Anatomy and Physiology of the Ear

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The external ear collects sound pressure waves and funnels them toward the tympanic membrane.
The middle ear ossicles transmit the sound waves to the inner ear (cochlea).

The middle ear acts to match the impedance difference between the air of the external environment to the fluid within the cochlea.
This permits maximal sound transmission.
The outer ear includes the portion of the ear that we see—the pinna/auricle and the ear canal.
The pinna or auricle is a concave cartilaginous structure, which collects and directs sound waves traveling in air into the ear canal or external auditory meatus.
The ear canal is 1.25 inches long and .25 inch in diameter.
The inner two-thirds of the ear canal is imbedded in the temporal bone. The outer one-third of the canal is cartilage. the canal forms an elongated "s" shape curve. The ear canal directs airborne sound waves towards the tympanic membrane .
The ear canal maintains the proper conditions of temperature and humidity necessary to preserve the elasticity of the tympanic membrane. Glands, which produce cerumen (earwax) and tiny hairs in the ear canal, provide added protection against insects and foreign particles from damaging the tympanic membrane.
The tympanic membrane or eardrum serves as a divider between the outer ear and the middle ear structures.
It is gray-pink in color when healthy and consists of three very thin layers of living tissue.
The eardrum transmits the airborne vibrations from the outer to the middle ear and also assists in the protection of the delicate structures of the middle ear cavity and inner
The middle ear cavity is located in the mastoid process of the temporal bone. The middle ear cavity extends from the tympanic membrane to the inner ear.
The eustachian tube acts as an air pressure equalizer and ventilates the middle ear. Normally the tube is closed but opens while chewing or swallowing. When the eustachian tube opens, the air pressure between the outer and middle ear is equalized. The transmission of sound through the eardrum is optimal when the air pressure is equalized between the outer and middle ear
The middle ear is connected and transmits sound to the inner ear via the ossicular chain. The ossicular chain amplifies a signal as it transfers signals from the tympanic membrane to the inner ear.
The ossicular chain consists of the three smallest bones in the body: the malleus, incus, and stapes. The malleus is attached to the tympanic membrane. The footplate of the stapes inserts into the oval window of the inner ear. The incus is between the malleus and the stapes.
Attached to the ossicular chain are two tiny muscles, the stapedius and tensor tympani muscles. These muscles contract to protect the inner ear by reducing the intensity of sound transmission to the inner ear from external sounds
The inner ear is composed of the sensory organ for hearing—the cochlea, as well as for balance—the vestibular system. The systems are separate, yet both are encased in the same bony capsule and share the same fluid systems.
The balance part of the ear is referred to as the vestibular apparatus. It is composed, in part, of three semicircular canals located within the inner ear. The vestibular system helps to maintain balance, regardless of head position or gravity, in conjunction with eye movement and somatosensory input. The semicircular canals are innervated by the VIIIth cranial nerve.
Cochlea
The hearing part of the inner ear is the cochlea
The cochlea is composed of three fluid-filled chambers that extend the length of the structure. The two outer chambers are filled with a fluid called perilymph. Perilymph acts as a cushioning agent for the delicate structures that occupy the center chamber. It is important to note that perilymph is connected to the cerebrospinal fluid that surrounds the brain and the spinal column. The third fluid filled chamber is the center chamber, called the cochlear duct. The cochlear duct secretes a fluid called endolymph, which fills this chamber.
The cochlear duct contains the Basilar membrane upon which lies the Organ of Corti. The Organ of Corti is a sensory organ essential to hearing. It consists of approximately 30,000 finger-like projections of cilia that are arranged in rows. These cilia are referred to as hair cells. Each hair cell is connected to a nerve fiber that relays various impulses to the cochlear branch of the VIIIth cranial nerve or auditory nerve. The "pitch" of the impulse relayed is dependent upon which areas of the basilar membrane, and hence, which portions of the Organ of Corti are stimulated. The apical portion of the basilar membrane (the most curled area of the cochlea) transfers lower frequency impulses.
The basal end relays higher frequency impulses.
The VIII cranial nerve carries the impulses generated from the Organ of Corti to the brainstem.

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