Transmission of sound vibrations through the middle and inner ear. % EQ]
position of the stapes in the oval window. These muscles help to protect the delicate receptor apparatus of the inner ear from continuous intense sound stimuli and improve hearing over certain frequency ranges.
The entire system described thus far has been concerned with the transmission of sound energy into the cochlea, where the receptor cells are located. The cochlea is almost completely divided lengthwise by a fluid-filled membranous tube, the cochlear duct, which follows the cochlear spiral (see Figure 9-35). On either side of the cochlear duct are fluid-filled compartments: the scala vestibuli, which is on the side of the cochlear duct that ends at the oval window; and the scala tympani, which is below the cochlear duct and ends in a second membrane-covered opening to the middle ear, the round window. The scala vestibuli and scala tympani meet at the end of the cochlear duct at the helicotrema (see Figure 9-35).
Sound waves in the ear canal cause in-and-out movement of the tympanic membrane, which moves the chain of middle-ear bones against the membrane covering the oval window, causing it to bow into the scala vestibuli and back out (Figure 9-37), creating waves of pressure there. The wall of the scala vestibuli is largely bone, and there are only two paths by which the pressure waves can be dissipated. One path is to the helicotrema, where the waves pass around the end of the cochlear duct into the scala tympani and back to the round-window membrane, which is then bowed out into the middle ear cavity. However, most of the pressure is transmitted from the scala vestibuli across the cochlear duct.
One side of the cochlear duct is formed by the basilar membrane (Figure 9-38), upon which sits the organ of Corti, which contains the ear's sensitive receptor cells. Pressure differences across the cochlear duct cause vibration of the basilar membrane.
The region of maximal displacement of the vibrating basilar membrane varies with the frequency of the sound source. The properties of the membrane nearest the middle ear are such that this region vibrates most easily—that is, undergoes the greatest movement, in response to high-frequency (high-pitched) tones. As the frequency of the sound is lowered, vibration waves travel out along the membrane for greater distances. Progressively more distant regions of the basilar membrane vibrate maximally in response to progressively lower tones.
The receptor cells of the organ of Corti, the hair cells, are mechanoreceptors that have hairlike stereocilia protruding from one end (Figure 9-38c). The hair cells transform the pressure waves in the cochlea into receptor potentials. Movements of the basilar membrane stimulate the hair cells because they are attached to the membrane.
The stereocilia are in contact with the overhanging tectorial membrane (Figure 9-38c), which projects inward from the side of the cochlea. As the basilar membrane is displaced by pressure waves, the hair cells move in relation to the tectorial membrane, and, consequently, the stereocilia are bent. Whenever the stere-ocilia bend, ion channels in the plasma membrane of the hair cell open, and the resulting ion movements depolarize the membrane and create a receptor potential.
Efferent nerve fibers from the brainstem regulate the activity of certain of the hair cells and dampen their response, which protects them. Despite this protective action, the hair cells are easily damaged or even completely destroyed by exposure to high-intensity noises such as amplified rock music concerts, engines of jet planes, and revved-up motorcycles. Lesser noise levels also cause damage if exposure is chronic.
Hair cell depolarization leads to release of the neu-rotransmitter glutamate (the same neurotransmitter released by photoreceptor cells), which binds to and activates protein binding sites on the terminals of the 10 or so afferent neurons that synapse upon the hair cell. This causes the generation of action potentials in the neurons, the axons of which join to form the cochlear nerve (a component of cranial nerve VIII). The greater the energy (loudness) of the sound wave, the greater the frequency of action potentials generated in the afferent nerve fibers. Because of its position on the basilar membrane, each hair cell and, therefore, the nerve fibers that synapse upon it respond to a limited range of sound frequency and intensity, and they respond best to a single frequency.
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
The Sensory Systems CHAPTER NINE
The Sensory Systems CHAPTER NINE
Neural Pathways in Hearing
Cochlear nerve fibers enter the brainstem and synapse with interneurons there, fibers from both ears often converging on the same neuron. Many of these in-terneurons are influenced by the different arrival times and intensities of the input from the two ears. The different arrival times of low-frequency sounds and the difference in intensities of high-frequency sounds are used to determine the direction of the sound source. If, for example, a sound is louder in the right ear or arrives sooner at the right ear than at the left, we assume that the sound source is on the right. The shape of the outer ear (the pinna, see Figure 9-34) and movements of the head are also important in localizing the source of a sound.
From the brainstem, the information is transmitted via a multineuron pathway to the thalamus and on to the auditory cortex (see Figure 9-6). The neurons responding to different pitches (frequencies) are arranged along the auditory cortex in an orderly manner in much the same way that signals from different regions of the body are represented at different sites in the somatosensory cortex. Different areas of the auditory system are further specialized, some neurons responding best to complex sounds such as those used in verbal communication, whereas others signal the location, movement, duration, or loudness of a sound.
Electronic devices can help compensate for damage to the intricate middle ear, cochlea, or neural structures. Hearing aids amplify incoming sounds, which then pass via the ear canal to the same cochlear mechanisms used by normal sound. When substantial damage has occurred, however, and hearing aids cannot correct the deafness, electronic devices known as cochlear implants may restore functional hearing. In response to sound, cochlear implants directly stimulate the cochlear nerve with tiny electric currents so that sound signals are transmitted directly to the auditory pathways, bypassing the cochlea.
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