October 17, 2010

How The Ear Hears (Mechanism Of Hearing)

Hearing, or auditory perception, is the ability to perceive sounds by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear. Sound may be heard through solid, liquid, or gaseous matter. And there are three main components of the human auditory system: the outer ear, the middle ear, and the inner ear.

In this article:
Mechanism of hearing
USMLE exam: clinical scenerio
USMLE exam: answer
USMLE exam: explanation

Mechanism of hearing

In order for a sound to be transmitted to the central nervous system, the energy of the sound undergoes three transformations. First, the air vibrations are converted to vibrations of the tympanic membrane and ossicles of the middle ear. These in turn become vibrations in the fluid within the cochlea. Finally, the fluid vibrations set up traveling waves along the basilar membrane that stimulate the hair cells of the organ of Corti. These cells convert the sound vibrations to nerve impulses in the fibres of the cochlear nerve, which transmits them to the brainstem, from which they are relayed, after extensive processing, to the primary auditory area of the cerebral cortex, the ultimate centre of the brain for hearing. Only when the nerve impulses reach this area does the listener become aware of the sound.

Outer ear, middle ear and inner ear showing mecahnism of hearing
The mechanism of hearing. Sound waves enter the outer ear and travel through the external auditory canal until they reach the tympanic membrane, causing the membrane and the attached chain of auditory ossicles to vibrate. The motion of the stapes against the oval window sets up waves in the fluids of the cochlea, causing the basilar membrane to vibrate. This stimulates the sensory cells of the organ of Corti, atop the basilar membrane, to send nerve impulses to the brain.
Credit: Encyclopædia Britannica, Inc.

USMLE exam: clinical scenerio

A physician is explaining to his student that the process of hearing requires that amplified mechanical energy is transmitted from the middle ear to the cochlea. He claims that the process of amplification is necessary in order to amplify weak vibrations in the air before reaching the inner ear.

Which of the following factors has the greatest amplifying influence on sound?

a) the ratio of the size of the oval window to the size of the eardrum
b) the shape of the middle ear
c) the bone lever system
d) the pressure gradient between middle ear and the nasopharynx
e) the physician is wrong, amplification does not occur within the middle ear

MCQ questions & answers on medicalnotes.info

USMLE exam: answer

The correct answer is A.
The ratio of the size of the oval window to the size of the eardrum has the greatest amplifying influence on sound.

USMLE exam: explanation

The amplified mechanical energy transmitted from the middle ear to the cochlea is transferred into hydraulic pressure. This hydraulic pressure creates movement to the cochlear duct and to the organ of Corti (the seat of hearing). The oval window is the interface between middle and inner ear. The stirrup passes on the vibrations to the oval window, a small thin membrane covering an opening in the bony case of the cochlea.

The size of the window (15-30 times smaller than the eardrum), produces critical amplification needed to match the vibrations in the air and the fluid in the cochlea. The sound is amplified at the window another 15 to 30 times.

Amplification from outer to inner ear

There are three distinct areas where the incoming sound vibrations are amplified on the way to the inner ear. The reason for this is to magnify weak vibrations in air so that they can establish pressure waves in liquid.
  • The shape of the ear canal may increase pressure times 10.
  • The stirrup, anvil, and hammer bones of the middle ear may nearly triple the pressure.
  • The arrangement between the eardrum and oval window provide up to another 30 times magnification.
The results in amplification is 800 times before the sound wave sets the liquid of the inner ear into motion.

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