088 – Highlights of the Auditory System

TPA Contributor Madeleine shares some amazing knowledge from her audiology studies.

Written by Madeleine Campbell

This week I’m wrapping up my first semester of coursework towards my doctorate in audiology at the CUNY Graduate Center. Learning about the anatomy and physiology of the human auditory system has been fascinating. Here are some highlights of the outer, middle, inner ear, and the central auditory nervous system.

The outer ear starts at the auricle, which is also called the pinna. This is the part of our ear we can actually grab onto. It attaches to our head at a 15-30° angle, which is helpful for sound collection. In a nutshell, the outer ear gathers sounds from the outside world and funnels them through the ear canal to the middle ear at the tympanic membrane, a.k.a. the eardrum. If we block the entrance to the ear canal with an ear plug, much less sound is able to reach the middle ear. The outer ear also provides important cues about timing and intensity that help us discern and localize where the sounds are coming from.

The middle ear starts with the tympanic membrane (the eardrum). Once a sound reaches the middle ear, the eardrum transmits the signal to a chain of three very tiny bones - the smallest bones in the human body - called the ossicles (the malleus, the incus, and the stapes) which in turn pass it on to the entrance of the cochlea. The middle ear also includes the eustachian tube, which regulates and controls the pressure of our head. In addition to the continued transmission of sound, the middle ear has a hugely important physiological function - it acts as a transformer. Sound enters the ear through the air-filled ear canal and is passed through the middle ear into the fluid-filled cochlea. This presents a big impedance mismatch that the mechanical actions of the middle ear structures collectively overcome by boosting the acoustic signal nearly 30dB before it reaches the cochlea.

This presents a big impedance mismatch that the mechanical actions of the middle ear structures collectively overcome.

Now we’ve reached the inner ear! Fair warning: the anatomy and physiology of the inner ear and beyond is extremely complex, but this highly abridged, simplified overview will underscore the basic ideas of what occurs. The main purpose of the cochlea, a small, sort of snail-shaped organ, is to transduce vibratory and mechanical input received from the middle ear into chemical and electrical impulses that are then sent to the brain. In this process, it encodes important information about the frequency, intensity and temporal elements of the sound.

The cochlea is full of sensory hair cells which are connected to auditory nerve fibers. When the hair cells are stimulated by cochlear movement from incoming signals, they incite ionic responses which actually alter the voltage of the hair cell and build up to the release of neurotransmitters. Once these neurotransmitters are released, nerve fibers send information about the auditory stimulus up the central auditory nervous system to the brain where it is interpreted as sound. One thing that’s especially cool about the cochlea is that it acts as a frequency analyzer. This means an incoming auditory signal will stimulate the hair cells on the region of the cochlea that best correlates to the frequency of that signal. Damage to these hair cells from prolonged exposure to loud noises can cause permanent hearing damage.

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The central auditory nervous system acts as a series of relay stations that subtly refine and repackage a sound so that our brain can make better sense of it. This is where binaural hearing begins and sound stimuli from both ears are fused to sound as one.

Below is a diagram of the inner, middle, and outer ear from Encyclopaedia Britannica. This overview is really just the tip of the iceberg, but the auditory system is a truly remarkable entity that works at lightning speed. As live sound engineers, we must actively preserve our auditory health with adequate hearing protection and regular hearing evaluations.