What transmits sound to the inner ear?

The outer ear is called the pinna and is made of rigid cartilidge covered by skin. Sound goes into the pinna down the external auditory canal until it reaches the eardrum, known as the tympanic membrane.

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What do the different parts of my balance organ do?
How can they tell when I am moving?
Anatomy of the ear
The process of hearing
Related Stories
Pitch and intensity
Hearing loss

Sound causes the eardrum and the tiny bones in the middle ear - the hammer, anvil and stirrup bones to vibrate. The stirrup fits in the oval window and that transmits sound to the cochlea in the inner ear. The middle ear space contains air, which gets in through the eustachian tube and this opens at the back of the nose.

The inner ear contains the organs of hearing and balance.

The hearing organ is known as the cochlea. When sound passes through the middle ear, the fluids of the inner ear vibrate and trigger the little hair cells within the cochlea. The hair cells fire off electrical pulses. These pulses go down the nerve and are processed by the brain which is what we perceive as hearing.

The balance organ is known as the labyrinth and is made up of three fluid-filled chambers, known as the semicircular canals which send information on balance and head position to the brain, and two otolith organs, the saccule and utricle, which are responsible for sensing gravity and movement.

What do the different parts of my balance organ do?

The three semicircular canals (called the ‘horizontal’, ‘posterior’, and ‘anterior’ canals) are all at right angles to each other so that together they can detect movement across all the different angles you can move through. The ‘horizontal’ semicircular canal detects movements such as turning your head from left to right (e.g. when you are crossing the road). The ‘posterior’ and ‘anterior’ semicircular canals often work together and detect movements such as nodding your head up and down (e.g. when you are looking up at a high shelf), and tilting your head sideways (e.g. holding a phone between your ear and your shoulder). The two otoliths are called the utricle and the saccule. The utricle detects whether you are upright or lying down (using gravity), and the saccule detects whether you are moving forwards and backwards (e.g. in a car).

How can they tell when I am moving?

Each of the semicircular canals and the two otoliths are covered in tiny sensory hair cells. It is these hair cells that send balance signals back to the brain. The whole inner ear is filled with endolymphatic fluid. This fluid moves around your inner ear when you move. The hair cells in your balance organ are activated to send signals to your brain when they are moved by this fluid.

Reviewed by Afsaneh Khetrapal, BSc

The ear is the organ of hearing and balance. One of the very advanced sensitive organs of the human body, the ear detects, transmits, and transduces sound to the brain and maintains a sense of balance. Just as ripples are spread out in circles from the point where a stone is dropped in water, the sound source creates pressure variations in the air, which are perceived by the ear as sound.

Anatomy of the ear

The parts of the ear include the outer ear, the middle ear, and the inner ear.

The outer ear consists of the pinna or auricle, the part that we see on the side of the head, and the tube or external auditory canal that connects the outer ear to the middle ear through which sound travels.

The tympanic membrane or the eardrum lies between the outer and middle ear.

The middle ear is composed of ossicles and the eustachian tube. Ossicles consist of three small bones–malleus, incus, and stapes—that are coupled to transmit sound waves to the inner ear. The eustachian tube, lined with mucous, is a canal that helps to equalize pressure in the middle ear so that the sound waves are transferred properly. Sound enters the outer ear and makes the eardrums vibrate, and the vibrations are passed along the ossicles.

The inner ear consists of the cochlea, vestibule, and semicircular canals. The cochlea is the hearing part, comprising a coiled spiral tube containing nerves for hearing. The cochlea, a snail-shaped and fluid-filled structure, is in the inner ear. An elastic partition, which is the basilar membrane (serves as a base on which the important hearing structures resides), runs from the start to the end of the cochlea and splits it into an upper and a lower part.

Vestibule and semicircular canals are part of the balance system. The semicircular canals provide information to the brain about the direction of the head movement.

The process of hearing

Many stages are involved in the process of transduction i.e. the complex process by which sound waves are transformed into electrical signals, which are then conveyed by the auditory nerve to the brain.

When we hear a sound, it is transmitted as a wave and reaches the outer ear. The sound waves pass through the ear canal, a slender passage, leading to the eardrum. When the eardrum is struck, the vibrations are sent to the ossicles in the middle ear. These small bones are responsible for amplifying the vibrations and sending them to the cochlea. A rippling effect is caused by the vibrations in the cochlea, and this results in the formation of a traveling wave along the basilar membrane. The sensory cells present on the top of the basilar membrane, called hair cells, recognize the sound waves.

Detection of a sound is dependent on its pitch - while high-pitched sounds are detected by the hair cells near the wide end of the cochlea, low-pitched sounds are identified by the hair cells closer to the center of the cochlea.

Stereocilia are vital for the process of electro-mechanical transduction. They are the microscopic hair-like projections that rest on and protrude from the top of the hair cells. These knock against a covering structure and are deflected when the hair cells move up and down. This process opens up the stretch-sensitive ion channels present at the tips of the stereocilia and allows the influx of specific cations, namely K+ and Ca2+, which generates an electrical signal. This signal travels through the auditory nerve to the brain, which is ultimately responsible for the processing of sounds, and recognition of inter-sound variability.

Pitch and intensity

It is important to understand the terms pitch and loudness, as together they are used to describe characteristics of a sound. The pitch, or sound frequency, is measured in hertz (Hz) and the loudness, or the intensity, of the sound is measured in decibels (dB).

Typically, the most important sounds that humans hear daily are in the range of 250–6,000 Hz where the normal ranges are 20-20,000 Hz. However, hearing becomes most sensitive in the 2000–5000 Hz frequency range.

The audible range for humans is 0-140 dB. While 0 decibels is the quietest, a whisper is around 25–30 dB and conversations are usually 45-60 dB, as speech is a combination of low- and high-frequency sounds.

Hearing loss

If there are obstacles that stop the sound passing through the outer or inner ear, there could be conductive hearing loss. In sensorineural hearing loss, the problem could be with the inner ear or the hearing nerve. A combination of both may result in mixed hearing loss.

The sound information is passed from the ear to the brain via the auditory nerve, with the information processed by the auditory pathway as it travels along the auditory hearing system. This indicated the great role of brain pathways in the hearing process. Although sound can enter the ears, if there are damages to the inner ear or the auditory nerve, the brain cannot ‘understand’ the sound, resulting in Auditory Neuropathy Spectrum Disorder.

Exposure to more than 85 dB for long periods is dangerous to the hearing and may lead to temporary or permanent hearing loss.

Sources

https://www.nidcd.nih.gov/health/how-do-we-hear
https://www.cdc.gov/ncbddd/hearingloss/sound.html

Last updated Feb 26, 2019

Hearing depends on a series of complex steps that change sound waves in the air into electrical signals. Our auditory nerve then carries these signals to the brain. Also available: Journey of Sound to the Brain, an animated video.

Source: NIDCD

Sound waves enter the outer ear and travel through a narrow passageway called the ear canal, which leads to the eardrum.
The eardrum vibrates from the incoming sound waves and sends these vibrations to three tiny bones in the middle ear. These bones are called the malleus, incus, and stapes.
The bones in the middle ear amplify, or increase, the sound vibrations and send them to the cochlea, a snail-shaped structure filled with fluid, in the inner ear. An elastic partition runs from the beginning to the end of the cochlea, splitting it into an upper and lower part. This partition is called the basilar membrane because it serves as the base, or ground floor, on which key hearing structures sit.
Once the vibrations cause the fluid inside the cochlea to ripple, a traveling wave forms along the basilar membrane. Hair cells—sensory cells sitting on top of the basilar membrane—ride the wave. Hair cells near the wide end of the snail-shaped cochlea detect higher-pitched sounds, such as an infant crying. Those closer to the center detect lower-pitched sounds, such as a large dog barking.
As the hair cells move up and down, microscopic hair-like projections (known as stereocilia) that perch on top of the hair cells bump against an overlying structure and bend. Bending causes pore-like channels, which are at the tips of the stereocilia, to open up. When that happens, chemicals rush into the cells, creating an electrical signal.
The auditory nerve carries this electrical signal to the brain, which turns it into a sound that we recognize and understand.

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May 2015

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