overlying tectorial membrane.
An enlargement of the organ of Corti is shown in Figure 7. The organ of Corti is the primary auditory receptor structure and houses the sensory receptor cells which are known as hair cells because each has about 100 hair-like stereocilia extending from its apical end which are embedded in the tectorial membrane. You can see that the hair cells are of two types: outer hair cells and inner hair cells, which are separated by a rigid inverted V-shaped structure known as Corti's arch.
We shall return to the organ of Corti and the hair cells and their involvement in the transduction of an auditory signal into neural information in Section 3.4. For now, we need to consider the basilar membrane and in particular, its response to vibrations in the cochlear fluid.