How the Ear & Hearing Works

The Ear

How the Ear & Hearing Works

We describe the ear in four main parts:
1. The Outer Ear
2. The Middle Ear
3. The Inner Ear
4. Retro-cochlear that includes the eighth nerve and the brain

The following is a very simplified and general description of the path of sound energy through our auditory system that creates our unique perception of sound.

 

1. The Outer Ear

The Outer Ear consists of the Pinna or Auricle which is the part of the ear on the outside of the head. The function of the Pinna is to channel sound energy (mechanical vibration) into the ear toward the Tympanic membrane (ear drum). The pinna and ear canal are considered the outer ear.

 

2. The Middle Ear

The middle ear consists of the tympanic membrane (ear drum), the three middle ear bones; Incus, Malleus and Stapes connected in series known as the “ossicular chain.” The ossicular chain sits in the middle ear cavity an air-filled space with a pressure ideally the same as the atmosphere around us. This pressure is being equalized via a eustachian tube connecting each middle ear space to the back of the throat (naso-pharynx).

Sound vibrations set the ear drum vibrating and this mechanical energy is transferred via the ossicular chain to the oval window of the cochlea which forms the interface to the inner ear. The larger surface area of the tympanic membrane combined with the mechanical action of the ossicular chain provides a mechanical amplifying effect. There’s a protective “acoustic reflex” that protects the inner ear from hazardously loud sounds when working normally.

 

Inner Ear

The footplate of the most inner ear bone the Stapes pushes in and out of the oval window of the Cochlea setting up a hydraulic vibration in the fluids within the cochlea. The Cochlea is the tiny snail shell shaped organ of the inner ear where the hydraulic vibration is converted to neural impulses. It contains what are called “hair cells” or stereo cilia. These microscopic cells connect to approximately 24,000 nerve fibres necessary for hearing.

The vibration of fluids within the inner ear set up a whip like wave of movement on the tectorial membrane which sits on the stereo cilia. The peak of this wave corresponds to the frequency of the sound vibration. The movement of this membrane creates a shearing action on the stereo cilia which initiates them to send neural impulses via their connecting nerves. The cochlea is connected with the semi-circular canals that form the vestibular system which contributes to our perception of balance and motion.

 

Retro-Cochlear Auditory Pathway

Beyond the cochlear the nerve fibres converge into the auditory nerve which carries information to the brain stem and on to the auditory cortex of the brain. Throughout these pathways neural impulses are processed and interpreted into an understanding of what’s being heard. Change to any of these retro-cochlear functions can affect the ability to hear and interpret sound.

Inner Ear

The footplate of the most inner ear bone the Stapes pushes in and out of the oval window of the Cochlea setting up a hydraulic vibration in the fluids within the cochlea. The Cochlea is the tiny snail shell shaped organ of the inner ear where the hydraulic vibration is converted to neural impulses. It contains what are called “hair cells” or stereo cilia. These microscopic cells connect to approximately 24,000 nerve fibres necessary for hearing.

The vibration of fluids within the inner ear set up a whip like wave of movement on the tectorial membrane which sits on the stereo cilia. The peak of this wave corresponds to the frequency of the sound vibration. The movement of this membrane creates a shearing action on the stereo cilia which initiates them to send neural impulses via their connecting nerves. The cochlea is connected with the semi-circular canals that form the vestibular system which contributes to our perception of balance and motion.

 

Retro-Cochlear Auditory Pathway

Beyond the cochlear the nerve fibres converge into the auditory nerve which carries information to the brain stem and on to the auditory cortex of the brain. Throughout these pathways neural impulses are processed and interpreted into an understanding of what’s being heard. Change to any of these retro-cochlear functions can affect the ability to hear and interpret sound.

How the ear works