The mechanoreceptors are in the ears. The ear accomplishes two sensory functions: equilibrium (balance) and hearing. The receptors for both of these functions are located in the inner ear and consist of hair cells with cilia that respond to mechanical stimulation. Each hair cell has from 30 to 150 cilia. When the cilia of any particular hair cell are displaced in a certain direction, the cell generates nerve impulses, which are sent along the eighth cranial nerve to the brain.
The ear has three divisions: external, middle, and inner.
External Ear
The external ear consists of the pinna (external flap) and external auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. In the upper wall are ceruminous glands, modified sweat glands that secrete earwax, which helps guard the ear against the entrance of foreign materials, such as air pollutants.
Middle Ear
The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall with two small openings covered by membranes. These openings are called the oval window and the round window. Between the tympanic membrane and the oval window are three small bones called the ossicles. The ossicles are the malleus (mal' e-us), also called the hammer, incus (ing'kus), also called the anvil, and stapes (sta'pez), also called the stirrup. The ossicles are named for the objects they resemble. The malleus adheres to the tympanic membrane, and the stapes touches the oval window. The posterior wall of the middle ear also has an opening that leads to mastoid sinuses of the skull.
The auditory (eustachian) tubes, which extend from the middle ear to the nasopharynx, permit equalization of air pressure. Chewing gum, yawning, and swallowing help move air through the auditory tubes during ascent and descent in elevators and airplanes.
Infections of the middle ear (otitis media) can occur frequently during childhood. As a precautionary measure to prevent perforation of the tympanic membrane, children with frequent infections may need to have an incision of the tympanic membrane, called myringotomy (mir"ingot'o-me), followed by the insertion of a tiny tube into the membrane. The tube ensures that the pressure is equal on each side of the tympanic membrane, regardless of whether the auditory tube is blocked by the pus that results from the infection. With time, the tubes are sloughed out of the ears or surgically removed.
Inner Ear
The inner ear, anatomically speaking, has three areas: the first two-a vestibule and the semicircular canals-are concerned with balance; the third-the cochlea-is concerned with hearing.
The semicircular canals are arranged so that there is one canal in each dimension of space. The base of each canal, called an ampulla, is slightly enlarged. Within the ampullae, the cilia of small hair cells insert into a gelatinous medium.
A vestibule, or chamber, lies between the semicircular canals and the cochlea. It contains two small sacs callea the utricle and the saccule. Within both of these are the cilia of small hair cells that protrude into a gelatinous substance. Otoliths, calcium carbonate granules, rest on this gelatinous material.
The spirals of the cochlea (kok'le-ah) resemble a snail shell. The cochlea contains the organ of Corti (kor'te), which sends nerve impulses to the brain stem by way of the eighth cranial nerve. Eventually, the nerve impulses are relayed to the temporal lobe of the cerebrum, where they are interpreted as sound.
The external ear, middle ear. and cochlea are necessary for hearing. The vestibule and semicircular canals are concerned vvith the sense of balance.
Hearing
The process of hearing begins when sound waves enter the auditory canal. Just as ripples travel across the surface of a pond, sound travels by the successive vibrations of molecules. Ordinarily, sound waves do not carry much energy, but when a large number of waves strike the eardrum, it moves back and forth (vibrates) slightly. The ossicles receive the vibrations from the eardrum and transmit them to the oval window, having amplified them about 20 times. Vibrations of the oval window cause pressure waves in the fluid of the cochlea.
The tubular cochlea has three canals: the vestibular canal, the cochlear canal, and the tympanic canal. Along the length of the basilar membrane, which forms the lower wall of the cochlear canal, are little hair cells whose cilia are embedded in another membrane, called the tectorial membrane. The hair cells of the cochlear canal compose the organ of Corti.
If the cochlea is unwound, the vestibular canal is seen to connect with the tympanic canal; therefore, as the figure indicates, pressure waves move from one canal to the other toward the round window, a membrane that can bulge to absorb the pressure. As a result of the movement of the fluid within the cochlea, the basilar membrane moves up and down, and the cilia of a portion of the at least 20,000 hair cells are bent. This bending of the cilia initiates nerve impulses that pass by way of the cochlear branch of the vestibulocochlear nerve to the brain stem. Eventually, the impulses reach the temporal lobe of the cerebrum, where they are interpreted as a sound.
The sense receptors for sound are hair cells on the basilar membrane (the organ of Corti). When the basilar membrane vibrates, the delicate hairs that are embedded in the tectorial membrane bend, and nerve impulses begin in the cochlear nerve and are transmitted to the brain.
