Show Sound waves traveling through the air are collected through the auricle or pinna, which directs them into the external auditory canal. These two structures comprise the outer ear. The sound waves pass to the end of the canal and cause pressure changes on the ear drum or tympanic membrane. The tympanic membrane moves back and forth in response, reproducing the vibrations of the sound wave source. The auditory ossicles, consisting of the malleus or hammer, incus or anvil, and stapes or stirrup are attached to the wall of the tympanic cavity by tiny ligaments. The malleus is attached to the ear drum, so when the ear drum vibrates; the malleus vibrates in unison with it. The malleus causes the incus to vibrate and it passes the movement onto the stapes. Vibration of the stapes at the oval window causes motion in a fluid within the scala vestibuli of the inner ear, called perilymph. In short, the ossicles help amplify incoming sound waves and convert the sound vibrations into mechanical energy. The vibrations now pass through the vestibule membrane and enter the endolymph of the cochlear duct, where they cause movement in the basilar membrane. The hearing receptors, called hair cells, are located in the organ of Corti on the upper surface of the basilar membrane. As the sound vibrations pass through the inner ear, the hair cells are bent, or pulled, across the tectorial membrane, which lies over them. As the hair cells bend, they mediate the vibrations into nerve impulses; converting mechanical energy into electrochemical energy. The impulse is then transmitted along the cochlear nerve to the vestibulocochlear nerve. This nerve carries the impulses to the auditory cortex in the temporal lobe of the brain where interpretation of the sound, or hearing, occurs. Next StepsThe tympanic membrane, or eardrum, serves as the window into the middle ear. Direct observation of the tympanic membrane and external auditory canal though an otoscope, offers valuable information about possible disease within the middle ear. Ear Pathologies Learning Outcomes
Vibrating objects, such as vocal cords, create sound waves or pressure waves in the air. When these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse (electrical signal) that the brain perceives as sound. The pressure waves strike the tympanum, causing it to vibrate. The mechanical energy from the moving tympanum transmits the vibrations to the three bones of the middle ear. The stapes transmits the vibrations to a thin diaphragm called the oval window, which is the outermost structure of the inner ear. The structures of the inner ear are found in the labyrinth, a bony, hollow structure that is the most interior portion of the ear. Here, the energy from the sound wave is transferred from the stapes through the flexible oval window and to the fluid of the cochlea. The vibrations of the oval window create pressure waves in the fluid (perilymph) inside the cochlea. The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal (as illustrated in Figure 1). Inside the cochlea, the basilar membrane is a mechanical analyzer that runs the length of the cochlea, curling toward the cochlea’s center. Figure 1. A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined by the changes in amplitude and frequency of the sound waves entering the ear. The mechanical properties of the basilar membrane change along its length, such that it is thicker, tauter, and narrower at the outside of the whorl (where the cochlea is largest), and thinner, floppier, and broader toward the apex, or center, of the whorl (where the cochlea is smallest). Different regions of the basilar membrane vibrate according to the frequency of the sound wave conducted through the fluid in the cochlea. For these reasons, the fluid-filled cochlea detects different wave frequencies (pitches) at different regions of the membrane. When the sound waves in the cochlear fluid contact the basilar membrane, it flexes back and forth in a wave-like fashion. Above the basilar membrane is the tectorial membrane. Practice QuestionCochlear implants can restore hearing in people who have a nonfunctional cochlear. The implant consists of a microphone that picks up sound. A speech processor selects sounds in the range of human speech, and a transmitter converts these sounds to electrical impulses, which are then sent to the auditory nerve. Which of the following types of hearing loss would not be restored by a cochlear implant?
The site of transduction is in the organ of Corti (spiral organ). It is composed of hair cells held in place above the basilar membrane like flowers projecting up from soil, with their exposed short, hair-like stereocilia contacting or embedded in the tectorial membrane above them. The inner hair cells are the primary auditory receptors and exist in a single row, numbering approximately 3,500. The stereocilia from inner hair cells extend into small dimples on the tectorial membrane’s lower surface. The outer hair cells are arranged in three or four rows. They number approximately 12,000, and they function to fine tune incoming sound waves. The longer stereocilia that project from the outer hair cells actually attach to the tectorial membrane. All of the stereocilia are mechanoreceptors, and when bent by vibrations they respond by opening a gated ion channel. As a result, the hair cell membrane is depolarized, and a signal is transmitted to the cochlear nerve. Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated. The hair cells are arranged on the basilar membrane in an orderly way. The basilar membrane vibrates in different regions, according to the frequency of the sound waves impinging on it. Likewise, the hair cells that lay above it are most sensitive to a specific frequency of sound waves. Hair cells can respond to a small range of similar frequencies, but they require stimulation of greater intensity to fire at frequencies outside of their optimal range. The difference in response frequency between adjacent inner hair cells is about 0.2 percent. Compare that to adjacent piano strings, which are about six percent different. Place theory, which is the model for how biologists think pitch detection works in the human ear, states that high frequency sounds selectively vibrate the basilar membrane of the inner ear near the entrance port (the oval window). Lower frequencies travel farther along the membrane before causing appreciable excitation of the membrane. The basic pitch-determining mechanism is based on the location along the membrane where the hair cells are stimulated. The place theory is the first step toward an understanding of pitch perception. Considering the extreme pitch sensitivity of the human ear, it is thought that there must be some auditory “sharpening” mechanism to enhance the pitch resolution. When sound waves produce fluid waves inside the cochlea, the basilar membrane flexes, bending the stereocilia that attach to the tectorial membrane. Their bending results in action potentials in the hair cells, and auditory information travels along the neural endings of the bipolar neurons of the hair cells (collectively, the auditory nerve) to the brain. When the hairs bend, they release an excitatory neurotransmitter at a synapse with a sensory neuron, which then conducts action potentials to the central nervous system. The cochlear branch of the vestibulocochlear cranial nerve sends information on hearing. The auditory system is very refined, and there is some modulation or “sharpening” built in. The brain can send signals back to the cochlea, resulting in a change of length in the outer hair cells, sharpening or dampening the hair cells’ response to certain frequencies. Higher ProcessingThe inner hair cells are most important for conveying auditory information to the brain. About 90 percent of the afferent neurons carry information from inner hair cells, with each hair cell synapsing with 10 or so neurons. Outer hair cells connect to only 10 percent of the afferent neurons, and each afferent neuron innervates many hair cells. The afferent, bipolar neurons that convey auditory information travel from the cochlea to the medulla, through the pons and midbrain in the brainstem, finally reaching the primary auditory cortex in the temporal lobe. Try ItContribute!Did you have an idea for improving this content? We’d love your input. Improve this pageLearn More What vibrates against the cochlea?The cochlea is filled with a fluid that moves in response to the vibrations from the oval window. As the fluid moves, 25,000 nerve endings are set into motion. These nerve endings transform the vibrations into electrical impulses that then travel along the eighth cranial nerve (auditory nerve) to the brain.
What do hair cells bend against in the ear?The stapes pushes in and out against a structure called the oval window. This action is passed onto the cochlea, a fluid-filled snail-like structure that contains the organ of Corti, the organ for hearing. It consists of tiny hair cells that line the cochlea.
Which of the following parts of the ear sends vibrations to the cochlea?The eardrum vibrates. The vibrations are then passed to 3 tiny bones in the middle ear called the ossicles. The ossicles amplify the sound. They send the sound waves to the inner ear and into the fluid-filled hearing organ (cochlea).
What do hair cells in the cochlea detect?In the cochlea, receptor hair cells that detect stimuli produced by sound are short, goblet-like cells embedded in supporting cells (the phalangeal cells of Deiters). Their apical domain contains a U-shaped row of stereocilia (hairs) that are in contact with the tectorial membrane of the organ of Corti.
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