How do our ears work?

On the 3rd June we celebrate the birthday of Georg Von Békésy, born in 1899, who won the Nobel Prize in 1961 in 'Physiology or Medicine' for his work into the functions of the inner ear.

Von Békésy trained as a chemist but it was his interest in how the ear transforms vibrations into neuronal signals that became the focus of his work and later earned him the Nobel Prize.

The pathway of hearing is complex. Our ability to hear depends on a series of sophisticated steps that change incoming sound waves in the air into electrical signals, the language that the brain understands.

The outer ear collects sound waves, it is conveniently funnel shaped directing these sound waves through a narrow passage approximately 24mm in length called our ear canal. At the end of the ear canal is the eardrum. In response to these incoming sound waves, our eardrum vibrates which in turn vibrates three tiny little bones in our middle ear, known as the ossicles - malleus, incus and stapes. The ossicles amplify the sound vibrations across their length and to the stapes which is the third of these ossicles and also the smallest bone in the human body. The stapes is connected to a membrane of similar makeup to the eardrum, called the oval window. The vibration along the ossicles pushes on the oval window and drives fluid within the cochlea.

The cochlea is the organ of hearing and is complex in it's own right. A normal cochlea is filled with fluid and there is also an elastic partition separating it into an upper and lower part. This partition is called the basilar membrane because it serves as the base on which delicate hearing structures sit. So as the middle ear ossicles vibrate, they displace the fluids in the cochlea causing a ripple effect or what Von Békésy coined, the 'travelling wave' along the basilar membrane. Sensory hair cells sit on top of the basilar membrane and as each hair cell moves up and down they bend and change in structure releasing chemicals into the cells and creating electrical signals. Each sound we hear stimulates a particular place along the length of our cochlea and each place corresponds to a particular frequency of sound. The electrical signals that are generated are sent to the hearing nerve which then carries the information to the brain. The footprint of each sound we hear is stored in our memory bank ready to create responses both physical and emotional.

When we think of this impressive, sophisticated and complex journey it makes it easier to understand why we don't simply hear with our ears Instead, our ears collect and transfer sound across it's pathway. They translate the incoming sound information into a language that the brain understands. And it is the brain that interprets the sound and enables us to 'hear'.

Von Békésys work on the 'travelling wave' was a breakthrough. He built models to demonstrate how this delicate inner ear organ converts incoming sound waves into electrical impulses. He discovered that sensory hair cells respond to different frequencies as mentioned above, a phenomenon known as "place coding". He painstakingly dissected structures that make up our inner ears and was able to study them using microscopes and sequences of photographs. He also measured the variations in electrical charges in these sensory cell receptors. His research shed new light on our understanding of how the cochlea worked. A tiny pea-sized, snail shaped organ that could convert vibration into neural impulses.

Many of the mechanical models of the inner ear that Von Békésy built are now held at the Collection of Historical Scientific Instruments.

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