How is pitch encoded in the nervous system

Also because the arm of the malleus to which the tympanic membrane is attached is longer than the arm of the incus to which the stapes is attached, there is a slight amplification of the sound pressure by a lever action.

These two impedance matching mechanisms effectively transmit air-born sound into the fluid of the inner ear. If the middle-ear apparatus ear drum and ossicles were absent, then sound reaching the oval and round windows would be largely reflected. The cochlea is a long coiled tube, with three channels divided by two thin membranes. The top tube is the scala vestibuli, which is connected to the oval window. The bottom tube is the scala tympani , which is connected to the round window.

The middle tube is the scala media, which contains the Organ of Corti. The Organ of Corti sits on the basilar membrane, which forms the division between the scalae media and tympani. The three scalae vestibuli, media, tympani are cut in several places as they spiral around a central core. The tightly coiled shape gives the cochlea its name, which means snail in Greek as in conch shell. As explained in Tonotopic Organization , high frequency sounds stimulate the base of the cochlea, whereas low frequency sounds stimulate the apex.

This feature is depicted in the animation of Figure The activity in Figure The moving dots are meant to indicate afferent action potentials.

Low frequencies are transduced at the apex of the cochlea and are represented by red dots. High frequencies are transduced at base of the cochlea and are represented by blue dots. A consequence of this arrangement is that low frequencies are found in the central core of the cochlear nerve, with high frequencies on the outside. Sound waves cause the oval and round windows at the base of the cochlea to move in opposite directions See Figure This causes the basilar membrane to be displaced and starts a traveling wave that sweeps from the base toward the apex of the cochlea See Figure The traveling wave increases in amplitude as it moves, and reaches a peak at a place that is directly related to the frequency of the sound.

The illustration shows a section of the cochlea that is moving in response to sound. The traveling wave causes the basilar membrane and hence the Organ of Corti to move up and down.

The organ of Corti has a central stiffening buttress formed by paired pillar cells. Hair cells protrude from the top of the Organ of Corti.

A tectorial roof membrane is held in place by a hinge-like mechanism on the side of the Organ of Corti and floats above the hair cells. As the basilar and tectorial membranes move up and down with the traveling wave, the hinge mechanism causes the tectorial membrane to move laterally over the hair cells.

This lateral shearing motion bends the cilia atop the hair cells, pulls on the fine tip links, and opens the trap-door channels See Figure The influx of potassium and then calcium causes neurotransmitter release, which in turn causes an EPSP that initiates action potentials in the afferents of the VIIIth cranial nerve.

Most of the afferent dendrites make synaptic contacts with the inner hair cells. There are two types of hair cells, inner and outer. There is one row of inner hair cells and three rows of outer hair cells.

Most of the afferent dendrites synapse on inner hair cells. Most of efferent axons synapse on the outer hair cells. The outer hair cells are active. They move in response to sound and amplify the traveling wave. The outer hair cells also produce sounds that can be detected in the external auditory meatus with sensitive microphones.

These internally generated sounds, termed otoacoustic emissions , are now used to screen newborns for hearing loss. The mature human cochlea would look approximately the same. Superimposed schematically-depicted neurons show the typical pattern of afferent connections. Ninety-five percent of the VIIIth nerve afferents synapse on inner hair cells. Each inner hair cell makes synaptic connections with many afferents.

Each afferent connects to only one inner hair cell. About five percent of the afferents synapse on outer hair cells. These afferents travel a considerable distance along the basilar membrane away from their ganglion cells to synapse on multiple outer hair cells. The below micrograph is courtesy of Dr.

Reprinted with permission. Physical characteristics of the basilar membrane cause different frequencies to reach maximum amplitudes at different positions. Much as on a piano, high frequencies are at one end and low frequencies at the other. High frequencies are transduced at the base of the cochlea whereas low frequencies are transduced at the apex. Sensorineural hearing loss , which is caused by damage to the cilia or to the auditory nerve, is less common overall but frequently occurs with age Tennesen, Prolonged exposure to loud sounds will eventually create sensorineural hearing loss as the cilia are damaged by the noise.

People who constantly operate noisy machinery without using appropriate ear protection are at high risk of hearing loss, as are people who listen to loud music on their headphones or who engage in noisy hobbies, such as hunting or motorcycling.

Sounds that are 85 decibels or more can cause damage to your hearing, particularly if you are exposed to them repeatedly.

Sounds of more than decibels are dangerous even if you are exposed to them infrequently. People who experience tinnitus a ringing or a buzzing sensation after being exposed to loud sounds have very likely experienced some damage to their cilia.

Taking precautions when being exposed to loud sounds is important, as cilia do not grow back. While conductive hearing loss can often be improved through hearing aids that amplify the sound, they are of little help to sensorineural hearing loss. But if the auditory nerve is still intact, a cochlear implant may be used. A cochlear implant is a device made up of a series of electrodes that are placed inside the cochlea. The device serves to bypass the hair cells by stimulating the auditory nerve cells directly.

The latest implants utilize place theory, enabling different spots on the implant to respond to different levels of pitch. The cochlear implant can help children who would normally be deaf hear. Chisolm, T. The aging auditory system: Anatomic and physiologic changes and implications for rehabilitation. International Journal of Audiology, 42 Suppl. Corey, D. TRPA1 is a candidate for the mechano-sensitive transduction channel of vertebrate hair cells. Nature, , — Dettman, S.

Communication development in children who receive the cochlear implant younger than 12 months: Risk versus benefits. Ear and Hearing, 28 2, Suppl. Dorman, M. The design and function of cochlear implants. American Scientist, 92 , — Middlebrooks, J. Sound localization by human listeners. Annual Review of Psychology, 42, — Statistics Canada. Participation and activity limitation survey, Above this level, the variability inherent in neural firing becomes too great for such fine patterns to be resolved, and the frequency is probably coded for solely by the place code.

The central nervous system therefore gains information about stimulus frequency in two ways. First there is the place code: the fibres are arranged in a tonotopic map such that position is related to characteristic frequency. Second, there is the frequency code: fibres fire at a rate reflecting the frequency of the stimulus. Below 50 Hz, it appears that frequency is encoded solely by the frequency code.

Frequency coding is also of particular importance when the sound is loud enough to saturate the neural firing rate Section 6. Fibres of many characteristic frequencies will respond to a loud signal because it will be above threshold even for fibres with characteristic frequencies that are different from the signal frequency although they will respond less vigorously.

However, frequency information will still be encoded in the temporal firing pattern of all stimulated fibres. As you work through this course you will need various resources to help you complete some of the activities. Making the decision to study can be a big step, which is why you'll want a trusted University. Take a look at all Open University courses. If you are new to University-level study, we offer two introductory routes to our qualifications.

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