The science of hearing - Douglas L. Oliver
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All of the components of the auditory system work together to allow us to hear. The central auditory system consists of the cochlear nucleus, the superior olivary complex, the lateral lemniscus, the inferior colliculus, the medial geniculate body, and the auditory cortex. The Encyclopedia of Neuroscience Online includes articles about most components of the auditory system. In-depth information on the auditory system for the budding scientist is presented in the Springer Handbook of Auditory Research, a series of 65 books on all aspects of the auditory system, hearing, and hearing loss. These books continue to be published with new volumes coming out every year.
There are many interesting phenomena in the auditory system to explore. In the outer ear, sound is funneled by the pinna into the external auditory meatus, where it reaches the eardrum. So how does the size and shape of your ear affect what you hear? The ossicles in the middle ear sometimes freeze and are unable to conduct the vibrations from the eardrum to the cochlea. How is this hearing loss different from damage to the cochlea? The cochlea of the inner ear is complex and is the site of sensory transduction, where the mechanical signal is converted into a neural signal—but what are the molecules and channels involved in this process?
There are two types of hair cells. Inner hair cells connect to 95% of the fibers in the auditory nerve. How do inner hair cells transmit neural signals to the brain? Why are the synapses of the inner hair cells on the dendrites of the spiral ganglion cells among the fastest synapses in the nervous system? There are three times more outer hair cells than inner hair cells. Why do outer hair cells move up and down like little muscles during sound stimulation?
The auditory vestibular nerve, otherwise known as the eighth cranial nerve, sends signals from the hair cells to the cochlear nucleus in the brain. However, at the cochlear nucleus, many neural circuits begin to carry different types of information. Sound localization requires binaural sound stimulation with two ears. How does information from the two ears get combined and compared in the superior olivary complex so that we can localize sound? Why are the largest synapses in the brain found in the cochlear nucleus and superior olivary complex? Are there brain circuits that tell us what a sound is instead of where a sound source is located?
The inferior colliculus receives information from almost every part of the auditory brainstem, including the cochlear nucleus and superior olivary complex. This answers certain aspects of the what and some of the where, so how does it integrate all this information? There are multiple core auditory cortical areas surround by a belt of other auditory cortical areas. How is the information related to what and where distributed amongst the areas of auditory cortex?
When we have a hearing loss, it impairs our ability to communicate with others even when we are not deaf. However, some hearing loss is gradual and not noticed at first. This may be especially true for noise-induced hearing loss if we have chronic exposure to moderately loud sound, as opposed to extremely loud sound. Find out more about noise-induced hearing loss here.
Many people have a ringing in their ears called tinnitus, which never stops—even when there is no sound. This is a symptom rather than a disease. There may be many causes of tinnitus; exposure to loud sound may be among the most common causes. However, we are still not clear about what happens in the auditory system and how to cure the problem. Find out more about tinnitus here.
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