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ion of Sound | Biology for Majors II
- Describe the process of creating sound
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.
Cochlear 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?
- Hearing loss resulting from absence or loss of hair cells in the organ of Corti.
- Hearing loss resulting from an abnormal auditory nerve.
- Hearing loss resulting from fracture of the cochlea.
- Hearing loss resulting from damage to bones of the middle ear.
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.
The 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.
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Frequently Asked Questions About how are sound vibrations amplified in a vertebrate ear?
If you have questions that need to be answered about the topic how are sound vibrations amplified in a vertebrate ear?, then this section may help you solve it.
Which kind of receptor does sound use?
Mechanoreceptors in the inner ear are involved in the transduction of sound waves into neural signals during hearing or audition, detection of equilibrium, skin tactile sensing, deep tissue sensing, and sensing of arterial pressure.
A hair cell’s ls7c sensory receptor type,
Hair cells use chemosensory receptors to perform their functions.
Test your knowledge of sensory receptors.
Give examples of sensory stimuli, such as touch, pressure, pain, light, sound, position in space, and vibration, to illustrate what a sensory receptor is: a “specialized nerve cell that is designed to respond to a specific sensory stimulus.”
Which of the following describes how incoming vibrations and audition should happen in order?
Therefore, “External ear? Tympanic membrane? Cochlear canal? Sensory cells of the cortex? Cerebrum” is the right response.
Sound vibrations are amplified where?
The cochlea, a snail-shaped structure filled with fluid in the inner ear, receives the sound vibrations that have been amplified or made louder by the bones in the middle ear.
The ear’s sound receptors are located where?
About 4,000 specialized sound receptors called inner hair cells in the cochlea, the snail-shell-shaped inner ear structure, transform vibrations into electrical impulses that are transmitted to the brain.
Which nerve do hair cells stimulate?
In order to enable audition, the hair cells transform mechanical energy into electrical energy, which is then transmitted to the central nervous system via the auditory nerve.
Which four sensory receptors are there?
Sensory receptors can be divided into electromagnetic receptors (photoreceptors, thermoreceptors), mechanoreceptors (hearing, touch, balance, osmoreceptors), and chemoreceptors (odorant receptors, gustatory receptors), depending on the stimulus that activates them.
What is the sequence in which the cells are stimulated to produce the sense of hearing?
Auditory information is transferred to the inferior colliculus, the medial geniculate nucleus of the thalamus, and ultimately to the auditory cortex in the temporal lobe of the brain for processing as hair cells become activated.
Quiz: How do sound vibrations get to the inner ear?
The middle ear contains the smallest bones in the body: the hammer, anvil, and stirrup. Their function is to amplify the eardrum’s vibrations and transmit them into the inner ear. The sound wave strikes the eardrum, causing it to vibrate. The eardrum transmits these vibrations into the inner ear, where they are amplified.
Which is the proper order for the ear to receive sound energy?
The only visible part of the ear, the pinna, receives the sound waves first. 2. After passing the pinna, the sound waves travel into the auditory canal, or external acoustic meatus, before striking the tympanic membrane, or eardrum.
How does the ear process sound?
The fluid inside the cochlea vibrates in response to the vibrations from the oval window, setting into motion 25,000 nerve endings that convert the vibrations into electrical impulses that proceed to the brain along the eighth cranial nerve (the auditory nerve).
How does the cochlea amplify sound?
The cochlear amplifier is essentially a positive feedback loop within the cochlea that amplifies the traveling wave, allowing force to be generated in synchrony with the sensed vibrations in the organ of Corti.