How do we hear?

 

We all remember the basics taught to us in school, the process of sound entering into our ears, hitting our ear drums and sending the vibrations to the brain for it to determine the hidden message within and send it back to us in a language we can understand.

When asked to point at the ear we point to the mass of tissue affixed to the side of our heads and relish in knowing that we are aware as to the form and simplicity of our anatomy, that is until we dig a little deeper.

Cotton buds at the ready!

Many are aware that the ear extends past the ear we can see and it is formed of ear, canal and drum and whilst that is correct, the ear does so much more than that.

For now, lets start with the basics;

The part that is visible on the outside of our heads is actually one part of three that make up the ear and it is called the Pinna.

The part that is not as visible; commonly referred to as the ‘ear canal’ is actually the External Auditory Meatus.

And the drum is the Tympanic Membrane commonly abbreviated to TM.

The Pinna

Ever wondered why your ears look like they do?

The twists and folds of the pinna enhance high frequency sounds and also help us to determine the direction of the sound source. Sounds coming from the front and sides are slightly enhanced as they are directed into the ear canal while sounds from behind are slightly reduced.

This helps us to hear what we are looking at while reducing some of the distracting background noise coming from behind.

This is why cupping your hands behind your pinna provides additional amplification when listening to sounds coming from in front of you as it enlarges the collection surface area of the pinna.

Digging a little deeper

Connecting to the pinna is the tympanic membrane.

Whilst its interesting in terms of size and use, I find the cranial nerve branch hidden within a little more intriguing.

It passes just below the back canal wall surface and if the canal near it is touched, most will react by coughing, triggering a reaction in the network that is our body; much like static on a radio.

This area is also the source of ear wax or ‘Cerumen’ which is produced through glands found in the outer half of the Tympanic Membrane.

And lastly for the TM, as we’re in the business of sound it is useful to note that the ear canal has a natural resonance that can boost sound pitches around 3000 HERTZ (Hz.) by around 10-15 deciBels.

The Middle Ear

You know the old saying of having air between your ears?

Well, the next time you’re hit with that witty insult, you can fire back with the knowledge that there is in fact an air filled space between all of our ears. It’s found between the TM and the inner and contains three tiny bones (Ossicles) linked together that connect the TM to the opening in the inner ear.

The TM is a concave shaped layer of membrane at the end of the ear canal. Sounds travel down the ear canal and strike the Tympanic Membrane causing it to vibrate. These vibrations are then transferred though the Ossicles to the inner ear (Cochlea)

The first bone is named ‘Malleus’ (hammer) and is attached to the inside surface of the TM. The second bone is the ‘Incus’ ((anvil) anyone noticing a theme here?) and the innermost bone is the ‘Stapes’ (stirrup).

Sounds sets this hole structure into vibration and the footplate of the stapes vibrates within the oval window opening of the cochlea, transferring sound energy to the fluids and tissues of the inner ear.

The Eustachian Tube and the Pressure Pop

There is a small tube that connects the middle ear space to the back of the throat known as the Eustachian Tube. The tube, normally closed is opened momentarily when we yawn, swallow etc.

This periodic opening maintains equalization of the air pressure between the middle ear and the outside air pressure.

If it does not equalize, which is a common occurrence during rapid altitude change i.e – flying & diving, the sudden opening of the tube produces a ‘pop’ and improved hearing because pressure balance is restored.

The Inner Ear

The inner ear actually comes in two parts. The first; ‘Vestibular’ or balance part and the ‘Cochlea’, which is the hearing part.

Both connected and both equally vital in the process.

The Vestibular portion is to help us sense acceleration/ deceleration of both rotational and linear motion and to sense head position in relation to gravity.

Equally important, it is part of a reflex arc that makes it possible for us to maintain sharp visual focus with the many small and rapid motions of the head that occur as we engage in walking, riding, chewing etc.

The Cochlea is a coiled canal in the dense bone tissue of the skull.

This canal houses three fluid filled membranous canals. The central of which houses the Organ of Corti which is comprised of specialised cells and their supporting tissues.

Vibratory energy propagated through the fluid produces deformation of the organ of Corti, in turn resulting in shearing forces on tiny tufts of hairs.

This shearing action triggers an electro – chemical signal that travels upward through the auditory nervous pathway which passes through the internal auditory canal to the brainstem.

Once its reached its destination it is passed upwards towards the auditory processing centres in the temporal lobes of the brain and this, in its incredibly simplistic form is the process of ‘hearing.’

 

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