Audition, the sense of hearing, involves the transduction of sound waves into electrical energy, which then can be transmitted in the nervous system. Sound is produced by waves of compression and decompression, which are transmitted in elastic media such as air or water. These waves are associated with increases (compression) and decreases (decompression) in pressure. The units for expressing sound pressure are decibels (dB), which is a relative measure on a log scale. Sound frequency is measured in cycles per second or hertz (Hz). A pure tone results from sinusoidal waves of a single frequency.
Most sounds are mixtures of pure tones. The human ear is sensitive to tones with frequencies between 20 and 20,000 Hz and is most sensitive between 2000 and 5000 Hz. A reference, 0 dB, is the average threshold for hearing at 1000 Hz. Sound pressure, in dB, is calculated as follows:
= Sound pressure being measured
= Reference pressure measured at the threshold frequency
Therefore, if a sound pressure is 10 times the reference pressure, it is 20 dB (20 × log 10 = 20 × 1 = 20 dB). If a sound pressure is 100 times the reference pressure, it is 40 dB (20 × log 100 = 20 × 2 = 40 dB).
The usual range of frequencies in human speech is between 300 and 3500 Hz, and the sound intensity is about 65 dB. Sound intensities greater than 100 dB can damage the auditory apparatus, and those greater than 120 dB can cause pain.
Structures of the Ear
Structures of the external, middle, and inner ear are shown in Figure 3-19 and are described as follows:
Figure 3–19 Structures of the external, middle, and inner ear. The cochlea has been turned slightly for visualization.
The external ear consists of the pinna and the external auditory meatus (auditory canal). The function of the external ear is to direct sound waves into the auditory canal. The external ear is air filled.
The middle ear consists of the tympanic membrane and a chain of auditory ossicles called the malleus, incus, and stapes. The tympanic membrane separates the external ear from the middle ear. An oval window and a round window lie between the middle ear and the inner ear. The stapes has a footplate, which inserts into the oval window and provides the interface between the middle ear and the inner ear. The middle ear is air filled.
The inner ear consists of a bony labyrinth and a membranous labyrinth. The bony labyrinth consists of three semicircular canals (lateral, posterior, and superior). The membranous labyrinth consists of a series of ducts called the scala vestibuli, scala tympani, and scala media.
The cochlea and the vestibule are formed from the bony and membranous labyrinths. The cochlea, which is a spiral-shaped structure composed of three tubular canals or ducts, contains the organ of Corti. The organ of Corticontains the receptor cells and is the site of auditory transduction. The inner ear is fluid filled, and the fluid in each duct has a different composition. The fluid in the scala vestibuli and scala tympani is called perilymph, which is similar to extracellular fluid. The fluid in the scala media is called endolymph, which has a high-potassium (K+) concentration and a low-sodium (Na+) concentration. Thus, endolymph is unusual in that its composition is similar to that of intracellular fluid, even though, technically, it is extracellular fluid.
Auditory transduction is the transformation of sound pressure into electrical energy. Many of the structures of the ear participate, directly or indirectly, in this transduction process. Recall that the external and middle ears are air filled, and the inner ear, which contains the organ of Corti, is fluid filled. Thus, before transduction can occur, sound waves traveling through air must be converted into pressure waves in fluid. The acoustic impedance of fluid is much greater than that of air. The combination of the tympanic membrane and the ossicles serves as an impedance-matching device that makes this conversion. Impedance matching is accomplished by the ratio of the large surface area of the tympanic membrane to the small surface area of the oval window and the mechanical advantage offered by the lever system of the ossicles.
The external ear directs sound waves into the auditory canal, which transmits the sound waves onto the tympanic membrane. When sound waves move the tympanic membrane, the chain of ossicles also moves, pushing the footplate of the stapes into the oval window and displacing the fluid in the inner ear.
Cochlea and Organ of Corti
The cochlea contains the sensory transduction apparatus, the organ of Corti. The structures of the cochlea and the organ of Corti are shown in Figure 3-20.
Figure 3–20 Structure of the cochlea and the organ of Corti.
The cross-section of the cochlea shows its three chambers: scala vestibuli, scala media, and scala tympani. Each chamber is fluid filled, the scala vestibuli and scala tympani with perilymph and the scala media with endolymph. The scala vestibuli is separated from the scala media by Reissner’s membrane. The basilar membrane separates the scala media from the scala tympani.
The organ of Corti lies on the basilar membrane of the cochlea and is bathed in the endolymph contained in the scala media. Auditory hair cells in the organ of Corti are the sites of auditory transduction. The organ of Corti contains two types of receptor cells: inner hair cells and outer hair cells. There are fewer inner hair cells, which are arranged in single rows. Outer hair cells are arranged in parallel rows and are more numerous than inner hair cells. Cilia, protruding from the hair cells, are embedded in the tectorial membrane. Thus, the bodies of the hair cells are in contact with the basilar membrane, and the cilia of the hair cells are in contact with the tectorial membrane.
The nerves that serve the organ of Corti are contained in the vestibulocochlear nerve (CN VIII). The cell bodies of these nerves are located in spiral ganglia, and their axons synapse at the base of the hair cells. These nerves will transmit information from the auditory hair cells to the CNS.
Steps in Auditory Transduction
Several important steps precede transduction of sound waves by the auditory hair cells on the organ of Corti. Sound waves are directed toward the tympanic membrane, and, as the tympanic membrane vibrates, it causes the ossicles to vibrate and the stapes to be pushed into the oval window. This movement displaces fluid in the cochlea. The sound energy is amplified by two effects: the lever action of the ossicles and the concentration of sound waves from the large tympanic membrane onto the small oval window. Thus, sound waves are transmitted and amplified from the air-filled external and middle ears to the fluid-filled inner ear, which contains the receptors.
Auditory transduction by hair cells on the organ of Corti then occurs in the following steps (Fig. 3-21):
Figure 3–21 Steps in auditory transduction in hair cells. Circled numbers correspond to steps described in the text.
1. Sound waves are transmitted to the inner ear and cause vibration of the organ of Corti.
2. The auditory hair cells are mechanoreceptors, which are located on the organ of Corti (see Fig. 3-20). The base of the hair cells sits on the basilar membrane, and the cilia of the hair cells are embedded in the tectorial membrane. The basilar membrane is more elastic than the tectorial membrane. Thus, vibration of the organ of Corti causes bending of cilia on the hair cells by a shearing force as the cilia push against the tectorial membrane.
3. Bending of the cilia produces a change in K+ conductance of the hair cell membrane. Bending in one direction produces an increase in K+ conductance and hyperpolarization; bending in the other direction produces a decrease in K+ conductance and depolarization.
4. These changes in membrane potential are the receptor potentials of the auditory hair cells. The oscillating receptor potential is called the cochlear microphonic potential.
5. When hair cells are depolarized, the depolarization opens voltage-gated Ca2+ channels in the presynaptic terminals of the hair cells. As a result, Ca2+ enters the presynaptic terminals and causes release of glutamate, which functions here as an excitatory neurotransmitter, causing action potentials in the afferent cochlear nerves that will transmit this information to the CNS. When the hair cells are hyperpolarized, the opposite events occur, and there is decreased release of glutamate.
6. Thus, oscillating depolarizing and hyperpolarizing receptor potentials in the hair cells cause intermittent release of glutamate, which produces intermittent firing of afferent cochlear nerves.
Encoding of Sound
Encoding of sound frequencies occurs because different auditory hair cells are activated by different frequencies. The frequency that activates a particular hair cell depends on the position of that hair cell along the basilar membrane, as illustrated in Figure 3-22. The base of the basilar membrane is nearest the stapes and is narrow and stiff. Hair cells located at the base respond best to high frequencies. The apex of the basilar membrane is wide and compliant. Hair cells located at the apex respond best to low frequencies. Thus, the basilar membrane acts as a sound frequency analyzer, with hair cells positioned along the basilar membrane responding to different frequencies. This spatial mapping of frequencies generates a tonotopic map, which then is transmitted to higher levels of the auditory system.
Figure 3–22 Frequency responses of the basilar membrane.
Information is transmitted from the hair cells of the organ of Corti to the afferent cochlear nerves. The cochlear nerves synapse on neurons of the dorsal and ventral cochlear nuclei of the medulla, which send out axons that ascend in the CNS. Some of these axons cross to the contralateral side and ascend in the lateral lemniscus (the primary auditory tract) to the inferior colliculus. Other axons remain ipsilateral. The two inferior colliculi are connected via the commissure of the inferior colliculus. Fibers from nuclei of the inferior colliculus ascend to the medial geniculate nucleus of the thalamus. Fibers from the thalamus ascend to the auditory cortex. The tonotopic map, generated at the level of the organ of Corti, is preserved at all levels of the CNS. Complex feature discrimination (e.g., the ability to recognize a patterned sequence) is the property of the auditory cortex.
Because some auditory fibers are crossed and some are uncrossed, a mixture of ascending nerve fibers represents both ears at all levels of the CNS. Thus, lesions of the cochlea of one ear will cause ipsilateral deafness. However, more central unilateral lesions do not cause deafness because some of the fibers transmitting information from that ear have already crossed to the undamaged side.