Function of Cochlea
Anatomy of the Cochlea and Hearing Function
Briefly reviewing the anatomy of the cochlea, focusing on functional aspects relevant to hearing.
Random Facts
Human hearing is not as good as a dog's, but it is still quite remarkable. It has a huge dynamic range, a great ability to discriminate between two frequencies, and can respond to tiny vibrations of the membrane. The range of human hearing is approximately to . The most sensitive region is between and , which includes the frequencies needed to hear and understand speech.
Anatomy of the Cochlea
The spiral organ within the cochlea includes outer and inner hair cells. The stereocilia of the outer hair cells are embedded within a membrane, while the stereocilia of the inner hair cells are moved by fluid between the membranes. The cilia bundles have a chevron shape and are connected with links. Ionic concentrations differ in various compartments: the scala tympani and scala vestibuli have high sodium and low potassium concentrations, while the scala media has high potassium and low sodium concentrations.
Function of the Basilar Membrane
The basilar membrane vibrates when the cell membrane bounces up and down, translating into a side-to-side movement of the cilia. The membrane has different structures along its length. At the base, it is narrower and stiffer, resonating to higher frequencies. At the apex, it is broader and fluffier, resonating to lower frequencies. A pure tone of will cause a ripple of vibration close to the apex, while a higher frequency tone will cause vibration closer to the base. This creates a place code for different frequencies.
The basilar membrane resonates to different frequencies at different locations, creating a place code. However, the place code alone is insufficient to explain the ability to distinguish so many frequencies. There are inner hair cells in each cochlea, yet we can resolve approximately unique frequencies. This would mean almost two hair cells for every unique frequency. However, there is no way the waves of vibration within the membrane can distinguish something happening microns apart. Thus, the frequency is more complex than just a place code.
Afferents and the Spiral Ganglion
The cochlea is embedded in bone, consisting of tunnels through the bone. Nerves and axons spread out as they move up the spiral. The swollen region contains the spiral ganglion, where the cell bodies of the afferents are located. There are two types of afferents: type one and type two.
Type one afferents: Receive input from the inner hair cells (95% of afferents). Thicker axon, myelinated, work quickly, local connections (input from one location). Transmit information really quickly.
Type two afferents: Receive input from the outer hair cells (5% of afferents).
There are three to five times as many outer hair cells than inner hair cells, but the afferents that go to the inner hair cells are by far the most numerous. This indicates a lot of divergence in this pathway.
Type one afferents are the true afferents of the auditory system, carrying information to the cerebral cortex. They contact only a single hair cell, preserving frequency information. The axons are fast and powerful, transmitting information rapidly. Each type one afferent receives input from a single hair cell, but each cell gives output to up to 20 type four afferents. There are also afferents with different sensitivities. Some are very sensitive to quiet sounds, while others only respond to very loud sounds, extending the dynamic range of the system.
If a type one afferent is tested with different tones at different volumes to find its threshold, the quietest sound to which it responds at that frequency will be that. There is a sharp peak of high sensitivity called the characteristic frequency for that particular afferent. A hair cell's response to a tone exhibits both a DC component (depolarization) and an oscillation at the tone's frequency. Individual action potentials are about milliseconds long, so most nerve cells can fire at about in very brief bursts. The oscillations may work at the very lowest end of the frequency range, but as frequency increases, the DC component gets bigger and the oscillations get smaller and smaller because it has to follow the higher frequencies. Type two afferents are mysterious, with thin axons. It has been suggested that they could be a form of nociceptive, as they don't respond to sound unless it is very loud.
Feedback and Horizontal cells analogy
Type two afferents receive input from a wide range of outer hair cells. They are feeding back to the outer hair cells. The receptors and horizontal cells in the retina are all doing centers around inhibition. The long-ranging dendrites at the end of the type two afferents are working a bit like horizontal cells, quite independently of any axon that they happen to have because the axon doesn't fire action potentials to normal sound levels.
Type two axons pick up information from a broader range of frequencies, and feed it back onto the outer hair cells. Both type one and type two have axons that go to the dorsal and ventral cochlear nuclei within the brainstem.
Cochlear Amplifier
The basilar membrane isn't a passive structure; the cochlear amplifier enhances its response. Destroying the outer hair cells (and thus the cochlear amplifier) reduces the level of vibration in the membrane. The cochlear amplifier takes the tiny ripple created by the sound wave and amplifies it into a larger response. This amplification is due to the outer hair cells, which grow longer and shorter as they depolarize and hyperpolarize. The stereocilia may also contribute to the amplification effect.
The sharp peak in the type one afferent response curve is due to the cilia membrane movement. Removing the outer hair cells eliminates that peak. The cochlear amplifier amplifies membrane vibrations. Without the cochlear amplifier, massive amounts of sensitivity would be lost, and whatever sensitivity they had left, their frequency resolution, their ability to tell apart the frequencies would be destroyed as well.
Volume Control
The cochlear amplifier amplifies the membrane vibrations. the cochlear nerve comes into the cochlear nuclei on the brainstem. This signal is taken up to the colliculus and to the left and up to the auditory cortex. There is another output from the cochlear nuclei which goes inside the superior olivary nuclei. The medial superior olivary nuclei feedback with afferents that respond well to sound. They are the source of efferent input back down into the inner ear and right down to the hair cells. The nuclei do well to sound, nice, fast, well-myelinated axons. Good for sound is narrowly tuned to frequency (good auditory response). They feedback roughly equally to both ears, though they get their input contralaterally.
The medial superior olivary nuclei outputs to the cochlea inhibit outer hair cells. They get their input from the inner hair cells via the type one afferents. They feedback to the same point on the membrane. This feedback is inhibitory, hyperpolarizing the cells, stopping them from depolarizing as much and therefore not shortening as much. This turns down the cochlear amplifier, reducing the vibration of the membrane. Stimulating the medial input reduces the response of type one afferents. The cell becomes less sensitive, and the sharp peak is reduced.
When sound gets very loud, medial afferents are activated more strongly, hyperpolarizing the outer hair cells and turning down the response of the inner ear. This has a protective effect against noise-induced hearing damage. They may also have a protection for hearing.
Sound and Noise Adaptation
In a quiet room, a type one afferent responds to a tone with increasing loudness until it saturates. With background noise, the cell responds at a higher frequency of action potential firing. The cell is less sensitive to the tone because the noise has caused it to adapt. The hair cells adapt to the noisy environment and become less sensitive.
Medial feedback turns down the response to the noise. The hair cell is less strongly activated by the noise, and therefore they do not adapt as much. They are still capable of responding to the sound. Medial feedback changes the signal-to-noise ratio, making it easier to hear a signal in noise. Sound and noise disappears with age because without the medial feedback, your ability to hear sound and noise would be much, much worse.
When testing signal against background noise to see how good they were hearing, younger people responded much better than older people. If the medial cochlear system is lesioned, hearing goes downhill much faster. It protects you from the gradual degradation of your hearing, which will occur because of all the insults that life sends you. So they help adaptation to different sound levels signals noise in louder environments. There may be a role in selective attention, and it's thought to be protective against noise trauma.
The lateral cochlear system is again strange, mysterious because it has thin axons. They sound, they do respond to sound, but these thin, slow axons are not perhaps taking part in immediate processing of sound. The feedback loop is primarily ipsilateral these things feed back onto the bulbs. Post-synaptic sort of part of the type one afferents. They use different neurotransmitters. It's obviously a heterogeneous feedback system. Some of them may facilitate, some of them may suppress. They are feeding back to the same location that they are getting the auditory input from.
Noise-Induced Hearing Loss
Loud noises can cause damage to the cochlea. The outer hair cells are the first to go, and then you lose your amplifier, so the inner hair cells are useless. High-noise causes damage to the outer hair cells. Afferents fan out and go to innervate those hair cells. The louder the sound and longer their duration causes hearing damage. Temporary loss of hearing can cause permanent damage.
Exposure to loud sounds can cause a temporary loss of hearing, but it can also cause a permanent loss of type one afferents. Even if hearing recovers, the number of type one afferents may be reduced. Animals can have a temporary loss of hearing and a possible permanent loss of type one afferents. A temporary shift of hearing will come back to normal. They explained that they have lost their afferents.
Animals with a temporary loss of hearing and loss of afferents have normal hearing at threshold, so they didn't lose their sensitivity to quiet sounds. The sensitivity to louder and louder sounds will show that they have lost sensitivity to the louder sounds. You will have to turn up the music because it is not loud enough. Thus, you cause even more damage. So it has difficulty understanding speech and noise. You get worse and worse at being able to distinguish sound and noise.
Medial feedback may protect against hair cell loss. Medial feedback may protect against afferent loss, but the experiments are ambiguous on both of those points.