Understanding Auditory Transduction: How We Hear
Hearing is a complex process that allows us to identify and recognize objects based on the sounds they produce, and it enables communication through 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. This process, known as auditory transduction, is essential for our ability to perceive the world of sound.
Auditory transduction is the process by which sound waves are converted into electrical signals that can be interpreted by the brain. This process begins when sound waves enter the ear and cause vibrations in the eardrum, which are then transmitted through the ossicles to the cochlea. Inside the cochlea, specialized hair cells convert these mechanical vibrations into neural impulses, allowing the brain to perceive sound.
The visible part of the ear, or pinna, collects the changes in air pressure that carry sound and funnels them down the external auditory canal to the tympanic membrane, or eardrum. The eardrum vibrates in response, which in turn moves three tiny bones (the ossicles: malleus, incus, and stapes) in the Eustachian tube in succession. The surface of the tympanic membrane is several times larger than that of the stapes’ footplate, so the ossicles concentrate vibrational energy on it. The lengthened end of the incus acts as a lever in transmitting force to the stapes, which also concentrates the vibrational energy on the oval window.
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. Here, the energy from the sound wave is transferred from the stapes through the flexible oval window and to the fluid of the cochlea.
As the footplate of the stapes presses on the oval window at the base of the cochlea, it pressurizes the fluid in the scala vestibule or vestibular canal, a channel that runs the length of the cochlear spiral. Given that the cochlea is a closed system, this pressure has to be dissipated somehow, or the stapes footplate would be pushing an incompressible fluid against the unyielding walls of the cochlea. As a first approximation, what happens is that pressure bows the inner wall of the vestibular canal, which in turn bows the inner wall of a channel on the other side, the scala tympani or tympanic canal. Pressure so-transferred to the tympanic canal runs down the cochlea and bows the round window, a flexible membrane at the base of the cochlea. Since the round window does not impinge on anything else, its winds up absorbing whatever pressure is left, effectively draining it from the system.
The structures of the inner ear are found in the labyrinth, a bony, hollow structure that is the most interior portion of the ear. The vibrations of the oval window create pressure waves in the fluid (perilymph) inside the cochlea. All of this is to get you to the point of understanding what happens to the strip of tissue that separates the vestibular and tympanic canals and thus bends with the transfer of pressure from one side to the other. Known as the basilar membrane, it is fixed at the base of the cochlea and free to move at the apex. Any vibration impinging on it creates what is known as a traveling wave. There is one way in which the basilar membrane is very different from a towel, however. A towel is of uniform width and stiffness throughout its length. The basilar membrane is not; it is narrower and stiffer at the base, and wider and more flexible at the apex. The result is that the basilar membrane’s elasticity decreases with distance from the base. A specific frequency of vibration thus preferentially resonates at a specific distance from the base.
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). 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.
Cochlea cross section
The Role of Hair Cells
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. When the sound waves in the cochlear fluid contact the basilar membrane, it flexes back and forth in a wave-like fashion.
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. About 90 percent of the afferent neurons carry information from inner hair cells, with each hair cell synapsing with 10 or so neurons. 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. Outer hair cells connect to only 10 percent of the afferent neurons, and each afferent neuron innervates many hair cells. 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. The bending of the stereocilia opens pores at their tips which allows positively charged potassium ions (\(K^+\)) to enter and depolarize the cell. This receptor potential opens voltage-gated channels which allow calcium ions (\(Ca^2+\)) to enter the cell and trigger the release of neurotransmitters at the bottom of the cell. As a result, the hair cell membrane is depolarized, and a signal is transmitted to the cochlear nerve. The neurotransmitters diffuse across the narrow space between the hair cell and a cochlear nerve terminal, where they then bind to receptors and thus trigger action potentials in the nerve.
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.
Since the stereocilia only open potassium pores as long as they are bent, a hair cell only produces neurotransmitters for as long as the frequency lasts. For a faint sound, the basilar membrane barely moves, so only the tallest stereocilia brushes up again the tectorial membrane and so only opens its few potassium pores and ultimately producing just a handful of action potentials. On the other hand, for a loud sound, the basilar membrane moves brusquely, forcing all the stereocilia up again the tectorial membrane and so opening all the potassium pores. This produces a cascade of action potentials. This is how the intensity of a sound can be transduced to the auditory nerve.
Place Theory and Pitch Detection
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. This fixed array of frequency sensitivity down the length of the cochlea is know as tonotopy or a frequency-to-place mapping. The basic pitch-determining mechanism is based on the location along the membrane where the hair cells are stimulated.
The drawing in the top half of the image shows four frequencies that peak at segments A to D of the basilar membrane. The graph in the bottom half measures the response of a hair cell that is closely tuned for each frequency. The x-axis measures frequency logarithmically; the y-axis measures intensity in db SPL. The four curves are roughly V shaped. By way of example, the yellow, high-frequency curve has its sharpest response at something like 5 dbSPL; its sensitivity gradually gets broader (less precise) until it hits about 60 db SPL, when it essentially collapses and starts responding to almost any frequency.
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.
Auditory Nerve and Brain Processing
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 inner hair cells are most important for conveying auditory information to the brain.
The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor. The change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? As an example, a type of receptor called a mechanoreceptor possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential.
The primary centers in the auditory brainstem in order of their anatomical location from the cochlea to the cortex are: cochlear nucleus, olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate. The outer, middle, and inner ears along with the auditory nerve make up the peripheral auditory system, and the brainstem and brain constitute the central auditory nervous system. Together the peripheral and central nervous systems are responsible for hearing and auditory perception.
Sound Intensity and Perception
Intensity is the attribute of a sound that allows it to be ordered on a scale from quiet to loud. Sound intensity is also known as sound pressure, sound power or sound strength. It is usually measured as the sound pressure level or SPL in decibels. A decibel is one tenth of a bel. A bel is the ratio of the absolute measure of power or intensity to a reference value. For sound pressure level, the reference value is the threshold of human hearing. The ratio is measured logarithmically, so that an increase of 10 db means that intensity has increased 100 times.
Even though sound is measured physically in three ways, it is perceived in at least six: pitch, loudness, phase, direction, distance, and timbre.
Cochlear Implants
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.
Key Terminology
Below are some definitions of terms and measures used to describe sound.
- Sound pressure (p): sound pressure is equal to the force (F) produced by the vibrating object divided by the area (Ar) over which that force is being applied: p = F/Ar.
- Sound intensity (I): sound intensity is a measure of power. Sound intensity equals sound pressure squared divided by the density (po) of the sound-transmitting medium (e.g., air) times the speed of sound (c): I = p2/poc.
- Energy: Energy is a measure of the ability to do work and is equal to power times the duration of the sound, or E = PT, where P is power and T is time (duration) in seconds.
- Decibel (dB): dB = 10*log10(I/Iref) or 20*log10(p/pref), where I is sound intensity, p is sound pressure, ref is a referent intensity or pressure, and log10 is the logarithm to the base 10. When pref is 20 micropascals, then the decibel measure is expressed as dB SPL (sound pressure level).
- Hertz (Hz): Hertz is the measure of vibratory frequency in which “n” cycles per second of periodic oscillation is “n” Hz.
- Phase (angular degrees): One cycle of a periodic change in sound pressure can be expressed in terms of completing the 360 degrees of a circle.
- Tone (a simple sound): A tone is a sound whose amplitude changes as a sinusoidal function of time: Asin(2 πft + θ), where sin is the trigonometric sin function, θ = peak amplitude, f = frequency in Hz, t = time in seconds, and θ = starting phase in degrees.
- Complex sound: Any sound that contains more than one frequency component.
- Spectrum: The description of the frequency components of sound; amplitude spectrum describes the amplitude of each frequency component; phase spectrum describes the phase of each frequency component.
- Noise: A complex sound that contains all frequency components, and whose instantaneous amplitude varies randomly.
- White noise: A noise in which all of the frequency components have the same average level.
These three aspects of sound transduction are the physical grounding of the cognitive attributes of sound, especially linguistic sounds.