The Structure of the Human Ear
Figure 1: Labeled structures of the outer, middle and inner ears.
Image courtesy of http://en.wikipedia.org/wiki/File:HumanEar.jpg This image is in the public domain and thus free of any copyright restrictions.
The outer ear
In the outer ear, a structure called pinna collects sounds from a wide area and funnels them into the internal ear through the auditory canal.
The middle ear
The canal ends at the tympanic membrane (eardrum), which is located in the middle ear. Connected to the membrane on the other side is a series of tiny bones called ossicles. The first ossicle attached to the tympanic membrane is the malleus, which connects with the incus. The third ossicle called the stapes is connected with the incus. The footplate of the stapes can move in or out at a membrane covering a hole in the skull bone called the oval window. The ossicles are important for amplifying the sound force received from the tympanic membrane. It is worth to note that the middle ear is continuous with the nasal cavities via the Eustachian tube which equalizes the air pressure in the middle ear with the ambient air pressure.
The inner ear
Behind the oval window is the fluid-filled cochlea present in the inner ear. The cochlea derived its name from the Latin for “snail” due to its spiral shape of two and a half turns. The round window, which is another membrane-covered hole, is located below the oval window in the structure.
Figure 2: Cross section of the cochlea
Image courtesy of http://en.wikipedia.org/wiki/File:Cochlea-crosssection.png under the terms of the GNU Free Documentation License.
In cross section, the tube is seen divided into three fluid-filled chambers: the scala vestibule, the scala media, and the scala tympani. The scala vestibule is separated from the scala media by the Reissner’s membrane, while the basilar membrane separates the scala vestibule from the scala media. At the apex of the cochlea, the scala vestibule becomes continuous with the scala tympani via a hole called helicotrema; at the base of the cochlea, the scala vestibule encounters the oval window while the scala tympani meets the round window.
The scala vestibule and scala tympani are filled by fluid called perilymph, which has low K+ (7 mM) and high Na+ (140 mM) concentrations. In contrast, endolymph that fills the scala media has an unusual ionic concentrations similar to intracellular fluid, i.e. high K+ (150 mM) and low Na+ (1mM). This difference is generated by active transport processes at the endothelium lining, stria vascularis of the scala media. As a result of the ionic gradient differences and the permeability of reissner’s membrane, the endolymph is 80 mV more positive than perilymph. This electrical potential, called the endocochleaer potential, is important for the enhancement of sound transduction.
The organ of corti
Lying on the basilar membrane is the organ of Corti (illustrated in Figure 2 above), which contains auditory receptor neurons. The structure is of particular importance for transducing sound energy into neural activity. It comprises hair cells, the rods of Corti, and various supporting cells.
The auditory receptors are named hair cells due to the presence of stereocilia protruding from the top of cell surface. These cells are covered by the basilar membrane on one side and a thin tissue membrane called the reticular lamina on the other side where stereocilia are present. The rods of Corti span between basilar membrane and reticular lamina to provide structural support.
There are two types of hair cells: inner hair cells of 3,500 in number that form a single row and outer hair cells totaling 15,000-20,000 arranged in three rows. The stereocilia, protruding into the endolymph, are attached to the tectorial membrane (the outer hair cells) or extend to just below the tectorial membrane (the inner hair cells). Hair cells form synapses onto neurons that project axons along the auditory nerve into the brain (the cochlear nuclei in the medulla).