What? ~ The mechanics of hearing.
The mechanism of hearing is a truly remarkable process that we take for granted until one day it starts to fail. This article will look at how the system works and a follow-up article will look at some of the ways that it stops working.
The process of hearing is like playing a drum….backwards.
To play a drum a person must first have the idea that they want to make a sound. The idea turns into an electrical impulse that travels along nerves to muscles that make the arm bones move which in turn moves a drumstick. The large movement of the drumstick becomes tiny vibratory movements in a drum skin which makes the air molecules next to the drum vibrate also. As the air molecules dance they bump into their neighbours and in this way the sound wave spreads outwards away from the drummer.
The ear does all of this in reverse. First, vibrations in the air are collected up and passed to a drum skin which changes the tiny air molecule vibrations into the physical movements of a series of small bones. These bones amplify the movement so that it can be translated into electrical impulses which then travel along nerves into the brain for processing.
In essence, it is a four step process: collect, amplify, electrify and analyse. But how does it happen?
To go any further we need to consider the anatomy of the human ear, which can be divided into 3 regions:
– the outer (or external ear),
– the middle ear and
– the inner ear.
Each region has a specific role in hearing and we’ll consider them in turn.
The Outer Ear (the collector)
The most obvious part of the outer ear is the large, flappy piece of skin and cartilage that we call the ‘pinna’, or ‘auricle’. This acts as a trumpet-like receiver, collecting sound waves and funnelling them down into the other parts of the outer ear, namely the ear canal and the ear drum.
The pinna’s sound-gathering can be enhanced by simply using a cupped hand next to your ear. Try it and you’ll notice how much the sound volume increases, apart from when the sound is coming straight from the side. This is the idea behind ear trumpets that were used in the past to help the hard of hearing. Fortunately modern hearing aids are much more discrete.
The adult ear canal is about 25mm long and about 7mm wide. It functions best when empty of everything except air. This means that hearing is much more difficult when the ear canal contains water or excessive ear wax or when there is extreme narrowing of the ear canal or other obstructions are in the way, such as bony growths, peas, mashed potato, bits of Q-tip or lego for instance.
At the inner end of the ear canal lies the ear drum, a 10mm diameter sheet of very thin (0.1mm) skin. It’s medical name is the tympanic membrane – from the Latin ‘tympanum’ (drum) or the Greek ‘tympanon’.
Like so many bits of human anatomy, the ear drum was named during the Renaissance Period. In this case it was Gabriele Fallopio, an Italian catholic priest and professor of anatomy at the university of Padua, who gave the name in the mid 1500s. You may already know the name Fallopio; he also described the pipe (that we call the ‘Fallopian tube’) that connects the ovary with the womb, along which each egg must pass if it wants to ‘go to the dance’, so to speak.
As the sound wave moves down the ear canal the air molecules next to the ear drum start to vibrate which makes the ear drum vibrate also.
To review: the outer ear collects sound waves and funnels them inwards, transferring the movement of the air molecules into vibrations of the ear drum.
The Middle Ear (the amplifier)
The middle is a small space inside the skull that starts and ends with a membrane. At one side is the ear drum and at the other there is a much smaller membrane called the oval window (or to continue the classical theme: fenestra ovalis).
The middle ear is an amplifier. It uses the three smallest bones in the body connected together in such a way that each acts as a lever upon the next. This process turns small movements in the 10 mm diameter ear drum into bigger movements in the 0.5mm diameter membrane of the oval window.
Without this amplification mechanism it is said that only 0.1% of the sound wave energy would reach the inner ear, but to work well the middle ear must also be air-filled. Fluid in the middle ear dampens down the movements of the bones and membranes making it much harder to hear, as any parent of a child with “glue-ear” will know.
To ensure that the middle ear remains air-filled we have a small vent. This narrow tube – about 35mm long and only 3mm wide – connects the inner ear to the back of the throat.
This structure too was first described during the Renaissance Period. This time it was another Titan of Italian anatomy, Bartolomo Eustachi, who wrote about it in his bestseller, “Anatomical Engravings”. Unfortunately for Eustachi it took over a hundred years for “Anatomical Engravings” to became a bestseller, but the payback was immortality – the small air vent he described is called the Eustachian tube.
The tiny diameter of this ventilation tube means that it is easily blocked, and this prevents the automatic equalising of air pressure on each side of the ear drum. When there is a pressure difference between one side and the other the ear drum either gets sucked in or pushed out. Not only is this very painful but the drum may stretch too far and tear, making a perforated ear drum.
The three middle ear bones that amplify the sound wave are: the hammer (Latin: malleus – think of the English word ‘mallet’), the anvil (Latin: incus) and the stirrup bone (Latin: stapes). They are all tiny but the 3mm long stapes bone is the smallest, and therefore wins the smallest-bone-in-the-human-body competition.
The handle of the hammer rests against the centre of the ear drum and picks up it’s movement, passing it to the anvil which transmits the movement on to the stirrup bone. The footplate of the stirrup bone rests against the oval window and as the bone moves it pushes the membrane of the oval window in and out like a piston in an engine.
Not all animals have this fancy arrangement of middle ear bones. Birds have only one middle ear bone, making them less able to amplify sound and therefore not as good as we humans at hearing higher pitched sounds. They can fly though, so things even out.
To review: the bones of the inner ear take the vibrations of the ear drum, amplify them using mechanical leverage and transfer them into piston-like movement in the much smaller membrane of the oval window.
The Inner Ear (the electrifier)
The oval window marks the interface between middle and inner ear and here we change from movement in air to movement in fluid. The inner ear contains a spiral shaped organ called the cochlea (Latin: cochlea / Greek: kokhlias, meaning spiral or snail shell).
You may have heard of a ‘cochlear implant’. These are electrical listening aids that essentially function as artificial ears. They have a microphone part that sits on the outside of the skin to detect sound waves and turn them into a digital signal. The signal passes to the second part of the device that is implanted underneath the skin and this is wired directly to the nerves inside the cochlea.
The helix-shaped cochlea is a about 30mm long and is divided into three chambers each of which contains fluid that is similar to the juice that normally flows around the brain and spinal cord (the fluid that is removed for examination in a ‘spinal tap’ procedure).
Each chamber in the cochlea is called a scala, (Latin for ladder), because to the Renaissance anatomists the membranes and cross-fibres within these structures were reminiscent of a ladder. (The pre-requisites for being a Renaissance anatomist were sharp eyes and a wild imagination).
The term scala in relation to matters of hearing may bring to mind Milan’s famous opera house, La Scala. It would be super cool if the name was given because it looks like or is designed in some way similar to the inner ear but sadly not. The opera house was built on the site of a former church, Santa Maria alla Scala, which was named to honour the family name of the sponsor’s wife. (For some people flowers just aren’t enough.)
The pitch of the sound wave that enters the ear sets the vibration frequency of the ear drum which is then passed on through the middle ear bones, ultimately ending with the stirrup bone driving the membrane of the oval window in and out of the cochlea at the same frequency.
This sets off a wave of motion in the fluid within the cochlea which then travel the full length of the scala chambers up to the apex of the coil.
It is thought that the cross-fibres in the ladders (scala) can detect waves of different frequencies in different sections of the cochlear spiral, and in this way nerve fibres at different positions will fire off according to the pitch of the sound coming in.
These nerve fibres from all along the cochlea then join together into a larger nerve that is rather unimaginatively called the cochlear nerve. This nerve then leaves the inner ear and travels deeper, delivering its payload of information to the auditory centres within the brain where magical things will happen.
To review, the inner ear takes the amplified vibrations of the ear bones and changes them into electrical nerve impulses in the fluid-filled cochlea before sending them to the brain for processing.
Whether each of the elaborate structures described above developed through a long series of random, fortunate accidents or whether it was by design, the precise interplay between each part really is an amazing piece of bio-engineering.
And yet, as intricate and elegant as the ears mechanisms are, what happens in the auditory centres of the brain is in a different league again, as somehow the raw data is reconstituted into something actionable, emotional and beautiful. Or screechy.
It’s really quite amazing that we can hear anything at all.
Thank you for reading. I hope that you have enjoyed this post. In the next article, we will take a look at some of the common medical conditions that interfere with the mechanics of hearing.
In the meantime, don’t forget that every month Be Well Medical Centre runs a hearing clinic with audiology specialists on site to investigate and address hearing problems. The date of the next clinic is published on our homepage www.bewell.co.th and appointments are easily made by a telephone call (02 111 6644) or email.