About Hearing

HOW WE HEAR

Hearing is one of the five important senses and the ear is the organ of hearing. The ear performs two sensory functions, hearing and maintenance of body balance. Anatomically ear can be divided into three main sections namely, OUTER EAR, MIDDLE EAR AND INNER EAR. Sound passes through all the three of the ear before it goes to the brain. The brain interprets the sound and tells us what we are hearing.

OUTER EAR: The outer ear consists of pinna (AURICLE) and the ear canal( also called external auditory meatus).
Pinna is the visible part of the ear that protrudes from the sides of the head. It made of elastic cartilage. The purpose of pinna is to direct sound waves into the ear canal. The sound travels through the S shaped ear canal to the ear drum (tympanic membrane).

MIDDLE EAR: Middle ear is a small air filled 2cc cavity consisting of three middle ear bones and muscles. The eardrum (tympanic membrane) separates the outer ear from the middle ear.
There are three bones in the middle ear connects the tympanic membrane to the inner ear. The three ossicles are the
* MALLEUS
* INCUS
* STAPES
The stapes is the smallest bone in the human body. The footplate of stapes fits into the oval window of the inner ear.
The two muscles in the middle ear contracts in the presence of loud sounds and protects the inner ear from damage due to loud sounds.

INNER EAR: The inner ear is in the temporal bone of the skull and contains the organs of both hearing and balance. The balancing mechanism is composed of vestibular system (semicircular canals, utricle and saccule) and superior portion of 8th cranial nerve. The hearing mechanism is composed of the cochlea and VIIIth cranial nerve. Cochlea is a snail shaped organ which consists of fluids separated by membranes and haircells which is responsible for conversion of sound energy into electrical impulses which is picked up by the hearing nerve.

The outer ear receives sound waves and directs them to the ear drum. The ear drum vibrates in response to the sound waves and these vibrations are transmitted through the middle ear bones to the oval window of cochlea. The vibrations are passed through the oval window on to the fluid of the cochlea, where they generate waves in the fluids. These waves induce a ripple in the membrane causes movements (shearing action) of the hair cells. As a result, nerve impulses are generated in the 30,000 associated nerve cells. These impulses are transmitted by the fibers of the auditory nerves via the central auditory pathway to the auditory area in brain. The impulses are analyzed and the sound is recognized and understood.