Physiology of Hearing

To truly understand the physiology of hearing we need to go into a bit more detail in terms of the inner ear anatomy and get to grips with the internal anatomy of the cochlea in general.

We already know that inside the membranous labyrinth there is a fluid known as the endolymph, this region is sometimes referred to as the scala media. Surrounding the scala media is the perilymph which is encased in the bony labyrinth. There are two channels filled with perilymph which are part of the bony labyrinth: the scala vestibuli, which end at the oval window, and the scala tympani which ends at the round window (below the oval window).

Cochlea cross-section

The picture above shows that:

The vestibular membrane seperates the scala vestibuli from the scala media and;

The basilar membrane seperates the scala media and the scala tympani. Resting on the basilar membrane is the organ of corti.

The organ of corti

This is a coiled sheet of epithelium which contain the receptors to hearing, the hair cells. At the tip of each hair cell is a hair bundle, made up of many stereocilia which extend into the endolymph of the scala media. The hair cells synapse with the cochlea branch of the vestibulocochlea (VIII) nerve. The tectorial membrane is a flexible gel-like substance that covers the hair cells of the organ of corti.

The Key Events Involved in Hearing

1. The Auricle

    The auricle directs sound waves into the external auditory canal

2. The Eardrum

    When soundwaves hit the eardrum, the waves cause the eardrum to vibrate.

3. The Ossicles

    The auditory ossicles move as the vibrations are transmitted across the middle ear.

4. The Oval Window

    As the stapes taps on the cochlea, its pushes the membrane of the oval window vigorously, as the vibrations are magnified onto the smaller surface area of the stapes.

5. Perilymph

    The movement of the oval window creates waves of pressure in the perilymph of the cochlea. As the oval window is pushed inwards by the stapes, it pushes on the perilymph of the scala vestibuli.

6. The Round Window

     Pressure waves are transmitted from the scala vestibuli to the scala tympani and eventually to the oval window which then bulges out into the middle ear.

7. The Endolymph

    The pressure waves deform the walls of the scala vestibuli and the scala tympani and they also push the vestibular membrane back and forth, creating pressure waves in the endolymph inside the scala media

8. The Hair Cells

    The pressure changes in the endolymph cause the basilar membrane to vibrate, which moves the hair cells against the tectorial membrane. Bending of the hair cell stereocilia produces receptor potential which lead to nerve impulses.

Check out the video below to see these events in action!


Image courtesy of WikiCommons

Video courtesy of YouTube

A Handy Analogy

An easy way to get your head around some pretty daunting concepts is to image that you've got two plates and a jelly in between!

If you move one plate (the basilar membrane) you will move the jelly (the hair cells) against the other plate (the tectorial membrane). This rubbing on the second plate causes a receptor potential.