How Do We Hear
Hearing is one of our basic sense and forms an integral part of our sense of surroundings. Hearing is the ability of an individual to percept different sounds to which he is exposed. Before we move into understanding the direct mechanism by which we hear, it is imperative that we look into the structure of ear and the anatomical organization of its various contents.
(Image credit: http://www.md-health.com)
The ear is divided into 3 parts:
- External Ear
- Middle Ear
- Inner Ear
External Ear Middle Ear Bones
Inner ear (Notice the snail like cochlea – Left to right: Malleus, Incus, Stapes)(Image credits:http://archive.xmlprague.cz/, http://me.hawkelibrary.com/, http://greymattersjournal.com)
External Ear consists of the outer visible portion of the ear which we normally see and the ear canal which goes inwards and at the end of the canal, the tympanic membrane which is more commonly known as the ear drum. Middle ear consists of the ear bones and the middle ear cavity (air filled) and the inner ear consists of the hearing organ (the cochlea) and the vestibular system which is dedicated to balance.
Concept of Hearing
Sound waves are actually vibrations and they generate mechanical vibration within the ear which are then converted to electrical impulses and will be perceived by the brain as “sound”.
In short, the mechanism of hearing can be broadly divided into:
- Conduction of sound to ear
- Conversion of sound to mechanical energy of vibration
- Conversion of this mechanical energy into electrical impulses
- Conduction of the electrical impulses to brain
The following text allows a better elaboration of the above mentioned points.
ORGANIZATION OF EAR AND THE PATHWAYS TAKEN UP BY THE SOUND WAVES
Sound waves are generated outside the ear and they travel and strike the external ear also known as the Pinna or the Auricle. The outer visible part of the ear facilitates the sound waves by converging them towards the outer opening of the ear canal. The sound waves then travel through the ear canal which is about 2.5 cm in length and 0.7 cm in diameter. At the end of the ear canal, there is the ear drum. This thin specialized membrane is the receiver of all the sound waves transferred to it by the external ear. Sound waves strike this membrane and cause it to vibrate and these vibrations are transmitted to stapes footplate (part of ear bone in middle ear) through the chain of small ear bones (ossicles) that are present in the middle ear. Ahead of the stapes footplate is the inner ear composed of cochlea. Movements of the stapes footplate causes the fluid contained in the cochlea to move with the vibrations.
Thus sound waves outside the body travel into the ear and give rise to vibrations in the inner ear fluid.
Sensation and Perception of Sound
Inner ear fluid moves with sound energy and causes pressure changes in the inner ear which moves the basilar membrane. Present on this basilar membrane are numerous hair cells which are the actual transducers and convert the mechanical energy into electrical impulses which are then transmitted through the auditory nerve pathway (vestibulocochlear nerve) to the brain. Hair cells undergo distortion due to rocking of the cochlear fluid back and forth due to mechanical energy generated from the vibration of the sound waves. Each hair cell has neural connection at its base, this rocking of hair cells generates nerve impulses which travel to brain. Brain then tells us what kind of sound we are hearing and about its various characteristics.
Nerve pathways that tell the brain about hearing
Impulses generated at cochlea were transferred via the VIIIth cranial nerve. After passing through various nuclei, the impulses reach their final destination in the auditory cortex which is located in the temporal lobe of the brain.