Biophysics of the auditory organ

Introduction

 * Audition is the sense of hearing in which a stimulus is present. This stimulus is a pressurized sound wave that goes through the air towards the head and the first thing they hit is the outer visible part of the ear called the pinna (auricle). Hair cells are the receptors which are located inside the ear enabling us to hear.


 * http://www.biographixmedia.com/human/ear-anatomy.jpg

How is the stimulus perceived inside the ear?

 * 1) The Pinna (auricle) receives a sound wave which travels through the external auditory canal to the tympanic membrane (eardrum).
 * 2) The tympanic membrane starts to oscillate in rhythm with the sound wave. The central part of the eardrum is connected to a small bone of the middle ear called the malleus.
 * 3) As the malleus vibrates, it transmits the sound vibrations to the other ossicles of the middle ear, the incus and the stapes.
 * 4) The tympanic-membrane oscillations are then transmitted via ossicles (malleus or hammer, incus or anvil, and stapes or stirrups) – to another elastic pellicle and oval window, into the inner ear filled with liquid – the cochlea.

How does fluid behave inside the ear?

 * 1) The stapes is attached to the elliptical (oval) window. As the stapes rocks back and forth against it, the oval window also starts to vibrate back and forth and is attached to a round snail-like structure called the cochlea that contains the receptor organ for hearing. This organ is called the spinal organ of Corti.
 * 2) Inside the cochlea, there is a fluid and the oval window gets pushed inside and outside the cochlea by the stapes; it actually pushes the fluid so it causes the fluid to be pushed towards the cochlea and goes all around the cochlea till it reaches tip of cochlea.
 * 3) When it reaches tip of cochlea, the fluid moves back to the circular (round) window and causes round window to get pushed out.
 * 4) The fluid keeps moving toward cochlea and back out and forth until the energy of the sound wave causes the fluid to stop moving as the energy is dissipated.

Receptor’s reaction to stimulus

 * Hair cells in the cochlea move back and forth and translate the fluid vibrations of sounds from its surrounding ducts into electrical impulses that are connected to the brain by sensory nerves. These electrical impulses are interpreted as sound.

Conclusion

 * Sound waves in the air are directed by the outer ear into the ear drum causing it to vibrate very slightly. The three bones of the middle ear (the smallest bones in the human body) work together as a lever system to amplify the vibrations as they pass them along to a smaller vibrating membrane on the surface of the cochlea of the inner ear. This vibration is passed on by the fluid filled spiral tube of the cochlea to tiny hair cells along the inside. As the hairs move, nerve cells at their base change this motion into electrical signals that are passed on to the brain. The brain in turn interprets those signals as sound.