Structure of Human Ear
Human ear is divided into three parts;
- outer ear
- middle ear
- inner ear
Outer ear consists of pinna and auditory canal.
Auditory canal is a slightly ‘‘S’’ shaped tube and lined by hairy skin with numerous modified sweat glands which secrete ear wax.
Auditory canal extends to the tympanic membrane (ear drum) which is located in between the middle and the outer ear.
Middle ear (tympanic cavity) is an air-filled cavity within the temporal bone.
It is lined by simple epithelium.In the medial wall of the middle ear, there are two openings called oval window and round window.
Oval window is covered by a small bone called stapes.
Round window is covered by a fine fibrous tissue.
Three very small bones (ear ossicles) called malleus, incus and stapes extend across the middle ear from tympanic membrane to the oval window.
They form movable joints with each other and the medial wall of the cavity at the oval window.
Malleus is in contact with the tympanic membrane and form a movable joint with the incus.
Incus articulates with the stapes which fits with the oval window.
A long tube called Eustachian tube connects the middle ear to the pharynx.
Inner ear is formed from a network of channels and cavities in temporal bone which are called bony labyrinth.
Within the bony labyrinth, a network of fluid filled membranes called membranous labyrinth is present which lines and fills the bony labyrinth.
Inner ear is composed of three main regions:
Vestibule is the expanded part near the middle ear.
Oval and round windows are present in its lateral walls.
Vestibule contains two membranous sacs called utricle and saccule.
- Vestibule
- three semicircular canals
- cochlea
Vestibule is the expanded part near the middle ear.
Oval and round windows are present in its lateral walls.
Vestibule contains two membranous sacs called utricle and saccule.
Semicircular canals are three tubes arranged at right angles to one another so that one is situated in each of the three planes of space.
They are continuous with the vestibule.
Cochlea is a coiled structure with the broad base which is continuous with the vestibule.
Cochlea has three compartments:
- an upper vestibular canala
- lower tympanic canal
- middle cochlear duct
Middle cochlear duct which is a small canal that separates the upper and lower canals.
Vestibular canal originates at the oval window and tympanic canal ends at the round window.
The two canals are continuous with each other and filled with perilymph.
The cochlear duct is a part of the membranous labyrinth and filled with endolymph.
The floor of the cochlear duct is called the basilar membrane which bears the organ of Corti (spiral organ).
It contains supporting cells and specialized cochlear hair cells containing mechanoreceptors (auditory receptors) of the ear.
Hairs of the cochlear hair cells project into the cochlear duct.
Many hairs are attached to the tectorial membrane that hangs over the organ of Corti.
Auditory receptors are dendrites of sensory nerves that combine to form the auditory nerve to the brain.
Functions of the human ear
Hearing
Vibrating objects produce pressure waves in the surrounding air.
In hearing, the ear transduces these pressure waves (mechanical stimulus) into nerve impulses that are transmitted to the brain which perceives as sound.
The outer ear collects and concentrates the sound waves and directs them along the auditory canal towards the tympanic membrane.
This causes the tympanic membrane to vibrate.
Tympanic membrane vibrations are transmitted and amplified through the middle ear by the movement of three jointed ear ossicles.
The ear ossicles transmit the vibrations to the oval window which is located on the membrane of the cochlear surface.
When the stapes vibrates against the oval window, pressure waves are created in the perilymph inside the cochlea.
When the fluid pressure waves enter the vestibular canal, they push down on the cochlea duct and the basilar membrane.
As a result, the basilar membrane and attached hair cells vibrate up and down.This causes bending of hair projecting from the hair cell against the fixed tectorial membrane which lies above the hair cells.
This results in the stimulation of auditory receptors in the auditory hair cells and generation of nerve impulses.
These nerve impulses are passed to the auditory area of the brain (temporal lobe of the cerebrum) for sound perception.
After the sound perception, the fluid wave is finally dissipated into the middle ear by vibration of the membrane of the round window.
Eustachian tube maintains the air pressure on both sides of tympanic membrane at the atmospheric pressure level.
Equilibrium
Semicircular canals and vestibule located in the inner ear - provide information about the position of the head in space and contribute to maintain the posture and balance.The semicircular canals arranged in three spatial planes detect angular movements of the head.
Within each canal, hair cells form a cluster with the hairs projecting into a gelatinous cap.
Changes in the position of the head causes movements in the perilymph and endolymph.
As a result, hair cells are stimulated and resulting nerve impulses are transmitted to the brain.
Utricle and saccule of the vestibule - perceive position with respect to gravity or linear movements.
Each of this perilymph filled chambers contain hair cells that project into a gelatinous material in which small calcium carbonate particles (otolith) are embedded.When the head is tilted otoliths press on the hairs projecting into the gels. Hair cell receptors transform this deflection into an electrical signal and pass into cerebellum.