31 Auditory System: The Ear

Our nervous system is equipped with a variety of specialized biological “tools” that can detect much more than just photons of light. We can detect the shape of air waves, and interpreting those signals give us sound information and the perception of music. In this chapter we will trace how sounds travel through the structures of the ear, ultimately causing the auditory receptors to alter their activity and send their signals to the brain.

Properties of Sound

Unlike photons of light, sound waves are compressions and rarefactions of a medium. For us land animals, that medium is usually air, but sound waves can propagate very well in water or through solids. Before we get to the anatomical structures involved in sound perception, it is important to first understand the physical nature of sound waves. All sounds, from the clattering of a dropped metal pan to the melodies of a Mozart violin concerto, are contained in their corresponding sound waves. Two components of sound waves are frequency and amplitude.

1. Frequency, or “How often do the sound waves compress?” The greater the frequency, the higher the pitch. The highest notes humans are able to hear is around 20,000 Hz, a painfully-shrill sound for those who can hear it. People often tend to lose their high frequency hearing as they age. On the opposite end of the spectrum, low frequency sounds are the deep rumbles of bass, and the human ear can hear sounds down in the 20 Hz range.
2. Amplitude, or “How much do the waves displace the medium from baseline?” The larger the amplitude of the wave, or the greater distance between the peak and the trough of the signal, the louder the sound is. Loudness is measured in decibels (dB). To give you an idea of approximate sound intensities, the background noise of a quiet library is about 40 dB, and a typical conversation is close to 60 dB. A rock concert or lawnmower is between 100 and 110 dB, which is right around the pain threshold. Prolonged exposure to these high amplitude sound waves can lead to permanent damage to the auditory system resulting in hearing loss or tinnitus (a ringing in the ear, even in the absence of a sound stimulus).

Physical Structures of the Auditory System: Outer Ear

Our auditory system is a series of physical structures and nervous system components that are responsible for conveying sound waves into meaning and context.

The external component of the auditory system begins with the pinna. Its shape functions as a funnel, capturing and channeling sound waves into the auditory canal. The pinna and the auditory canal are parts of the outer ear. Also, because the pinna is asymmetrical, its shape helps us determine where a sound is coming from. In some nonhumans, the pinna serves these functions and more. For instance, some animals are able to disperse excess heat through their ears (elephants), and some even use them to display emotion (dogs, horses).

At the end of the auditory canal is the tympanic membrane, or ear drum. This membrane is a very delicate piece of tissue at only 0.1 mm thin and is subject to damage by physical injury such as head trauma, nearby explosions, or even changes in air pressure during scuba diving. When incoming sound waves reach the tympanic membrane, it vibrates at a matching frequency, and amplitude. The tympanic membrane also represents the boundary between the outer ear and the middle ear.

Physical Structures of the Auditory System: Middle Ear

The middle ear is an air-filled chamber. Physically attached to the tympanic membrane are the ossicles, a series of three bones that convey that vibrational sound information. These bones in order, called the malleus, incus, and stapes, conduct vibrations of the tympanic membrane through the air-filled middle ear. The stapes has a footplate that attaches to a structure called the oval window, which serves as the junction between the middle ear and the inner ear. The middle ear serves important functions in both sound amplification and sound attenuation.

Sound Amplification

The tympanic membrane and the ossicles function to amplify incoming sounds, generally by a tenfold difference. This amplification is accomplished through 2 mechanisms:

1. Due to the ossicles being connected, the ossicles act in a lever-like fashion to amplify the movements of the tympanic membrane to the oval window
2. The stapes has a smaller area on the oval window than the tympanic membrane, thus movements of the larger tympanic membrane must be transformed into smaller and stronger vibrations at the oval window.

This amplification is important because the inner ear is filled with liquid rather than air, and sound waves do not travel very well when moving from air into a denser medium – think about how muffled sounds are when you submerge your head underwater.

Sound Attenuation

The movement of the ossicles are partially regulated by two different muscles, the tensor tympani muscle which connects with the malleus, and the stapedius muscle which connects to the stapes. When these muscles contract, it causes the ossicles to be more rigid and for the ossicles to move less, which decreases the intensity of loud sounds. This response, called the acoustic reflex, dampens incoming sound by about 15 dB. (This is why we talk much louder than normal when we first leave a concert: we have lessened auditory feedback from our ears, so we tend to talk louder to compensate.) The muscles contract at the onset of loud noises with a slight delay of 50-100ms.

Physical Structures of the Auditory System: Inner Ear

The inner ear is a fluid-filled structure made up of two structures: the cochlea that functions in hearing, and the semicircular canals that function in balance.

The auditory part of this structure is a small spiral-shaped structure about the size of a pea, called the cochlea (cochlea is named for the Ancient Greek word “snail shell”.) The cochlea has two small holes at its base: the oval window and the round window.

Cross Section of the Cochlea

When we examine a cross-section of the cochlea, we can see that there are 3 distinct fluid-filled chambers called the scala vestibuli, the scala media, and the scala tympani. These chambers are separated from each other by membranes.

The fluid found within the scala vestibuli and scala tympani is called perilymph and it has a low concentration of potassium and a high concentration of sodium. The scala media is filled with a fluid called endolymph that has a high concentration of potassium and a low concentration of sodium.

Reissner’s membrane separates the scala vestibuli from the scala media. The basilar membrane separates the scala tympani from the scala media.

Basilar Membrane

Think of the cochlea as a rolled-up cone. If this cone was theoretically unrolled, the widest diameter portion, called the base, would be closest to the oval window, while the narrowest portion, called the apex, would be at the center of the spiral.

The basilar membrane runs down the middle of the cochlea. The width of the basilar membrane changes as it runs down the length of the cochlea from the base to the apex. This change in shape from the base to the apex is important: objects with different stiffness vibrate at different frequencies. The base of the cochlea is stiff and rigid and will vibrate at high frequencies. Whereas the apex is wider and less stiff, so it vibrates at lower frequencies.

Because different frequencies of sound affect different areas of the basilar membrane, the basilar membrane is what is referred to as tonotopically organized. In fact, you can think about the way that frequencies are mapped to the basilar membrane similar to a backwards piano.

Let’s consider how sounds will affect the cochlea. For simplicity’s sake, the figure here shows only the scala vestibuli and the scala tympani (not the scala media) with the basilar membrane running down the middle. Keep in mind that due to the flexibility of Reissner’s membrane that separates the scala vestibuli from the scala media, we can assume that pressure changes in the scala vestibuli are transferred through the scala media, ultimately affecting the basilar membrane.

1. When sounds move through the outer and middle ear, it causes vibration of the stapes footplate at the oval window. The movement of the stapes at the oval window is similar to a piston pushing at the oval window at the same frequency and amplitude as the incoming sound.
2. When the stapes pushes into the oval window, it causes movement of the perilymph within the scala vestibuli.
3. This causes the sounds to move through the cochlea as a wave from the base to the apex, displacing the flexible basilar membrane at different locations dependent on the frequency of sound.
4. At the apex of the basilar membrane is a hole called the helicotrema that connects the perilymph of the scala vestibuli to the perilymph of the scala tympani and allows for the pressure to be transferred from the scala vestibuli to the scala tympani.
5. The pressure then moves through the scala tympani back towards the base of the cochlea until it pushes out at the round window.

The Organ of Corti

Embedded within the basilar membrane is a structure called the Organ of Corti. The Organ of Corti is the first nervous system structure that is responsible for processing physical vibrations and converting them into signals that the nervous system can interpret. The Organ of Corti contains the components necessary for converting sound waves into action potentials. Recall that the scala media that surrounds the Organ of Corti contains endolymph, a fluid that has a high concentration of potassium and a low concentration of sodium.

A separate membrane hangs over the Organ of Corti called the tectorial membrane. Recall that “tectum” means roof. The tectorial membrane acts as a roof over the Organ of Corti.

Embedded along the interior surface of the Organ of Corti are the somata of hair cells, the primary sensory neurons that interpret physical movement. They are named “hair cells” because of their cellular structure; each hair cell has somewhere between 30 and a few hundred hair-shaped stereocilia that protrude away from the Organ of Corti, reaching into the endolymph.

Importantly, hair cells are not neurons. They do not produce action potentials and do not have axons. In fact, hair cells are a specialized type of epithelial cell. We have two different populations of hair cells, the inner hair cells and outer hair cells.

• The outer hair cells are arranged in three rows. Their stereocilia extend into the endolymph  and are physically embedded within the tectorial membrane.
• The inner hair cells are arranged in a single row. Their stereocilia extend into the endolymph but are not embedded within the tectorial membrane. Instead, the stereocilia of the inner hair cells freely float within the endolymph.

Although the hair cells themselves are not neurons, the mechanical bending of the hair cell stereocilia will be converted into a neural signal. Hair cells synapse on spiral ganglion cells (neurons). The axons of the spiral ganglion cells make up the auditory nerve, which will project into the cochlear nuclei of the medulla.

Neural Components of the Auditory System

As we have learned, when a vibration reaches the oval window, this causes the basilar membrane to move in response to the change in pressure. The basilar membrane is located at the base of the hair cells, and as the basilar membrane moves up and down, it pushes the hair cell stereocilia into the tectorial membrane above, causing the stereocilia to bend.

Hair Cells

Let’s take a closer look at the structure of the hair cell.

The stereocilia of the hair cell have different lengths and are arranged from shortest to tallest. At the tip of each stereocilium are mechanically-gated ion channels. These are mechanically-gated because they open and close in response to physical movement. The stereocilia are linked together with spring-like proteins called linker proteins. These linker proteins are specifically attached to small covers that block movement through mechanically-gated ion channels when closed.

The stereocilia can bend in two different directions, toward the shortest stereocilium or towards the tallest stereocilium. At rest (no sound), the stereocilia are not bent. Some of the mechanically-gated ion channels are open, and some are closed.

When the stereocilia bend towards the tallest stereocilium, the distance between the stereocilia increases and the spring-like linker proteins are stretched, causing the opening of the covers on the mechanically-gated ion channels.

Recall that the stereocilia are surrounded by endolymph, which is high in potassium and low in sodium. When the mechanically-gated ion channels are opened, potassium ions will flow into the hair cells, moving down its concentration gradient. As the positively charged potassium ions move into the hair cell, it causes the hair cell to depolarize.

If the stereocilia instead bend towards the shortest stereocilium, the distance between the stereocilia gets smaller. The mechanically-gated ion channels cannot open and the channels that were previously opened when the cell was at rest, will now be shut. As a result, there is less potassium influx into the hair cell and thus less positive charge in the hair cell than there was at rest, leading to hair cell hyperpolarization.

Hair Cell Signaling

Located within the hair cell are vesicles that are filled with glutamate (an excitatory neurotransmitter). When hair cells are depolarized, this causes the opening of voltage-gated calcium channels in the hair cell membrane. The influx of calcium will ultimately cause the vesicles full of glutamate to fuse with the hair cell membrane and release the glutamate into the synapse with the spiral ganglion cell.

Spiral Ganglion Cell Innervation

Most of the spiral ganglion cells (>95%) collect information from the inner hair cells. In fact, one inner hair cell will synapse onto many different spiral ganglion cells. Whereas a small number of spiral ganglion cell (<5%) collect information from the outer hair cells. Thus, the inner hair cells are responsible for sending the majority of auditory signals from the cochlea into the brain for processing.

Cochlear Amplifier

Although the outer hair cells are not responsible for the majority of the signal to the spiral ganglion cells, they serve another function. The outer hair cells function as the cochlear amplifier to increase the intensity of vibrations within the cochlea. It is estimated that the outer hair cells increase sound by anywhere between 20 and 80 dB.

There is a motor protein called prestin within the outer hair cell membrane. As a motor protein, prestin can mechanically contract and elongate within the outer hair cells, changing the length of the outer hair cell. The membrane of the outer hair cell shortens and lengthens with the movement of the basilar membrane. When the basilar membrane is displaced upwards, this causes the prestin protein to contract, further shortening the length of the outer hair cell. When the basilar membrane is displaced downward, the prestin protein elongates the length of the outer hair cell. Essentially, the prestin within the outer hair cells amplifies the movement of the basilar membrane in both directions.

When the cochlear amplifier is functional, it doesn’t only change the displacement of the basilar membrane, but it also has effects on the inner hair cells. The cochlear amplifier will cause the inner hair cells to bend more than they would without the amplifier present. In this way, although the outer hair cells have fewer direct signals to the spiral ganglion cells, they still contribute to the firing of the auditory nerve through their influence on the inner hair cells.

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Attributions

Portions of this chapter were remixed and revised from the following sources:

1. Open Neuroscience Initiative by Austin Lim. The original work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.